Advanced Polymer Composites

MATERIALS FORNUCLEAR SYSTEMS

KEYWORDS: spent nuclear fuel,disposal containers, polymercomposites

ADVANCED POLYMER COMPOSITESFOR THE FABRICATION OF SPENTNUCLEAR FUEL DISPOSALCONTAINERSHUGUES W. BONIN,* IAN MIEDEMA, and VAN TAM BUIRoyal Military College of Canada, Department of Chemistry and Chemical EngineeringP.O. Box 17000, Station FORCES, Kingston, Ontario K7K 7B4, Canada

Received July 11, 2007Accepted for Publication February 27, 2008

In Canada, the spent nuclear fuel disposal methodproposed is to permanently isolate the spent fuel in deepunderground vaults carved in stable granite rock forma-tions within the Canadian Shield, with the integrity of theisolation to be assured for a minimum period of 500 yr.The present work aims at determining the feasibility ofusing a consolidated composite material made of an ad-vanced polymer called PEEK (Poly Ether Ether Ketone)and continuous graphite fiber to fabricate a containerdesigned to isolate the spent nuclear fuel from the bio-sphere for such very long time periods. The researchfocused on submitting the PEEK-based composite mate-rial to a thermal and radioactive environment compara-ble to, and, in some aspects, more aggressive than, theconditions of exposure in the disposal vault. The changesto the physical, mechanical, and chemical properties ofthe material following prolonged exposure were then de-

termined. The simulation of the environment was achievedby irradiating numerous test specimens in a mixed radi-ation field produced by a SLOWPOKE-2 nuclear re-search reactor at controlled ambient temperatures rangingfrom;20 to 758C. The specimens were characterized viaseveral methods: tensile and flexural testing, differentialscanning calorimetry, scanning electron microscopy, andwide-angle X-ray scattering. The results confirmed thatthe PEEK-based composite material was resistant to ex-posure to high radiation doses (1 MGy), at temperaturesbetween;20 and 758C. The mechanical and other prop-erties were barely affected, with values rarely exceeding1s of the properties of nonirradiated samples, suggest-ing that the PEEK–graphite fiber composite material canindeed be considered as a very good candidate for thisdemanding application.

I. INTRODUCTION

The issue of the ultimate disposal of spent nuclearfuel and high-level radioactive waste is of paramountimportance for the success of nuclear energy as a majorreliable, safe, and environmentally benign source of elec-trical power. Since some of the radioisotopes present inthe spent nuclear fuel need several centuries for theirradioactivity to decay back to background levels, theproblem consists of not only protecting the present gen-eration of humans and living beings from the hazardscaused by the radiation but also isolating the radioactive

waste from future generations for the following severalcenturies.

The fuel element ~called fuel bundle! currently usedin Canada deuterium uranium ~CANDU! nuclear reac-tors consists of 37 natural uranium fuel rods grouped andwelded together to end plates. In the Canadian approachfor the spent fuel disposal, the fuel bundles remain intactas no reprocessing operations are carried out on them.They would then be simply stored in specially designedcontainers ~Fig. 1!, the current design proposed byAtomicEnergy of Canada Limited1,2 ~AECL! foreseeing a ca-pacity of 72 bundles per container, which would be madeof copper, although titanium alloys have been consid-ered. The containers in turn are to be inserted in cavitiescarved in the floor and0or sides of galleries dug as an*E-mail: [email protected]

286 NUCLEAR TECHNOLOGY VOL. 164 NOV. 2008

extensive network at a depth of some 500 to 1000 m ingranite rock formations within the Canadian Shield geo-logic province. The selected rock formations are knownas “plutons” and are huge monoliths that have “survived”intact the numerous geologic upheavals that have oc-curred during the last several billions of years.

The deep underground approach for the spent nu-clear fuel disposal, as proposed in Canada, is based onthe assumption that no matter the barrier, given enoughtime, it is bound to fail. The design proposed by AECL ispresented in Table I and Fig. 2. Since the time frameconsidered here is of the order of several centuries, theengineering effort aims at ensuring that when all thebarriers have failed, the escaping radioisotopes wouldhave decayed back to natural background levels by thetime they manage to physically reach the biosphere.Each one of the barriers has a probability of failure thatvaries with time. The event consisting of the radio-isotopes reaching the biosphere can occur only when allthe barriers have failed simultaneously and the overallprobability for this is equal to the product of the individ-ual probabilities. Since the barriers are designed suchthat their probabilities of failure are much smaller thanone, an extremely low probability of occurrence forthe event is thus obtained. The approach proposed byAECL was recommended by the Nuclear Waste Manage-ment Organization ~NWMO! in its final study report,3

except that the NWMO recommended that a provisionFig. 1. Spent CANDU fuel disposal container as proposed by

AECL ~Ref. 2!.

TABLE I

Multibarrier System*

Nature of Barrier Main Function~s!

Engineered ~Designed! Barriers

Uranium dioxide fuel Traps most of radioisotopesZircaloy sheath Contains the spent fuel radioisotopes; impedes or delays contact to groundwaterFiller material ~glass beads!a Provides heat transfer mediumContainer walls Delays access to groundwater; contains radioisotopesClay buffer Delays access to groundwater and migration of radioisotopesBackfill Delays migration of radioisotopes

The Geosphere ~Selected!

Plutonic rock Impedes or delays migration of radioisotopesNatural aquifers May disperse the radioisotopes

The Surface Environment ~Biosphere! ~Out of Human Control!

Surface water, plants, animals, air Disperses and dilutes the radioisotopes

*Reference 2.aInformation above from Ref. 3. In the present work, thorium dioxide is proposed to replace glass beads as filler material in thecontainer. In addition to improving the heat transfer, ThO2 would serve as an excellent gamma radiation absorber to protect thecontainer walls.

Bonin et al. POLYMER COMPOSITES USED FOR SPENT NUCLEAR FUEL CASKS

NUCLEAR TECHNOLOGY VOL. 164 NOV. 2008 287

for eventual retrieval of the spent nuclear fuel be in-cluded in the disposal concept. On June 14, 2007, theMinister of Natural Resources of Canada announced thatthe Government has accepted the approach recom-mended by NWMO ~Refs. 4 and 5!.

II. CONTEXT AND GOALS OF THE RESEARCH

Providing a solution to the issue of the permanentsafe disposal of the spent nuclear fuel and radioactivewaste will certainly help in improving the public accep-tance of nuclear energy since this issue is one of the mainarguments used by nuclear activists opposing the devel-opment of nuclear energy. The present work aims at pro-viding a valuable contribution to the field of spent nuclearfuel management by focusing on the container housingthe spent nuclear fuel bundles. The container representsone of the multiple barriers mentioned above. The reader

must be aware that the present work is much more than asimple research on materials intended to determine howsome materials behave after having been irradiated underspecific conditions. The goal is to demonstrate that apolymer-based composite material can effectively be usedinstead of metals to fabricate the containers for the spentnuclear fuel disposal. The intent of the work is thereforefocused on the application rather than on the materialitself. The paper thus uses an extensive set of experimen-tal data obtained and presents them in support of theapplicability of the Poly Ether Ether Ketone ~PEEK! andcontinuous graphite fiber ~PEEK-GF! composite mate-rial to the container fabrication. In such a context, thelength of the paper is thus well justified.

The container must be fabricated with materials dis-playing exceptional performance such as high mechani-cal strength, resistance to radiation, and excellent chemicalresistance. Because of the large number of containersrequired over the years, consideration must be given tothe production costs, the environmental impact of theproduction, the availability of the materials, and the fab-rication technology as well as transportation issues. Thecontainers must keep their integrity under adverse con-ditions for at least 500 yr ~Ref. 2!. Some of the bestmaterials meeting these requirements are a few metalssuch as copper and titanium, certain alloys based on thesemetals, and stainless steel. However, the main concernwith all metals is corrosion. Even if several metals andalloys display very good resistance to corrosion for up tomodest time ranges, it is not well known how these ma-terials can withstand several centuries of exposure toaggressive environments such as found in the deep un-derground vaults in contact with alkaline or acidic ground-water while being bombarded by the radiation emitted bythe spent nuclear fuel. Another factor present at the dis-posal site is the relatively elevated temperatures. Thespent nuclear fuel continues to generate heat long afterremoval from the nuclear reactor core because of theenergy released by residual fissions ~spontaneous andstimulated! and by the radioactive decay processes of thefission products. Even after extended dwelling periods inthe spent fuel bays, the spent fuel bundles can still gen-erate enough heat to bring the temperature of the con-tainer walls close to 1008C. In addition, the mechanicalforces due to the hydrostatic pressure of the pluton’sgranite at such depths must be accounted for.

In its proposal,2 AECL suggests the use of the metalsand alloys mentioned above to fabricate the disposal con-tainers. These choices are based on the knowledge thatcopper artifacts have been found in a relatively goodstate of preservation in archaeological sites several cen-turies, millennia in some cases, after their fabrication. Inthe case of titanium-based alloys, they are known to dis-play good resistance to aggressive corrosive agents, butthe knowledge acquired through experimentation is onlyrelatively recent, and doubts persist on their long-rangeresistance to corrosion.

Fig. 2. Multibarrier system: Vault.



The present research therefore aims at providing soundanswers to the following questions. Are there alternativematerials to the metals for the fabrication of the spentnuclear fuel disposal containers? If yes, then what are themost promising materials? How does one know that thesematerials indeed meet the requirements for such a de-manding application? Since decisions must be made onthe selection of the materials for the fabrication of thedisposal containers within the next decade or so, exper-imental work must be carried out over a realistic span oftime and certainly much less than the required 500 yr inorder to ascertain the perfect integrity of the container.Therefore, the present research aims at providing an ac-ceptable answer to the following question: How does onedesign an experimental procedure for the simulation ofthe exposure over a minimum of 500 yr of the containermaterial to the aggressive environment present at thedisposal site deep underground?

Previous studies6–8 have shown that PEEK and PEEK-based composites display excellent radiation stability.No research has been previously reported on the com-bined effects of irradiation and elevated temperatures onPEEK composites. High temperatures enhance the mo-bility of the polymer chains and radiation-induced chem-ical radicals, affecting either the rate of chain scission oreven the cross-linking rate by freer movement of thepolymer backbones, thus affecting the physical, mechan-ical, and chemical properties of the irradiated material.In the present study, mechanical, chemical, and thermalproperties of PEEK-based composites are investigatedafter exposing the composite material to a mixed radia-tion field at temperatures ranging from ;20 to 758C.From the information collected experimentally using avariety of analytical methods and tests, the research workthus aims at determining whether or not a PEEK-basedcomposite represents a suitable alternative to metals andmetallic alloys for the fabrication of spent nuclear fuelcontainers and, in the case of a positive conclusion, towhich extent this composite may be even superior forsuch an application.

III. PEEK AND COMPOSITE MATERIALS

The composite material investigated here is based onPEEK, also known as polyaryletheretherketone, in con-junction with continuous graphite fiber as reinforcingmaterial ~PEEK-GF!. The polymer was first synthesizedin the early 1980s, but because of its costs, it has not yetbeen used in many applications. Its chemical and me-chanical properties are outstanding and well comparableto those of metals and alloys for the mechanical proper-ties while the chemical resistance is much superior to thatof the metals. While metals and metallic alloys undergocorrosion-type chemical degradation, by their nature, poly-mers do not experience corrosion. However, other types

of chemical aggression may lead to the degradation ofthe material, but PEEK has proven itself to have a veryhigh resistance to these chemical reactions.9

Another factor affecting polymers and metals is theeffect of ionizing radiation such as gamma rays, high-energy electrons, and neutrons. Since the disposal con-tainer has to isolate the spent nuclear fuel for extendedperiods of time, the container walls have to accumulatesignificant doses of radiation. Because of their crystal-line nature, metals display high resistance to the effectsof radiation, while most polymers are much less resis-tant. However, PEEK and composite materials based onthis polymer have proven themselves to be little affectedby radiation because of their chemical structure based onmultiple and unusually stable benzene-like structure anddelocalized pi electrons.10 Figure 3 shows the basic struc-ture of PEEK. The regularity and linearity of the merstructure and the lack of bulky groups are to be noted.The backbone of this polymer is especially rigid becauseof the dominant phenyl groups. The side groups and theregularity of the polymer provide it with a semicrystal-line morphology.11 Polymers rarely completely crystal-lize, and most end up after curing with regions having acrystallized structure ~called crystallites! embedded in anamorphous matrix. Like their mineral and metallic coun-terparts, the crystallites have their molecules regularlyarranged as repetitive cells, but in the case of polymers,these cells grow in an axial direction mostly and rarely inthe other directions at the same time.

This semicrystalline structure of PEEK, coupled withthe presence of dominating phenyl functional groups alongthe polymeric chain, provides PEEK with exceptionalresistance to radiation. The energy of the radiation is thusdissipated within the crystallites through a phenomenonknown as the “cage effect”12 and within the many aro-matic structures of the phenyl groups, thus avoiding dam-aging the structure and the bonds of the molecules. It isthus the weakest bonds ~C-H and C-O! that are targetedgenerally by the ionizing radiation, leaving the aromaticrings and the ketone side groups essentially unaffected.In addition to the aromatic structures providing a multi-ple resonance effect, the presence of the delocalized pielectrons contributes to spreading the effects of the de-posited energy throughout the whole structure.10 Delo-calized pi electrons are electrons that are not solely withina given chemical bond. In benzene, the pi electrons con-stitute two identical electron “clouds” above and below

Fig. 3. Chemical structure of poly~oxy-1,4-phenyleneoxy-1,4-phenylnecarbonyl-1,4-phenylene!.



the plane of the aromatic ring. The delocalization effec-tively reduces the repulsive forces between the electronsand lowers the energy of the molecule thus making theindividual bonds more difficult to ionize and enhancingthe resistance of the molecule to ionizing radiation.

A major concern, especially for polymer-based ma-terials, is that of creep. This is a phenomenon commonfor most polymers submitted to high temperatures. It isknown13 that creep does not become an important factorin a polymer for temperatures below one-third of themelting temperature. In the case of PEEK, the meltingtemperature is 3438C, and therefore, creep is not ex-pected to be important below one-third of this value, i.e.,1148C. The maximum value for the temperature of thecontainer walls is predicted to be below 1008C, clearlybelow the critical value above which creep would repre-sent a problem. The creep rate is therefore not retainedamong the properties investigated in this study.

IV. EFFECTS OF IONIZING RADIATION ON POLYMERS

The types of ionizing radiation making the mixedradiation field to which the composite material is sub-jected in this work are the fast and thermal neutrons,X-rays and gamma photons, and charged particles ~pro-tons and electrons!. Having no electrical charge, neu-trons are not technically ionizing radiation, but they knockout other particles such as protons, which in turn travel athigh velocities as positive ions thus becoming them-selves highly effective ionizing agents. Charged particlestraveling at high velocities within some material stripelectrons from the atoms encountered, thereby ionizingthem. The photons ~X-rays and gamma rays! also ionizethe atoms with which they interact but through differentmechanisms: Compton and photoelectric effects, and pairproduction.

While this is true in any kind of material, the overalleffects differ according to the type of material. In poly-mers and polymer-based composite materials, the cre-ation of ions triggers series of chemical reactions as theions themselves often become chemically active radi-cals. In these materials, the overall effects from the ra-diation are the occurrence of cross-linking and chainscission chemical reactions. Cross-linking is the recom-bination of adjacent polymer chains with bonds that linkthem to one another, resulting in higher-weight mol-ecules and higher mechanical resistance. Quite differ-ently, chain scission results in the division of long chainsinto smaller components when weaker bonds are brokeneither directly by the incident radiation or, more fre-quently, indirectly as a result of chemical attack by thefree-moving radicals produced by the interaction withradiation. In this case, the mechanical strength of thematerial is lessened significantly. Depending on the am-bient conditions, one type of reaction is predominant.

These ambient conditions include the molecular struc-ture, the crystallography, the reactive species created as aresult of the interaction with the incident radiation, thefreedom of movement of these species, the temperature,the ambient pH, and the accumulated radiation dose. Thiswould determine whether the overall effects of the radi-ation will strengthen or weaken the material and willmodify its physical and chemical properties in a favor-able or baneful manner.

Many studies have been carried out on the effects ofradiation on PEEK and PEEK composites, with a focuson space applications since these materials are promisingcandidates for the fabrication of space vehicle compo-nents and would be exposed to demanding environmentsin which the bombardment by cosmic rays and high ra-diation fields in the Earth’s magnetosphere would be animportant component. Yoda14,15 exposed these materialsto high-energy electron beams, while Hegazy et al.16,17

used both electrons and gamma radiation. In both stud-ies, radiation effects tended to occur preferably in theamorphous region of the polymer rather than within thecrystalline region, mostly evidenced by the cross-linkingoccurrence. A more recent investigation by Pagé6,8 fo-cussed on the radiation-induced effects on PEEK whensubjected to a mixed field of radiation produced by asmall nuclear reactor. Both the material’s morphologyand viscoelasticity were studied in addition to other prop-erties. Again, both cross-linking and chain scission wereevidenced, with chain scission being the dominant factorin the polymer’s degradation, with the interface betweenthe crystalline and amorphous regions being the mostaffected regions. When compared to the effects of elec-tron radiation alone, the mixed field of radiation pro-duced a more rapid degradation of the PEEK.

Sasuga et al.18–22 have also done several investiga-tions on the radiolysis of PEEK, using gamma, electron,or ion radiation in various environments. Tensile testingand differential scanning calorimetry ~DSC! enabled themto assess the extent of modifications brought in the ma-terial by the radiation, observing the simultaneous occur-rence of cross-linking and chain scission as the dosesaccumulate. The room temperature mechanical proper-ties, as well as the high-temperature properties, wereonly very slightly affected by the radiation doses up to180 MGy, with only a slight increase in mechanical prop-erties due to cross-linking at the higher temperatures. Asfor the carbon fibers, Jones and Peggs23 studied the ef-fects of neutrons on the carbon fibers. Their investigationwas conducted over a range of fluence and temperaturefrom 4 � 1017 n cm�2 at 4208C to 5.4 � 1020 n cm�2

at 6658C. It was found that both the fracture strength andthe elastic modulus tended to increase with larger radia-tion doses. These results were similar to those from testsdone at much lower temperatures ~,808C! under similarfluences. The density and the crystallite size were alsoobserved to increase. This was attributed to a recrystal-lization process during the irradiation.



V. METHODOLOGY

The PEEK-GF composite used in this work was ob-tained from Applied Fiber Systems, Clearwater, Florida,under the trade name of Towflex�. It consists of wovencontinuous graphite fiber in a 2 � 2 twill using a matrixof Victrex Grade 150 PEEK. The composite was pro-duced by stacking plies of woven fabric preimpregnatedwith PEEK resin powder and compression moulding intoa consolidated sheet. The final property data are listed inTable II ~Ref. 24!. The PEEK-GP composite sampleswere initially cut from a 75-cm2 panel into 2.54-�7.62-cmsamples, themselves further cut into specimens of dimen-sions 1.27 � 7.62 � 0.32 cm using a liquid-cooled bandsaw. Figure 4 shows one of these samples of the com-posite material. The dark gray graphite fibers are clearlyvisible, with the PEEK matrix appearing as the light graycomponent. The samples of the PEEK-GF composite werefirst analyzed for purity using neutron activation analy-sis. The results showed that there were very few impuri-ties, with only traces of calcium, sodium, titanium, zinc,and even gold detected.

The SLOWPOKE-2 nuclear reactor at the Royal Mil-itary College of Canada ~RMC! is a 20-kW~thermal! in-herently safe pool-type nuclear reactor that produces abroad-spectrum mixed radiation field. This research re-actor was readily available for this work as a convenient

source of the mixed radiation field that included a neu-tron component in addition to major gamma-ray and elec-tron components, quite similar to the actual radiationfield emitted by the spent nuclear fuel but with a largerintensity. For this work, the SLOWPOKE-2 reactor wasoperated at half power because it served several otherresearchers simultaneously, most of them using neutronactivation analysis and for which the reduced reactorpower is more convenient. The PEEK-GF samples werepositioned by an “elevator” device in the reactor pool at32 cm from the core center line, against the outer side ofthe reactor vessel wall. Table III presents the results ofthe mixed radiation field obtained from a previousstudy,25,26 at this position and at half power. The cor-responding total dose rate was determined to be 3.7 �104 Gy h�1. While the previous study referenced abovehas attempted at determining energy spectra for the var-ious particles composing the mixed field produced by theSLOWPOKE-2 nuclear reactor at the irradiation site, bothexperimental data and computer simulation codes did notproduce energy spectra with reasonable accuracy. As itwill be discussed below, the spectral effects of the parti-cles have only a marginal importance in the context ofthe present work.

TABLE II

Property Data for PEEK-GF Composite*

FiberVolume~%!

Density~g cm�3 !

ProcessingTemperature~8C!

FlexuralStrength~MPa!

FlexuralModulus~MPa!

53.0 1.56 371 to 391 944.6 53.09

*Reference 24.

Fig. 4. PEEK-GF composite material ~with graphite fibers!.

TABLE III

Ionizing Particles Produced by SLOWPOKE-2,Steady-State Half-Power Operation at Reactor

Midheight and at 32.0 cm from Core Centerline*

Ionizing Particles %

Electrons 88.6Gamma photons 7.2Recoil protons 2.8Fast and thermal neutrons 1.4

Total 100

*References 25 and 26.



The irradiation of the samples took place in a spe-cially designed heated chamber that was immersed in thepool of the SLOWPOKE-2 nuclear reactor and posi-tioned against the reactor vessel with the midsections ofthe samples in line with the midplane of the reactor core,at 32 cm from the core center line and ;6-m depth.Figure 5 presents a photograph of the irradiation cham-ber at its irradiation position. The chamber is an acrylicbox, open at the bottom to allow installation and removalof the sample holder and the samples. The system uses acarbon heater connected to aluminum terminals and wiresand controlled through a Hewlett Packard 6264B DCvariable 400-W direct-current ~dc! power supply. Thechamber is fed by pressurized air at a rate set up to keepthe pressures both inside and outside the chamber equal-ized, thus preventing pool water from entering thechamber.

Table IV presents the irradiation and temperaturecombinations carried out. At the end of the irradiationperiod, the chamber containing the irradiated sampleswas moved away from the reactor vessel and positionedat midway up in the reactor pool at a position not reachedby the neutrons emitted by the reactor. The samples wereleft there for a cooldown period in order to bring theradiation dose rate from the “elevator components,” thechamber’s materials, and the samples to values belowthe maximum permissible dose rate of 2.5 mSv h�1

~2.5 mrem h�1 ! at the surface of the chamber. Thisway, the samples were handled safely for subsequentanalyses that included tensile and flexural testing, DSC,scanning electron microscopy ~SEM!, and wide-angleX-ray scattering ~WAXS!. All the tests were conducted

Fig. 5. Irradiation setup for the PEEK samples.

TABLE IV

Irradiation Profile

Irradiation Temperature~8C!

TotalExposure~h!

AccumulatedDose~kGy!

13 to 15 0 0~Pool water temperature! 1 37

7 25927 999

50 0 01 377 259

27 999

75 0 01 377 259

27 999



on computer-controlled devices for a high level of accu-racy, precision, and repeatability.

The composite samples were tested as rectangularflexural samples using an Instron Model 4206 equippedwith the Series IX Automated Materials Testing SystemRevision K. The data collection was done with an IBM-compatible computer system through an IEEE interface.ASTM International ~ASTM! Standard D 790, “Stan-dard Test Methods for Flexural Properties of Unreinforcedand Reinforced Plastics and Electrical Insulating Mate-rials,”27 was followed. The three-point bend test ~TestMethod 1! was the test method followed. The length-to-thickness ratio was 1601, with a support span of 50 mm~measured to 61%!, and the crosshead speed was1.3 mm min�1. Each specimen was measured for widthand thickness to the nearest 0.03 mm at the center of thesupport span using a digital micrometer. For each run~dose0temperature combination!, a total of six sampleswas tested, with the software reporting the propertieswith a mean and a standard deviation for all the sampleswithin the run.

Differential scanning calorimetry is a thermal methodfor measuring transitions in materials. In most applica-tions, the transitions evidenced by the DSC method arethe glass transition temperature Tg, the crystallizationand recrystallization temperatures Tc and Trc, and themelting temperature Tm. In addition, the technique canprovide information on the heat of crystallization and theheat of fusion for the materials investigated. For theseanalyses, a TA Instruments DSC 2010 apparatus was usedin conjunction with an IBM-compatible Instrument Con-trol V 2.5 operating system. The procedure followedASTM Standards D 3418 ~Ref. 28! and D 3417 ~Ref. 29!.The PEEK-GF nonirradiated and irradiated samples werecut into pieces of ;10 mg and placed into sealed DSCpans, followed by a thermal relaxation period of;10 minat 4008C, then quenched to 508C below the Tg ~being;1458C!, and placed into the DSC chamber. For the Tg

and Tc determination, the samples were heated at a rate of208C min�1 until 2508C under a nitrogen atmosphere.For the measurements of Tm and Trc, the heating rate usedwas 108C min�1 until 4008C, held for 10 min, then cooleddown at the same rate. All relevant data were recorded inthe data acquisition system and sent as ACSII files forfurther analysis.

Wide angle X-ray scattering is an analytical methodusing diffracted X-ray radiation to determine the crystal-line structure of materials. Generally, the sample to beanalyzed is irradiated by an X-ray beam at a certain angle,and the diffracted X-ray photons are detected via a de-tector that is swung around the sample through an angle2u. The detector may consist basically of a flat photo-graphic plate onto which the diffracted beam is projectedor as a strip of film encircling the sample. In the firstcase, the diffracted beams are recorded as circles, and inthe latter case, the beams appear as arcs. The detector’sresponse is displayed as a plot of X-ray diffraction ~XRD!

intensity versus 2u, called an XRD pattern. The higher-intensity peaks indicate preferred diffraction angles andprovide information on the type of crystals and their Millerindices. These well-defined peaks are due only to thecrystalline portions of the materials studied. Amorphoussubstances yield only diffuse responses from the detec-tor, known as an amorphous halo. In this work, the graph-ite fibers within the composite material are also detectedby the method and produce a well-identifiable intensityplot, as is the case for the amorphous component ofthe composite material. The amount of crystallinity ofthe polymer can be quantitatively determined though theintensity-2u graph, via the integration of the XRD pat-tern, after the pattern has been split into three compo-nents: crystalline, amorphous, and Compton scattering.Using the Ruland30 method, the volume fraction of crys-tallinity can be found. In semicrystalline polymers, thecrystals are oriented randomly and, consequently, do notdiffract into single points. To overcome this difficulty,the Debye-Scherrer method31 is used for the analysis.

The WAXS analysis was carried out using a Scintagdiffractometer with a u-u geometry goniometer coupledwith DMS2000 diffraction management system soft-ware. The samples used for this analysis were 2-cm-longpieces cut from the narrow section of the tensile samplesto ensure that the same portion was tested for all otheranalysis techniques. There was one sample for WAXSanalysis for each of the dose0temperature sample groups.The samples were scanned from 2u� 2 to 40 deg at a rateof 1.8 deg min�1 and a step size of 0.03 deg. The X-rayswere produced by a copper target, and their wavelengthwas 1.54 Å. Disregarding the carbon halo, the strongestpeak observed was the 18.9-deg peak, which was used todetermine the crystallite size.

The final analytical method used in this work wasSEM. The method essentially detects backscattered elec-trons from a small sample of the material in order toproduce a greatly magnified image that can be visuallyanalyzed. The specimens examined here are not electri-cally conductive, and for this technique to be appliedsuccessfully, their surface must be plated with a conduct-ing material such as gold. This was done for this workusing a Polaron SC7640 sputter coater with gold as plat-ing material. The SEM apparatus used was a Phillip XL30CP scanning electron microscope. Each sample wasfirst cut to a length of ;1 cm and attached to an SEManode stand. They were fixed to the stand with carbonglue and then gold plated. Then, they were placed insidethe SEM vacuum chamber, and several images of themwere taken with a variety of magnifications ~typically20X, 29X, 42X, 482X, and 1017X!.

VI. EXPERIMENTAL RESULTS

The results of the mechanical testing for the samplesfor the PEEK-GF composite material are presented in



Figs. 6, 7, and 8. Figure 6 presents a typical load-extension graph, for a PEEK-GF “0DRT” composite sam-ple ~zero dose, room temperature!. The designation “roomtemperature” here is synonymous with “pool tempera-ture” and indicates the temperature of the pool watersurrounding the sample’s irradiation box near the bottom

of the reactor pool. Since the reactor pool water is inthermal equilibrium with both the reactor vessel and theouter wall of the pool itself in contact with the under-ground materials ~mostly limestone!, the pool tempera-ture would then vary with the season and the power levelof the reactor, from ;138C in the middle of the winterjust before the reactor start-up early in the morning, to;238C in summer with the reactor at full power.As shownby the results, any variations of the measured data caused

Fig. 6. Typical load-extension graph for a PEEK-GF composite sample ~nonirradiated at reactor pool temperature!.

Fig. 7. Flexural strength of PEEK-GF composite material withrespect to the duration of irradiation ~at half reactorpower!, for the various temperatures investigated.

Fig. 8. Flexural modulus of PEEK-GF composite material withrespect to the duration of irradiation ~at half reactorpower!, for the various temperatures investigated.



by the variation of the pool temperature are indeed min-imal and well within the error bars.

The curve in Fig. 6 is produced by the software of theInstron flexural testing apparatus, and its features areused for determining the final results such as the modu-lus. While it could have been preferable that the softwareproduces a curve in terms of the strain instead of theextension, the purpose of inserting Fig. 6 here is to pro-vide the reader with an example of intermediate results inorder to help with the understanding of this part of theexperimental work. The strain can be calculated from theextension. Since the flexural method is a well-knownanalytical technique, it is felt as appropriate in the presentpaper to limit the depth of the description of the methodand invite the reader to consult the appropriate reference~Ref. 26!. The curve in Fig. 6 displays the sharp breakingpoint that is typical of composites and brittle materials ingeneral. The linear portion of the curve permits the de-termination of the rupture stress. Several further jaggedbreaking points are observed after the yield point: Theserepresent the breaking of the fibers, thus indicating thatthe sample is no longer consolidated. The matrix andfibers work in unison to produce the high strength of thecomposite material, but once past the rupture point, thesecomponents must absorb the energy separately. With moreductile materials ~such as the virgin PEEK!, there is usu-ally a flat linear portion of the load-extension curve fol-lowing a local maximum, the flat portion ending abruptlyat the break point. For the nonirradiated composite sam-ples at room temperature, a flexural modulus of 49.7 61.1 GPa and a yield strength of 852 6 44 MPa are ob-tained, and these results are the basis for comparisonwith the results for irradiated samples.

In the graphs depicted in Figs. 7 and 8, it is evidentthat the strength and modulus are only minimally af-fected by the radiation doses. For the composite samples,one observes a very slight drop following the initial ra-diation dose ~37 kGy! for each of the temperature sub-groups. Such a decrease in the mechanical strength andmodulus is indicative of the predominance of chain scis-sion, an effect already known from previous experiencewith polymers.9 Figures 7 and 8 also provide informationon the effects of the temperature of the samples duringtheir irradiation in the pool of the SLOWPOKE-2 reac-tor. For the PEEK-GF composite material, higher tem-peratures tend to decrease the values of the flexuralstrength and modulus, but only slightly as the dose in-creases. In some cases, the effect is marginal at best.Since the thermal expansion coefficients are different forthe graphite fibers and the PEEK matrix in the compositematerial, higher temperatures create additional stressesat the interface between the fibers and the matrix, therebyenhancing the chain scissions caused by the radiation.

However, one should note that all the changes occurwithin the error bars such that from the accuracy of theresults, it is not possible to come to a conclusion onwhether these changes are real or not. If these are actual

changes due to the combined effects of the radiationdose and the temperature, then the changes are minimalat best, but in the context of the present research, theseresults are indeed positive as they demonstrate that thePEEK-GF composite material can withstand the com-bined aggressive action of the radiations and thetemperature.

Figure 9 presents a typical output scan and analysisfrom the computer software controlling the differentialscanning calorimeter. This particular graph is for a DSCscan of a glass transition Tg point for a nonirradiatedvirgin PEEK sample at room temperature, but the DSCscans for PEEK-GF samples are quite similar. Althoughthe effects of the accumulated doses and the ambienttemperature were investigated for the glass transition,crystallization, recrystallization, and melting tempera-tures with this method, only the effects for the glasstransition temperatures are reported here since the otherthree temperatures are too high to be relevant for thescope of the present study. The crystallization, recrystal-lization, and melting temperatures were measured at 179658C, 2916 18C, and 335.76 0.48C, respectively. Thesetemperatures are significantly higher than the maximum1008C target decided upon in the design of the spentnuclear fuel disposal facility.

The results from DSC reveal strong evidence of cross-linking in the PEEK-GF composite material ~Fig. 10!.For the composite material, the glass transition temper-ature slightly increased with increasing accumulated ra-diation dose, except for the composite material irradiatedat 758C, which displays a slight drop of Tg with the max-imum dose absorbed. The changes in the properties re-mained relatively modest ~,3.5%!.

Figure 11 is that of a PEEK-GF composite materialsample fracture area following a flexural test. Note thatthe magnification is 1017� for the top picture ~zero doseand pool temperature! and 482� for the bottom one~1-MGy dose and 758C temperature!. It is apparent thatthe primary mode of failure is a fiber failure. While thesamples having received the largest radiation doses dis-played some actual signs of fiber pullout, this was notreflected in the mechanical strength and modulus of thecomposite material.

Figure 12 presents the diffraction patterns for thePEEK-GF samples, focusing on the 18.9-deg peak, ob-tained via the wide-angle XRD method. This peak waschosen as it shows qualitatively the amount of crystal-linity of the samples. Except for the composite sample atpool temperature, the peaks reveal in all other cases adecrease in crystallinity for the samples that have accu-mulated high radiation doses when compared to thenonirradiated samples. In composite samples at pool tem-perature, one observes an increase in crystallinity beforethe ultimate reduction. The influence of the temperatureis made evident in these results as there is no initialincrease of the crystallinity observed for the samplesirradiated at 50 and 758C.



VII. DISCUSSION

The changes witnessed in the mechanical and thermalproperties of the PEEK-GF composite material are attrib-uted to two main factors in the experimental procedure:the radiation dose and the ambient temperature at the ir-radiation site. By keeping the factors constant, it was pos-sible to see the effects that each of these factors had on theproperties. As with all polymers, when PEEK is exposedto ionizing radiation, there are two types of reactions thatoccur simultaneously: chain scission and cross-linking. De-pending ~among other effects! on the magnitude of theradiation dose absorbed, one of the two reactions is pre-dominant and controls the nature of the property change.The graphite fibers are themselves very little affected bythe radiation,11 but the interface between the fibers andthe polymer matrix is sensitive, and the bonding can bealtered by weakening or, in some cases, strengthening.

For similar radiation doses, but in different temper-ature environments, the experimental work evidencedsome slight variations in the physical properties of thesamples. For example, the results of the elastic modulusshow that after a low dose of radiation at pool tempera-ture, minimal changes are observed. The largest variationof the flexural modulus observed was a 7.2% decrease ofthe modulus for the samples having received a low dose~37 kGy! at 658C. A change in modulus is indicative ofradiolytic effects.

Fig. 9. Typical DSC scan for Tg ~nonirradiated sample at pool temperature!.

Fig. 10. Glass transition temperature for PEEK-GF compositematerial with respect to the duration of irradiation ~athalf reactor power!, for the various temperaturesinvestigated.



Most of the results for the yield stress were verysmall variations ~61%! around the value at room tem-perature for the not irradiated samples. The largest vari-ation observed was a 5.8% decrease of the yield strengthfor the samples irradiated at 758C and having received amedium dose ~260 kGy!. This barely exceeds the valueof 1s for the experimental results, which is equal to5.4%.All these results lead to the conclusion the PEEK-GFcomposite material is very modestly affected after beingirradiated at various temperatures from the flexural test-ing viewpoint.

As evidenced by the DSC and XRD method analy-ses, the thermal properties and the crystallinity of thematerials are affected by both radiation and elevated tem-peratures, albeit to a relatively modest degree, as de-scribed in Sec. IV. Finally, an examination of the SEMmicrographs of the fractured areas of the PEEK-GF sam-ples reveals that the primary mode of failure is a fiberfailure, and for the samples having accumulated the larg-est radiation doses, there are some signs of fiber pullout,but not to the extent of affecting the results of the me-chanical strength and modulus tests on the compositematerial.

VIII. RELEVANCE OF THE RADIATION DOSES

IN THE SLOWPOKE-2 REACTOR POOL

How can these results be put in the proper context ofthe actual sojourn of the polymer composite-based con-tainer filled with spent CANDU nuclear fuel bundles inthe disposal site deep underground in the Canadian Shield?Previous research at the SLOWPOKE-2 Facility25,26 atRMC has determined that the total dose rate at the irradi-ation site in the reactor pool is 37 kGy h�1. The sameresearch also determined the relative contributions to thedose from the different particles as presented in Table III.An extensive investigation carried out at AECL ~Ref. 32!has produced tables of results for the inventories of theradioisotopes present in the CANDU fuel at various timessuch as at the fuel discharge from the reactor core, and aseveral times after, covering many centuries. The resultspresented in Ref. 32 are for a standard 37-element fuelbundle irradiated in a Bruce-A reactor under a thermalneutron flux of 1.26�1014 n cm�2 s�1 during 228.72 days,thus having accumulated a burnup of 685 GJ kg�1 initialU ~7928 MW days tonne�1 initial U!.

From the inventories in Ref. 32, the dose calcula-tions could be carried out on the basis of the activities ofthe actinides for which the activities after a 10-yr decayperiod are larger than 3.7 �105 Bq kg�1 ~10�5 Ci kg�1!.As for the fission products, those included in the calcu-lations had their activities after a 10-yr decay period.3.7 � 107 Bq kg�1 ~10�3 Ci kg�1!. For the dose cal-culations, it was assumed that the filling materials in thecontainer ~fuel bundle cladding, plus glass beads or tho-rium dioxide! could absorb the alpha and beta particlesemitted by the radioactive isotopes in the fuel elements,as well as the recoil protons from the neutron collisions,and most of the neutrons. The only particles affectingthe container walls were then the gamma photons andthe energetic electrons created by the Compton, photo-electric, and pair production effects in close proximity ofthe container walls. The dose rates due to the gammaphotons alone were computed by the MicroshieldTM

software.33

The details of these calculations can be found inRef. 34. Typical dose rates on the container walls havebeen determined as 19 Gy h�1 when no filling materialis present in the container, 11 Gy h�1 when glass beadsare used as the filling material in the container, and0.05 Gy h�1 when ThO2 is used as the filling material.Recall that these dose rates are for fuel having undergonea 10-yr decay period after discharge from the CANDUreactor core. Now, the contribution of the stripped ener-getic electrons resulting from the interactions ofthe gamma photons in the vicinity of the container wallcan be accounted for using the results of an analysis donefor the remote elevator position in the SLOWPOKE-2reactor pool.35 At this position, the contributions of bothfast and thermal neutrons and of the recoil protons ~andassociated stripped electrons! are truly negligible. The

Fig. 11. SEM micrograph of PEEK-GF composite material sam-ple. Top: zero dose, pool temperature. Bottom: 1 MGydose, 758C.



results of this analysis indicate that the relative contribu-tion of the energetic electrons produced by the Comptonscattering, the photoelectric effect, and the pair produc-tion amounted to;51% of the total dose rate. In the caseof the container wall, the situation is quite comparablesince the neutron and recoil proton contributions to thedose rate are also negligible. Therefore, the total dose ratesin the container walls are essentially equal to the doserates due to the gamma photons alone, divided by 51%.The total dose rates would then be 22 Gy h�1 and 0.1Gy h�1 for the container filled with glass beads and tho-rium dioxide, respectively. The actual dose rate receivedby the PEEK-GF samples in this work was 37 kGy h�1,

representing indeed an “overkill” when compared to theactual conditions to which the container walls are submit-ted in the deep disposal site. Yet, the composite materialhas behaved with only quite modest changes in its prop-erties in such an aggressive environment.

Another important consideration is the total accumu-lated dose over the 500 yr of exposure to the radiationproduced by the spent fuel. In order to determine thisdose, the dose rate is integrated over the time period,from a time zero defined as 10 yr after the withdrawal ofthe fuel from the reactor core and over a period of 500 yrafterward. The hourly dose rates given just above ~22 and0.1 Gy h�1! correspond to yearly dose rates of 1.93�105

Fig. 12. Diffraction patterns for PEEK-GF composite material samples. Top: Pool temperature. Middle: 508C. Bottom: 758C.



Gy yr�1 and 877 Gy yr�1 for the container filled withglass beads and thorium dioxide, respectively. The inte-gration over the years was carried out based on the datain Ref. 32 and using a numerical method. The cumulativedoses to the container walls over the 500-yr period werethen found as 12 MGy and 56 kGy for the glass beads andthe thorium dioxide filling materials, respectively.

If thorium dioxide is used as the filling material inthe container, the cumulative dose over the 500-yr periodis well below the dose imparted to the PEEK-GF samplesin the SLOWPOKE-2 reactor pool irradiation site, andthe results show clearly that the PEEK-GF propertieswould not be altered to compromise the compositematerial’s integrity. Irradiations of PEEK-GF samples inthe SLOWPOKE-2 reactor pool of ;324 h would havebeen needed to produce the 12-MGy dose determined forcontainers filled with glass beads. Such long irradiationscould not be obtained with the equipment used for theresearch. If glass beads have to be used as the fillingmaterial, then it is not difficult to alter the design of thecontainer by providing it with an internal layer of lead.For such a layer, a minimum thickness of ;4 cm wouldbe sufficient to keep the cumulative dose below 1 MGy atthe end of the 500-yr period.

The knowledge gained with this research indicatesthat using thorium dioxide as the filling material not onlywould reduce significantly the cumulative dose to thecontainer wall but also would contribute greatly to re-ducing the temperature of the container walls since theheat conduction coefficient of ThO2 ~8.4 W m�1 K�1!~Ref. 36! is much better than that of the glass beads ~0.78to 1.09 W m�1 K�1! ~Ref. 37! for solid glass, and muchless for glass bead filling material. In either scenario~thorium dioxide or glass beads with a lead liner as fillingmaterial!, the maximum doses received by the PEEK-GFsamples in this work are very representative of the actualdoses imparted to the container walls in a real-life sce-nario. The extremely long half-life of 232Th ~1.405 �1010 yr! results in an extremely small contribution of thisradioisotope making the natural content of thorium to thedose rates imparted to the container walls.

Another aspect of the validation of using theSLOWPOKE-2 nuclear reactor to simulate the radiationfield to which the container walls would be submitted atthe disposal site deep underground is related to the gammaspectrum. In a critical nuclear reactor at power, the gammaphoton field is made of two contributions: that of theprompt gammas emitted by the fission process itself andthat of the delayed gammas emitted by the fission prod-ucts. The following equation38 represents the gamma spec-trum within the 1.0- to 7.0-MeV range:

N~E ! � 8.0e�1.10E MeV�1

with a 615% error for the 1.0- to 4.5-MeV and 640%error for energies between 4.5 and 7.0 MeV. For energiesbelow 1.0 MeV, the following equation38 is more accurate:

N~E ! � 26.8e�2.30E MeV�1 ,

and for energies above 7.0 MeV, the gamma population islow enough to be neglected. As for the delayed gammas,their total activity reaches a saturation point a few hoursafter the start-up of the reactor. The energy spectrum ofthese gammas may be given by the following equation:

N~E ! � 6.0e�1.10E MeV�1 .

The total spectrum in the 1.0- to 7.0-MeV range isthen given by

N~E ! � 14.0e�1.10E MeV�1 .

Once the reactor is stopped, the gamma flux comesfrom the radioactive decay of the long-lived fission prod-ucts. Their contributions have been studied by severalgroups of researchers39 who presented their results withthe spectrum subdivided into six or seven energy groups.These results show that at some 104 h ~417 days! afterreactor shutdown, the populations of gammas within theenergy groups have all decreased by four or five orders ofmagnitude and more, and the only significant contribu-tion left at the end of that decay period was that of thegroup of photons with energies below 1.0 MeV. Thistrend is also valid for after 10 yr after removal of the fuelfrom the reactor.

It can then be concluded that the irradiation of thePEEK-GF composite material in proximity of the core ofthe SLOWPOKE-2 nuclear reactor is achieved not onlyat a significantly higher dose rate but also with a muchharder energy spectrum than the conditions under whichthe container walls would be subjected to at the disposalsite deep underground. In the actual container case, thesource of gammas is made of very long-lived radioiso-topes producing lower dose rates and a softer energyspectrum, leading to less damaging effects per inter-action of the photons with the composite material. Whilethe SLOWPOKE-2 nuclear reactor does not quite pro-duce a radiation field that represents very faithfully thatof the disposal site, it does produce a much more aggres-sive and damaging radiation environment permitting oneto assess the resistance of the polymer composite mate-rial in a definitely conservative manner.

IX. ERROR ANALYSIS

The analysis of the results from mechanical testingmethods shows very small standard deviations. The In-stron instrument used for this work is of excellent qualityand has excellent repeatability. Most of the error herecomes from variations in the preparation of the samples.The small standard deviations are attributed mostly to thegreat care given to the sample preparation, and as Fig. 13shows, the maximum standard deviations of the strengthand modulus do not exceed 8%, with most values reported



being below 2.5%. The standard deviation was calcu-lated for each set of samples ~six samples per set! usingthe usual equation40 for the sample variance for samplesizes ,20:

s 2 �1

n � 1 (j�1

n

~xj � Sx!2 ,

where xj is the datum for the j ’th sample and Sx is theaverage value for the n samples. The standard deviationis simply the positive square root of the sample variance.

The SLOWPOKE-2 nuclear reactor at RMC is pro-vided with an excellent automatic control system thatmaintains the neutron flux level ~hence the reactor power!to within 61% of the set point, and this for extendedperiods of time that can extend to several days of uninter-rupted operation. This is the reason why some of theexperimental data are reported as functions of irradiationduration since the error bars on these values are verysmall. The abscissae of the graphs have been providedwith a second scale in terms of accumulated dose to helpthe reader with the interpretation of the results. However,problems are experienced when interpreting the irradia-tion durations in terms of radiation doses. Past researchhas been done at RMC to address this issue, and the dose

rates were correlated to the reactor power levels and thethermal neutron flux by means of experimental data inconjunction with computer simulations of the nuclearreactor and surrounding pool.25,26 The uncertainty of thedose rate at the irradiation site used for this work wasreported as 628%, which is a conservative figure asexplained in detail in Refs. 25 and 26. This large uncer-tainty comes from three major sources: ~a! the difficultyin obtaining accurate experimental measurements of theflux of particles due to the high levels of these fluxes andthe limitations of the detectors, ~b! the accuracy of thecomputer models used for the calculation of the particlefluxes, and ~c! the conversion of the values of the fluxesdetermined experimentally and computationally at theirradiation site into radiation dose rates through the en-ergy transfer coefficients. There is also an uncertaintyrelated to the actual time required to position and retrievethe samples at and from the irradiation site in the reactorpool. This last uncertainty is assessed at most to be on theorder of only 3%.

During the irradiations, the temperature of the cham-ber containing the samples was monitored and controlledthrough the combination of a thermocouple with digitalreadout and fully adjustable dc power supply. The ther-mocouple resided inside a “dummy” sample that wasplaced alongside the other samples within the irradiationchamber for accurate measurement of the temperature ofthe samples proper, not just the temperature of the am-bient air. The thermocouple0readout system had an ac-curacy of 0.18C. The temperature was adjusted by meansof an electrical current supply that could provide up to20 A, although the current usually needed for this workdid not exceed 10 A. The accuracy of the power supplyreadouts was 0.5 V and 0.5 A. At any one time, the tem-perature of the sample chamber never exceeded638C ofthe set point for this work ~50, 65, and 758C!. The re-sponse of the thermocouple did not reveal any adverseeffects on the equipment from the radiation. The irradi-ation chamber was not provided with measurement de-vices to assess the uniformity of the temperature withinits volume. This was assessed as not necessary since thedimensions of the chamber were rather modest ~some30 � 10 � 6 cm!; therefore, the temperature gradients inthe chamber could be assumed as small.

The Instron mechanical testing machine producedhighly repeatable results due to the computer control anddata acquisition system. Most of the error for these testscame from the fact that the strain was not measured di-rectly from the samples but from recording the exactposition of the crosshead, at every 0.3 s during the tests.Grip slippage and machine slack also could have alteredthe accuracy of the results, but in the present work, thiswas found to be insignificant causes of error.

The DSC testing was carried out using an instrumentthat has an accepted error of 60.5%, thanks to thecomputer-controlled data acquisition system and thermalmethods. This instrument is of superior quality, and the

Fig. 13. Standard deviation of the mechanical properties.



procedure included weighing the samples with an ana-lytical balance with an accuracy of 1 mg.

This error analysis confirms that the results obtainedhere are highly repeatable with very reasonable uncer-tainties. The major source of error thus comes from theinterpretation of the exposure times in terms of accumu-lated radiation doses, and the second source of error inimportance comes from the statistical spread of the re-sults within each group of samples.

X. CONTAINER DESIGN CONSIDERATIONS

At this point in the research, the dimensions of theproposed containers in the AECL study are retained forthe calculation of the stresses and the thickness of thePEEK-GF composite material container walls. The con-tainer proposed by AECL has a 2.25-m height and a0.620-m inner diameter and contains 72 standard CANDU37-rod fuel bundles. If the titanium alloy making up theAECL container were to be replaced with PEEK-GF com-posite material, then the minimum thickness of the wallsmust be determined. At the repository site deep within apluton in the Canadian Shield, it is estimated that thecontainer must withstand a 13-MPa hydrostatic pressure,plus the weight of other containers stacked above it andthe impact stress from the container being dropped dur-ing the transportation of the container to the disposal siteand its placement within the vault.

The purpose of this part of the research is at thepresent time to obtain a first assessment of the mechan-ical properties of the container made with the PEEK-GFcomposite material. In a subsequent phase of the re-search, actual mechanical testing is foreseen on modelsof the container in order to ascertain its mechanical be-havior more accurately. In the calculations to determinethe thickness of the container walls, the lowest values ofstrength and modulus determined experimentally are used,namely, a strength of 805 MPa ~dose of 259 kGy at 758C!and a modulus of 46.1 GPa ~dose of 37 kGy at 658C!. Thecontainer is first considered as a thin-walled right cylin-der, i.e., for which the ratio of the interior diameter to thewall thickness must be .20. Since the interior diameteris 0.620 m, the wall thickness must be ,0.031 m. How-ever, the thin-walled cylinder model assumes that theradial stress sr is zero. This is not assumed here, and theradial stress must be calculated with respect to the con-ditions presented above. It is also assumed that the tan-gential or hoop stressst and the axial stresssa are uniformthroughout the wall thickness. The analysis must be con-ducted along the most probable mode of failure of thethin-walled cylindrical container, i.e., through buckling.The first buckling criterion considered here is caused bythe tangential stress. This is explained by the so-called“plastic hinge model.”41 Four nodes are identified asequidistant points along the circumference of the cylin-

der. Under the tangential stress, the cylinder then col-lapses until two opposite nodes touch each other, thecross section of the cylinder changing from a perfectcircle to two “ellipse”-shaped forms, with the long axeslinking opposite nodes. In the plastic hinge model, a pro-cedure is followed permitting the calculation of the pres-sure on the cylinder’s side to produce the work necessaryto collapse the cylinder, this pressure being proportionalto the yield stress and the square of the wall thickness,and inversely proportional to the square of the insidediameter of the cylinder.

With all the forces and stresses accounted for throughbalance equations, it is possible to determine the mini-mum wall thickness for the container to resist to the netforces against the walls, these net forces being the dif-ference between the external forces onto the containerand the resistance forces offered by the materials withinthe container. For a 13-MPa external hydrostatic pres-sure, the results are reported in Table V. It is thereforeseen that the minimum wall thickness is the value corre-sponding to the tangential buckling criterion. In the prac-tical design of the container, some safety margins mustbe considered, and in this particular application, an out-side pressure 25 to 50% larger than the pressure consid-ered in these calculations should be used, yielding valuesfrom 49.7- to 54.4-mm wall thickness retained for thefinal design of the container.

In a second step of this analysis, the maximum staticload before buckling in the axial direction is determined.This is proportional to the elastic modulus multiplied bythe second moment of area and inversely proportional tothe square of the length of the cylinder. If the container ispositioned vertically, this load is due to the hydrostaticpressure, unless other containers are stacked onto thecontainer under study. In this latter case, every one ofthese containers would have a weight of 1774 or 1906 kgif glass beads or thorium dioxide is used as the fillermaterial in the container. These values are determinedfrom the assumption that a single container includes 72CANDU spent-fuel bundles for a total of 1728 kg; adensity of 2550 kg m�3 or 10 000 kg m�3 for the glassbeads and the thorium dioxide, respectively; and a vol-ume occupied by the filling material of 0.0178 m3. If one

TABLE V

Minimum Wall Thickness Required fora 13-MPa Hydrostatic Pressure

Failure CriterionMinimum Wall Thickness

~mm!

Tangential buckling 44.5Tangential stress 5.01Axial buckling 3.05Axial stress 2.40



considers that the bottom container has a thickness of44.5 mm for its walls, the total area onto which the stressfrom the stacked containers is going to act can easilybeen computed as 0.0929 m2. It is then possible to com-pute the total and axial stress on the bottom container forup to four stacked containers, and Table VI presents thesevalues. It is then obvious that the safety margins men-tioned above would be sufficient to cope with the con-tainer stacking issue.

The possibility of dropping the container during itstransportation to the repository site and its placementinto the cavity in the underground disposal gallery mustalso be considered. The scenario considered in the presentstudy is a vertical drop of 2 m such as inside the boreholecarved in the floor of the underground gallery. In such acase, the maximum stress experienced in the containercan be approximated by the static stress in the containermultiplied by an impact factor n calculated by means ofthe following equation42:

n � 1 � �1 � 2� h

Dst� ,

where Dst is the static elastic strain, and h is the height ofthe container drop. For the PEEK-GF composite mate-rial, the impact factor is calculated as n � 2.09 �102. Forthis calculation, the container is considered outside therepository borehole ~i.e., without the radial stresses!, suchthat this impact factor acts as a stress concentration onthe axial stress for a single container ~i.e., not stacked!.The axial stress on the dropped container from a 2-mheight would then be equivalent to the axial stress at rest~0.201 MPa for the container using the ThO2 filler ma-terial!multiplied by the impact factor, resulting in a totalstress of 42.0 MPa on impact.

XI. CONCLUSIONS

In this work, a polymer-based composite materialmade of PEEK and graphite fibers is proposed for the

fabrication of a container that would be suitable for theultimate disposal of CANDU reactor spent fuel bundles.The nuclear research reactor SLOWPOKE-2 at RMC wasused to reproduce a mixed radiation field not only com-parable to but also more damaging than the radiationfield generated by the spent nuclear fuel on the container’swalls. Samples made of a PEEK-GF composite materialwere irradiated in the pool of the reactor inside atemperature-controlled chamber and left to accumulateradiation doses up to nearly 1 MGy.

At all the dose levels tested, the dominant effectobserved was cross-linking, with some chain scissionbeing a concurrent phenomenon caused by the radia-tions. However, the changes in the mechanical properties~strength and modulus! that were observed were rela-tively minor and certainly not enough to cause any con-cern about the integrity of the PEEK-GF compositematerial.

The radiation doses received by the samples in thisstudy are significantly more than the radiation doses re-ceived by an actual container after 500 yr in the under-ground disposal site. Also, within the deep undergrounddisposal site, the temperature of the container’s wallswould decrease relatively rapidly to values close to thoseof the ambient temperature of the plutonic granite rock,well below the 758C investigated here. The results ob-tained by the present study at this elevated temperatureand high accumulated doses correspond to a scenario inwhich the container would remain at 758C for severalcenturies. This represents a very conservative assump-tion indeed. Yet, the results obtained demonstrate that theintegrity of a container made of a PEEK-GF compositematerial would be ensured. Replacing the titanium alloyand the copper metal presently proposed with thePEEK-GF composite material investigated here thusrepresents a very promising option.

The research considered using glass beads ~the op-tion proposed by AECL! and also using sintered thoriumdioxide as the filling material. The latter material wouldpromote better heat transfer ~from its much higher den-sity!. Most importantly, it would shield the container

TABLE VI

Total and Axial Stress Caused by Container Stacking, 13-MPa Hydrostatic Pressure

Glass Beads Filler Thorium Dioxide Filler

Number ofContainers

StackingStress~MPa!

TotalStress~MPa!

AxialStress~MPa!

StackingStress~MPa!

TotalStress~MPa!

AxialStress~MPa!

0 0 13 55.2 0 13 55.21 0.187 13.2 56.1 0.201 13.2 56.12 0.374 13.4 56.9 0.402 13.4 56.93 0.561 13.6 57.8 0.603 13.6 57.8



walls from the radiation emitted by the CANDU fuelbundles, thanks to the high-Z value ~90! for thorium.Finally, some aspects of the design of the containers wereexamined, such as determining the thickness of thecontainer’s walls assuring the integrity of the container.A thickness of ;50 mm would be a conservative valueand is reasonable both in the context of the fabrication ofthe container and in keeping the costs at affordable levels.

XII. RECOMMENDATIONS

In order to offset the uncertainties associated withthe experimental data, this research can be extended byirradiating PEEK-GF composite material samples in orderto accumulate higher radiation doses under higher tem-peratures, provided that the control problems encoun-tered with maintaining temperatures .758C in theirradiation chamber could be solved. The present re-search is part of a larger research program in which thenext step consists of irradiating the PEEK-GF samples inan even more aggressive environment in which the sam-ples would be not only subjected to the mixed radiationfields at elevated temperatures but also in contact withacidic and alkaline solutions representing the ground-water at the disposal site deep underground. This wouldpermit the determination of the chemical resistance ofthe polymer-based materials in addition to their mechan-ical and thermal resistance, again in a simulation of thecontainer’s dwelling time at the spent fuel disposal siteover 500 yr and more.

The mechanical behavior of the container fabricatedwith PEEK-GF composite material must be determinedexperimentally to validate the theoretical predictionsachieved in this work. For this, scale models of the con-tainer have recently been fabricated with the PEEK-GFcomposite material to ascertain any problems related tothe fabrication of actual containers with this material andto carry out mechanical testing to confirm the theoreticalmodels used in this work.

It is planned to continue the study to address thelarge reported error of the value of the dose produced bythe SLOWPOKE-2 reactor at RMC, as a parallel project.Although this has actually a minor effect on the conclu-sions reached from this work, it would be appropriate tomake use of improved computer codes now available tosimulate the SLOWPOKE-2 nuclear reactor and of moreaccurate radiation measurement devices that have be-come available since the last study25,26 was carried out.

ACKNOWLEDGMENTS

The authors wish to thank K. Nielsen and the staff of theSLOWPOKE-2 Facility at RMC and the technicians of theRMC Department of Chemistry and Chemical Engineering fortheir sustained and enthusiastic assistance with this demanding

research project. In addition, the financial assistance of theNatural Science and Engineering Research Council of Canadais gratefully acknowledged.

REFERENCES

1. “Summary of the Environmental Impact Statement on theConcept for Disposal of Canada’s Nuclear Fuel Waste,” AECL-10721, p. 10, Atomic Energy of Canada Limited ~1994!.

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Advanced Polymer Composites

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