CER

N-I

NTC

-201

6-03

9/

INTC

-P-4

7402

/06/

2016

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH Proposal for an Experiment to the ISOLDE and Neutron Time-of-Flight Committee

Irradiation of prototype tungsten blocks for test of

Halogen Release Fraction from the future ESS Helium cooled Tungsten Target. (WHALE project).

Submitted 29. May 2016

By: Mikael Jensen Professor of Applied Nuclear Physics, Hevesy Lab, DTU-NUTECH,Risø, Roskilde, Denmark

Spokesperson(s): Mikael Jensen ,kmje@dtu.dk Local contact: Karl Johnston, karl.johnston@cern.ch

Abstract

The European Spallation Source (ESS) presently under construction in Lund, Sweden, will be based on a pulsed 2.5 GeV proton linear accelerator delivering on average 5 MW beam power to a neutron generating target based on solid tungsten. The target will be made up by a large rotating array of matchbox size tungsten blocks and cooled by rapidly circulating helium. Such target at these power levels have never been realized before. Considerable effort is put into the assessment of safety and durability of the target during expected life span of 5 years for each target. Tungsten is expected to withhold the majority of the generated spallation radionuclides, based both on diffusion calculations and experimental observations at spallation sources at lower power. However, a few elements defied such theoretical and bibliographical approach and among these most notably the halogens. Because of the high radiotoxicity and the high volatility of iodine, the release fraction of spallation induced radioiodine in the tungsten target to the helium cooling circuit needs an independent experimental verification. For this purpose, we wish to activate one or two tungsten blocks by the proton beam (1.4 GeV) at ISOLDE. The primary radionuclide studied will be I-125. The release experiment will be performed at Hevesy lab in Denmark, after 2- 3 weeks of cooling and 1-2 days transport. We ask for irradiation of the tungsten blocks inside a sealed ISOLDE target assembly with about 1E18protons corresponding to one day. The timing of the experiment is not critical and the target may be standing at ISOLDE waiting for an unexpected hole in the ISOLDE science schedule.

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Proposal The tungsten target is regarded to be a very safe material in terms of containment the many radionuclides produced by spallation over the lifetime of such target (5 years). As part of the ESS target design update process, we have calculated the release factors for all potential radioelements in the tungsten, for a range of temperatures and surface chemistries of the tungsten blocks. We found, based on available literature, all the necessary information on the diffusion coefficients for minority elements in solid tungsten to perform such a release assessment both in the normal operation and in the accident scenario. Many elements needed scaling of the measured diffusion to the relevant lower temperatures encountered at ESS, but that could be safely done by application of Arrhenius formula. A few elements defied such theoretical and bibliographical approach and among these most notably the halogens. Because of the high radiotoxicity and the high volatility of iodine, the release fraction of radioiodine to the helium cooling circuit has been identified as needing an experimental evaluation before approval and final construction of the ESS target. My group at DTU-Nutech and the Hevesy lab has taken on the responsibility to do such measurements soonest possible. We want to subject a real size, prototype metallic tungsten block (80x20x10 mm) made for the actual ESS target to activation by proton induced spallation and subsequent (after weeks of cooling, and then transported to a hot cell in our lab) subject the block to gradual heating in a tube furnace under slow helium flow. Radioisotopes released to the gas will be captured, identified and quantified as function of time and temperature. Of top interest is the isotope I-125, which is known to be produced in the spallation process. It has a suitable half-life for the experiment and is of primary facility safety relevance for ESS. The capture of iodine (and bromine isotopes) will be done by bubbling the furnace helium gas flow through sodium hydroxide solution, followed by radiochemical workup and counting (x-ray spectrometry, LSC). We will also preserve and analyze samples for later analysis of the I-129 release by ACMS. Other radioisotopes suspected to appear to some degree in the gas stream will likewise be collected. We have built an experimental setup at the Hevesy Lab with a quartz tube furnace capable of finely regulated temperature control up to 1000 deg C, located in a 100 mm lead shielded hot-cell with manipulators. We want to have the tungsten activated by spallation with a rather uniform depth profile (mimicking the future ESS block conditions). A good possibility is to irradiate one or two tungsten blocks along the short axis inside an otherwise empty ISOLDE Ions source assembly. The tungsten blocks shall be in good thermal contact with the water-cooled studs in the ion source assembly. The ion source assembly shall be sealed and leak tested before irradiation, then partially filled with helium (100-200 mbar). The assembly shall stay sealed and at under-pressure during irradiation, cooling and transport. We are interested in sampling the atmosphere inside the assembly upon arrival at our lab. The irradiation time and the total number of protons are a balance between sensitivity of the experiment ( the activity of I-125 induced) and the radiological safety of the transport of the assembly. However, irradiation by 1E18 protons in either a single days or a few days interrupted irradiation will be enough for our purposes.

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In front of this proposal, the yield expected yield of I-125 and the radiologically important inventory of spallation isotopes has been calculated in two ways:

1) Using published experimental yields in ref. 1 and 2 (1.2 GeV) we get 300-600 MBq I-125 after irradiation of 10 mm thick tungsten with 1E18 protons (about 24 hours at 2 uA) and after decay of 2-3 weeks.

2) Using a MCNP/CINDER calculation performed at ESS (Günter Mührer, personal communication) for 1.4 GeV 1E18 protons incident on 10 mm tungsten and 2 weeks cooling we get 690 MBq I-125.

The two calculations agree reasonably well also on the radiologically most important isotopes. These isotopes are identified by multiplying the accumulated activity (after cooling) by the corresponding dose rate constants taken from reference 3. The dominating contributors to external dose after both 2 and 3 weeks cooling are Lu-171+172, Eu 145 and Ta-182. But the calculations show that the target assembly can handled and transported after 2-3 weeks cooling. The target assembly delivers at 1 meters distance from a 10 mm lead lined transport drum container less than 900 microSv/h at 1 meter. This can easily and safely be transported. We intend to use a transport company and an approved transport container previously used to transport irradiated complete Isolde target assemblies from CERN to PSI. After cooling of 2-3 weeks, the entire target assembly shall be transported by road to our lab at Risø, Roskilde, Denmark for subsequent analysis. Size and nature of the tungsten block to be activated The prototype tungsten blocks to be tested have the expected purity, finish and crystal structure as the real ESS target blocks. They are rectangular blocks 10 x 20 x 80 mm, with an approximate mass of 320 grams. The blocks are foreseen to be activated by protons traversing the blocks along the shortest axis (10 mm). We wish irradiate two such blocks (one behind the other in a single target assembly and in a single Isolde run. In a given run, the beam profile and beam monitoring is not critical. To our application, it is only the basic assumption of rather uniform spallation activation with depth that is important, not the distribution across the beam direction. In the end, we need to know the total amount of radioiodine in the block before – or after- the release experiment. Instead of subjecting the entire block to the cumbersome and bulky wet chemistry work-up, we propose to insert two tungsten monitor foils (0.5 mm thick, 20x80 mm size) with one in front and one behind the thick block. The “thin” W-foils can be used to gauge both the total iodine activation and the assumption of activation uniformity. The monitor foils will be clamped together and mounted in good thermal contact to the water-cooled studs inside the target assembly. Loss of activity by sputtering out of block. A minor topic of our experiment could be to capture the activity sputtered (or recoiled) out of the tungsten block during bombardment. We therefore, as an add-on to the experiment propose to insert a 0.1-0.2 mm aluminum foil after the thick tungsten blocks, but in front of the last monitor tungsten foil. I am aware of the additional activity produced by this foil (most importantly Na-22, 24), but this will be a lesser contribution to the total activity. The majority of the Na-24 will have decayed before transport. The aluminum foils can be used to give an independent measurement of the accumulated proton dose during the irradiations.

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Fixation of the irradiation block sandwich during bombardment It is our wish not to have the tungsten heated to any significant diffusion temperature during the irradiation- This means it should be kept below 200 deg C at any time. The proton energy loss at 1.4 GeV on transversal of 11 mm tungsten is roughly 25 MeV (SRIM) giving a stopping power heat of 25 Watt/uA beam/ block. Assuming a peak current of 2 uA it gives a maximum power to the block assembly of 100 watt. To keep temperature down during irradiation, it is proposed that the sandwich block is firmly bolted to an aluminum cooling flange in good thermal contact with the three water cooled studs inside the ion-source assembly.

Summary of requested shifts:

Requested shifts: About 24 hours total irradiation or equivalently 1E18 protons accumulated in one or more relatively close lying shifts. Timing and irradiation continuity is not critical.

References:

Reference 1): ” Activation by Protons in Range-Thick Lead and Tungsten Spallation Targets” C. E. Laird et al., published in NUCLEAR SCIENCE AND ENGINEERING: 130, 320–339 (1998) Reference 2) : "Measurement and Simulation of the Cross Sections for Nuclide Production in nat-W and 181-Ta Targets Irradiated with 0.04- to 2.6-GeV Protons" Yu. E. Titarenko et al., translated from Russian in ISSN 1063-7788, Physics of Atomic Nuclei, 2011, Vol. 74, No. 4, pp.551–572 (2011). Reference 3): “ “Exposure rate constants and lead shielding values for over 1100 radionuclides.” David S. Smith and Michael G. Stabin* Health Phys. 102(3):271-291; (2012)

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Appendix

DESCRIPTION OF THE PROPOSED EXPERIMENT

The experimental setup comprises: (name the fixed-ISOLDE installations, as well as flexible elements of the experiment)

Part of the Choose an item. Availability Design and manufacturing

Main ISOLDE ion source setup. Existing To be used without any modification

none Existing To be used without any modification To be modified

New Standard equipment supplied by a manufacturer CERN/collaboration responsible for the design and/or

manufacturing

none Existing To be used without any modification To be modified

New Standard equipment supplied by a manufacturer CERN/collaboration responsible for the design and/or

manufacturing

[insert lines if needed]

HAZARDS GENERATED BY THE EXPERIMENT

The target assembly will never be open to the ISOLDE separator, beam line or detector setup, and cannot pose a risk to other experiments. It will be handled by the robotic system like any other target, using the same experimental setup.

Additional hazards:

Hazards

[Part 1 of the experiment/equipment]

[Part 2 of the experiment/equipment]

[Part 3 of the experiment/equipment]

Thermodynamic and fluidic Pressure 200 mBar absolute , > 2 litres.

Vacuum no

Temperature No levated temperatures are expected.

Heat transfer By contact to water cooled studs, < 100 Wats in total.

Thermal properties of materials

Solid tungsten-

Cryogenic fluid none

Electrical and electromagnetic Electricity none

Static electricity nine

Magnetic field None/not critical.

Batteries

Capacitors

Ionizing radiation Target material Solid tungsten., supplied by

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Goodfellow metals.

Beam particle type (e, p, ions, etc)

p

Beam intensity 1e18 p/ day , < 2 uA

Beam energy Standard Isolde

Cooling liquids water

Gases None

Calibration sources: NONE

Open source NONE

Sealed source NONE

Isotope NONE

Activity Spallation products in W.

Use of activated material:

Description The tungsten blocks inside the standard ISOLDE ion sorce assembly.

Dose rate on contact and in 10 cm distance

Dose rate at 1 meter after 2 weeks cooling < 1000 uSv/h. Contact dose and dose at 10 cm distance is not relevant.

Isotope All spallation products from W.

Activity

Non-ionizing radiation Laser none

UV light none

Microwaves (300MHz-30 GHz)

none

Radiofrequency (1-300MHz) N.a.

Chemical Toxic none

Harmful none

CMR (carcinogens, mutagens and substances toxic to reproduction)

none

Corrosive none

Irritant None

Flammable none

Oxidizing none

Explosiveness none

Asphyxiant none

Dangerous for the environment none

Mechanical Physical impact or mechanical energy (moving parts)

none

Mechanical properties (Sharp, rough, slippery)

none

Vibration [location]

Vehicles and Means of Transport

[location]

Noise

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Frequency [frequency],[Hz]

Intensity

Physical Confined spaces [location]

High workplaces [location]

Access to high workplaces [location]

Obstructions in passageways [location]

Manual handling [location]

Poor ergonomics [location]

0.1 Hazard identification

3.2 Average electrical power requirements (excluding fixed ISOLDE-installation mentioned above): (make a rough estimate of the total power consumption of the additional equipment used in the experiment)

(none).