Helium Chemistry in High-Temperature Gas-Cooled Reactors - Chemical Impurity Behaviour in the Secondary Helium Coolant of the HTTR -

Hamamoto S. 1, Sakaba N. 1 1 Japan Atomic Energy Agency, Higashiibaraki, Ibaraki, Japan 1. Introduction The High Temperature Gas-cooled Reactor (HTGR), which is a graphite moderated, helium gas cooled reactor, is particularly attractive for its capability of producing high temperature helium gas as well as its inherent safety characteristics. The HTGR is appealing as an option to efficiently burn weapons-grade plutonium for energy production. These interesting aspects make the HTGR worthy of further discussion for the future advanced reactors, along with the advanced light water reactor (LWR). The HTGR is also expected to contribute to solving the current global environmental issue of CO2 emission, since it can be an alternative or a supplement to fossil-fuel energy sources for process heat application. Increasing interest has been given to the HTGRs in the world falling under the Generation IV system concept as represented by the VHTR. Under these circumstances, the HTGR development activity in Japan is becoming more active than before with the progress of the HTTR (High-Temperature Engineering Test Reactor) project [1]. It is widely recognized in the nuclear community that the timely and successful operation and tests of the HTTR are major milestones in development of the HTGR and high temperature nuclear process heat application. This paper gives an overview of the status of the HTTR project, and chemical impurity behaviour in the secondary helium coolant during the initial 950°C operation of the HTTR. 2. HTTR equipment 2.1 Outline of the HTTR Based on the research and development of HTGR, the HTTR was built by the JAEA for the purpose of: (1) Establishment of basic HTGR technologies, (2) Demonstration of HTGR safety operations and inherent safety characteristics, (3) Demonstration of nuclear process heat utilization, (4) Irradiation of HTGR fuels and materials in an HTGR core condition, and, (5) Provision of testing equipment for basic advanced studies.

Figure 1 The bird’s-eye view of the reactor building of the HTTR

Air cooler

Spent fuel storage pool

Reactor pressure vessel

Secondary pressurized water cooler Intermediate heat exchanger Primary pressurized water cooler Reactor containment vessel

The reactor core, composed of graphite blocks, is so designed as to keep all specific safety features. In the cooling system, the intermediate heat exchanger (IHX) which pipes made by Hastelloy XR high-temperature alloy [2] is equipped to supply high-temperature helium gas to some process heat application system being coupled to the HTTR in the future. A bird’s-eye view of the reactor building is shown in Figure 1. The HTTR has a thermal power of 30MW and 950˚C maximum reactor-outlet coolant temperature. The detailed HTTR design has already been reported elsewhere [3] and so only the equipment relevant to the secondary helium chemistry is described in this chapter. 2.2 Helium purification system The helium purification system is installed to the primary and secondary helium circuits in order to reduce the quantity of chemical impurities such as hydrogen, carbon monoxide, water vapour, carbon dioxide, methane, oxygen, and nitrogen [4]. These impurities are emitted from the core graphite and thermal insulator. The secondary helium purification system mainly contains the following: an inlet heater, two copper oxide fixed beds to oxidize carbon monoxide and hydrogen, a water-cooled helium cooler, two molecular sieve traps to remove carbon dioxide and water by adsorption, a cold charcoal trap to remove impurities such as nitrogen and methane, gas circulators, and a dust filter to protect the helium circulator. Figure 2 shows a schematic diagram of the secondary helium purification system. The flow diagram of the primary helium purification system is almost same as that of the secondary system except for the presence of a pre-charcoal trap, etc. The main processes of metallic corrosion in coolant helium circuits are oxidation, carburization, and decarburization of high-temperature materials used in the heat exchanger.

2.3 Helium sampling system The helium sampling systems detect chemical and radioactive impurities in the primary and secondary helium cooling systems. The concentration of chemical impurities, hydrogen, carbon monoxide, water vapour, carbon dioxide, methane, nitrogen, and oxygen are measured by gas chromatograph mass spectrometers. The primary helium sampling system consisting of sampling equipment, a carrier gas supply system, and a standard gas supply system automatically transmits the impurity concentration measurement to the main control room, as does the secondary helium sampling system. The purpose of the secondary helium sampling system is: (1) To monitor the chemical impurity level of the secondary helium coolant, (2) To detect rupture of a heat exchanger tube of the secondary pressurised water cooler, and, (3) To monitor the performance of the traps in the secondary helium coolant purification system. The sampling locations of impurities other than water vapour in the secondary system are the inlet and outlet of the IHX, the outlet of the secondary helium coolant purification system, the inlet of the molecule

Figure 2 Schematic diagram of the secondary helium purification system of the HTTR

(a) (b)

(c)

(a) Copper oxidation trap (b) Molecular sheave trap (c) Cold charcoal trap AHX: Auxiliary Heat Exchanger IHX: Intermediate Heat Exchanger

RPV

IHX SPWC

PPWCAHX

sheave trap, and the inlet of the cold charcoal trap. Two detectors are installed for detecting water vapour and their sampling locations are: the inlet and outlet of the secondary pressurised water cooler, and the inlet and outlet of the primary purification system for the ‘No. 1’ detector; the outlet of the IHX, the outlet of the secondary helium purification system, and the inlet of the cold charcoal trap for the ‘No. 2’ detector. The sources of the initial impurities in the secondary coolant can be traced back to the heat insulator in the secondary hot-gas-duct and to original impurities of the helium gas supplied to the secondary helium cooling system. 3. Secondary coolant chemistry 3.1 Purification system performance Since the removal efficiency of the traps were set as 95% at the design stage, the evaluation results of the primary helium purification system showed that the removal efficiency of the actual equipment was depended on the trap entrance impurity concentration [5]. In the copper oxide trap following chemical reaction is occurred.

2H2 + O2 → 2 H2O (1) 2CO + O2 → 2 CO2 (2)

Therefore, removal efficiency of a copper oxide trap can be evaluated by the hydrogen and carbon monoxide concentration at the inlet and outlet of the copper oxide trap. The removal efficiency can be evaluated as following equation (3).

100×−

=i

io

CCC

η (％) (3)

where iC = copper oxide trap inlet concentration (vppm)

oC = copper oxide trap outlet concentration (vppm) The removal efficiency of carbon monoxide by the copper oxide trap during the operation is shown in Figure 3. A least-squares curve is constructed using a scatter chart of measurement results of impurities concentration. The approximate curve is expressed as equation (4) with normalized constants a, b. For this evaluation of carbon monoxide, a is 99.974 and b is - 0.9996.

iCba −=η (4)

0

25

50

75

100

0.01 0.1 1

Inlet concentration [vppm]

Rem

oval

effi

cien

cy [%

]

Figure 3 Removal efficiency of carbon monoxide by the copper oxide fixed bed during the 950°C

operation of the HTTR.

3.2 Chemical impurity behaviour The coolant chemistry deals with the behaviour of the gaseous impurities in the secondary cooling circuit and their reactions with high temperature materials. A main concern is the integrity of structures against corrosion. The coolant chemistry was monitored by the helium sampling system continuously between the reactor start-up and shut-down. Figure 4 shows the chemical impurity behaviour at the IHX secondary outlet during the 950°C operation period.

Each impurity was steadily removed by the purification system. In the operations below 950°C which were previously performed, impurities other than water did not increase rapidly. The hydrogen level rises with the temperature of the secondary system helium at the IHX outlet. Figure 5 shows the amount of impurities removed from secondary helium coolant and it is classified by temperatures. The gaseous output is the integrated amount of removed impurities. Hydrogen is the most common molecule in the discharged impurities. Large amount of hydrogen was emitted from the temperature range of 600°C to 800°C. Hydrogen and water are discharged from the thermal insulator. Figure 6 shows the amount of impurities removed from primary helium [5] and it is sorted by reactor outlet temperature. Comparing the impurity emission characteristics between the primary and secondary coolants, it is shown that the impurities in the secondary coolant are characterized by large amounts of nitrogen. The source of nitrogen is the new helium which is supplied frequently so as to control the regulated differential pressure with primary helium. Since methane was not detected in the secondary coolant helium, it is constantly detected in the primary helium. Methane in the primary circuit was thought to be generated by the radical reaction between the core graphite which contains water as an impurity.

0.0

0.2

0.4

0.6

0.8

1.0

23-May 28-May 2-Jun 7-Jun 12-Jun 17-Jun 22-Jun 27-Jun 2-Jul 7-Jul 12-JulDate (year: 2004)

Impu

rity

conc

entr

atio

n [p

pm]

0

200

400

600

800

1000

Tem

pera

ture

[ ℃]

H2

H2O

N2

IHX 2nd-outlet temp

Reactor-outlet temp

Figure 4 Chemical impurity behaviour at the reactor inlet observed during the HTTR operation.

3.3 Chromium stability A convenient way of representing the production of a corrosion compound as a function of gas chemistry is to construct thermochemical diagrams that depict the stability ranges of various condensed phases as functions of the thermodynamic activities of the two components of the reactive gas [6,7,8]. Figure 7 shows the thermochemical stability diagram for the Cr–C–O system developed for a temperature of 858°C. In the construction of this diagram, the thermodynamic activities of the metal and corrosion–product phases are assigned a value of unity.

0.0

1.5

3.0

4.5

H2 CO H2O CO2 CH4 N2

Impurity

Am

ount

of e

mis

sion

[g] ～500℃

500℃～600℃600℃～800℃800℃～

H2 CO H2O CO2 CH4 N2

Figure 5 Amount of removed impurities from the secondary helium coolant of the HTTR

0

500

1000

1500

H2 CO H2O CO2 CH4 N2

Impurity

Am

ount

of e

mis

sion

[g]

～500℃500℃～600℃600℃～800℃800℃～

H2 CO H2O CO2 CH4 N2

Figure 6 Amount of removed impurities from the primary helium of the HTTR [5]

-8

-6

-4

-2

0

-28 -24 -20 -16Log pO2 (atm)

Log

a C (carburization)

Date: 2004/6/24Operation: Parallel-loaded2nd He Tmp. : 860 oC

HTTR　2nd He(decarburization)

Cr2O3

CrxCy

Cr

Figure 7 Thermochemical diagram for Cr-C-O system at 858°C, indicating the stability region of

various phases.

This evaluation shows that the secondary helium at inside of the heat transfer pipes of the IHX has a slightly carburizing atmosphere which atmosphere can be expected to maintain structural integrity of Hastelloy XR better than that of decarburizing atmosphere. 4. Concluding remarks Chemical impurity behaviour in the secondary helium coolant of the HTTR during the initial 950°C operation is evaluated in this paper. The results of this evaluation are: (1) Removal efficiency of the helium purification system is dependent on inlet concentrations. (2) The emitted impurity to the secondary helium from the thermal insulator was calculated by using

chemistry concentrations measured by the helium sampling system. (3) The secondary helium at inside of the heat transfer pipes of the IHX has a slightly carburizing

atmosphere which atmosphere can be expected to maintain structural integrity of Hastelloy XR. 5. Acknowledgements The authors would like to express their appreciation to Mr. Koichi Emori and Mr. Yoshiaki Komori of the JAEA Department of HTTR for their useful comments and advice. 6. References

[1] S. Saito, T. Tanaka, Y. Sudo, O. Baba, et al., JAERI 1332, 1994. [2] Y. Tachibana, T. Iyoku, Nuclear Engineering and Design, Vol.233, Iss.1-2, 2004, pp.261-272 [3] K. Hada, Y. Motoki, O. Baba, JAERI-M 90-148, 1990 (in Japanese). [4] N. Sakaba, T. Furusawa, T. Kawamoto, Y. Ishii, Y. Oota, “Short design descriptions of other

systems of the HTTR”, Nuclear Engineering and Design, Vol.233, Iss.1-2, 2004, pp.147-154 [5] N. Sakaba, S. Nakagawa, T. Furusawa, K. Emori, Y. Tachibana, “Coolant Chemistry of the High

Temperature Gas-Cooled Reactor ‘HTTR’ ”, Nihon-Genshiryoku-Gakkai Shi (J. At. Energy Soc. Jpn.), Vol.3, No.4, 2004, pp.388-395

[6] Quadakkers, W. J., H. Schuster, “Thermodynamic and Kinetic Aspects of the Corrosion of High-Temperature Alloys in High-Temperature Gas-Cooled Reactor Helium”, Nuclear Technology, Vol.66, 1984, pp.383-391

[7] Y. Kurata, Y. Ogawa, H. Nakajima, “Effect of Decarburizing Helium Environment on Creep Behaviour of Ni-base Heat-resistant Alloys for High-temperature Gas-cooled Reactors”. Tetsu-to-Hagane, Vol.74, No.2, 1988, pp.380-387 (in Japanese)

[8] Y. Kurata, Y. Ogawa, H. Nakajima, “Effect of carburizing Helium Environment on Creep Behaviour of Ni-base Heat-resistant Alloys for High-temperature Gas-cooled Reactors”. Tetsu-to-Hagane, Vol.74, No.11, 1988, pp.2185-2192 (in Japanese)