A study on the methodology for tritium behavior in the gas cooled reactor for hydrogen production system

In Korea, a nuclear hydrogen program has been established to develop and demonstrate mass production system for hydrogen generation. The objective of this study is to establish the evaluation methodology and procedure for the tritium behavior in the NHDD(Nuclear Hydrogen Development and Demonstration) reactor. In order to achieve this objective, tritium generation, diffusion, and permeation are considered and the simulation code predicting the tritium behavior was developed. The newly formulated and developed TBEC (Tritium Behavior Estimation Code) program can estimate the tritium generation and the corresponding tritium diffusion and permeation with respect to the temperature distribution in the Pebble and Prism type reactor core. The tritium generated by the fission reaction can be leaked into the helium coolant from the coated ceramic particles and fuel elements. The annual release rate of tritium diffused from coated fuel to the primary helium coolant through the encapsulated graphite were evaluated about 0.47 percent in case of the Pebble, 0.39 percent in case of the Japan prism type, and 10 percent in case of the U.S.A Prism type compared with the generated tritium. There is a big difference between the Japan and the U.S.A Prism type gas cooled reactor. In case of the Japan Prism type fuel element, Graphite plugs are densely screwed into the tops of the fuel holes so any other gas can not permeate these graphite plugs. But in case of the U.S.A, Graphite plugs cemented into the tops of the fuel holes enclose the fuel compact stacks. These graphite plugs density is very low so the tritium gas can be permeated into the primary coolant. Tritium attributed by ⁶Li, existing as impurities in the reflector, can be released to the helium coolant by the diffusion process and the total annual release rates of the tritium are estimated as 5.3 percent in case of Pebble type and 4.2 percent in case of the Prism type reactor through the reflector to the helium coolant. Based on the Siverts' law, tritium permeation from the primary coolant to the hydrogen production system is also evaluated and the result is calculated as 0.23 ∼ 76 Bq/g-H₂ with respect to the PRF (Permeation Reduction Factor =10∼1000) in case of the normal operation of the 300 MW_th Pebble type reactor. Also, tritium specific activity in the U.S.A. type Prism reactor is only 0.6 ∼ 497.5 Bq/g-H₂ with respect to the PRF. Tritium activities in each case of reactor types with respect to the variation of input parameters are also performed. As the results of sensitivity analysis are shown, tritium generation amount is lager than that of the any other source even if the variation of the ternary fission value is small. Lithium impurities in the graphite material can be contained up to 100ppm then tritium activities generated by ⁶Li can be the largest source in the first year of operation. But the Li impurities in the graphite can be sufficiently controlled as low as possible so the probability that the ⁶Li is the largest source is very low. In view of the conservatism, there are little differences of the maximum tritium specific activities between each case of reactor types. No oxide film layer effect can cause the relatively fast permeation of tritium through IHX and this phenomenon result in successively the low effect of He purification system. Average and minimum tritium specific activities taken into account of PRF which has up to 100 with He purification system is one order lower than that of the case without He purification system. Therefore, the level of tritium specific activities are generally evaluated as 10 ∼ 100 Bq/g-H₂ regardless of reactor types. Also, the minimum tritium specific activities can be reached a few Bq/g-H₂ by using the He purification system even if the PRF value is not maximized up to 1000