Development of an evaluation method for the HTTR-IS nuclear hydrogen production system

A thermochemical water splitting hydrogen production system based on the iodine sulphur (IS) process is presently under development in JAEA. The hydrogen production system is to be connected to the HTTR operating test reactor in JAEA. An important development goal for the HTTR-IS system is design and construction of the IS process to the standards of a conventional chemical industrial plant in order to simplify the cost and operation of the overall nuclear hydrogen production. The present study is conducted to meet a key technology requirement to achieve the goal, that is, to establish a method to evaluate the effects of anticipated abnormal load changes in the IS process on the reactor operation. The numerical models are developed with capability to compute abnormal transient behavior of process heat exchangers in the IS process. Furthermore, interface schemes between the RELAP5 code and the models are developed to quantify the impact of IS process load change events on the reactor operation. A preliminary analysis in case of inadvertent opening of pressure control valve in the H2SO4 decomposition procedure of the HTTR-IS system has been conducted. The results of the analysis show that the abnormal load increase of the IS process due to the decrease in boiling and dew point in the H2SO4 decomposer and to the chemical equilibrium shift of the H2SO4 decomposition is successfully simulated by the chemical reaction and H2SO4–H2O aqueous system models developed. Furthermore, the mitigation effect of a steam generator installed alongside the IS process against abnormal load transient in the reactor is clarified by the developed algorithm interfacing the chemistry model calculation with the system analysis that considers the heat conduction in heat transfer tubes. It is shown that the thermochemical models and model interface schemes developed in the present study make possible the system analysis of the HTTR-IS nuclear hydrogen production during the IS process abnormal events.