A qualification of the thermal hydraulic system code RELAP5 (reactor excursion leak analysis program) is conducted for the analysis of helically coiled heat exchangers used in a high-temperature environment. A set of closure models based on the past separate effect test are suggested and validated against the measured data in the high temperature engineering test reactor (HTTR). The modified code is then tested for the analysis of a representative nuclear hydrogen production system.
The comparison of calculated and measured data with steady-state operation showed that the original heat transfer package in RELAP5 significantly underpredicts the peak heat transfer tube temperature in the intermediate heat exchanger (IHX), particularly when the inlet helium gas temperature exceeds 600 degrees C. In contrast, the prediction with the suggested model generally agreed well. Lower prediction of peak temperature was observed in outer shell due to the modeling capability in system codes. However, those and the other results from the analysis of the HTTR-IS system revealed that the simplification of the heat structure geometry in IHX with low operating shell temperature is acceptable. In conclusion, this study explores the applicability of the system code to safety analysis of nuclear hydrogen production systems by means of a newly introduced closure model.