Development of Core Dynamics Analysis of Coolant Flow Reduction Tests of HTTR

The high temperature engineering test reactor (HTTR) is a graphite-moderated gas-cooled reactor with a thermal power of 30 MW and a reactor outlet coolant temperature of 950°C. Safety demonstration tests using the HTTR are in progress to verify the inherent safety features and to improve the safety technology and design methodology of high temperature gas-cooled reactors (HTGRs). The coolant flow reduction test is one of the safety demonstration tests for the HTTR. This test simulates a rapid decrease in reactor power by tripping one or two out of three gas circulators without operating the reactor power control system. The experimental results revealed the inherent safety features of HTGRs, such as the negative reactivity feedback effect. A numerical analysis code was developed to analyze the reactor dynamics including the flow behavior in the HTTR core. We used a conventional method that utilizes a fuel channel model and a reactor kinetics model with a single temperature coefficient taking into account the temperature variation in the core. This paper describes the validation results of the analysis code using the experimental results of the coolant flow reduction tests. It was confirmed that the analysis code could not only analyze the reactor core dynamics accurately but also simulate the core dynamics during the abnormal reactivity insertion events postulated in the HTGR safety analysis. This code can also be applied for the safety analysis of the very high temperature reactor (VHTR), which is one of the Generation IV reactor concepts. (author)