The mechanical behavior and reliability prediction of the HTR graphite component at various temperature and neutron dose ranges

Abstract

In a pebble-bed high temperature gas-cooled reactor (HTR), nuclear graphite serves as the main structural material of the side reflectors. The reactor core is made up of a large number of graphite bricks. In the normal operation case of the reactor, the maximum temperature of the helium coolant commonly reaches about 750°C. After around 30 years’ full power operation, the peak value of in-core fast neutron cumulative dose reaches to 1×1022ncm−2 (EDN). Such high temperature and neutron irradiation strongly impact the behavior of graphite component, causing obvious deformation. The temperature and neutron dose are unevenly distributed inside a graphite brick, resulting in stress concentrations. The deformation and stress concentration can both greatly affect safety and reliability of the graphite component. In addition, most of the graphite properties (such as Young's modulus and coefficient of thermal expansion) change remarkably under high temperature and neutron irradiations. The irradiation-induced creep also plays a very important role during the whole process, and provides a significant impact on the stress accumulation. In order to simulate the behavior of graphite component under various in-core conditions, all of the above factors must be considered carefully. In this paper, the deformation, stress distribution and failure probability of a side graphite component are studied at various temperature points and neutron dose levels. 400°C, 500°C, 600°C and 750°C are selected as the core-side temperature of the graphite component, while the range of neutron dose is 0–1×1022ncm−2 (EDN). It is shown in the numerical analysis that temperature strongly affects the histories of both stresses and failure probabilities as fast neutron dose increases. In addition, the behavior of graphite brick at 750°C is observed obviously different from that at other temperatures.