Fracture Mechanical Assessment of VVER Reactor Internals

Abstract

Reactor internals of VVER reactors are heavy components made of austenitic steel 08Kh18N10T. As they are close to the reactor core, they receive very high dose during their operation, which causes significant embrittlement. Their brittle fracture initiation has to be excluded during all operating conditions including emergency conditions to maintain its functionality and to avoid mechanical damage of fuel assemblies. During the emergency event the reactor internals are loaded by highly non-uniform temperature field due to thermal shock and also by dynamical loading due to non uniform pressure load. The temperature loading is relatively slow (critical time is about 100 s after beginning of the emergency event), while the mechanical loading is very fast (critical time is about 0.1 s after beginning of emergency event). The worst emergency event for both types of loading is LB LOCA. Due to this fact, both loading types can be assessed separately. This paper deals with the temperature loading. The procedure for fracture mechanical assessment of VVER reactor internals is given in “Guidelines for Integrity and Lifetime Assessment of Components and Piping in VVER Nuclear Power Plants (IAEA-NULIFE-VERLIFE)”, Appendix C “Integrity and Lifetime Assessment Procedure of RPV Internals in VVER NPP’s During Operation”. The procedure prescribes postulated cracks in the components and allowable value of J-integral in dependency on dose and temperature. The assessment of brittle fracture initiation is based on comparison of J-integral calculated for postulated crack for the assessed emergency regime with its allowable value. In the paper, brief description of both the VVER reactor internals design and methodology of the assessment are presented, and also some examples of the results are shown.