Safety Characteristics of the Circular and Annular Pebble-Cores for a 200MWth Prototype VHTR for a Hydrogen Production

Korea Atomic Energy Research Institute (KAERI) has proposed the conceptual design of a very high temperature gas-cooled reactor (VHTR) of a pebble-bed type with 200MWth (PBR200) for demonstrating a nuclear hydrogen production. The PBR200 is a simple and inherently safe nuclear system. The only safety related system of the PBR200 is an air-cooled reactor cavity cooling system (RCCS) which has a capability of removing decay heat by cooling the reactor pressure vessel (RPV) wall with a natural circulation of air. Two types of pebble-bed cores are considered: annular core with a central reflector and a circular core. Since the safety characteristics during an accident strongly depend on the reactor core shape, fuel temperature behaviors of both pebble-core candidates are investigated for the limiting Low-Pressure Conduction Cooldown (LPCC) accident. The annular core with a large heat storage capability of a central reflector might be expected to have a large enough temperature margin for the fuel failure limit, but it is doubtful whether the circular core may have enough safety margin or not. In particular, since the thermal conductivity and heat capacity of the graphite pebbles and reflectors are a strong function of a fast neutron irradiation level, its impact on the degree of a safety margin is examined and discussed for both PBR200 candidates