Response of the ISER for a beyond-design-basis hypothetical accident

This paper describes the simulation results of two cases of beyond-design-basis hypothetical accident (BDBHA) of the intrinsically safe and economical reactor (ISER) (IAEA-TECDOC-479, 1988, pp. 152-156; H. Wakabayashi, Nucl. Eng. Int. June (1988) 45-46; H. Wakabayashi, T. Yoshida and Y. Asahi, Nucl. Eng. Des. 126 (1991) 89-103; H. Wakabayashi, Proc. 1st JSME/ASME Joint Int. Conf. on Nuclear Engineering (ICONE-1), Tokyo, 1991, pp. 305-310) due to a hypothetical failure of the steel-made inner vessel; the ISER-200 is a steel reactor pressure vessel (RPV) type PIUS (process inherent ultimate safety) reactor of ABB-Atom. ISER's RPV is submerged in a medium pressure (2 MPa) borate water pool made of prestressed concrete to protect passively the pressure boundary and to assure a longer grace period (period needed for assurance of the core safety without the assistance of active components and human intervention) of over 40 days. It was concluded from the present simulations that the ISER with a low pressure relief valve adequately ensures the core integrity even for a BDBHA involving a break of the reactor pressure vessel. (orig.)