PASSIVE HEAT REMOVAL BY VESSEL COOLING SYSTEM OF HTTR DURING NO FORCED COOLING ACCIDENTS

The high temperature engineering test reactor (HTTR) being constructed by the Japan Atomic Energy Research Institute is a graphite-moderated , helium-cooled reactor with an outlet gas temperature of 950 degrees C. Two independent vessel cooling systems (VCSs) of the HTTR cool the reactor core indirectly during depressurized and pressurized accidents so that no forced direct cooling of the reactor core is necessary. Ea ch VCS consists of a water cooling loop and cooling panels around the reactor pressure vessel (RPV). The cooling panels, kept below 90 degre es C, cool the RPV by radiation and natural convection and remove the decay heat from the reactor core during these accidents. This paper de scribes the design details and safety roles of the VCSs of the HTTR du ring depressurized and pressurized accidents. Safety analyses prove th at the indirect core cooling by the VCSs and the inherent safety featu res of the reactor core prevent a temperature increase of the reactor fuel and fission product release from the reactor core during these co nditions. Furthermore, it is confirmed that even if VCS failure is ass umed during these accidents, the reactor core and RPV can remain in a safe state.