Patent Number: 054426685
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to passive decay heat removal in a pressure tube light water cooled and moderated reactor with a large power rating on the order of 1,000 MWe. A large amount of decay energy is continually generated by nuclear fuel after the reactor is shut down. Conventionally, actively pumped redundant cooling systems provide the emergency coolant to remove this decay heat under abnormal conditions such as a loss of coolant accident. Conventional reactors have the additional disadvantage that their power density profile has a maximum average power ratio or peaking factor of about, 2, even when measures are employed to minimize this peaking. SUMMARY OF THE INVENTION One object of the present invention is to insure that the fuel remains undamaged and within safety limits even in the total absence of coolant in the primary system of the reactor. Another object of the present invention is to provide a heat path to ambient air which is provided by natural heat transfer mechanisms such as, conduction, radiation and free convection and which eliminates the reliance on the availability of pumped cooling. A further object of the present invention is to considerably flatten the power density profile of the reactor across the core to increase safety margins or increase power rating. These and other features of the present invention are achieved in accordance with the present invention in one embodiment wherein a wet calandria design utilizes a fuel matrix in a pressure tube with light water as the coolant, the calandria is surrounded by a solid reflector having a mixture of a light water moderator and a void or graphite in the calandria space and arranged such that the light water moderator is always in contact with the calandria tube and provides a heat sink, and the optional use of a thermal switch in the gap between the pressure tube and the calandria tube which provides thermal insulation during normal operation to minimize heat loss and enhance heat transport across the gap during accidents. In accordance with another embodiment of the present invention wherein a dry calandria is utilized, the present invention is carried out utilizing a fuel matrix in a pressure tube with light water as the coolant, a dry calandria surrounded by a solid reflector connected by passages to a light water pool kept outside the calandria via a gas lock and self-actuated means which initiates calandria flooding by releasing the gas pressure within the calandria space during accidents which could lead to temperatures exceeding the safety limits. As a result of the above-mentioned features of the present invention, both the wet calandria and the dry calandria embodiments can survive the loss of coolant in an accident without a scram and without replenishing primary coolant, there is a super flat power density profile which considerably increases safety margins or allows operation at higher power ratings, very tight neutronic core coupling is achieved which allows for reactor control to be performed from outside the core and eliminating the possibility of local criticality and giving absolute stability towards xenon spatial oscillations, long prompt neutron lifetime in comparison with conventional light water reactors which reduces potential concerns with prompt reactivity excursions and negative coolant void coefficient is attained. With respect to the dry calandria embodiment, other advantages include the fact that it can operate in post-critical heat flux without exceeding safe fuel and matrix temperature limits and has one additional barrier in the silicon carbide matrix coating to fission product release. While all of the above-mentioned embodiments utilize water as a coolant, the wet calandria design can also be adapted to use gas coolants and both versions can employ organic coolants. These and other features and advantages of the present invention will be seen from the following detailed description of the invention taken with the attached drawings, wherein: