PROPOSAL OF HIGH RELIABILITY DHR SYSTEM FOR NPPs WITH NEVER ENDING CAPACITY

The present paper aims at describing a totally innovative top-reliability Decay Heat Removal (DHR) system suitable for application in Nuclear Power Plants (NPPs). It is completely passive and with a never ending capacity. The configuration here described is mainly addressed to liquid metal cooled nuclear reactors, but is based on a sub-system also applicable to any type of nuclear power plant and even to some chemical plants. The main subsystems of the innovative DHR System are patent pending. The paper is mainly based on the work carried out in the framework of the CP ESFR (European Sodium Fast Reactor) project (7th R&TD FP of the EU); the research leading to these results has received funding from the European Community’s Seventh R&TD Framework Programme (FP7/2007-2011) under grant agreement n° 232658. The innovative DHR system is based on a Direct Reactor Cooling System (DRC): the DRC is basically made of a loop (in this case, with oil as coolant) receiving nuclear decay heat through a Radiation-based bayonet Decay Heat Exchanger (DHX) and transferring the heat through natural circulation to an innovative external Air cooled Heat Exchanger (AHX), using water and air as heat sink during the cooling phase. The patent pending DHX is a sodium / vacuum / oil Bayonet-tubes type. The thermal power is transferred from primary nuclear coolant (in the paper, sodium) to oil of the DRC loop through radiation in the vacuum gap of the bayonet. The patent pending AHX is an oil/ water-air pool heat exchanger, which transfers thermal power from oil to different fluids: water at the beginning of the transient, when the decay thermal power to remove is the highest; then, following the decrease of the decay heat, and after the vaporization of the boiling water, thermal power is transferred from oil to air (that replaces water in the pool without any external intervention). This AHX was conceived and developed for top-reliability cooling in PWRs, but has a generalized applicability. Preliminary analyses have demonstrated the reliable operation of the two innovative sub-systems (DHX and AHX) coupled in the SFR project, in an absolutely reliable natural circulation DRC . The absence of risks due to anticipated failure of the barriers either/both on the primary or/and on the secondary side of the DHX, due to the vacuum gap presence as well as the perfect compatibility of the secondary oil with the primary coolant (sodium), make this concept a very attractive solution for the diversified ESFR DRC passive system. The same concept is applicable to Lead Fast Reactors as well. It is interesting to underline that the innovative DHR concept is easily implemented in every industrial plant where, should an hazardous accident occur, a heat removal system can avoid more dangerous consequences (i.e.: in Nuclear Power Plants, in severe accident management, eventually substituting oil with other coolant; in chemical plants to manage run-away reactions).