Natural-circulation decay heat removal

An integral sodium experiment has been carried out with a partial core model composed of seven subassemblies, inter-wrapper gaps, an upper plenum and dipped cooler. The tests for core-plenum and cooling system interaction were almost completed. A series of tests is under way focusing inter-wrapper flow under conditions of natural circulation decay heat removal. 

Kamide, H., et al.. Multi-bundle Sodium Experiments for Thermohydraulics in Core Subassemblies during Natural Circulation Decay Heat Removal Operation, Proc. of the IAEA Specialists Meeting on Evaluation of Decay Heat Removal by Natural Convection, Oarai, Japan, 1993. 

Watanabe, et al., 2015. Development of evaluation methodology for natural circulation decay heat removal system in a sodium cooled fast reactor. Journal of Nuclear Science and Technology 52 (9), 1102-1121. 

The in-conlainmeni refueling water storage tank provides 500,000 gallons (about 1900 cubic meters) of water with a gravity head above the core. This water inventory is sufficient to flood the containment above the level of the reactor core and provide decay heat removal by natural circulation. 

Long term coolant Low pressure coolant injection system (2) Re-circulation coolant system (1) Static residual heat removal system (2) (Natural circulates) Emergency decay heat removal system (3) (Natural circulates)... 

The engineered safety system of MRX is greatly simplified by adoption of the water-filled containment and the passive decay heat removable system relied on the natural circulation. Comparing the numbers of the sub-systems and equipment in the engineered safety system with other nuclear plants, the number of is decreased significantly. 

In the 4S, following the loss of off-site power, the primary loop shifts to natural circulation. The pump in the secondary loop of the decay heat removal system does not work assuming the loss of emergency power following loss of off-site power. Under such conditions, the secondary coolant operates in natural circulation mode and natural air-cooling operates in the air cooler. Thus, a passive heat removal circuit is established. 

For plant designs in which natural circulation is an integral part of the decay heat removal process, loss of natural circulation has sometimes been shown to be an important event. Thus, for those plants which require natural circulation, detailed analyses should be performed to identify how natural circulation can be lost or degraded. 

Safety evaluation studies have been conducted for confuming the physical phenomena and integrity of the fuel subassemblies, the core internal structures and the heat transport systems during the normal operation, scram transients and the early stage of postulated accidents. On this account, thermohydraulic experiments related to the decay heat removal by natural circulation have been carried out, and the development and validation of the thermohydraulic safety analysis codes is also in progress. 

During October 1996, 4 experiments on research of heat exchange and process of natural circulation of the heat-carrier of 1 and 2 circuits are conducted. In experiments results are obtained confirming an opportunity of long-duration of decay heat removal after drainage of the steam-generators for a level of capacity 520 MW The experiment 4 proceeded more than 30 hours and allowable levels of temperatures 420 °C for cold part of the secondary circuit pipeline were not exceeded. 

Improved decay heat removal. Improved heat transfer by natural circulation of the molten salt allows the design of larger reactors with passive safety (see Sect. 3.3). 

Thae has also been important work on the natural circulation behaviour under decay heat removal ccmditions. The very good potential for a sodium cooled reaaor has been demonstrated without severe loadings to the structures. 

The potential to remove decay heat by natural circulation is one of the important safety features of LMFBRs. To enhance the decay heat removal capability under the case of loss of all electrical power, experimental studies on natural convection in a reactor vessel are in progress using sodium as a working fluid. 

If feed water systems or steam generator heat removal is not available, two PRHR heat exchangers provide decay heat removal. The system is located above the Reactor Coolant System (RCS) and forms a closed loop at full system pressure using natural convection circulation. The... 

The IC/PCC pool has an installed capacity that provides at least 72 hours of reactor decay heat removal capability. The heat rejection process can be continued indefinitely by replenishing the IC/PCC pool inventory. The ICS passively removes sensible and core decay heat from the reactor (i.e., heat transfer from the IC tubes to the surrounding IC/PCC pool water is accomplished by natural convection, and no forced circulation equipment is required) when the normal heat removal system is unavailable. 

Emergency decay heat removal channel permanently connected to the secondary circuit by valves having different principles of action (natural coolant circulation in primary and secondary circuits and forced circulation of service water or evaporation of service water in the tanks). 

Decay heat removal Residual heat Removal system (RHRS) passive containment coolmg system (PCCS) passive passive Natural circulation... 

The emergency decay heat removal system (EDRS) is a closed system which transfer decay heat from the core to the containment water It includes four trains each of which has the ability to remove the core decay heat Each train consists of a water reservoir tank, a cooler, two valves and piping In the case of accidents, the valves are opened after the main feed water/steam isolation valves are closed Coolant will be circulated by natural convection, heated by the steam generator and cooled by the cooler An alternative EDRS is also being studied... 

Two pipes between Pressuri2 r and Reactor Vessel connect hydraulically the top and the bottom of the respective cold water plena in order to create a common plenum. The choice of two connection levds makes natural circulation possible in case of temperature difference between cold plena. If the normal decay heat removal route (i.e. the active steam/water system) is lost, the uninsulated wall portion of the Pressurizer would thus help by conducting the decay heat to the Reactor Pool. 

Decay heat removal PRHRS/AHRS Passive Natural circulation of water/jur AHRS is permanently connected... 

Four primary reactor auxiliary cooling systems (PRACS) are used A cooling coil is installed in the inlet plenum of each IHX and a heat transfer coil is installed in the mr cooler of ultimate heat sink Coolant is circulated by EM pumps supported by emergency AC power The air cooler consists of a blower, a stack, vanes and dampers The blower is supported by the emergency AC power The vanes and the dampers are operated by the emergency DC power Decay heat removal by natural circulation is possible to mitigate a total blackout event (loss of all AC power)... 

The decay heat is removed by a system consisting of the decay heat removal coil installed in the reactor and the natural ventilation of air from outside the guard vessel. Heat removal by natural circulation takes place inside the reactor after shutdown, eliminating necessity for the operation of active components. 

The arrangement and relative elevation of the IHTS piping and components, shown in Figure 6.5, are designed to promote natural circulation for decay heat removal. 

Further development of the UNITHERM layout was defined by the decision not to employ operating personnel for reactor control. A sudden reduction or even cessation of heat transfer to the user should not cause shutdown of the reactor and overshooting of the system parameters. Such situation can be mitigated through the heat exchanger- evaporator in the continuously operated independent circuit for heat dump, which is added to the intermediate circuit. In addition to the evaporator, the circuit consists of the radiator (7) connected to the evaporator and cooled by atmospheric air under natural circulation, see Fig. II-1 and II-2. The independent circuit for heat dump allows transfer of the reactor to a hot standby mode without the need for shutdown. In emergency situations the circuit acts as the decay heat removal system. 

The RVACS removes shutdown heat with natural circulation of air. In the IRACS operation for shutdown heat removal, dampers are adjusted for the required capacity of heat removal. In case of a long-term operation for decay heat removal, IRACS is directed into a natural circulation mode via the adjustment of the dampers. 

The levels of natural circulation in the heat-removal circuits are provided sufficient to ensure decay heat removal without unacceptable over-heating of the core ... 

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