Natural-circulation decay heat removal system

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. 

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. 

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. 

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... 

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. 

Second, the AHTR operates 200 to 500 C hotter the S-PRISM (500 to 550 C for S-PRISM versus 750 to 1000+ C for the AHTR). Since natural circulation of cooling air increases with temperature and heat transfer across the argon gap varies with T, the higher temperatures allow for more efficient removal of decay heat, with heat removal rates adjusted by design of the decay-heat removal system. 

Decay heat removal. Several types of passive decay heat removal systems have been used in liquid-metal reactors. The AHTR, like S-PRISM, uses RVACs. There are other options such as DRACs, a secondary natural circulation loop to remove heat from the reactor vessel to the environment. This provides multiple longer-term cooling options including the options that may ultimately allow larger power outputs. 

Increased reliability of decay heat removal, achieved through a passive decay heat removal system, which transfers the decay heat to GDWP by natural circulation. 

Primarily the radiative heat transfer through the argon gas from the reactor vessel to the guard vessel controls the rate of heat removal. Radiative heat transfer increases by the temperature to the fourth power (T ) thus, a small rise in the reactor vessel temperature (as would occur upon the loss of normal decay heat removal systems) greatly increases heat transfer out of the system. Under accident conditions such as a loss of forced cooling accident, natural circulation flow of liquid salt up the hot fuel chaimels in the core and down the edge of the core rapidly results in a nearly isothermal core with about a 50 C temperature difference... 

In terms of passive decay heat removal systems, a major difference is noted between the liquid cooled AHTR and gas cooled reactors. The AHTR can be built in very large sizes (>2400 MW(th)), while the maximum size of a gas cooled reactor with passive decay heat removal systems is limited to -600 MW(th). The controlling factor in decay heat removal is the ability to transport this heat from the center of the reactor core to the vessel wall or to a heat exchanger in the reactor vessel. The AHTR uses a liquid coolant, where natural circulation can move very large quantities of decay heat from the hottest fuel to the vessel wall with a small coolant temperature difference ( 50°C). Unfortunately, under accident conditions when a gas cooled reactor is depressurized, the natural circulation of gases is not efficient in transporting heat from the fuel in the center of the reactor to the reactor vessel. The heat must be conducted through the reactor fuel to the vessel wall. This inefficient heat transport process limits the size of the reactor to -600 MW(th) to ensure that the fuel in the hottest location in the reactor core does not overheat and fail under accident conditions. 

A schematic of the reactor and cooling system is shown in Fig. 11.2. Two intermediate reactor auxiliary cooling systems (IRACSs) and one direct reactor auxiliary cooling system (DRAGS) have been applied as a decay heat removal system (DHRS) suitable for the two-loop cooling system and the adopted type of SG. These systems are passive type by natural circulation. 

Ohyama, K., et al., 2009. Decay heat removal system by natural circulation for JSFR. In Proceedings of International Conf. Fast Reactors and Related Fuel Cycles (FR09), Kyoto, Japan, December 7—11, 2009. 

Gen4 or HYPERION s safety system can remove the decay heat in two ways (1) dumping the steam to the condenser and (2) if first decay heat removal way is not sufficient, back up decay heat removal system is used. This system utilizes natural circulation of primary coolant through bypass path in the core. The surface of Gen-IV module is cooled with latent of heat via water sprays provided by emergency cooling tank. The water inventory in this tank can be injected due to gravitational force to remove the decay heat for 2 weeks. The second system works as a passive safety system. 

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. 

Residual Heat Removal System The residual heat removal sy stem (RHRS) of the NHR 5 consists of two independent trains which assigned to two groups of primary heat e.xchangers. There are three natural circulation cycle for each train. Figure 4 show s the schematic s stem diagram of the RHRS. After reactor shut-down the decay heat will be transferred to the... 

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. 

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

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. 

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. 

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