Active decay heat removal systems

KALIMER has enhanced safety features with the use of metallic fuel, Self-Actuated Shutdown System (SASS), Gas Expansion Module (GEM) in the core, and Passive Safety Decay Heat Removal System (PSDRS). Utilization of these enhanced safety features eliminates the need for diverse and redundant engineered safety systems and KALIMER accommodates unprotected anticipated transients without scram (ATWS) events without operator action, and without the support of active shutdown, shutdown heat removal, or any automatic system without damage to the plant and without jeopardizing public safety. 

Inherent (i.e. intrinsic) safety feature" is a specialized term used in this case to describe the fact that the reactor itself reacts to certain malfunctions without the actuation of active systems or external controlling interventions in such a way that no inadmissible or even dangerous situations can be reached. These reactions are governed by the laws of nature, i.e. they always function regardless of the condition of active systems. This means that they cannot malfunction or fail. The technical and nuclear physical design of the HTR-Module is such that the maximum fuel element temperature always stabilizes itself below 1600°C even in the case of assumed failure of all active shutdown and decay heat removal systems. 

Later on, research and design activities concerned the development of special components as the improved check valves and the atmospheric-pressure condenser of the passive decay heat removal system. Experimental tests were also performed on prototypes. 

Safety system independent of emergency power and active decay heat removal... 

Morgenstem, F.H., "Safety Characteristics of Decay Heat Removal Systems", Proc. of the IAEA Specialists Meeting on Passive and Active Safety Features of LMFBRs, Oarai, Japan, 1991. 

Regarding the control of accidents within the design basis (DID Level 3 in Table 4), the design of AHTR incorporates a mechanical reactivity control and shutdown system based on control rods with the external drives, and two diverse decay heat removal systems, of which one is passive and one is active. The reference AHTR design uses passive reactor vessel auxiliary cooling (RVAC) systems similar to that developed for decay heat removal in the General Electric sodium cooled S-PRISM reactor. Different from its prototype, the RVAC system of the AHTR relies not only on the processes of convection and conduction but on the radiation also. 

Decay heat removal system 1 Accumulator injection (passive) and 2 Flooding systems (active) long-term... 

Active decay heat removal provided by residual heat removal (RHR) and clean up water (CUW) systems. 

There are two decay heat removal systems when the reactor is shut down. The first is a passive decay heat removal system used for all shutdowns (planned, unplanned, accident) and the second is an active non-safety grade system used for refuelling and other shutdown operations that require close control of the reactor core temperature. 

Korea has been actively engaged in international collaborative research activities. As part of this effort, Korea has been actively participating in collaborative research and development (R D) activities of the Gen-IV International Forum (GIF). Large experimental facihties have been constmcted to conduct various experiments to validate thermal—hydrauhc phenomena and a large sodium loop, called Sodium Test Loop for Safety Simulation and Assessment (STELLA)-1, for the test of key decay heat removal system (DHRS) components, started its operation in 2014. Design work started in early 2015 for STELLA-2, which is an integral test loop for a simulation of the thermal—hydraulic characteristics of the PGSFR primary and intermediate heat transport systems. 

Two activities related to Japanese SLP PSAs have been reported to-date. One is a MITI-sponsored Level 1 PSA for Typical Japanese 1.100 Class PWR and BWR Plants During Low-Power and Shutdown Operation , and another is the activities related to assessment of the decay heat removal system at LMFBR plant. 

Accident sequences investigated were decay heat removal without / with main cooling system. In the former case, both core and reformer are cutoff from the coolant circulation and a gradual temperature balance is obtained by heat conduction, radiation zind small internal convection. The activation of the main cooling system delayed by 1 h encounters a merely changed temperature distribution allowing for a smooth and gentle restart. 

The number and location of the top entry control rods and the diverse reserve shutdown control have been specified to assure that the reactor thermal power is controlled both for normal and off-normal conditions. The radial thickness of the active core annulus was specified on the basis of assuring that the control rod worths of the reflector-located rods would meet all shutdown and operating control worth requirements. The choice of reflector control, coupled with the choice of a control system withdrawal sequence and safety classification was made to assure that the control rod integrity is maintained during passive decay heat removal. 

Tl e reactor vessel is surrounded by a reactor cavity cooling system, which provides totally passive decay heat removal. A separate cooling system provides decay heat removal for refuelling activities. 

Low stored energy is a characteristic in small reactOTs which permits use of passive saf systems for safety functions such as decay heat removal. This in turn eUminates the redundancy needs of active components. This is believed to improve plant safety, while simultaneously reducing plant costs (frx- components and building volume). The simplicity attained by these features will be conducive to increasing plant availability and lowering the rize of the qroiating crew, as well as reducing the probability of serious accidents. 

Emergency water injection system for reactor flooding under LOCAs is designed as active one because of certain size limitations in the FNPP. It includes 3 high pressure and 2 low pressure pumps Water recirculation system for long term decay heat removal is also provided It is essential that even in case of all emergency heat removal and water injection systems failure under LOCA the reactor core uncovery starts only after 3 hours of the accident initiation... 

A key feature of LEADIR-PS, shared with the Modular High Temperature Gas-Cooled Reactor (MHTGR) under development by General Atomics, is that radionuclide releases are prevented by retention of the radionuclides within the fuel particles under all design basis events without operator action or the use of active systems. Thus, the control of radionuclide releases is achieved primarily by reliance on the inherent characteristics of the coolant, core materials, and fuel. Specifically, the geometry and size of the reactor core, its power density, coolant, and reactor vessel have been selected to allow for decay heat removal from the core to the ultimate heat sink through the natural processes of radiation, conduction and convection, while the negative temperature coefficients of the fuel and moderator assure reactor shutdown. 

The physical configuration of the reactor power module assures decay heat removal by passive means in the event that all normal heat sinks are lost, without the action of the operator or any active system, for all credible events. 

Decay heat removal Via the mam heat sink or via the RHR system, exceptionally, via coolant evaporation active Heat removal via mrbme condenser to heat sink Heat removal to heat sink or to containment... 

Decay Heat Removal Residual Heat Removal System (RIIRS) SIS/Containment Containment Spray System (CSS) Active Active/Passive Active ... 

Decay heat removal Secondary circuit cooling system (using main condenser) Tertiary circuit cooling system (using cooler) ECCS (using heat exchanger) Active Active Passive ... 

Decay heat removal Shutdown cooling system Reactor cavity cooling system Active Passive - If cavity cooling fails, heatup of environment without overheating fuel... 

Decay heat removal Main heat transfer system Cavity cooling system Active Activc/passive ... 

Decay heat remo al - Residual heat removal system (RHR) Active ... 

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 Primary reactor auxiliary cooling system (PRACS) Steam/water system Active Active N/C operation available Non-safety grade... 

A natural air cooling system is employed for cooling the reactor cavity. While its main purpose is removal of heat from the reactor in normal operation, it has also a function of decay heat removal. Points to be noted in the natural air cooling system include uniform temperature distribution circumferentially low pressure loss, and prevention of air activation. Particular attention is placed on the uniformity of circumferential temperature distribution. 

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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