Safety-grade decay heat removal system

The passive safety grade decay heat removal system is the reactor vessel air cooling system (RVACS). The steam generators (immersed in the secondary coolant pool) provide another heat rejection path for decay heat removal. 

Passive safety-grade decay heat removal system (PSDRS). 

Highly reliable and diversified decay heat removal Emphasis should be given not only to safety grade decay heat removal systems but also to non-safety grade decay heat removal systems. 

The design features of the KAMADO provide a passive decay heat removal capability with all components of the reactor core and water pool acting as a passive decay heat removal system. In this, the residual heat removal (RHR) system is reduced to a water pool cooling system, which could be made non-safety-grade and passive. 

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. 

The inherent safety characteristic against postulated events is the most remarkable superiority of a liquid metal cooled reactor (LMR) to other type of reactors. One of the major threats to the safety of LMR is a loss of flow event accompanied a failure of reactor shutdown systems. This situation is usually referred to as an unprotected loss of flow (ULOF). The inherent safety of the Korean Advanced Liquid Metal Reactor (KALIMER) during the ULOF [I] has been assessed for the situation of all pump trips followed by coastdown. It was assumed that the decay heat is removed by four intermediate heat exchangers (IHXs) and the safety grade system of passive safety decay heat removal system (PSDRS). The results showed that the power was stabilized by the reactivity feedback of the system even though the effect of the gas expansion module (GEM) was not taken into account. 

The advanced light water reactor (ALWR) passive plants, employ safety grade passive decay heat removal (PDHR) systems in order to enhance the capabihty (relative to current plants) of maintaining the plant in a safe shutdown condition following non-LOCA events. 

There is a passive system for providing make up water in the event of a LOCA, the Emergency Coolant Injection System (ECIS). This uses steam injectors to pump water from the containment tanks into the RPV. The steam is taken from the top of the pressuriser. There is two times redundancy. This system can be combined with a bleed system to provide a second route for decay heat removal. Fig. 7.5.2. gives a schematic diagram of the safety systems and the nonsafety grade heat removal systems. 

The normal decay heat removal function of the steam and feedwater systems is not safety related. The closure of both steam and feedwater lines isolation valves allows a complete separation of the NSSS from the non safety grade BOP. No need for pressure relief is anticipated, since the system is designed for the maximum expected pressure. 

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

The ECCS and CSS installed in conventional PWRs are eliminated. The residual heat removal system (RHRS), CCWS, ESWS, emergency AC power system, and heating, ventilating, and air conditioning (HVAC) system of the main control room are designed as non-safety grade, since the SDHS directly removes decay heat from core to the atmosphere through the SGs without operator actions or external supports such as power, water, etc. 

The GT-MHR has two non-safety grade, diverse active heat removal systems, the power conversion system and the shutdown cooling system that can be used for the removal of decay heat. 

In case off-site power is available, the decay heat is removed through normal heat transport path of secondary sodium and water/steam circuits. Additionally, an independent safety grade passive direct reactor cooling system consisting of 4 independent circuits of 6 MWt nominal capacity each has been provided. Each of these circuits comprises of one sodium to sodium heat exchanger dipped in reactor hot pool, one sodium to air heat... 

The RHRS is a non-safety grade system (2 trains). The RHRS circulates water in the RPV to a heat exchanger outside the CV to remove decay heat under the scheduled shutdown... 

In hot conditions, residual heat is removed through the steam generator. The steam is discharged to the atmosphere and the SG is fed by the start-up shutdown system (SSS). The system is not safety grade. Then, at low temperatures, the RRP with the air-cooling tower (RRPa) removes the decay heat. 

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