Pressurized water reactors residual heat removal system

The low-pressure ECCSs consist of two separate and independent systems, the CS system and the LPCI mode of the residual heat removal system. The CS system consists of two separate and independent pumping loops, each capable of pumping water from the suppression pool into the reactor vessel. Core cooling is accomplished by spraying water on top of the fuel assemblies. The LPCI mode of the residual heat removal system provides makeup water to the reactor vessel for core cooling under LOCA conditions. 

Flow proceeds from the lower plenum, through the core. The steam and water are separated the steam is then dried and passed to the turbine. Other flow (see above) returns to the recirculation system. Feedwater is introduced to the annulus between the core shroud and reactor vessel (Fig. 4). The recirculation system piping is a primary pressure boundary for the high-pressure, high-temperature reactor coolant. Type 304 stainless steel was selected for recirculation system piping and numerous other auxiliary systems (such as the reactor water cleanup system, residual heat removal system, core spray, and other emergency core cooling systems) for its corrosion resistance and adequate mechanical properties. Failures of weld heat affected zones... 

For loss of coolant accident (LOCA) scenarios core makeup tarrk (CMT) draining will signal actuation of the first three stages of the automatic depressurisation system (ADS), depressurising the reactor and allowing the accumulators to inject more borated water into the RCS. If the pressure falls further, the residual heat removal system (RNS) can also be ahgned to remove heat. This latter action provides additional defence in depth but is not claimed in the fault schedule. 

The accident sequence corresponding to the loss of the Residual Heat Removal System in Operating Mode 4 (OM 4) during cool down operations has been studied (Table 1). The analysis of this sequence is framed within the Low Power Shutdown PRA (LPRA) of a typical Pressurized Water Reactor (PWR). 

Containment, designed with pressure suppression pools and with emergency cooling and residual heat removal systems, can he significantly smaller than those of current water-cooled reactors. 

The pressure-relief valves were manually opened from the control room, and the steam was transferred from the pressure vessel to the pressure-suppression pool (still within primary containment) and condensed. By this method the pressure in the vessel was reduced to about 1.8 MPa (260 psi) in 20 min the condensate booster pumps were then used to maintain an adequate water level in the realtor vessel. During the depressurization period the water level in the core decreased but did not drop below a point 1.2 m (4 ft.) above the top of the fuel. Normal level is 5.08 m (200"), but the 1.2 m (4 ft.) level is still 0.76 m (2.5 ft.) above the level at which the core spray and residual-heat-removal systems are actuated. Once the reactor pressure was reduced below 2.4 MPa (350 psi), one condensate booster pump and one condensate pump provided adequate makeup water, and the normal water level above the fuel was attained. 

Several other systems are available to control room operators to keep the reactor cool during an emergency. These include the High Pressure Core Spray System, Low Pressure Core Spray System, Residual Heat Removal System, Low Pressure Coolant Injection, and Suppression Pool Cooling. All of these systems require both AC and DC power, plus a sufficient flow of coolant water connected to the ultimate heat sink. At Fukushima this ultimate heat sink was the Pacific Ocean (US Nuclear Regulatory Commission, n.d.a). 

Pressurized Water Reactor. The PWR contains three coolant systems (1) the primary system, which removes heat from the reactor and partially controls nuclear criticality (2) the secondary system, which transfers the heat from the primary system via the steam generator to the turbine-driven electric generator (3) the service water system (the heat sink), which dumps the residual coolant energy from the turbine condenser to the environment. The service water is recirculated from a river, lake, ocean, or cooling tower. In the primary system (Fig. 31.21), dissolved boron is present to control nuclear criticality. Fixed-bed ion exchange units are used to maintain the water quality in both the primary and the secondary systems. In addition, the chemical and volume control system reduces boron concentration during the power cycle to compensate for fuel burnup. These operations are carried out continuously though bypass systems. A more complete... 

The reaction takes place at low temperature (40-60 °C), without any solvent, in two (or more, up to four) well-mixed reactors in series. The pressure is sufficient to maintain the reactants in the liquid phase (no gas phase). Mixing and heat removal are ensured by an external circulation loop. The two components of the catalytic system are injected separately into this reaction loop with precise flow control. The residence time could be between 5 and 10 hours. At the output of the reaction section, the effluent containing the catalyst is chemically neutralized and the catalyst residue is separated from the products by aqueous washing. The catalyst components are not recycled. Unconverted olefin and inert hydrocarbons are separated from the octenes by distillation columns. The catalytic system is sensitive to impurities that can coordinate strongly to the nickel metal center or can react with the alkylaluminium derivative (polyunsaturated hydrocarbons and polar compounds such as water). 

---