maintains the integrity of the core under the reactor transient or accident conditions.

4. It has no failures that jeopardize the system re­flooding capability in the RPV.

The PLR consists of two external recirculation loops, both installed inside the PCV. Each loop has a recirculation pump, two gate valves before and after the pump, and the related piping including the suction pipe, discharge pipe, header and riser lines. Each recirculation pump pressurizes the coolant discharged from the RPV downcomer (an annulus space between the RPV wall and the core shroud) and returns it to the RPV through five riser lines and 10 internal jet pumps. An elbow meter is set up on the suction line of each recirculation pump in order to measure the coolant flow rate in each loop. The total core flow rate is measured at the diffuser section of the internal jet pumps.

The coolant flow in the PLR system is controlled by the speed control of the recirculation pumps, which is done by the frequency control of the AC electric power supply to the pump motors. The AC electric power is supplied from the variable­frequency electric power sources (stationary or M-G set). At normal operating conditions, both of the two recirculation pumps are in service. Even if one of the pumps is tripped, the reactor can continue its operation at the reduced power level (approximately 80% of the rated power). The natural circulation capability of the coolant in the RPV allows the reactor to continue its operation at approximately 35% of the rated power (see Figure 2.3.22), even if both recirculation pumps trip.

The PLR system has the following two supplementary (ancillary) systems.

1. Recirculation pump seal purge system

Part of the high pressure low temperature water from the control rod drive hydraulic control system is constantly injected at a fixed rate into the mechanical seal assembly of the recirculation pumps. This keeps the mechanical seal clean, extends the seal life, and provides a back-up cooling function for the mechanical seals.

2. Reactor water sampling system

The reactor water is usually sampled from the reactor water cleanup system. When the reactor water cleanup system is not in operation, samples of the reactor water are taken from the sampling

piping connected to the discharge line of the recirculation pumps.

In an ABWR, the PLR system is completely housed in the RPV, entirely different from the conventional BWR plant configurations. Hie ABWR PLR system circulates the coolant water using the reactor internal pumps (RIPs), which are installed at the bottom of the RPV. The external recirculation loops, which are a source of radiation exposure to workers on duty in conventional BWRs, are eliminated.

The RIPs are the vertical single-stage mixed- flow type, driven by wet motors. In a 1,350 MWe plant, ten such RIPs are installed. Each RIP is driven by a variable frequency electric power source (stationary), thus enabling the control of the reactor coolant flow by adjusting the RIP speeds.

2. Main steam (ms) system

The MS system is designed based on the following philosophies.

1. It transports the steam to the turbine with minimum pressure loss and maximum separation of moisture from the steam.

2. It maintains the reactor pressure within the permitted values during reactor transients.

3. It restricts the steam release from the RPV and the PCV in an accident

The MS system has four main steam lines that transport the steam generated in the RPV to the turbine system. Each line has one steam flow restrictor (venturi type) and multiple SRVs inside the PCV. Two main steam isolation valves (MSIVs) are installed in series on each main steam line before and after the line penetrates the PCV wall. Each SRV is directly connected to the suppression chamber through a discharge line. The discharge line ends with a quencher for accelerating steam condensation in the pressure suppression pool (S/P) water.

The steam flow restrictor is installed near the RPV and restricts the maximum steam outflow below the 200% level of the rated flow in case of the main steam pipe guillotine break. The steam flow restrictor is also used to measure the steam flow rate during normal operation. The safety relief valves (SRVs) are provided for suppressing the in-vessel peak pressure under all operational conditions.

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NSRA, Japan