Normal Residual Heat Removal System

APIOOO normal residual heat removal system. 

Design Bases for Normal Residual Heat Removal System Operation ... 

This building contains the component parts of the primary coolant system, the passive decay heat removal system and the passive containment cooling system. Also present are parts of the normal residual heat removal system and the chemical and volume control system. The reactor pressure vessel holds Ihe bulk of the radioactive material present on site, in the form of the irradiated fuel within it. The principal contents of the contaimnent/shield building are the ... 

Pipes and valves between the reactor coolant system and the normal residual heat removal system. 

The only identified loss of electrical supply faults would be a loss of ac power to station auxiliaries (see subsection 5.3.5.3), or a loss of power to the normal residual heat removal system... 

Loss of electrics while shut down would result in the loss of the normal residual heat removal system. Two situations are considered one with the RCS intact, and one with the RCS open. 

The starting point for the intact circuit assessment is immediately following the ehangeover from steam generator cooling, when the decay heat burden on the normal residual heat removal system is at its maximum. The analysis assumes the worst case availability of safety measures permitted by the Technical Specifications for any intact circuit shutdown mode (see Chapter 16 of Reference 5.6) ... 

The analysis concludes that the consequences of a loss of the normal residual heat removal system while shutdown are acceptable, depressurisation and IRWST injection (and CMT injection, in the intact circuit case) provide cooling capability. 

A sensitivity analysis is made for the unavailability of all five of the standby non-Class 1 safety systems (chemical and volume control system (CVS), start-up feedwater system (SFW), normal residual heat removal system (RNS), diverse actuation system (DAS), diesel generators (DGs)). The plant CDF obtained is 7.4Qx 10, which is a factor of 31 increase over the base case. This sensitivity analysis shows that if no credit is taken for non-Class 1 safety t systems then the plant CDF, and hence any impact on the workers of the public, would increase to just above the BSO, confirming that they are only marginally important and rated appropriately. 

The reactor coolant system interfaces with a number of auxiliary systems, principally the ehemical and volume control system, the normal residual heat removal system, the steam generators, the primary sampling system, the liquid radwaste system and the eomponent cooling water system. 

When the reactor is shut down, the steam generators provide eooling until system pressure and temperature are reduced to a level that can be accommodated by the normal residual heat removal system. The normal residual heat removal system takes its suetion from one of the hot legs, the same one that has the surge line connection to the pressuriser, and it returns its cooled flow into both direct vessel injection lines. 

When the reactor is to be refuelled, the refuelling cavity is flooded, following which Ihe reactor vessel closure head is removed and then parked. The normal residual heat removal system continues to cool the reactor core but now also the refuelling cavity, by virtue of the reactor vessel being open to the water in this cavity. 

The inclusion of the direct vessel injection nozzles allows the inlet nozzles to be positioned at a higher vertical elevation than the outlet nozzles, resulting in dry cold legs during mid-loop operation and thereby enabling the removal of a reactor coolant pump with the normal residual heat removal system in operation. 

As a result of the design of the core, the reactor coolant system, the steam generators and the normal residual heat removal system it has been possible to demonstrate that there is at least a 95 % probability, at a 95 % confidence level, that the fuel rods will not exceed the uranimn dioxide melting temperature ... 

Reactor coolant system overpressure protection during power operation is provided by the pressuriser safety valves, discussed in Section 6.3.2. The provided overpressure protection is compliant with the requirements of the ASME III Paragraphs NB-7300 and NC-7300 for pressurised water reactor systems. Low temperature overpressure protection is provided by a relief valve in the suction line of the normal residual heat removal system, as described in Section 6.5.3.3. 

The passive residual heat removal heat exchanger is designed to cool the reactor coolant system to 216°C (420°F) in 36 hours, with or without reactor coolant pumps operating (Reference 6.1, Section 6.3.1.1.1). This allows the reactor coolant system to be depressurised and the stress in the reactor coolant system and connecting pipe to be reduced to low levels. This also allows plant conditions to be established for initiation of normal residual heat removal system operation. 

The steam generators must remove fission product heat from the reactor core in the initial phase of shutdown operation before the normal residual heat removal system can be... 

Nitrogen supply in the passive core cooling system Normal residual heat removal system... 

Actuation of the automatic depressurisation system reduces the reactor coolant system pressure to below the operational pressure of the accumulators. Following actuation of stage 3, the operational procedure is to start the normal residual heat removal system when the reactor coolant system... 

During plant shutdown periods, where the reactor coolant pumps are stopped, the normal residual heat removal system provides the motive force for the chemical and volume control system purification. Purification flow Ifom the normal residual heat removal system heat exchanger is routed directly through the normal chemical and volume control system purification loop. Boron changes and dissolved gas control are still possible by operating the chemical and volume control system in a semi-closed loop arrangement. 

The normal residual heat removal system eonsists of two mechanieal trains of equipment, each comprising one pump, one heat exchanger and the associated pipes, valves and instrumentation. 

The normal residual heat removal system must be able to remove the fission product heat from the reactor coolant system from the time after shutdown when the reactor coolant system pressure falls below the design operating pressure of ihe normal residual heat removal system of 62 bar (900 psig). 

The normal residual heat removal system must be protected against spurious and inadvertent over-pressurisation, because, once cormected, the reactor coolant system pressure boimdary is extended to include the entirety of the normal residual heat removal system. That part of the system outside the containment has a design pressure that is lower than the main part of the reactor coolant system. 

The normal residual heat removal system also has the capability to provide the additional defence in depth during and following fault conditions ... 

Operation of the system with both subsystems of normal residual heat removal system pumps and heat exchangers available. 

Initiation of normal residual heat removal system operation at four hours following reactor shutdown, after the first phase of cool down by the main steam system has reduced the reactor coolant system temperature to less than or equal to 176.7°C and 3.1 MPa gauge. 

The component cooling water system supply temperature to the normal residual heat removal system heat exchangers is based on maximum normal ambient wet bulb temperature as defined in Table 2-1 of Reference 6.1. The maximum normal ambient temperature is assumed for shutdown cooling ... 

With the normal residual heat removal system in service when shutdown, the only over-pressure protection claimed by the fault schedule (Section 4.10.5.9 of Reference 6.13) is the normal residual heat removal system safety rehef valve (Section 19E.4.10.1 of Reference 6.1). This valve is located inside the containment on the suction header, and set to open at 35.5 bar (500 psig). The capacity of the relief valve is 3200 litres per minute (850gpm) (Section 4.10.5.10.6 of Reference 6.13). 

Indication is provided to alert the operator to the actuation of the normal residual heat removal system relief valve. Positive position indication is provided for the normal residual heat removal system relief valve. Temperatures in the safety valve discharge lines are measured, and an... 

Containment isolation valves are provided on either side of containment penetrations. The normal residual heat removal system has two containment penetrations the normal residual heat removal system suction line penetration, and the normal residual heat removal system discharge line penetration. Both penetrations are provided with contaimnent valves inside and outside containment, which are actuated by the containment isolation system. 

The component cooling water system can remove, in conjimction with the normal residual heat removal system, both residual and sensible heat ftom the core and the reactor coolant system and reduce the temperature of the reactor coolant system during the second phase of cool down. 

The first phase of plant cool down is accomplished by transferring heat from the reactor coolant system via the steam generators to the main steam systems (see Section 6.6.6). The component cooling water system, in conjunction with the normal residual heat removal system (see Section... 

The fire protection system also has the following capabihty to provide the additional defence in depth support during and following fault conditions it can deliver an alternative supply of cooling water to the normal residual heat removal system after a loss of normal component cooling water system function. 

Connections are provided between the fire protection system and the normal residual heat removal system. A permanent eonneetion is provided to allow the fire protection system to fiimish water to cool a normal residual heat removal system pump and heat exehanger following a fire that disables the normal component cooling water system cooling function (see Table 9.5.1-4 of Reference 6.1). 

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