Because the process of fission product decay leads to the generation of significant quantities of heat, even when the reactor is shut down, continuous cooling qf the reactor core is required in order to achieve and maintain the low temperatures (well below 100°C) required for refuelling, maintenance or repair of the reactor and its coolant system. As explained in Section 12 of this chapter, this cooling is achieved by the residual heat removal system (RHRS) when the re­actor coolant temperature is about 177°C or less.

The RHRS (Fig 2.142) comprises two parallel heat transfer loops, each containing a centrifugal pump and a vertical shell and U-tube heat exchanger. The pumps each draw reactor coolant leaving the core from one of the RCS hot legs and pass it through the heat exchanger tubes, in which it is cooled by component cooling water flowing through the heat exchanger shell before returning it to a pair of RCS cold legs, and thence to the core inlet. The pumps and heat exchangers are located on the lower floors of the auxiliary building and the suction and delivery pipework therefore penetrates the wall of the reactor containment building. All RHRS components and pipe­work are constructed from austenitic stainless steel and the design pressure is approximately 39 bar.

The RHRS equipment is sized so that with both loops operating, the primary circuit can be cooled from 177°C to 60°C (the temperature at which re — — actor. dismantling can commence) over a period of about 16 hours, this representing an acceptably small impact on the critical path of the refuelling and main­tenance operations planned during the shutdown peri­od whilst leading to reasonable equipment sizes. To maximise system reliability, each RHRS loop uses se­parate services and supplies of electrical power and cooling water. Furthermore, two additional identical pumps normally serving as containment spray pumps are provided with cross-connecting pipework, which enables. either to be used in place of an RHR pump if necessary. All electrical and control equipment is diesel-backed.

Because the RHRS equipment is located outside the containment and has a relatively low design pressure, very stringent precautions are taken against inadvertent exposurlfof the RHRS to the primary circuit when the latter is at high temperature and pressure. Each of the suction lines contains three motorised gate valves in series, and any one remaining closed will isolate the RHRS from the primary circuit.

These valves are separately interlocked to prevent opening when pressure is too high. On the delivery

t

side, multiple non-return (check) valves provide re­liable isolation.

Heat exchanger performance is dependent on the coolant flow rate and the temperature difference be­tween the primary coolant and the component cooling water; to aoid too rapid cooling, which could in­crease thermal stresses in reactor components, the heat exchanger How-rate can be throttled using a manually controlled butterfly valve, A bypass line, containing an automatic butterfly valve, ensures that the pump flow rate remains roughly constant at 800 m’/h per RHRS loop.

Reactor cooling via the RHRS would also be needed in the event of a fault which necessitates repairs to the reactor itself, the primary and secondary circuits or equipment inside the reactor building. One RHRS loop has sufficient cooling capacity to match the de­cay heat production rate about 4 hours after reactor trip or shutdown, to prevent reactor temperature ris­ing above 177°C and to cool the primary circuit to 93°C within a further 12 hour period. This enables the circuit to be completely depressurised and cold shutdow-n conditions to be reached. The RHRS equip­ment is designed to withstand or be protected from all significant external and internal plant hazards, including a safe shutdown earthquake, and rigorous fire segregation is provided between the two equipment loops.

In the special case of a loss of coolant accident (LOCA) causing rapid emptying and depressurisation of the primary circuit, the RHR pumps act as part of the emergency core cooling system which is de­scribed in the next section.