Cooling the Reactor
The primary cooling system in the OPAL reactor dissipates fission heat by means of light water, circulating the water upwards through the reactor core. This design guarantees the thermal stability of the cooling water intake to very close tolerances.
Outside the pool, the primary cooling system divides into three circuits. Each of these has a pump and a heat exchanger able to handle 50% of the full reactor power. Thus, one of the circuits becomes a redundant backup system. There is a separate cooling system serving the reactor and service pool. This system has the following functions:
- Dissipation of heat from the irradiation facilities, during normal operation, by forced downwards circulation of light water.
- Dissipation of core heat and continuous cooling of the irradiation facilities when the reactor is shutdown, by natural circulation.
- Keeping the pool water temperature within its prescribed range, for all foreseeable operating conditions.
- Dissipation of heat generated by spent fuel elements.
Both the primary cooling circuit and the reactor and service pool cooling circuit have flap valves that open when the associated pumps stop so as to allow natural circulation of the water.
The neutron reflector surrounding the core also has its own independent heavy water circulation, cooling and purification system.
The secondary cooling system extracts heat from the primary cooling circuit, the reactor and service pool cooling system, the neutron reflector cooling system and from other thermal sources within the reactor. This heat is finally dissipated to the environment by means of external cooling towers.
Cooling water is maintained within strict purity standards thanks to conventional purification systems that include mixed ion-exchange resin beds. The neutron reflector heavy water quality standards are maintained by similar means, through it's own purification system. The deuterium and oxygen concentrations within this system are maintained within limits by a deuterium-oxygen recombination system.
A hot water layer is established in the top 1.5 metres of the pool water and maintained by means of yet another circulation and purification system. This system further reduces the radiation dose rate above the pools.
All the pipes connecting to the reactor pool water penetrate that pool liner above the core level. Siphon-breaking mechanisms are provided, to prevent the emptying of the pool by syphoning.