Reactor Coolant System

The function of a reactor coolant system is to keep cooling during normal and transient operations. It keeps the reactor core temperature in check, preventing it from overheating. In other words, it is what makes the use of nuclear power plant safe as it is an essential tool to control fission reaction. There are three main types of coolant: heavy water (deuterium), light water, and gas (carbon dioxide).

In order for a reactor coolant system to work adequately, there must be enough water inventory as well as safety water injection systems to ensure that water reaches the core. The heat is then transferred by forced or natural circulation to the ultimate heat sink. In many plants, water to refill any reduced water inventory in the main circuit is stored completely inside the containment; this provides additional protection against any external event. This is when water is used as coolant.

There are other features used to ensure protection of the primary circuit inventory. They are a pressurizer relief to the water storage tank; heat rejection to a water storage tank through heat exchangers; water storage tank joined with the containment sump; water storage tank set up high above the core for gravity-driven injection; and core make-up tanks at full circuit pressure to provide high pressure injection.

Passive injection from accumulators is available in advanced passive designs. Passive low-pressure injection from the water storage tank is set up at high elevation across the core. Discharge can only occur when the reactor system pressure is at the last stage of depressurization. This type of design can be seen in the AP1000/600 and the VVER-1000/W-407.

Containment

Steel containments have the advantage of good heat removal characteristics as it often includes passive heat removal concepts. Concrete containments, on the other hand, have a proven capability to withstand greater loads but at the expense of poorer heat transfer characteristics. Therefore, concrete containments need additional heat transfer systems, such as heat exchangers or condenser systems, which assist in heat removal from the interior to the exterior of the containment.