Water is used as a moderator for boiling water reactor and pressurized water reactors. Control rods steer the chain reaction of the nuclear fission and therefore also the thermal output of the reactor. By entirely lowering the control rods, the nuclear fission process stops and the power plant is switched off.
Boiling water reactor (BWR) and pressurized water reactor (PWR)
The BWR and PWR differ in their constructive design: In a BWR the water enclosing the fuel rods is heated until evaporation. The steam is then directed to the turbine and used for the generation of electricity via the steam power process. Potentially contaminated water is thus in direct contact with the turbine and all other components of the steam power process.
The PWR design makes use of two separate circuits of water. A primary circuit (water that flows around the fuel rods) and a secondary circuit (water used in the steam power process). The water in the primary circuit is pressurized to avoid evaporation. In that way heat transfer between the fuel rods and the medium water is optimized compared to the BWR. A heat exchanger facilitates the heat and thus energy transfer from primary to secondary circuit.
In both designs the reactor is enclosed by a double-walled container made of steel and concrete. This so-called “containment” should prevent the release of radioactive material in case of internal as well as external impacts.
Just as fossil-fuel fired power plants, the BWR and PWR power plants also need a cooling water circuit, which might be supplied by a nearby river and provides the condenser with cool water. In big cooling towers, hot water is being sprayed and thus able to cool down to ambient air temperature.
Compared to the BWR the PWR is inherently safer due the utilization of an additional water circuit. As the water stays in its liquid form for the whole time, the control rods can be inserted from above. That means they are not dependent on motors, which might fail or be damaged during an accident.
If the moderating substance, in that case water ceases to be in place, the chain reaction stops. However the previously created fission products still generate considerable heat. This heat has to be discharged in order to prevent a core meltdown.
