Nuclear Safety

The main safety problem in the design of a nuclear plant is to assure that the large amounts of radioactive materials which produced in the reactor remain safely confined during the operation of the plant, refueling of the reactor, and preparation and shipping of spent fuel. 

The main objective of nuclear safety is the achievement of proper operating conditions and the prevention or mitigation of accident consequences, resulting in protection of workers, the public and the environment from undue radiation hazards.      

To prevent the escape of radioactivity, nuclear plants are designed using the concept of multiple barriers.

These barriers represent a sequence of obstacles to block the passage of radioactive atoms from the fuel, or wherever they may originate, to the surrounding population.

The barriers normally present are the following:

1.        The fuel; Retention of fission products in the nuclear fuel itself.

2.        Cladding; to prevent escape of fission product gases and to confine fission fragments emitted near the surface of the fuel, the fuel is surrounded by a layer of cladding.

3.        Closed coolant system; in all modern power reactors, the primary coolant, that is, the coolant comes in contact with the fuel, moves in one or more closed loops. Fission products that have escaped from the fuel, activated atoms picked up by the coolant, and activated atoms of the coolant itself are thus confined within the coolant system.

4.        Reactor vessel; because they represent an obvious barrier to the release of radioactivity, reactor vessels are required to be designed, manufactured, and tested to meet the highest standards of quality and reliability.

5.        Containment; all reactors are required to be entirely enclosed by a structure of one type or another to contain radioactivity, should this be released from either the coolant system or from within the reactor vessel itself. Most PWR containment structures are made of reinforced concrete with a steel liner.

6.        Site location; nuclear plants must be constructed at locations that are relatively remote from large masses of people and where the plant is not likely to be damaged by natural phenomena such as earthquakes.

7.        Evacuation; the final barrier is the evacuation of the local populace from areas receiving or likely to receive excessively high radiation doses.

Finally; in order to assure that none of these barriers is compromised as the result of such as equipment failure, human error, or natural phenomena, the NRC has adopted as its safety philosophy the concept of the three levels of safety.