Radioactive substances are generally considered hazardous and they should be kept within a nuclear power plant as far as possible. They are produced mainly in the reactor core during nuclear fission, but with a minute amount found in the adjacent reactor structure due to neutron activation, and in the reactor cooling water in the case of the Pressurised Water Reactor that the chemical additives it carries for reactor control produces radioactive substances upon neutron bombardment.
A typical plant will provide physical barriers to keep these substances inside, by means of metallic cladding to contain the nuclear fuel at the reactor core, a pressure vessel to house the reactor that is surrounded by cooling water which takes heat away with pipework, and finally by a containment structure that houses the pressure vessel and the major pipework for the cooling water. Radioactive substances thereby produced will be effectively kept within the plant.
Yet it is necessary to maintain the radioactive inventory at the plant within a certain quantity, to avoid an inventory increase in the long term. This is achieved mainly by replacing used nuclear fuel with new, but because the reactor cooling water may carry radioactive substances produced from its chemical additives, from the reactor structure which comes in contact, or even by possible egress from the nuclear fuel through any cladding imperfections, it becomes necessary to recycle the reactor cooling water for purification. The purification facility will remove from the cooling water the bulk of the radioactive substances which are then packed as a solid waste for subsequent long term storage.
No engineering facility is completely efficient and a minute quantity of radioactive substances that are extracted from the cooling water, such as gases, may not be solidified or permanently stored. They are therefore stored within the plant for their radiation to decrease naturally with time, typically for weeks, before they are released into the environment.
Regulations are in place to limit the releases so that the nearby public will receive a radiation dose within a fraction of that from the environment. A well-managed plant will release up to a small percent of the limits, so that the dose given to the public will actually be next to imperceptible.
This article is contributed by Ir Richard Fung with the coordination of the Nuclear Division.