2.7 Fission Products
When a uranium atom undergoes fission the two, occasionally three, smaller atoms produced are known as fission products. Many of these fission products will be radioactive species of the particular elements produced. Once a reactor has been running for some time the core will contain a cocktail of uranium together with mixed fission products. The radiation given out by these fission products will be intense and heavy shielding is required around the reactor core. The levels of radiation are actually high enough to heat up the core of the reactor even after the chain reaction has ceased. This 'residual heat' is removed by the main coolant or, in the event of failure, by emergency cooling systems. A wide variety of fission products are produced, many will be solids but some will be. gaseous or volatile. In the event of a nuclear reactor accident it is these gaseous and volatile elements which will be of primary concern, since it is these elements which are more likely to escape into the environment. Such elements include xenon, krypton, iodine, caesium, tellurium, ruthenium, etc.
2.8 Containment
It is important that the fission products produced in the uranium fuel are not allowed to escape into the environment. The fission products must be contained. In a PWR there are usually four barriers to prevent such an escape. The uranium fuel, in the form of uranium oxide, is contained in ceramic fuel pellets, which are contained inside a zirconium alloy cladding. In normal circumstances most of the fission products produced in the fuel will remain in the ceramic pellets and all will be retained by the cladding. If however the fuel pin had been subjected to major mechanical damage, or allowed to overheat, then the gaseous and volatile elements could escape from the cladding into the primary circuit. The primary circuit is a piece of well engineered plant designed to withstand high pressure. For example the pressure vessel itself is made of steel over 20 cm thick. If however there was a rupture of the primary circuit then the gaseous and volatile elements could escape into the containment building. This massive re-enforced concrete structure is designed to contain radioactive material in the event of a major accident to the reactor plant. There are, however, some major accident scenarios in which some of the gaseous fission products and a very small fraction of the volatiles could escape into the environment.
The importance of the containment in the event of a major reactor accident is clearly illustrated by experience at Three Mile Island. The PWR there suffered major core damage due to a loss of coolant accident. Both the primary circuit and the containment building functioned well and the release of radioactivity into the environment was very small.
2.9 Reactor Safety
Reactors are designed using a defence in depth philosophy. Components and whole systems have a large amount of redundancy such that they are tolerant to failures. For example it is assumed that pumps, instruments, etc., will fail and therefore back up systems are provided, often operating on a completely different principle in order to provide diversity as well as redundancy. By this means the chance of a major accident occurring is made very remote. Notwithstanding this, it is assumed that a major core melt accident could still occur and therefore a high integrity containment building is provided. Moreover in France all PWRs are now being installed with pressure relief systems, using sand filters, to prevent over-pressurisation of the containment building even following extreme accidents. It is understood that consideration is currently being given to adopting this pressure relief system for the PWRS at Daya Bay.
12