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Firstly some definitions1:
Criticality Safety: 'Protection from the consequences of a criticality accident, preferably by prevention of the accident'
Criticality Accident: 'The release of energy as a result of accidentally producing a self-sustaining or divergent fission chain reaction.’
1 from LA-11627 Glossary of Nuclear Criticality Terms, H C Paxton. 1989.
But what does that mean?
Certain nuclides, such as uranium-235 and plutonium-239 are capable of undergoing fission, that is the atoms can be split giving off heat and radiation. Each time these atoms split, particles called neutrons are also given off which can cause splitting of further atoms.
In a nuclear reactor materials capable of undergoing fission are assembled under well defined conditions to produce a controllable chain reaction (or controlled criticality) where neutrons from one fission go on to cause other atoms to fission. Splitting atoms releases large amounts of heat (used to generate electricity) as well as radiation. Very thick biological shields (usually a combination of concrete and steel walls) are present around the reactor to protect the workers from the radiation.
When the same materials are handled outside of a reactor it is possible that fissile material can accumulate in certain conditions and result in an accidental and uncontrolled chain reaction, or criticality accident. When a criticality accident occurs it can give off high levels of radiation in a similar way to a nuclear reactor. This can present a dangerous or even lethal dose to anyone nearby if there is not sufficient shielding present.
The aim of criticality safety is preferably to ensure that the quantity of material and the conditions in which it is present remain such that a criticality cannot happen and the resultant high levels of radiation do not occur. In some circumstances it is necessary to provide shielding such that in the unlikely event of a criticality accident the workforce is protected from the radiation.
Have any criticality accidents occurred?
To date 22 criticality accidents have been reported in process facilities. A further 38 accidents have been reported in experimental facilities and research reactors.
Detailed descriptions of these can be found in:
You Tube videos (these may need to be viewed on a personal device):
A Brief History of The Wood River Junction Criticality https://www.youtube.com/watch?v=MuHD6VhrK5U
A Brief History of The Demon Core https://www.youtube.com/watch?v=VE8FnsnWz48&t=6s
A Brief History of The Y-12 Criticality Incident https://www.youtube.com/watch?v=HaOc8FhMdWc&t=2s
A Brief History of The Tokaimura Criticality Incident https://www.youtube.com/watch?v=acpz3CG1xi4&t=6s
A Brief History of the Sarov Criticality https://www.youtube.com/watch?v=s5HX_ALalnk
Further reading
Unsurprisingly there is rather more to the physics of the fission process, nuclear chain reactions and criticality safety than outlined above.
For a more detailed background to Nuclear Criticality Safety
The following pages may provide a useful starting point to learning more about the relevant topics.
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