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From the source to the dosimetric impact assessment

The risk of criticality

A criticality accident results from the triggering of an uncontrolled fission chain reaction. This occurs when the amount of nuclear material present (uranium or plutonium) accidentally exceeds a threshold known as “critical mass”, or whenever there is a neutron source that primes the reaction.

Published on 17 March 2015

Even aside from nuclear reactors, the fissile materials in laboratories, factories and transportation present a particular risk: criticality. This is the risk of combining priming conditions with the maintenance of a chain reaction. This occurs, for example, as soon as one gathers more than 60 kg of uranium enriched to 3.5% uranium-235, or (under certain conditions) more than 510 g of plutonium solution or 870 g of highly-enriched uranium solution. In nuclear reactors, this same risk can also lead to a reactivity accident.

A fission chain reaction is accompanied by a release of energy as heat. There is also the emission of gamma rays and neutrons, which can cause substantial irradiation to people nearby. The environmental impacts are limited since the radioactive discharges are weak.

Preventing the risk of nuclear criticality requires that special precautions are taken. This is the subject of research and analysis at all stages of implementing fissile material in the fuel cycle. Despite all of the preventive measures, one cannot completely exclude the possibility that a criticality accident might occur suddenly and without warning, when the amount of fissile material exceeds this critical mass. Since the circumstances and consequences of criticality accidents are so diverse, research programs have been implemented to improve the understanding of their conditions for occurrence and to model them, in order to limit their impacts on man, the environment, and on the technical facilities.

Dosimetric estimation and prognosis

After a criticality accident, the priorities of medical treatment are determined by the absorbed dose and its distribution; this requires the means to measure high-sensitivity activity.

​Assistance of victims of a criticality accident at Tokaï-mura in Japan (September 1999)

Measuring the activities of sodium-24 in the blood and phosphorus-32 in the skin appendages (hair and nails) of criticality accident victims provides an estimate of the neutron dose and its distribution within the body. Analyzing skin appendage samples taken from the anterior, posterior, right and left sides of the body and the head provides essential information regarding the orientation of the victim at the time of the event. Both the dose estimation and its heterogeneity are essential data for the appropriate treatment of accidentally irradiated persons by specialized medical teams.