When handling fissile material (1), particularly enriched fissile materials
(those that have been concentrated to remove some of the non-fissile companion material) great care is taken to avoid the system going super-critical
(the situation where each fission causes on average more than one further fission). In a super-critical system the rate of fission and therefore the rate of
energy production rises exponentially (very quickly).
The controlled fission chain reaction depends on delayed neutrons (those emitted by decay of fission products) for control. If the system reaches the stage
called "prompt critical", where the growth is only dependent on prompt neutrons, the exponential growth rate is significantly increased leading to an
explosion and an intense flash of neutron and gamma radiation.
A criticality accident (or "criticality excursion") is an accidental uncontrolled rapidly increasing nuclear fission chain reaction.
A criticality accident is accompanied by an intense emission of neutron and gamma radiation and releases of radioactive products in the form of gas and aerosols,
generating further irradiation and contamination risks.
In some circumstances, generally where fissile liquids are being processed in robust containers, the criticality can be prolonged or cyclic. Other systems may destroy
themselves in the first pulse and thus terminate the event.
(1)"fissile material" is material capable of sustaining a nuclear fission chain reaction. It includes the U-235 fuel used in nuclear
reactors and the uranium and plutonium used in nuclear bombs.
The avoidance of criticality
A criticality excursion depends on a high chance that a neutron produced in the [[Fission chain reaction | fission process]] goes on to initiate another fission.
It is generally avoided by being deliberately wasteful of neutrons. Thus
- Enriched fissile materials are stored in limited quantities - below a minimum mass (the "critical mass") criticality is impossible.
- Enrichment is generally no higher than needed because enrichment is expensive and the more highly enriched materials are the less is needed for criticality.
- Enriched materials are stored in long thin containers rather than more spherical ones. This enhances the leakage of neutrons from the assembly.
- Enriched fissile materials are kept away from potential reflectors that may bounce leaked neutrons back into the assembly. Water and oil are particular targets for
exclusion since they are good reflectors and good moderators.
- Fission is more likely to be induced by slow neutrons whereas fission neutrons are fast (thermal reactors use a "moderator" to slow neutrons down). In the storage and
transport of fissile material efforts are made to exclude moderators such as water and oil.
- Enriched materials can be stored with neutron absorbing materials (neutron poisons) to preferentially remove neutrons from the system.
See Criticality Safety Basics, A Study Guide and Nuclear Criticality
Safety Controls for more information and the ONR Guide
Criticality Safety: ONR Guide Criticality Safety for the regulator's view.
The consequences of criticality excursions
There have been a depressing number of criticality accidents recorded. 60 such accidents are described in a
Los Alamos National Laboratory report
from 2000. Some of these relate to experiments to measure
parameters associated with critical that went badly wrong, some relate to accidents or operator error at sites handling enriched material.
One example is the Tokaimura Criticality Accident 1999 which
World Nuclear News summarised as:
In 1999 three workers received high doses of radiation in a small Japanese plant preparing fuel for an experimental reactor.
The accident was caused by bringing together too much uranium enriched to a relatively high level, causing a 'criticality'
(a limited uncontrolled nuclear chain reaction), which continued intermittently for 20 hours.
A total of 119 people received a radiation dose over 1 mSv from the accident, but only the three operators' doses were above permissible limits. Two of the doses proved fatal.
The cause of the accident was "human error and serious breaches of safety principles," according to the International Atomic Energy Agency''.
The general phenomenology of a criticality accident in solution is described by
Barbty and Fouillaud ,
An uncontrolled chain fission reaction starts when the
quantities of nuclear materials (uranium or plutonium) present accidentally exceed the critical mass. At this stage the chain reaction increases exponentially at a
rate determined by the overall reactivity of the system which is a function of the materials present and their shape. The result is a rapid increase in the number of fissions,
a rapid increase in energy production (mainly heat), intense emission of neutron and gamma radiation and the release of fission gases. The resultant rise in temperature and
bubble formation leads to a reduction in the reactivity and the system becomes sub-critical. However, when the bubbles disperse the power excursion can restart leading to a
cyclic or pulsed criticality. How the event ends depends on the circumstances. The assembly can destroy itself, ending the event or can support further excursions.
The results of experiments gave first power peaks with a period of 0.9 ms to 4 minutes and a
maximum power ranging from 1012 to 3 x 1019 fissions.s-s.
It also reports that "''An analysis of past criticality accidents illustrates the wide variety of situations encountered :
media, configurations, causes and observed effects (power, energy, duration, etc.). The results show that the energy can vary
from a few 1015 fissions to 4 x 1019 fissions for fuel cycle installations, and the power during the first peak
can be as high as 1020 fissions.s-1 for a very short time. The duration can simply be a ‘flash’ of a few milliseconds, or it can continue for tens of hours".
Criticality Emergency Response
The immediate response to a criticality alarm is to get away from the scene as quickly as possible. The initial criticality burst will have
resulted in intense neutron and gamma emission, giving a large and potentially fatal dose to those nearby and the possible continuation or
pulsing of the criticality could give further large doses as could exposure to the resultant fission products.
"When an evacuation is initiated, all personnel within the immediate evacuation zone shall evacuate without hesitation by planned evacuation routes to an established assembly station
or stations". (ANS, nuclear criticality accident emergency planning and response).
It then becomes a matter of providing medical assistance to those injured in the event
and those given large radiation doses by the event. The latter should be identified by logging their positions when the event happened,
from readings from criticality lockers and by observation of the timing of on-set of any symptoms of Acute Radiation Syndrome (the higher the dose,
the quicker and more severe the symptoms).
Re-entry to the scene should be cautious taking into account the dose rates, the damage done and the potential to trigger further criticality excursions
if the system remains on the edge of criticality.
[A Tool for Criticality Accident Emergency Planning, Training and Response
[NCSP New NCSP 2 Week Hands-on Course Module: Emergency Response and the
Criticality Accident Slide Rule https://ncsp.llnl.gov/sites/ncsp/files/2021-05/Percher1-LLNL_TPR_2019TE_ERModuel.pdf]