What is Criticality: Definition and 21 Discussions

A criticality accident is an uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, or divergent chain reaction. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses, if they occur in an unprotected environment.
Under normal circumstances, a critical or supercritical fission reaction (one that is self-sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance.
Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical).
In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in 14 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions.

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  1. gmax137

    What is the current status of Vogtle Unit 3?

    https://www.georgiapower.com/company/news-center/2023-articles/vogtle-unit-3-reaches-initial-criticality.html#:~:text=Vogtle%20Unit%203%20reaches%20initial%20criticality%20March%206%2C,safely%20started%20the%20nuclear%20reaction%20inside%20the%20reactor. "March 6, 2023 Georgia Power today...
  2. Congrute

    B U235 Criticality: Neutron Capture and Alpha Emission

    How does U235 reach criticality via neutron capture when its natural decay process is via alpha emission. I know that U235 will reach criticality in a suitably shaped container after a specific concentration has been reached. In addition, I've read that the original U235 bomb (little boy) was...
  3. A

    MCNP Output Data: Tutorials & PDFs for SCWR Criticality Analysis

    Hi, Is there any tutorial or pdfs that can help me with the MCNP output data? I'm working on the criticality of the SCWR, and I designed the fuel assembly and run it on the MCNP, but I have no idea about data extraction.
  4. F

    Neutron quantity normalization in an eigenvalue computation

    Dear Community, I am having a question. I have developed a simple code to perform iteration power algorithm and find the keff value of a system. However, it is not still totally clear in my mind if I have to normalize all my scores by the eigenvalue, i.e. multiply by the keff (fluxes, power...
  5. B

    Question About "Engineered Criticality Devices" In A SciFi Scenario

    As a way of explanation I have to say I've been reffing RPG's since the mid seventies, some four months after D&D first came out, as it took that long back then for game ideas to move from the Great Lakes region to the West Coast. (I was Class of '78) So in the upcoming post vaccine era I'm...
  6. T

    A How far can one change a model before it leaves the universality class

    Usually, critical phenomena can be categorized in some kind of universality class which determines the critical exponent. A typical example is the class of the Ising model; adding a next-nearest-neighbour hopping term does not change the critical behavior. The typical explanation is that the...
  7. Delta Force

    Did the Tokaimura Criticality Incident Create an AHR?

    The Tokaimura Criticality Incident involved the improper mixture of 18.8% enriched uranium and nitric acid in a 100 liter precipitation tank that was 450 mm in diameter and 650 mm high. The tank was surrounded by a water filled cooling jacket. When enough of the mixture was in the tank a...
  8. T

    A Criticality, phase transitions and closing bandgap

    At critical points, when correlation lengths diverge near a phase transition, people often say that the spectral gap closes, which is to my understanding just the energy difference between the ground state and the first excited state (the first two eigenvalues of the hamiltonian). How should we...
  9. R

    "Magic Merv" and Criticality

    Homework Statement In terms of criticality "magic merv" stands for the nine parameters which determine the level of criticality in a system: mass, absorption, geometry, interaction, concentration, moderation, enrichment, reflection and volume. The topic is decomissioning plutonium nitrate...
  10. R

    Criticality safety assessment of an abandoned plutonium lab

    I'm assessing criticality safety of a waste removal plan for a (theoretical) abandoned russian laboratory. In the laboratory plutonium was recovered from fuel rods. The room is mainly empty except for a dissolver, a sump below the dissolver and spilled metal chips on the floor. The dissolver...
  11. C

    Triangular lattice and criticality

    Homework Statement Consider a two dimensional triangular lattice, each point of which can either contain a particle of a gas or be empty. The Hamiltonian characterising the system is defined in terms of the particle occupation numbers ##\left\{n_i \right\}_{i=1,...,N}## which can either be 0 or...
  12. sevenperforce

    Sustained nuclear criticality in liquid vortex

    In 1958, chemical operator Cecil Kelley was killed by a nuclear excursion in a mixing tank. A tank intended to reprocess trace amounts of dissolved plutonium-239 accidentally had dramatically more radioactive material dumped into it. The plutonium, being dissolved in a lower-density fluid than...
  13. Babatunde22

    Criticality calculation of an homogeneous finite reactor

    Please,I am working on the criticality calculation of an homogeneous finite cylindrical reactor core using four-group diffusion equations. I have been able to discretize the multigroup diffusion equations using the finite difference method(FED). But I am stocked on the iterative method to...
  14. J

    Relationship between the number of fuel elements and criticality

    I usually do not ask for this kind of stuff, but this time I really have no way to verify if this is correct(and it has to be). So here it goes: On a subcritical nuclear reactor with 100 fuel elements we read the signal of 10e-9 A. When we add 10 more fuel elements, signal jumps to 10e-8 A. How...
  15. S

    A question about terminology of phase state & criticality

    I'm looking at the diagram (Figure 1) at http://en.wikipedia.org/wiki/Supercritical_fluid I get that the region in which both the temperature & pressure are higher than the critical point is called a supercritical fluid, which really means it is both a gas & a liquid, thermodynamically...
  16. MTd2

    Is QEG's asymptotic safe point an example of self criticality?

    According to wikipedia: "In physics, self-organized criticality (SOC) is a property of (classes of) dynamical systems which have a critical point as an attractor. Their macroscopic behaviour thus displays the spatial and/or temporal scale-invariance characteristic of the critical point of a...
  17. N

    The Volume of the reactor and its criticality

    The Volume of the reactor and its criticality! What will happen to the "Keff" or to the criticality if the volume of the reactor was compressed to one-half its original volume and why ? if the reactor was operating at critical steady-state before changing the volume.
  18. V

    Programs/codes for criticality calculations

    What programs model nuclear reactors and can determine if the reactor goes critical or not? It would be a big advantage if the given programs are open source as well, since the ones that cost money evidently cost a lot. I might be able to get hold of SCALE 6.1, but that program/code would be...
  19. B

    Understanding Criticality: Unpacking K, $, and C for Safe Reactor Operation"

    Hi All. I have been reading up about criticality this weekend and I have come across some terms that I could do with some help on. I think I understand Keff which describes the 'amplification' of reaction and includes the physical nature of the reactor, i.e. mass, shape, spacing...
  20. T

    Adjoint flux for multi-group diffusion equation for criticality problem?

    Hi, I'm new to the entire neutronics field. I've learned about adjoints as a physics student in undergrad and I'm doing nuclear engineering for my graduate studies. I understand how to derive the adjoint operator for the diffusion equation, but I'm a bit confused as to how to calculate the...
  21. L

    First criticality and initial heat-up of the reactor core

    please help me in understanding the following scenarios: 1. how could one approach first criticality in practice? 2. what would happen if we start-up with full power conditions from first criticality? 3. what would the shutdown requirement be in comparison to an equilibrium core? thank...