Critical mass

This article is about nuclear fission reactions. For other uses, seeCritical mass (disambiguation).

Innuclear engineering,acritical massis the smallest amount offissilematerial needed for a sustainednuclear chain reaction.The critical mass of a fissionable material depends upon itsnuclearproperties (specifically, itsnuclear fissioncross-section), density, shape,enrichment,purity, temperature, and surroundings. The concept is important innuclear weapon design.

A re-creation of the 1945criticality accidentusing theDemon core:aplutonium pitis surrounded by blocks ofneutron-reflectivetungsten carbide.The original experiment was designed to measure the radiation produced when an extra block was added. The mass went supercritical when the block was placed improperly by being dropped.

Point of criticality

When a nuclear chain reaction in a mass of fissile material is self-sustaining, the mass is said to be in acriticalstate in which there is no increase or decrease in power, temperature, orneutronpopulation.

A numerical measure of a critical mass depends on theeffective neutron multiplication factork,the average number of neutrons released per fission event that go on to cause another fission event rather than being absorbed or leaving the material.

Asubcriticalmass is a mass that does not have the ability to sustain a fission chain reaction. A population of neutrons introduced to a subcritical assembly will exponentially decrease. In this case, called subcriticality,k< 1.

Acriticalmass is a mass of fissile material that self-sustains a fission chain reaction. In this case, called criticality,k= 1.A steady rate ofspontaneous fissioncauses a proportionally steady level of neutron activity.

Asupercriticalmass is a mass which, once fission has started, will proceed at an increasing rate.In this case, called supercriticality,k> 1.The constant of proportionality increases askincreases. The material may settle into equilibrium (i.e.become critical again) at an elevated temperature/power level or destroy itself.

Due tospontaneous fissiona supercritical mass will undergo a chain reaction. For example, a spherical critical mass of pureuranium-235(²³⁵U) with a mass of about 52 kilograms (115 lb) would experience around 15 spontaneous fission events per second.The probability that one such event will cause a chain reaction depends on how much the mass exceeds the critical mass. If there isuranium-238(²³⁸U) present, the rate of spontaneous fission will be much higher.Fission can also be initiated by neutrons produced bycosmic rays.

Changing the point of criticality

The mass where criticality occurs may be changed by modifying certain attributes such as fuel, shape, temperature, density and the installation of a neutron-reflective substance. These attributes have complex interactions and interdependencies. These examples only outline the simplest ideal cases:

Varying the amount of fuel

It is possible for a fuel assembly to be critical at near zero power. If the perfect quantity of fuel were added to a slightly subcritical mass to create an "exactly critical mass", fission would be self-sustaining for only one neutron generation (fuel consumption then makes the assembly subcritical again).

Similarly, if the perfect quantity of fuel were added to a slightly subcritical mass, to create a barely supercritical mass, the temperature of the assembly would increase to an initial maximum (for example: 1Kabove the ambient temperature) and then decrease back to the ambient temperature after a period of time, because fuel consumed during fission brings the assembly back to subcriticality once again.

Changing the shape

A mass may be exactly critical without being a perfect homogeneous sphere. More closely refining the shape toward a perfect sphere will make the mass supercritical. Conversely changing the shape to a less perfect sphere will decrease its reactivity and make it subcritical.

Changing the temperature

A mass may be exactly critical at a particular temperature.Fission and absorption cross-sectionsincrease as the relative neutron velocity decreases. As fuel temperature increases, neutrons of a given energy appear faster and thus fission/absorption is less likely. This is not unrelated toDoppler broadeningof the²³⁸U resonances but is common to all fuels/absorbers/configurations. Neglecting the very important resonances, the total neutron cross-section of every material exhibits an inverse relationship with relative neutron velocity. Hot fuel is always less reactive than cold fuel (over/under moderation inLWRis a different topic). Thermal expansion associated with temperature increase also contributes a negative coefficient of reactivity since fuel atoms are moving farther apart. A mass that is exactly critical at room temperature would be sub-critical in an environment anywhere above room temperature due to thermal expansion alone.

Varying the density of the mass

The higher the density, the lower the critical mass. The density of a material at a constant temperature can be changed by varying the pressure or tension or by changing crystal structure (seeallotropes of plutonium). An ideal mass will become subcritical if allowed to expand or conversely the same mass will become supercritical if compressed. Changing the temperature may also change the density; however, the effect on critical mass is then complicated by temperature effects (see "Changing the temperature" ) and by whether the material expands or contracts with increased temperature. Assuming the material expands with temperature (enricheduranium-235at room temperature for example), at an exactly critical state, it will become subcritical if warmed to lower density or become supercritical if cooled to higher density. Such a material is said to have a negative temperature coefficient of reactivity to indicate that its reactivity decreases when its temperature increases. Using such a material as fuel means fission decreases as the fuel temperature increases.

Use of a neutron reflector

Surrounding a spherical critical mass with aneutron reflectorfurther reduces the mass needed for criticality. A common material for a neutron reflector isberylliummetal. This reduces the number of neutrons which escape the fissile material, resulting in increased reactivity.

Use of a tamper

In a bomb, a dense shell of material surrounding the fissile core will contain, via inertia, the expanding fissioning material, which increases the efficiency. This is known as atamper.A tamper also tends to act as a neutron reflector. Because a bomb relies on fast neutrons (not ones moderated by reflection with light elements, as in a reactor), the neutrons reflected by a tamper are slowed by their collisions with the tamper nuclei, and because it takes time for the reflected neutrons to return to the fissile core, they take rather longer to be absorbed by a fissile nucleus. But they do contribute to the reaction, and can decrease the critical mass by a factor of four.Also, if the tamper is (e.g. depleted) uranium, it can fission due to the high energy neutrons generated by the primary explosion. This can greatly increase yield, especially if even more neutrons are generated by fusing hydrogen isotopes, in a so-calledboosted configuration.

Critical size

The critical size is the minimum size of a nuclear reactor core or nuclear weapon that can be made for a specific geometrical arrangement and material composition. The critical size must at least include enough fissionable material to reach critical mass. If the size of the reactor core is less than a certain minimum, too many fission neutrons escape through its surface and the chain reaction is not sustained.

Critical mass of a bare sphere

Top:Asphereof fissile material is too small to allow thechain reactionto become self-sustaining asneutronsgenerated byfissionscan too easily escape.

Middle:By increasing the mass of the sphere to a critical mass, the reaction can become self-sustaining.

Bottom:Surrounding the original sphere with aneutron reflectorincreases the efficiency of the reactions and also allows the reaction to become self-sustaining.

The shape with minimal critical mass and the smallest physical dimensions is a sphere. Bare-sphere critical masses at normal density of someactinidesare listed in the following table. Most information on bare sphere masses is considered classified, since it is critical to nuclear weapons design, but some documents have been declassified.

Nuclide	Half-life (y)	Critical mass (kg)	Diameter (cm)	Ref
uranium-233	159,200	15	11
uranium-235	703,800,000	52	17
neptunium-236	154,000	7	8.7
neptunium-237	2,144,000	60	18
plutonium-238	87.7	9.04–10.07	9.5–9.9
plutonium-239	24,110	10	9.9
plutonium-240	6561	40	15
plutonium-241	14.3	12	10.5
plutonium-242	375,000	75–100	19–21
americium-241	432.2	55–77	20–23
americium-242m	141	9–14	11–13
americium-243	7370	180–280	30–35
curium-243	29.1	7.34–10	10–11
curium-244	18.1	13.5–30	12.4–16
curium-245	8500	9.41–12.3	11–12
curium-246	4760	39–70.1	18–21
curium-247	15,600,000	6.94–7.06	9.9
berkelium-247	1380	75.7	11.8-12.2
berkelium-249	0.9	192	16.1-16.6
californium-249	351	6	9
californium-251	900	5.46	8.5
californium-252	2.6	2.73	6.9
einsteinium-254	0.755	9.89	7.1

The critical mass for lower-grade uranium depends strongly on the grade: with 20%²³⁵U it is over 400 kg; with 15%²³⁵U, it is well over 600 kg.

The critical mass is inversely proportional to the square of the density. If the density is 1% more and the mass 2% less, then the volume is 3% less and the diameter 1% less. The probability for a neutron per cm travelled to hit a nucleus is proportional to the density. It follows that 1% greater density means that the distance travelled before leaving the system is 1% less. This is something that must be taken into consideration when attempting more precise estimates of critical masses of plutonium isotopes than the approximate values given above, because plutonium metal has a large number of different crystal phases which can have widely varying densities.

Note that not all neutrons contribute to the chain reaction. Some escape and others undergoradiative capture.

Letqdenote the probability that a given neutron induces fission in a nucleus. Consider onlyprompt neutrons,and letνdenote the number of prompt neutrons generated in a nuclear fission. For example,ν≈ 2.5for uranium-235. Then, criticality occurs whenν·q= 1.The dependence of this upon geometry, mass, and density appears through the factorq.

Given a total interactioncross sectionσ (typically measured inbarns), themean free pathof a prompt neutron is$\ell ^{-1}=n\sigma$wherenis the nuclear number density. Most interactions are scattering events, so that a given neutron obeys arandom walkuntil it either escapes from the medium or causes a fission reaction. So long as other loss mechanisms are not significant, then, the radius of a spherical critical mass is rather roughly given by the product of the mean free path$\ell$and the square root of one plus the number of scattering events per fission event (call thiss), since the net distance travelled in a random walk is proportional to the square root of the number of steps:

$R_{c}\simeq \ell {\sqrt {s}}\simeq {\frac {\sqrt {s}}{n\sigma }}$

Note again, however, that this is only a rough estimate.

In terms of the total massM,the nuclear massm,the density ρ, and a fudge factorfwhich takes into account geometrical and other effects, criticality corresponds to

$1={\frac {f\sigma }{m{\sqrt {s}}}}\rho ^{2/3}M^{1/3}$

which clearly recovers the aforementioned result that critical mass depends inversely on the square of the density.

Alternatively, one may restate this more succinctly in terms of the areal density of mass, Σ:

$1={\frac {f'\sigma }{m{\sqrt {s}}}}\Sigma$

where the factorfhas been rewritten asf'to account for the fact that the two values may differ depending upon geometrical effects and how one defines Σ. For example, for a bare solid sphere of²³⁹Pu criticality is at 320 kg/m²,regardless of density, and for²³⁵U at 550 kg/m². In any case, criticality then depends upon a typical neutron "seeing" an amount of nuclei around it such that the areal density of nuclei exceeds a certain threshold.

This is applied in implosion-type nuclear weapons where a spherical mass of fissile material that is substantially less than a critical mass is made supercritical by very rapidly increasing ρ (and thus Σ as well) (see below). Indeed, sophisticated nuclear weapons programs can make a functional device from less material than more primitive weapons programs require.

Aside from the math, there is a simple physical analog that helps explain this result. Consider diesel fumes belched from an exhaust pipe. Initially the fumes appear black, then gradually you are able to see through them without any trouble. This is not because the total scattering cross section of all the soot particles has changed, but because the soot has dispersed. If we consider a transparent cube of lengthLon a side, filled with soot, then theoptical depthof this medium is inversely proportional to the square ofL,and therefore proportional to the areal density of soot particles: we can make it easier to see through the imaginary cube just by making the cube larger.

Several uncertainties contribute to the determination of a precise value for critical masses, including (1) detailed knowledge of fission cross sections, (2) calculation of geometric effects. This latter problem provided significant motivation for the development of theMonte Carlo methodin computational physics byNicholas MetropolisandStanislaw Ulam.In fact, even for a homogeneous solid sphere, the exact calculation is by no means trivial. Finally, note that the calculation can also be performed by assuming a continuum approximation for the neutron transport. This reduces it to a diffusion problem. However, as the typical linear dimensions are not significantly larger than the mean free path, such an approximation is only marginally applicable.

Finally, note that for some idealized geometries, the critical mass might formally be infinite, and other parameters are used to describe criticality. For example, consider an infinite sheet of fissionable material. For any finite thickness, this corresponds to an infinite mass. However, criticality is only achieved once the thickness of this slab exceeds a critical value.

Criticality in nuclear weapon design

If two pieces of subcritical material are not brought together fast enough, nuclear predetonation (fizzle) can occur, whereby a very small explosion will blow the bulk of the material apart.

Until detonation is desired, anuclear weaponmust be kept subcritical. In the case of a uranium gun-type bomb, this can be achieved by keeping the fuel in a number of separate pieces, each below thecritical sizeeither because they are too small or unfavorably shaped. To produce detonation, the pieces of uranium are brought together rapidly. InLittle Boy,this was achieved by firing a piece of uranium (a 'doughnut') down agun barrelonto another piece (a 'spike'). This design is referred to as agun-type fission weapon.

A theoretical 100% pure²³⁹Pu weapon could also be constructed as a gun-type weapon, like the Manhattan Project's proposedThin Mandesign. In reality, this is impractical because even "weapons grade"²³⁹Pu is contaminated with a small amount of²⁴⁰Pu, which has a strong propensity toward spontaneous fission. Because of this, a reasonably sized gun-type weapon would suffer nuclear reaction (predetonation) before the masses of plutonium would be in a position for a full-fledged explosion to occur.

Instead, the plutonium is present as a subcritical sphere (or other shape), which may or may not be hollow. Detonation is produced by exploding ashaped chargesurrounding the sphere, increasing the density (and collapsing the cavity, if present) to produce aprompt criticalconfiguration. This is known as animplosion type weapon.

Prompt criticality

Main article:Prompt criticality

The event of fission must release, on the average, more than one free neutron of the desired energy level in order to sustain a chain reaction, and each must find other nuclei and cause them to fission. Most of the neutrons released from a fission event come immediately from that event, but a fraction of them come later, when the fission products decay, which may be on the average from microseconds to minutes later. This is fortunate for atomic power generation, for without this delay "going critical" would be an immediately catastrophic event, as it is in a nuclear bomb where upwards of 80 generations of chain reaction occur in less than a microsecond, far too fast for a human, or even a machine, to react. Physicists recognize two points in the gradual increase of neutron flux which are significant: critical, where the chain reaction becomes self-sustaining thanks to the contributions of both kinds of neutron generation,andprompt critical,where the immediate "prompt" neutrons alone will sustain the reaction without need for the decay neutrons. Nuclear power plants operate between these two points ofreactivity,while above the prompt critical point is the domain of nuclear weapons and some nuclear power accidents, such as theChernobyl disaster.

See also

• Criticality (status)
• Criticality accident
• Nuclear criticality safety
• Geometric and material buckling