A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The critical mass of a fissionable material depends upon its nuclear properties, its density, its shape, its enrichment, its purity, its temperature and its surroundings.
What are Fissile materials?
There are different types of nuclear materials and one type is termed as fissile material. Fissile materials are able to sustain a reaction once it has begun. The most widely used fissile materials are Uranium-233, Uranium-235 and plutonium-239. These 3 materials meet the criteria of a fissionable material - persist for a reasonably long time, found in sufficiently large amounts to make using them for fuel practical.
What is the process of Nuclear Reaction?
In the phenomenon of Nuclear reaction, an atom of the fissile material, for instance, Uranium-235, captures a neutron as it moves past. This causes the atom to split into 2 smaller atoms and in the process, 2 or 3 more neutrons are released. Theses neutrons then fly-off and are captured by other atoms of uranium-235, which in turn split and send-off two or three more neutrons. All theses process happens in a very, very short span of time releasing enormous energy.
How to explain the criticality?
Critical mass: When a nuclear chain reaction in a mass of fissile material is self-sustaining, the mass is said to be in a critical state in which there is no increase or decrease in power, temperature or neutron(sub-atomic particle) population. A numerical measure of a critical mass is dependent on the effective neutron multiplication factor, k, the average number of neutrons released per fission event that continues to cause another fission event rather than being absorbed or leaving the material. When k = 1, the mass is critical, and the chain reaction is barely self-sustaining.
Sub-critical mass: A subcritical mass is a mass of fissile material that does not have the ability to carry on a fission reaction. A population of neutrons introduced to a subcritical gathering will exponentially decrease. In this case, k < 1. A steady rate of spontaneous fissions causes a proportionally steady level of neutron activity. The constant of proportionality increases as ‘k’ increases.
Super-critical mass: A supercritical mass is one where there is an increasing rate of fission. The material may settle into equilibrium (that is, become critical again) at a high temperature/power level or devastate itself, by which equilibrium is reached. In the case of super criticality, k > 1.
How is the point of criticality changed?
The point and therefore the mass where the criticality may be changed by modifying certain factors discussed below.
Varying 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 sub-critical mass to create an "exactly critical mass", fission would be self-sustaining for one neutron generation. If the perfect quantity of fuel were added to a slightly subcritical mass, to create a hardly supercritical mass, the temperature of the assembly would raise to an initial maximum (for example: 1 K above the ambient temperature) and then decline back to room temperature after a period of time, because fuel consumed during fission brings the assembly back to sub-criticality again.
Changing the shape: A mass may be exactly critical without being a perfect homogeneous sphere. Further refining the shape closely into a perfect sphere will make the mass supercritical. On the other hand, changing the shape to a less perfect sphere will decrease its reactivity and make it subcritical.
Altering the density of the mass: The higher the density, the lower the critical mass. An ideal mass will become sub-critical if expanded or conversely the same mass will become super-critical if compressed.
Using a Neutron Reflector: Surrounding a spherical critical mass with a any material that reflects neutrons (neutron reflector) reduces the mass further needed for criticality. A common material for a neutron reflector is beryllium metal. This reduces the number of neutrons which escape the fissile material, resulting in increased reactivity.
Using Tamper: In a bomb, a dense shell of material surrounding the fissile core will contain the expanding fissioning material through inertia. This increases the efficiency.
What is the Use of Criticality in Nuclear weapon design?
In order to use the fissile material in nuclear weapon, it is very important that the material must be kept below the critical mass; otherwise the bomb will detonate immediately.
Generally, two pieces of material are kept apart at sub-critical mass, and when it is the time for the bomb to be detonated, they are thrown together very hard and very rapidly.
Then they create a super-critical mass and the bomb explodes.
In case of not thrown together quick enough, a smaller explosion occurs first and blows the two pieces further apart, so that the large explosion never happens, which is often referred to as “fizzle”.
Will critical mass differ from material to material?
In the case of uranium-233, the critical mass is about 35 pounds (15 kg)
In uranium-235, the critical mass is 115 ponds (52kg)
In the case of Plutonium-239, the critical mass is about 22 pounds (10kg)
These amounts seems to be small, but these materials are extremely dense. With most nuclear materials, a sphere (with a few kilograms) will reach a critical mass and cause an enormous explosion