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Critical Mass. Critical Mass. The critical mass of fissile material is the minimum amount needed for a sustained nuclear chain reaction. (a fissile material is one that is capable of sustaining a chain reaction of nuclear fission). . Critical.
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Critical Mass • The critical mass of fissile material is the minimum amount needed for a sustained nuclear chain reaction. • (a fissile material is one that is capable of sustaining a chain reaction of nuclear fission).
Critical • “Critical" implies an equilibrium (steady-state) fission reaction; there is no increase in power/temperature/neutron population.
Subcritical • "Subcritical" implies an inability to sustain a fission reaction; a population of neutrons introduced to a subcritical assembly will decrease in number over time.
Supercritical • "Supercritical" implies an increasing rate of fission until natural feedback mechanisms cause the reactor to settle into equilibrium (i. e. be critical) at an elevated temperature/power level or destroy itself (disassembly is an equilibrium state).
A nuclear fission chain reaction. 1. A uranium-235 atom absorbs a neutron, and splits into two new smaller atoms (fission fragments), releasing three new neutrons and some binding energy.
A nuclear fission chain reaction. 2. One of those neutrons is absorbed by an atom of uranium-238, and does not continue the reaction. Another neutron is simply lost and does not collide with anything, also not continuing the reaction. However one neutron does collide with an atom of uranium-235, which then splits and releases two neutrons and some binding energy.
A nuclear fission chain reaction. 3. Both of those neutrons collide with uranium-235 atoms, each of which fission and release between one and three neutrons, which can then continue the reaction.
What does it depend on? The critical mass of a fissionable material depends upon: • its nuclear properties (e.g. the nuclear fission cross-section – a high one means that nuclear fission is highly likely) • physical properties (in particular the density), • its shape, and its • enrichment.
Critical Mass • The mass that allows the equilibrium of fissions to remain constant. • This means that the number of fissions produced remains steady. • One neutron from each fission goes on to produce another fission – the other neutrons are ‘lost’
Critical Mass • The number of fissions depends on the mass present. • The number lost depends on the surface area (from which neutrons can escape). • You therefore have to consider the mass/surface area ratio. The figure with the largest ratio is the sphere.
Best shape • The shape with minimum critical mass is a sphere. This can be further reduced by surrounding the sphere with a neutron reflector. • In the case of a bare sphere the critical mass is about 50 kg for uranium-235 and 10 kg for plutonium 239.
Self sustaining reaction • Top: A sphere of fissile material is too small to allow the chain reaction to become self-sustaining as neutrons generated by fissions can too easily escape. • Middle: By increasing the mass of the sphere to a critical mass, the reaction can become self-sustaining. • Bottom: By surrounding the original sphere with a neutron reflector, it can increase the efficiency of the reactions and also allow the material to become self-sustaining.
Enrichment • Enriched uranium is a sample of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. • Natural uranium is 99.284% 238U isotope, with 235U only constituting about 0.72 % of its weight. • 235U is the only isotope existing in nature (in any appreciable amount) that is fissionable by thermal neutrons.
Enrichment • The 238U remaining after enrichment is known as depleted uranium (DU), and is considerably less radioactive than even natural uranium, though still extremely dense.
Why? • The 238U remaining after enrichment is known as depleted uranium (DU), and is considerably less radioactive than even natural uranium, though still extremely dense. • U-235 half life: 7.038x108 years • U-238 half life: 4.468x109 years
Enrichment • It is useful for armour penetrating weapons, and other applications requiring very dense metals … can you think of problems with this?
How can you reduce the required mass? • Surrounding fissionable material by a neutron reflector reduces the needed mass for criticality. • (Beryllium is good at this) A (simulated) sphere of plutonium surrounded by neutron-reflecting blocks of tungsten carbide, as part of a re-creation of a 1945 criticality accident to measure the radiation produced when an extra block was added, making the mass supercritical.
Plutonium-239 – critical mass 10kg • Plutonium-239 is one of the three fissile isotopes used for the production of nuclear weapons and in nuclear reactors as a source of energy. Other fissile isotopes used are uranium-235 and uranium-233. • Plutonium-239 has a half-life of 24,110 years. • The nuclear properties of plutonium-239, as well as the ability to produce large amounts of nearly pure plutonium-239, led to its use in nuclear weapons and nuclear power.
Plutonium-239 – critical mass 10kg • The splitting of an atom of uranium-235 in the reactor of a nuclear power plant produces two to three neutrons, and these neutrons can be absorbed by uranium-238 to produce plutonium-239 and other isotopes. • Plutonium-239 can also absorb neutrons and fission along with the uranium-235. Plutonium fissions provide about one-third of the total energy produced in a typical commercial nuclear power plant. • The use of plutonium-239 in power plants occurs without it ever being removed from the nuclear reactor fuel, i.e., it is fissioned in the same fuel rods in which it is produced.
Nuclear bombs • Both types require ‘subcritical’ fissile material and a method of making it supercritical.
