Understanding Critical Mass and its Role in Nuclear Fission

In summary, critical mass refers to the amount of fissile material needed for a chain reaction to occur in a nuclear bomb. It must be kept below this amount to prevent spontaneous fission from occurring, which could cause a smaller explosion. Implosion is a method used to increase the density of the fissile core and make it more efficient. Spontaneous fission can occur in large amounts of Uranium, even without being struck by a neutron.
  • #1
Mr_Bojingles
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I can't quite understand critical mass. They say in a fission bomb they have to separate two blocks of fissile Uranium of subcritical mass so they won't undergo spontaneous fission.

What exactly is critical mass? Is it just the amount of Uranium held in one place at one time and if so why does it have to be under the critical mass. Even if it is a small amount and it spontaneously underwent fission would it not cause a smaller chain reaction and blow up that smaller amount of Uranium??

Why is it that a large amount of Uranium in one place is in danger of undergoing spontaneous fission? What would cause one of the atoms to split and cause a chain reaction?



Another question I have is when they propel the two blocks of fissile Uranium at each other what is it that causes the chain reaction? I thought the only way to cause the fission of a Uranium was bombarding it with neutrons. Is propelling Uranium particles at each other the same thing as propelling neutrons at the atoms?
 
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  • #2
Gun assembly
This more like the method you have described. Actually not that many bomb designs used it. Once implosion-type A-bombs became more compact, it became obsolete as the method has inherent safety issues and is difficult to make efficient in terms of yield for the amount of fissile material used.

Implosion
This is basically the better way to go about it.

Critical mass depends on the material configuration (geometry)(a sphere is optimal), the presence of a surrrounding tamper that can reflect neutrons back, and density. The "beauty" of implosion is that it causes the density of the fissile core to shoot way up. It also uses the optimal geometry too.
 
  • #3
Mr_Bojingles said:
Why is it that a large amount of Uranium in one place is in danger of undergoing spontaneous fission? What would cause one of the atoms to split and cause a chain reaction?

http://en.wikipedia.org/wiki/Spontaneous_fission

U-235 (and U-238 even) can simply fission at random even without absorbing a slow neutron or getting struck by a fast neutron. The rate isn't very fast, but it does happen.

http://en.wikipedia.org/wiki/Critical_mass
 
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1. What is critical mass and how does it affect nuclear fission?

Critical mass refers to the minimum amount of fissile material, such as uranium or plutonium, needed to sustain a nuclear chain reaction. In nuclear fission, atoms of these materials split apart, releasing energy and more neutrons, which can then collide with and split other atoms. If there is not enough fissile material present, the chain reaction will not continue.

2. How is critical mass determined?

The critical mass of a particular fissile material depends on several factors, including its density, shape, and level of enrichment. It is typically calculated through mathematical models and simulations, taking into account the material's properties and the design of the nuclear reactor.

3. What happens if critical mass is exceeded in a nuclear fission reaction?

If critical mass is exceeded, the nuclear chain reaction will rapidly accelerate, leading to a nuclear explosion. This is the principle behind nuclear weapons. In a controlled reactor, however, the reaction is carefully managed to prevent critical mass from being exceeded and causing an explosion.

4. How does critical mass differ between different types of nuclear reactors?

The critical mass for a particular fissile material may vary depending on the design and operation of the nuclear reactor. For example, a reactor using highly enriched uranium may require a smaller critical mass than one using low-enriched uranium. Additionally, some reactor designs have built-in mechanisms to maintain the reaction below critical mass, while others rely on external control systems.

5. Can critical mass be changed or manipulated?

The critical mass of a fissile material is a physical property that cannot be changed. However, the criticality of a nuclear reactor can be affected by various factors, such as the amount and distribution of fuel, the presence of neutron-absorbing materials, and the control mechanisms used. These factors can be manipulated to control the reaction and prevent critical mass from being exceeded.

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