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Induced Fission by Thermal Neutrons and Critical Mass

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**1. **Induced Fission by Thermal Neutrons and Critical Mass By Robert T Wilkin and David B Wyllie Esq.

**2. **Fission Reactions

**3. **Induced Fission Reactions By irradiating samples of heavy nuclides with slow-moving thermal neutrons it is possible to induce fission reactions.
When 235U absorbs a thermal neutron, for example, it splits into two particles of uneven mass and releases an average of 2.5 neutrons, as shown in the figure below.

**4. **Thermal Neutrons Neutrons from fission have very high speeds and must be slowed greatly by water "moderation" to maintain the chain reaction. This creates a thermal neutron
A thermal neutron is a free neutron with a kinetic energy level of ca. 0.025 eV (approx. 4.0e-21 J; 2.4 MJ/kg, hence a speed of 2.2 km/s). They are named 'thermal' as this level of kinetic energy is similar to the average kinetic energy of the molecules in a room-temperature gas
Thermal neutrons have a much larger effective cross-section than fast neutrons, and can therefore be absorbed more easily by any atomic nuclei that they collide with, creating a heavier - and often unstable - isotope of the element as a result.

**5. **Critical mass For a chain reaction of nuclear fission, such as that of uranium-235, is to sustain itself, then at least one neutron from each fission must strike another U-235 nucleus and cause a fission. If this condition is just met, then the reaction is said to be "critical" and will continue.
The mass of fissile material required to achieve this critical condition is said to be a critical mass.

**6. **What it depends on The critical mass depends upon:
The concentration of U-235 nuclei in the fissile material
Its geometry.
It also depends upon the moderation used to slow down the neutrons.