The phenomenon of radioactivity

Options

A. increases on applied pressure
B. is exothermic change which increases or decreases with temperature
C. is nuclear process which does not depend on external force
D. None of the aobve

Explanation

**Step-by-Step Solution:**

1. **Understanding Radioactivity**: Radioactivity is the process by which unstable atomic nuclei lose energy by emitting radiation. This can occur in the form of alpha particles, beta particles, or gamma rays.

2. **Role of Heavy Water**: Heavy water, or deuterium oxide (D2O), is used in certain types of nuclear reactors. In heavy water, the hydrogen atoms in regular water (H2O) are replaced by deuterium, which is a heavier isotope of hydrogen.

3. **Neutron Moderation**: In nuclear reactors, heavy water acts as a neutron moderator. A neutron moderator is a material that slows down fast neutrons produced during nuclear fission. By slowing down these neutrons, they can be more effectively captured by fissile materials, such as Uranium-235 (U-235).

4. **Fission Process**: When a slow (thermal) neutron collides with a U-235 nucleus, it can be absorbed, leading to the formation of Uranium-236 (U-236), which is an unstable isotope. This unstable nucleus will undergo further fission, splitting into two daughter nuclei and releasing additional neutrons and energy.

5. **Interaction with U-238**: Heavy water also allows for interactions with Uranium-238 (U-238). While U-238 does not undergo fission with thermal neutrons, it can capture them to form Uranium-239, which can eventually lead to the production of plutonium-239, a fissile material.

6. **Conclusion**: Therefore, heavy water is crucial in nuclear reactors because it effectively slows down neutrons, increasing the likelihood of fission reactions with U-235, thus enhancing the efficiency of the nuclear reaction.

**Final Answer**: The purpose of using heavy water in nuclear reactors is to act as a neutron moderator, slowing down neutrons so they can effectively react with fissile materials like Uranium-235.

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