A : For fusion, the light nuclei must have sufficient initial energy to cross the coulomb barrier.
Hence, fusion requires high temperature, however, the actual temperature required is somewhat less than expected clssically.
R: It is to mechanical tunneling of the potential barrier.

Answer

To analyze the given assertion (A) and reason (R) regarding nuclear fusion, we can break down the explanation into a step-by-step solution.

### Step-by-Step Solution:

1. **Understanding Nuclear Fusion**:
- Nuclear fusion is the process where light nuclei combine to form a heavier nucleus, releasing energy in the process. For example, hydrogen nuclei can fuse to form helium.

2. **Coulomb Barrier**:
- Light nuclei, like hydrogen, have a positive charge due to their protons. When they approach each other, they experience a repulsive force known as the Coulomb force, which creates a potential energy barrier that must be overcome for fusion to occur.

3. **Initial Energy Requirement**:
- To overcome the Coulomb barrier, the light nuclei must have sufficient kinetic energy. This energy is typically provided by high temperatures, which increase the thermal motion of the nuclei.

4. **High Temperature Requirement**:
- Classically, one might expect that a very high temperature is necessary to provide enough kinetic energy for the nuclei to overcome the Coulomb barrier. Thus, fusion is generally associated with high temperatures.

5. **Actual Temperature vs. Classical Expectation**:
- However, the actual temperature required for fusion is somewhat lower than what classical physics would suggest. This is because of the phenomenon of quantum mechanical tunneling.

6. **Mechanical Tunneling**:
- Quantum mechanics allows for the possibility of tunneling, where particles can pass through potential barriers even if they do not have enough energy to overcome the barrier classically. This means that nuclei can "tunnel" through the Coulomb barrier at lower energies than expected.

7. **Conclusion**:
- Therefore, both the assertion (A) that fusion requires high temperature and the reason (R) that this is due to mechanical tunneling of the potential barrier are correct. The tunneling effect allows fusion to occur at lower temperatures than would be required if only classical physics were considered.

### Final Answer:
Both the assertion (A) and reason (R) are correct. Fusion requires high temperatures to provide the necessary kinetic energy to overcome the Coulomb barrier, but the actual temperature needed is lower than expected due to the quantum mechanical effect of tunneling.