1.   Such was the case with the idea that hydrogen fusion might occur at moderate temperatures within solid materials.

2.   Although the fusion of ordinary hydrogen nuclei into helium atoms occurs within the Sun, pure hydrogen fusion would be very difficult to achieve on Earth.

3.   No one doubts that hydrogen fusion can be achieved.

4.   The flood of very high-energy neutrons produced by hydrogen fusion penetrates metal and disrupts its crystalline structure, causing it to expand and become dangerously brittle.