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Our current source of helium is the alpha decay of uranium and its daughter radioisotopes in the rocks of oil and gas fields ("Compound the error", Letters, 26 January, and "For He, just add electrons", 2 February). Renewing this source takes millions of years. Nuclear fusion is currently a poor method for making helium on Earth, but the alpha-emitting material (plutonium and minor actinides) produced in the current generation of fission reactors does emit the element.

My calculations suggest that one tonne of used reactor fuel would form enough helium to yield about 2ml of liquid helium a month. However, I suspect that the cost of collecting and purifying it away from the spent fuel would be exceptionally high.

One super-rare form of helium (helium-3) is typically made by man rather than nature: it is formed by the beta decay of tritium. Tritium (half-life 12 years) forms the very rare and non-radioactive helium-3, which can be separated from tritium gas.

In the past a great deal of tritium was made for nuclear bomb programmes, but I suspect arms reduction and the end of the Cold War reduced production. This reduction and increased demand for neutron detectors containing helium-3 after 9/11, when security agencies began to install more detectors at ports and border crossings, have made this rare helium isotope, vital for some research, even more expensive.

Mark R. StJ. Foreman, Chalmers University of Technology, Goteborg, Sweden