John Harris (generally known as Jack) was a nuclear physicist and metallurgist who was a powerful advocate both for the use of nuclear power and for the abolition of nuclear weapons. He was best known for his interest in metal corrosion and the strain it could place on the underlying material.

He began working to make nuclear power a more effective generator of electricity in 1959 when he joined the Central Electricity Generating Board (CEGB). The CEGB, created from the Central Electricity Authority in 1957, was the cornerstone of the UK’s electricity industry between 1947 when the industry was nationalised until the 1990s when it was privatised.

It had a large research and development section with three main laboratories at Leatherhead: the Central Electricity Research Laboratory, Marchwood (the Marchwood Engineering Laboratory) and the Berkeley Nuclear Laboratories (BNL).

When he joined the CEGB Harris was seconded for two years to Sheffield University. Afterwards, he went to work at BNL.

Berkeley was also the site of the Berkeley nuclear power station, at which two Magnox nuclear power reactors generated electricity; one of them was the first British reactor to produce electricity commercially.

The two reactors, fuelled with natural uranium, generated a total of 276 million watts of electricity, enough to supply a city with a population of about a quarter of a million. The name Magnox comes from the special magnesium alloy used to enclose the fuel rods inside the reactors, which were cooled with carbon dioxide gas. One of the reactors was shut down in October 1988 and the other in March 1989.

The decommissioned reactors are now encased in concrete and covered with soil — two grassy hills are all that remains of them.

In 1965, Harris was made the leader of a group at BNL responsible for examining highly radioactive spent fuel rods and for analysing the results.

As the uranium fuel in a Magnox reactor is used up, nuclear fission products that are produced accumulate in the fuel rods. The fission products reduce the effectiveness of the fission process and eventually the spent fuel rods are replaced with fresh ones.

As Magnox fuel elements are damaged by water they have to be kept in a dry store until the radioactivity in them has decayed enough to allow for their transportation from the reactor site to Sellafield for reprocessing.

Reprocessing involves the removal of plutonium from the spent (used) fuel elements in a chemical plant. As the uranium fuel undergoes nuclear fission in the reactor, plutonium is inevitably produced as a by-product. This plutonium can be used to fabricate nuclear weapons.

Harris argued that civil plutonium should be kept under the strictest possible system of international safeguards. He was aware that public acceptability of nuclear energy depended on effective control of plutonium.