Nuclear power is the cleanest available form of mass energy generation, producing no greenhouse gas emissions. Reactor design has improved, as has the technology of nuclear waste disposal. Greater recourse to nuclear power is a necessary ingredient of strategies to mitigate global warming. The ambitious emissions targets set for Britain by Labour are, as it has yet to admit, extremely unlikely to be met without a decision to replace at least some of this country’s ageing reactors.

Public confidence in every aspect of the civil nuclear sector is thus a vital national interest — and that means confidence not only about safety, but about security. Al-Qaeda records in Afghanistan showed that its leaders would have used weapons of mass destruction, had they had them. Cast-iron anti-theft safeguards are required, and nowhere more than in the nuclear reprocessing industry.

When the public learns, therefore, that 30kg of plutonium are listed as “unaccounted for” in the annual audit of the British Nuclear Group’s vast nuclear reprocessing complex at Sellafield, it is natural to assume that this is a euphemism for “lost” — that fissile material could fall, or have fallen, into dangerous hands. Reprocessed plutonium is not necessarily portable, existing not only in metal, but also in oxide powder and liquid nitrate forms, but that does not make it any easier to convince people that this is an inherently untroubling “accounting exercise”. Nor does it cut much ice that this quantity is only “0.1 per cent of throughput”, or that the apparent discrepancy is well within the 1 per cent ceiling permitted by Euratom. The thought that Sellafield might be “allowed” a 300kg leeway of error, enough for several dozen crude bombs, must naturally appal.

The point, however, is that the “unaccounted for” plutonium is not missing in the way common sense suggests. It probably never existed in the first place. Sellafield’s 30 kilos represent the difference between the quantity of plutonium calculated by nuclear physicists to exist inside the spent fuel rods brought to Sellafield for reprocessing, and the actual plutonium yield.

Plutonium is man-made, created when neutrons hit uranium atoms. The difficulty of measuring how much plutonium a spent fuel rod contains is that not every neutron hits a uranium atom and, even if hit, not every uranium atom turns into plutonium. Before reprocessing, nuclear physicists estimate the quantity of plutonium using factors such as the rod’s weight, the quantity of uranium burnt, how long the rod was in a reactor and where it was located, and operational intensity. They can do with with almost, but not total, accuracy. The aim must always be to reduce the margin of error.

Confidence requires openness — more of it than Britain’s nuclear industry has been known for in the past. One problem with BNG’s conversion to candour, as it has found this week, is that the science of nuclear reprocessing is not widely understood. The best course is not less information, but more. That is a commitment the Government should share with industry.