Sir

The UK government's Department of Trade and Industry has issued a document that claims to be “⃛part of the UK's renewed commitment to improve transparency and openness in its management of our national holding of civil plutonium”1. In fact the new format provides considerably less information than previously. It no longer gives the reactor-specific data which enabled outsiders like ourselves to make independent assessments. And the total plutonium produced by the UK civil Magnox reactors is still not published.

Until recently, the UK government provided a subtotal of the civil Magnox plutonium. The unpublished balance was sent to the United States before 1971 under “mutual defence agreements” (MDA). Previous Conservative administrations in the United Kingdom maintained that this plutonium was not used in weapons, but refused to quantify the balance.

In 1985, we estimated that this balance was 6.3 ± 0.8 tonnes (refs 2, 3). We revised this figure to 5.4 ± 0.8 tonnes (ref. 4) when it was revealed that the total plutonium in solid waste was much larger than official sources had indicated previously.

In February 1996, the US Department of Energy published an inventory of its plutonium stocks, stating that “⃛ from 1959 to 1980, the US acquired a total of 5.4 tonnes of plutonium in exchange for 6.7 kg of tritium and 7.5 tonnes of highly enriched uranium”5,6. The total of 5.4 tonnes is in remarkable agreement with our revised figure published four years earlier. However, refs 5 and 6 did not specify the end uses of the plutonium in the United States, nor did they clarify how much originated in the UK civil Magnox reactors rather than the military ones at Calder Hall and Chapel Cross.

The question of the UK origin of the plutonium is problematic because civil and military Magnox spent fuel was ‘co-processed’ at Sellafield until 1986. In December 1997, a US press release7 was issued stating that 0.1 tonne of very high purity plutonium (2% plutonium-240 content) was sent to the United States under the exchange programme and that no other plutonium with plutonium-240 content less than 10% was involved. This suggests that the amount of military-origin plutonium involved in the exchanges was small, consistent with estimates that the total UK inventory of weapons-grade plutonium is relatively small. (Calder Hall and Chapel Cross have not operated on a military cycle for many years.)

We believe that the amount of plutonium of military origin sent to the United States under the MDA was a small part of the 5.4 tonnes, possibly as little as the 0.1 tonne of 2% plutonium-240 purity referred to in ref. 7. We conclude that the United Kingdom provided the United States with around 5.4 tonnes of plutonium from the UK civil stockpile.

All this 5.4 tonnes plutonium is in the US defence stockpile. The UK government maintains it has not been used in weapons, but the US civil destinations that have now been listed contain at most 4 tonnes of plutonium2,3, leaving around 1.4 tonnes not accounted for. If sufficiently pure, this amount of plutonium could provide up to 300 warheads.

We call on the present (Labour) UK government to clarify fully the past history. The US disclosures have removed the previous (Conservative) UK government's excuse for secrecy, which was because it would reveal the quantity of highly enriched uranium received by the United Kingdom for defence purposes. The United States published the exact figure for the highly enriched uranium involved in the exchanges two years ago, and it does not appear that the security of the realm has been imperilled.