The Pantex workers had been removing high explosives from a relatively small nuclear warhead called a W48, which is designed to be fired as an artillery shell through the muzzle of a 155mm howitzer. They were running hot water over the hardened explosive to soften the adhesive used to glue it to the inner core of fissile nuclear material when they heard the noise. A crack had suddenly appeared on the surface of the stainless steel sphere containing the warhead's plutonium - creating a danger of nuclear contamination and possibly an explosion that would scatter radioactive material. 'During this process the unit became warm and it popped,' said Robert Bailey, a nuclear scientist at the Lawrence Livermore National Laboratory near San Francisco, the weapons centre that had originally designed the W48 device. 'When they heard the pop, they took a look and saw that something had gone amiss. They turned off the water and evacuated the cell. Then we were called in.'
The incident happened on 11 November and it took Lawrence Livermore scientists and Pantex workers a further two months to bring the accident to a safe conclusion, when the dismantled warhead was shipped from Texas to the Livermore laboratory in California. A detailed analysis of the metal sphere at the centre of the warhead, called a 'pit' (the American word for the stone at the centre of a peach - it is surrounded by the 'flesh' of high explosive), revealed a flaw in the weld used to stick the pit's two stainless steel hemispheres together. A microscopic analysis showed that the flaw was originally a mere 2mm long and 1.4mm deep, yet Dr Bailey said this was the root cause of the problem. The flaw made the weld weaker so when it expanded under the hot water, it cracked open further. 'If there had not been that flaw in the weld, the dismantlement would have been done successfully and we would not have had a failure.'
Nevertheless, the incident demonstrates how easy it can be for the unexpected to happen when nuclear weapons are being dismantled. Over the coming few years the United States and the former Soviet Union are expected to dismantle many thousands of their nuclear missiles and bombs as part of agreements made under the two Strategic Arms Reduction Treaties (Start). Although excluded from the treaties, many more thousands of 'tactical' warheads, such as the W48, are also being destroyed as part of a general reduction in arms. American scientist are engaged in an ambitious attempt to recycle everything they can from the US nuclear stockpile, including precious and semi-precious metals such as gold and platinum. Even the high explosive is being taken out and repackaged as a commercial explosive for mining.
A particular concern is for the safety of nuclear weapons in the former Soviet Union, where political and economic upheaval has increased the risk of accidents considerably, according to Dr Oleg Bukharin, an expert in Soviet nuclear weapons at Princeton University, New Jersey. A recent study of the risks found many problems can arise at any stage - from storage, transport and disassembly. Dr Bukharin is concerned that the former Soviet Union's economic difficulties will lead to a shortage of skilled and dedicated nuclear workers.
'The safety and health risks can be magnified by a human error,' he said last February at the annual meeting of the American Association for the Advancement of Science in San Francisco. 'In Russia, the human factor has been traditionally minimised through the implementation of the two-men rule: all operations with weapons have been conducted by two or three workers, one of them reading instructions and supervising correctness of their implementation.' He fears that that level of supervision may no longer be guaranteed.
Although Russia has so far accomplished much of its dismantlement programme successfully, Dr Bukharin said: 'Economic, political and technical problems make safe and secure dismantlement of many thousands of Russian nuclear weapons difficult. Any failure of the dismantlement - an accident with weapons, or, even worse, their diversion into the wrong hands - would be a disaster.'
He outlined one incident that occurred when the temperature inside a vault used to store warheads in Ukraine allegedly rose by a couple of degrees as a result of a 'violation of spacing requirements of storage' and an apparent lack of regular maintenance. The temperature rise would have increased the hydrogen content inside some warheads of the Soviet SS-24 nuclear missiles. 'Elevated temperatures, humidity and changes in the gas atmosphere inside warheads may in turn cause the high explosives component to age, degrading their physical integrity and increasing sensitivity. This may compromise safety.'
Little is known of the techniques used to dismantle Russian nuclear weapons but Dr Bukharin said a principal rule is that the four weapons factories that orginally assembled the weapons repeat the production process but in reverse order. Much the same principal is carried out in the US, where the critical stage is to separate the high explosives - used to implode or crunch the fissile material in the core of the warhead in order to trigger the nuclear chain reaction - from the radioactive material. This is what was happening at Pantex when the W48 warhead cracked.
'Every nuclear weapon has high explosive and every one has fissile material. Clearly one of the critical steps in dismantlement - to be assured that this is really no longer a weapon - is to remove the fissile material from the high explosive,' Dr Bailey said. 'There are people who work with high explosives all the time at Pantex who believe that this is a very benign step and the methods they use have really no chance of setting off a high explosive. The removal has to be done in a very cautious and systematic way, but they have been doing this for years and have never had a detonation of high explosives in all that time.'
The Pantex Plant has 13 special bunkers, called 'gravel gerties', which are buried in about five metres of gravel and sand to prevent the escape of radioactive material if there is an accidental explosion. Dr Bailey said it is forbidden to bring in more than a given amount of high explosives into the structure. 'It's got to be within those limits so that if that much high explosive did detonate all at one time it would all be totally contained.'
Weapons scientists insist that the risk of an accidental nuclear explosion - as opposed to a conventional chemical explosion that scatters radioactive material - is effectively so small as to be non-existent. To get a nuclear yield the high explosive has got to be detonated in a precise, pre-determined manner that could not occur, they say, during an accidental detonation. 'There is a difference between intentional and unintentional detonation. When you intentionally detonate, you will get a nuclear yield. All our weapons are designed so that if there is an accident there is no nuclear yield,' Dr Bailey said. 'So any detonation on the surface of the high explosive will not lead to a nuclear detonation.' But he admits that a high explosive blast, and the resulting radioactive contamination, and would be 'very dangerous'.
The current arms reduction treaties deal primarily with taking large 'strategic' nuclear warheads out of commission. But both the US and Russia are engaged in a programme to dismantle smaller 'tactical' warheads, such as the American W48 artillery shell. Dr Bailey said it is these smaller weapons that can pose the problems, and weapons scientists have singled out the W48 in particular. 'If you think about it, it makes sense because an artillery shell is small and light and so it is a very difficult system to build. It has to be both rugged to take the acceleration - 8,800 times the acceleration due to gravity - and yet you don't have much energy or space to get the yield. Some of our other systems are much larger and we have more design space to work in.'
Every W48 weapon now being dismantled is kept below 65C at all stages in the destruction process to avoid a cracked weld. 'Basically if you have a good weld, it won't crack,' Dr Bailey said. 'The thing we are concerned about is the fact that there may be another flawed weld in the stockpile. We've probably seen the only major flaw you are likely to see in a weld but we are taking a conservative viewpoint.'
The temperature of 65C is not arbitrary. Scientists calculate that the stress induced in the weapon at this temperature does not exceed the stress that every W48 was put through during pressure tests performed as part of the manufacturing procedure.
Concern over safety has begun to slow down the rate at which nuclear weapons are being dismantled and the rate of progress is beginning to worry the monitors of the Start agreements. 'One of the problems we are having is that in the name of safety we are slowing down our dismantlement process,' Dr Bailey said. 'That bothers a lot people because of commitments we've made to ourselves and the government that we would dismantle them at a given rate. We are having difficulty reaching that rate because we are trying to have a sufficient review process to make sure we're doing it safely, sanely and in the best way possible.'
That, he said, is probably the biggest problem the American weapons scientists face at present. 'Safety is winning out, that's the good news. The bad news is that that means we're probably not dismantling at the rate we would like to.' The lessons learnt after the W48 went pop that November morning will not be forgotten easily.
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