Professor Bernard Feld had a nightmare. He was one of the scientists of the Manhattan project who assembled the first atomic bomb, and he imagined the mayor of a big city sending for him: "He has received a note from a terrorist group telling him that they have planted a nuclear bomb somewhere in the city centre. He has confirmed that plutonium is missing. He shows me the crude diagram - and I know the device would work. Not efficiently, but with devastating effect. What should I advise? Surrender to blackmail, or risk the destruction?"
Professor Feld's nightmare may well come a long step closer in the next few months. Ministers have to decide now whether to turn Britain into a plutonium economy. This happens if they permit the removal of the raw material for nuclear bombs from closely guarded stores and allow it to be transported routinely through Britain and Europe like any other fuel - oil or coal.
The decision - potentially one of the most fateful ever before a British government - will be taken over a little-known plant at Sellafield with the innocent-sounding name of the Mixed Oxide Fuel Plant. But independent nuclear scientists in Britain and overseas believe that, if MOFP is allowed to start up, it will greatly increase the threat of nuclear terrorism.
The £ 300m plant stands ready for operation at the Cumbrian nuclear complex, waiting only for ministers to make up their minds. Towards the end of the summer, the Deputy Prime Minister, John Prescott, and Environment minister, Michael Meacher, are due to receive a lengthy report on the plant's pros and cons from the Environment Agency. They will then have two options, to wave it through immediately or call a public inquiry into what to do.
British Nuclear Fuels (BNFL), the publicly owned company that runs Sellafield, is pressing hard for an immediate go-ahead, saying that each week's delay costs it money. But Friends of the Earth insists that "a decision of this magnitude requires full public and democratic scrutiny". A battle royal looms.
Environmentalists may hate and fear the plant, but the nuclear industry insists it is "green" - a pioneer of recycling - because it takes plutonium from used nuclear fuel and sets it to work again.
Ordinary nuclear fuel uses only uranium; and, as it powers a reactor, some of it is turned into plutonium and highly radioactive wastes. The used fuel is sent to Sellafield to be "reprocessed", separating out the uranium, the plutonium and the wastes. The new plant would take some of the plutonium and mix it with uranium to make fresh reactor fuel. As every gram of plutonium contains as much energy as a ton of oil, this, says the industry, is a prudent and environmentally friendly way of using resources.
In truth, it is more a way of trying to reduce a growing plutonium mountain. Sometime in the next 18 months the amount of plutonium recovered from civilian reactor fuel by reprocessing will outstrip that already consumed by nuclear weapons and military stockpiles.
Most of this plutonium will be in Britain and France, the two main reprocessing nations which are becoming the world's twin nuclear capitals. Both now have about 61 tons of civilian plutonium - enough for about 4,700 bombs, dwarfing their nuclear arsenals - and the stockpiles are growing rapidly. Earlier this year, the Royal Society, Britain's scientific institution, added its voice to a growing chorus of alarm: the risk that terrorists might get hold of the plutonium, it said, was of "extreme concern".
It wasn't supposed to be like this. The original idea was that the plutonium would fuel fast breeder reactors, long seen as the next stage in nuclear power. By now there should have been 100 of them running commercially in the United States alone. Instead there are none, anywhere.
In the US the writing was on the wall more than 20 years ago and reprocessing of used fuel stopped. The US and most other nuclear nations planned to dispose of used fuel rods as they are, which meant that plutonium remained mixed up with - and protected by - the virulent nuclear waste. Britain and France, by contrast, built new reprocessing plants, both for their own fuel and for fuel from countries such as Germany and Japan which faced nuclear protests at home, and were anxious to get rid of it abroad.
Awash with plutonium, their fast breeder reactor programmes cancelled, nuclear industries in Britain and France turned to using reprocessed fuel in ordinary nuclear power stations. France already has a plant to make the fuel, and BNFL hopes that MOFP will soon to be competing with it. Both insist that this will decrease the risk of plutonium falling into the wrong hands, because it will be used up in reactors. Although this will slowly reduce the plutonium stocks, experts point out that it will make it more available - to be stolen while it is being made into fuel, and to interception when it is en route to the power stations.
"Plutonium is very vulnerable to both theft and sabotage when it is being transported," says Dr. Frank Barnaby, who worked at the Aldermaston Atomic Weapons Research Establishment before becoming director of the Stockholm International Peace Research Institute. He points out that it can be easily handled with only the minimum shielding: a single sheet of paper will stop its alpha radiation in its tracks (though it is deadly if breathed in). If the BNFL plant goes ahead, and reaches full capacity, it will be putting 7.2 tons of plutonium - enough for 550 nuclear bombs - through the transport system each year.
BNFL says it will be very difficult for a terrorist or criminal group to separate the plutonium from the uranium, but as long as two decades ago a massive Ford Foundation study, headed by one of the most eminent US bomb makers, described it as "fairly easy". BNFL used to insist that it would be impossible to make a bomb out of plutonium from reactors because it has more impurities than the material used in nuclear weapons. The French used to say the same: "No matter what you say, our plutonium is innocent," said one senior official.
But the US exploded a bomb made out of reactor-grade plutonium 30 years ago, and last year a major US government study concluded that a terrorist group could use plutonium to produce a bomb at least one-third as powerful as the one that devastated Hiroshima.
The technique of making the bomb is quite simple. The information has long been in the literature. As far back as 1970 the authorities of Orlando, Florida, received a demand for money accompanied by a drawing of the much more secret H-bomb, so convincing that they had to take it seriously. It turned out to be a false alarm. It had been drawn by a 14-year-old boy.
Is there anything to be gained in return for this risk? A study by the firm PA Consulting for the Environment Agency concluded that BNFL would make money from its new plant, but many other experts doubt it. Top officials from the pro-nuclear International Atomic Energy Agency, for example, calculate that plutonium fuel will be more expensive than the ordinary equivalent, even if the plutonium is free and the costs of reprocessing are omitted. If these costs are included, it is seven times more expensive.
Frans Berkhout, senior fellow at Sussex University's prestigious Science Policy Research Unit, reckons that the BNFL plant - if approved - "will never operate at more than half capacity" because not enough reactors will be licensed to use the new fuel.
The ultimate irony, Mr Berkhout points out, is that the new fuel, once used, will be disposed of without reprocessing because no one will want to incur the extra expense. If that is the case, why not just dispose of the spent fuel in the first place? This would save the billions it will cost to turn waste into plutonium fuel. It would also prevent Professor Feld's nightmare becoming a reality.