Building B30 is a large, stained, concrete edifice that stands at the centre of Sellafield, Britain's sprawling nuclear processing plant in Cumbria. Surrounded by a three-metre-high fence that is topped with razor wire, encased in scaffolding and riddled with a maze of sagging pipes and cabling, it would never be a contender to win an architectural prize.

Yet B30 has a powerful claim to fame, albeit a disturbing one. "It is the most hazardous industrial building in western Europe," according to George Beveridge, Sellafield's deputy managing director.

Nor is it hard to understand why the building possesses such a fearsome reputation. Piles of old nuclear reactor parts and decaying fuel rods, much of them of unknown provenance and age, line the murky, radioactive waters of the cooling pond in the centre of B30. Down there, pieces of contaminated metal have dissolved into sludge that emits heavy and potentially lethal doses of radiation.

It is an unsettling place, though B30 is certainly not unique. There is Building B38 next door, for example. "That's the second most hazardous industrial building in Europe," said Beveridge. Here highly radioactive cladding from reactor fuel rods is stored, also under water. And again, engineers have only a vague idea what else has been dumped in its cooling pond and left to disintegrate for the past few decades.

During the miners' strike of 1972, the nation's nuclear plants were run at full stretch in order to supply electricity to a beleaguered nation. As a result, it proved impossible to process all the waste that was being generated. Cladding and fuel were simply thrown into B38's cooling ponds and left to disintegrate.

But the building, like so many other elderly edifices at Sellafield, is crumbling and engineers now face the headache of dealing with its lethal contents.

This, then, is the dark heart of Sellafield, a place where engineers and scientists are only now confronting the legacy of Britain's postwar atomic aspirations and the toxic wasteland that has been created on the Cumbrian coast. Engineers estimate that it could cost the nation up to £50bn to clean this up over the next 100 years.

The figure is, by far, the largest part of the £73bn that has been committed to cleaning up Britain's nuclear-polluted past. It is also an acute embarrassment to the government, which is now anxiously promoting nuclear power as the solution to Britain's energy problems.

Last week ministers revealed a list of 11 sites for new nuclear plants around Britain. Atomic power will be the nation's salvation as it battles global warming and seeks to cut its carbon emissions, they insisted.

But the condition of edifices such as B30 and B38 - and all the other "legacy" structures built at Sellafield decades ago - suggest Britain might end up paying a heavy price for this new commitment to nuclear energy. After all, if it is going to cost that much to decommission early reactors, green groups and opponents of nuclear energy are asking, what might we end up paying for a second clean-up if we go ahead with new nuclear plants?

For its part, the nuclear industry is adamant. New reactors will produce little waste and pose few threats to the environment, say UK nuclear chiefs who point to the example of France where almost 80% of electricity is generated by atomic fission and waste is safely reprocessed. Atomic energy today is safe and

Sellafield's problems are merely a historic accident - the result of Britain's desperation to be a leading postwar power, they say.

But it will be a tricky job convincing the public that modern nuclear plants are the answer to Britain's energy worries, given that there are buildings in Sellafield filled with "appalling radioactive crap", as one senior nuclear physicist put it, and which will cost tens of billions of pounds to clean up.

"It is going to be a very difficult business," admitted Dr Paul Howarth, executive director of Dalton Nuclear Institute at Manchester University. "The taxpayer now has to pay around £1.5bn a year to clean up Sellafield's waste problems and will have to maintain that investment for years to come.

"That is a very large financial commitment. Nevertheless it would be wrong to dismiss nuclear energy out of hand. Modern reactors are indeed very different creations compared to the first reactors that were built at Sellafield in the 1940s and 1950s. New ones produce relatively little waste, will be easy to decommission and are intrinsically clean and safe. Convincing the public of these points will not be easy, however."

A former second world war ordnance factory, Sellafield was chosen to be the site for Britain's first atomic reactors - known as Pile 1 and Pile 2. These were not built to generate electricity, but to produce plutonium for the nation's independent nuclear deterrent. Construction was carried out at breakneck speed as political leaders pressed scientists to complete the project quickly.

As a result of these efforts, Britain was able to explode its own atomic bombs by 1952. The UK became a nuclear power and won itself a permanent seat on the UN security council, thanks to its nuclear engineers and scientists.

But success came at an appalling price. Those scientists had no time to think about the waste produced by their atomic bomb programme, a point starkly demonstrated by another Sellafield legacy building, B41. It still stores the aluminium cladding for the uranium fuel rods that were burnt inside Piles 1 and 2. That aluminium posed serious disposal problems when it was removed, in a highly radioactive state, from the two reactors as their fuel was decommissioned and their plutonium extracted.

So scientists hit on what seemed to be an ingenious solution: they would dump it in a silo. "If you drive across the plains of North America, you see these isolated grain silos where farmers store their grain," says Beveridge. "And that, essentially, is what B41 is - a grain silo."

Nuclear waste was tipped in at the top of B41 once it was erected and then allowed to fall to the bottom. Later, when it was realised that pieces of aluminium and magnesium among this waste could catch fire and cause widespread contamination, inert argon gas had to be pumped in to smother potential blazes. And so, for the past 60 years, building B41 has remained in this state, its highly radioactive contents mingling and reacting with each other. Now engineers have been told to clear it up.

They do, fortunately, have a plan. In a few years, vast metal-cutting machines will be brought into Sellafield and used to slice into the sides of the B41 silo before mechanical grabs pull out and sort through its contents. Then this radioactive debris will be mixed with liquid glass and allowed to solidify, a process known as vitrification, before it is kept for subsequent storage in underground vaults. Isolating this material will be immensely difficult, however: B41 will have to be covered and sealed to ensure no leakage of radioactive material. At the same time, the giant cutting machines employed to slice open the silo will have to negotiate the treacherous, tight concourses that separate Sellafield's different buildings. These are lined with cabling, ducts and, most worrying of all, elevated pipes, called pipe-bridges, that carry radioactive liquid waste around the site. Damaging or opening up one of these could have disastrous consequences.

Hence the care taken by engineers as they prepare their plans for B41 while their colleagues continue their work at the silo's sister plant, B29, where decommissioning work has already begun.

In effect, B29 is simply a huge covered cooling pond that once stretched between the heat stacks of Piles 1 and 2.

Fuel rods were removed from these two reactors, moved into the cooling pond of B29 and split open. Most of this material was removed for reprocessing but several tonnes of waste and old fuel still lies below the pond's thick milky waters and it is the task of Steve Topping, leader of the building's decommissioning team, to ensure that this is extracted and safely stored.

Calm, with greying hair, Topping has a reassuringly confident air about his work despite the fact he has to deal with tonnes of nuclear waste and old oxide fuel whose exact composition and location is unknown. "The trouble is there is no one left at Sellafield to tell us where things were put down there. The stuff in the pond has been down there for 50 years," says Topping.

Today B29 is showing its age and looks more like a dirty old dock than a pool with its crumbling grey concrete, grimy brickwork and old ducts and sections of corroding pipes. The water is filled with green algae and has the clarity of Milk of Magnesia, which defies all efforts to see what lies beneath.

To clean it up, robot machines will soon begin to split open the submerged skips in which old waste and fuel from Piles 1 and 2 are stored. The radioactive sludge at the bottom of the pool will then be pumped into a new tank that is now under construction beside B29. Then the internal linings of its walls will be scraped clean of radioactivity before the edifice is taken down, concrete section by concrete section. At the same time, the most dangerous waste will be vitrified ready for disposal.

The whole process will take at least 10 years to complete - and that is just for a single building. On top of the dismantling of B29 and B41, in which the waste from Britain's atom bomb programme is stored, there are the headaches that will be involved when dealing with the contents of B30 and B38.

These hold the leftovers from the nation's first civil reactor programme, a series of reactors known as Magnox plants. Eleven of these were built and two are still in operation. Piles of the waste they have generated is to be found around Sellafield awaiting the attention of engineers like Topping, who has spent his working life at the site.

"Sometimes I think this is the best job in the world," he said. "There are real skills needed to dismantle buildings like these. Every action has to be carefully planned. I love being among it all. On other days, though, it is really frustrating work. Everything has to be done in such a slow, safe and controlled manner."

The key problem for Sellafield is that so much of its highly radioactive waste has been stored in water. This was done to cool fuel rods and cladding as they emerged from reactors heated to hundreds of degrees celsius. But once in water, they disintegrated and immediately posed a hazard in case a pond wall became breached.

And that is why Sellafield is now undergoing its massively expensive clean-up. Those pond walls are getting old and their contents - forgotten by politicians for half a century - must be turned into solid waste that can be contained safely and buried once Britain has finally decided on the location of a deep underground repository.

"We are delivering the largest environmental restoration programme in Europe and making safe and disposing of some of the most hazardous material anywhere in the world, much of which originates from early nuclear research and military projects," says Richard Waite, acting chief executive of the Nuclear Decommissioning Authority. "At the same time we are providing essential services to enable current nuclear sites to 'keep the lights on'."

Nuclear opponents have less complimentary views about what goes on at Sellafield, of course. The place is "a slow-motion Chernobyl", according to campaigners from Greenpeace, a group which has a reputation for never missing out on the catchy phrase.

Nevertheless, Greenpeace has a point. Many of Sellafield's buildings are, essentially, no more than containers of highly radioactive scrap whose disposal is set to devour tens of billions of pounds of taxpayers' money.

The site has become the biggest, and mostly easily waved, stick in the armoury of the green movement. As one senior employee admitted: "If you want to object to anything nuclear, you just have to point to Sellafield."

In fact, Sellafield is a classic illustration of the failure of British industry. We were pioneers of nuclear power but in our desire to build our own atomic weapons, failed abysmally when it came to developing and managing our own civil reactors and reprocessing plants.

As a result, we have been left with a multibillion-pound clean-up bill and the prospect of buying either American or French reactors for our next generation nuclear plants. The lesson of Sellafield is not so much that nuclear power is dangerous but that Britain seems incapable of implementing any long-term engineering plan that comes its way, from high-speed trains to wind turbines or rocket launchers.