Once operations begin in earnest, Britain will become the world's largest producer of separated plutonium. Over the next 10 years, Thorp will add about 55 tonnes of plutonium to the existing 40-tonne civil stockpile which has accumulated from reprocessing fuel from first-generation Magnox reactors.
But some observers believe that within about five years BNFL will be forced to operate the plant at less than full capacity because the company will be unable to return plutonium and uranium to its foreign customers.
William Walker, at the Science Policy Research Unit of Sussex University, said: 'Britain will end up as Japan's offshore storage site for separated waste products.' He believes that overseas plutonium could be stockpiled at Sellafield for more than 25 years.
Yesterday's decision, Dr Walker said, 'forces the Government to permit long-term storage of foreign plutonium at Sellafield and, if they wish to draw down on those stocks, the Government will have to permit the construction of a mixed oxide fuel fabrication plant at Sellafield'.
BNFL hopes to be able to return plutonium to customers in the form of fresh fuel containing a mix of plutonium and uranium, but it needs planning permission to build a new plant at Sellafield to fabricate this fuel.
Thorp does not generate nuclear power and is a net consumer of energy. It exists to process spent fuel discharged, usually after three years, from nuclear power reactors. About 96 per cent of the spent fuel consists of unburnt uranium; 3 per cent is highly radioactive fission products; and 1 per cent is plutonium. The uranium and plutonium can, if there is a demand, be reused in power reactors, while the fission products have to be disposed of as waste.
About a quarter of the Sellafield site's 8,000 employees will be working directly on Thorp during normal operations. BNFL's plant represents a major source of employment in the economically blighted west of Cumbria.
The plant has been some 15 years in the building, since it was authorised by the Labour government in 1978. It was completed in February 1992, and has been waiting for the Government to authorise its operation.
To operate the plant, the company needed permission to discharge radioactivity from the chimney into the air, and down the pipeline into the Irish Sea. HM Inspectorate of Pollution took a very long time to draw up the new authorisations, which then had to go out for public comment and statutory consultations. This took until 25 January and then the inspectorate deliberated for months.
As time slipped by, costing BNFL pounds 2m each week, the rationale for the plant began to crumble under close scrutiny. Even BNFL's confidence began to slip: when Thorp was completed in March last year, Ken Jackson, director of international business for BNFL, proudly stated that it 'will underpin BNFL's profitability over the next 25 years'. By early January, senior BNFL staff were conceding privately that it might operate for about 10 years only, fulfilling the virtually unbreakable contracts foreign and domestic customers had signed in the late 1970s.
The first 10 years' capacity - to reprocess 7,000 tonnes of spent nuclear fuel - is virtually all taken up. BNFL maintains that this first 10 years will pay off the capital costs of building Thorp, make full provision for knocking down the radioactive building at the end of its life and still return a profit of about pounds 500m over the decade.
About 40 per cent of capacity for the second 10 years has already been sold to domestic customers and to Germany. But there is no longer any demand for the products that Thorp will provide.
The environmental lobby groups tapped an unexpectedly strong well-spring of opposition to Thorp and suddenly the Government realised that it faced a hot political issue. But Thorp is built and not putting it into operation would be to admit to the mistakes of the past decade when the project could have been terminated at a much lower cost.
If, in 10 years or so, BNFL runs out of customers and the highly radioactive plant costs more to knock down than the company had estimated, those who were responsible for yesterday's decision will be beyond the reach of any political retribution.