A hundred tonnes of uranium lying at the core of the Sizewell nuclear power station on the Suffolk coast will "go critical" later today as Lord Wakeham, the former Secretary of State for Energy, triggers the first nuclear chain reaction in the American-style pressurised water reactor. Water is already circulating through the reactor core, heated beyond boiling point to 290C but kept from flashing to steam by the immense pressure, equivalent to 154 times the pressure of the atmosphere. The nuclear fire latent in the uranium has been kept damped since the fuel was loaded into the reactor last September. Both a bank of safety rods and special chemicals dissolved in the water absorb the slightest hint of nuclear radiation.

Yesterday, the Nuclear Installations Inspectorate allowed the station's operating staff to start withdrawing the rods from the core. Today, as Lord Wakeham watches, the chemical composition of the circulating water will change. Gradually, the absorbers in the water will be diluted enough to allow sub-atomic particles called neutrons to fly freely through the reactor core. Travelling at speeds of nearly 5,000 miles an hour, they will occasionally hit the nucleus of a uranium atom, splitting it in two. Asit fissions, the uranium will release two or three more neutrons, which in turn will hit yet more uranium atoms. The nuclear chain reaction will have begun. Once the reaction is self-sustaining, when as many neutrons are produced as absorbed, Sizewell B, Britain's first American-style pressurised water reactor (PWR), will have achieved "criticality".

The completion of Sizewell B has slipped by only a month or so from the target date set in 1987, when the old Central Electricity Generating Board (CEGB) started the first major civil engineering works at the site. Its final cost will be tens of millionsof pounds less than the £2.03bn (in 1987 money) that was originally budgeted. When compared with the track record of big construction contracts in Britain (the Channel tunnel springs to mind), the completion of Sizewell B is a major achievement.

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And no one should be in any doubt about its magnitude. Sizewell B has taken nearly half a million cubic metres of concrete, reinforced with 70,000 tonnes of steel bars; the pipework alone would stretch from London to Birmingham, and the cables used simply for instrumentation and control from London to Brighton. Nearly 37,000 valves act to control the flow of the coolant water and safely shut off the reactor in the event of a malfunction. The four reactor coolant pumps drive nearly 20 tonnes of superheated water through the core every second. When it is up to full power, it will generate enough electricity to meet the needs of 1.5 million people.

If the engineers responsible for commissioning the plant are deft and swift, they may just be able to bring it up from criticality to feed power to the grid in time for the 59th birthday, on 5 February, of the man who has masterminded the whole complex project. Not since the 19th century have engineers been heroes in Britain, as a result, the man who has been the driving force behind this project, Brian George, is unknown to the public.

In the Thatcher era, during which the dominant personalities were slick young men in advertising and financial dealers in the City, Mr George represents an older, almost forgotten tradition: that of Britain as the workshop of the world. He is a reminder of the days when this country had a manufacturing industry, when the British economy employed workers in factories making manufactured products, rather than armies of insurance salesmen and financial advisers selling personal equity plans and pensions.

The man in charge of Sizewell B has to understand concrete and steel and how to shape hundreds of thousands of tonnes of such materials into the precision instrument that is a nuclear power reactor. More important still is the ability to manage a complexproject involving more than 10,000 workers, ranging from building labourers to PhD-level nuclear physicists.

One might expect a lean and hard executive handing down decisions to subordinates who are expected to obey. But with such a workforce, a military-style division into the officer class and other ranks would not work. Mr George's manner and appearance reflect a more measured style. Like Isambard Kingdom Brunel, one of the great engineers of the 19th century, he is of average height and a comfortable midriff speaks more of past lunch-time diplomacy than of competitive executive games on the squash court.

Mr George has been in charge of Sizewell since 1981, having joined the CEGB two years earlier from the National Nuclear Corporation, then Britain's reactor design and construction company. He has been a main board director of Nuclear Electric, the state-owned company which owns and operates nuclear power stations in England and Wales, since 1992. As the time comes to hand over from the construction to the operating division, Mr George remarked: "all project directors suffer withdrawal symptoms, but I feel very strongly that Sizewell B was the culmination of the work of the British nuclear industry. What we have achieved here makes it worthwhile."

The completion of Sizewell to time and budget comes just as the future of the British nuclear industry hangs once again in the balance. From an abysmal performance in the mid-Eighties, the output and efficiency of Britain's nuclear power stations has grown rapidly to the point that John Collier, the chairman of Nuclear Electric, has been talking about "creeping renationalisation of the electricity supply industry". The company has been generating about a quarter of all the electricity in England and Wales, surpassing the performance of PowerGen, its second-largest private-sector competitor. (North of the border, Scottish Nuclear accounts for 50 per cent of the electricity generated in Scotland.)

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It is a sign of the intensely commercial culture which now pervades Nuclear Electric that the company is making little public fuss about Sizewell B going critical - that is the psychological event for the purist nuclear engineers. The company is more interested in the moment when the generator will be synchronised to the grid. This, of course, is the accountants' moment, when the reactor generates not only electricity but also cash.

Against this background, the Government is once again considering the prospects for nuclear privatisation, and Nuclear Electric is hoping to make an immediate start on Sizewell C, which would be a twin reactor station, identical to the freshly minted Sizewell B design. The company needs this project, which would cost £3.5bn and generate 100,000 man-years of construction and manufacturing work, to maintain its share of the nation's electricity output. The elderly, first-generation Magnox reactors which it inherited from the old CEGB are coming to the end of their lives and most will have stopped by the year 2003, the earliest that the first of the two Sizewell C reactors could come on line (assuming they got permission to start construction in 1996).

Mr George says he is "personally convinced that there is a strategic element to Sizewell C. Over the next decade, fossil fuel prices will rise faster than inflation. In the long term, nuclear power will be needed, so the strategic question is, do we accept a lower margin on Sizewell C so as to be a good position next time round?"

He concedes that nuclear power stations cannot forecast high returns compared with Combined Cycle Gas Turbine plants but speculates that "in the real business world, if we had partners with the same strategic vision, that would dilute the problem". Couldthis mean a nuclear power station in Britain built with money from Electricite de France or one of the German utilities?

"I'm not expecting the Government to contribute anything to Sizewell C. We have to see a commercial way forward."

For the moment, Mr George has become Britain's nuclear salesman. He has just returned from a trip to Taiwan, where the authorities are considering bids for a new PWR. Together with Westinghouse, Nuclear Electric has submitted a proposal based on SizewellB. According to Mr George, "it will not be the cheapest bid, but it will be the most secure for the Taiwanese to get what they want. We have demonstrated that we know how to build nuclear power stations to tight programmes. We have available a good teamof people and a design that we know works. That is our strength."