For a first-time visitor to this idyllic county of West Cumbria, near where the Roman emperor Hadrian built his wall against Scottish invaders, "the site," as it is locally known, comes as a rude shock.

No grand highway approaches it. Until the last minute, there is no signpost along the narrow, hedge-bordered road that winds past picturesque lakes and villages, down the mountains and moors to the sea.

Sellafield, site of the world's largest nuclear reprocessing facility and the oldest commercial nuclear power plant, is perhaps the most controversial civilian nuclear installation. It rises unannounced from the coastal pastures, a massive, smoke-belching monument to the 20th century.

Most West Cumbrians find its presence comforting. The nuclear industry saved this remote district when the coal and iron-ore mines began to close. It employs 11,000 people, one out of every three workers in the area, and pours $3 million a week into the local economy in salaries alone.

Many residents can quote reports by government and industry on the relative safety of the nuclear industry as a whole, and the absence of conclusive medical evidence tying local ills to radiation. It is difficult to find anyone who believes what environmentalists and a minority of physicians say about increased cancer deaths in the district.

Even the tourists, who provide Cumbria's other main source of income, never seemed to mind the site, at least until an unintended discharge of radioactive sludge temporarily fouled the nearby beaches in 1983.

But for many in the rest of Britain, where nuclear power has long been a raging debate, Sellafield is much of the argument. With acres of spent fuel rods submerged in water awaiting reprocessing and the waste products of thousands more encased in concrete, pending a government decision on how to dispose of them, it is believed to be the largest repository of stored radioactive materials in the world.

Among its other superlatives, Sellafield -- known until 1981 as Windscale -- also emits more radioactive discharges, making the Irish Sea the most radioactive large body of water. Irish authorities have complained bitterly, and Swedish fisherman have hauled in radioactive fish that Stockholm says come from here.

Before the April 26 Soviet disaster at Chernobyl, this was also the site of the world's only known graphite-core reactor fire. The tall concrete cooling tower still stands on the site. Plutonium for British nuclear weapons was being produced under it at the time of the 1957 blaze. While most of the radioactivity it spewed across miles of countryside has long since dissipated, the tower-top air filters are still too "active" to contemplate tearing it down.

Sellafield has a record of what environmentalists say have been more than 300 radiation "incidents" during its 40-year history, including three minor but much reported accidents in January and February. The European Parliament voted to demand that Britain suspend operations at the site pending the outcome of safety investigations.

A British parliamentary select committee has suggested the possible cancellation of foreign reprocessing contracts that it said have made the site "a dumping ground to deal with other nations' spent fuel problems."

Government and industry officials say a dispassionate review of the facts simply does not support the case that Sellafield should be closed or that its risks outweigh its benefits to the public. Until recently, they have felt they were winning the battle for public opinion on behalf of nuclear power and reprocessing.

But that was before Chernobyl showed that catastrophic nuclear accidents can happen, posing dangers stretching far beyond even national borders. Now, the officials acknowledge, things will never be the same.

"Chernobyl will have a profound effect on public opinion and on the assessment of nuclear power," Environment Secretary Kenneth Baker told a concerned Parliament last week, "not just by the scientists or governments, but by the ordinary people in this country who vividly have been brought face to face with the possible consequences of a nuclear accident."

In the future, Baker said, "it is no good scientists or politicians simply asserting in a Panglossian way that everything is all right with nuclear power."

Pond 5, in the Fuel Handling Plant, is a massive indoor pool, 35 feet deep and of seemingly endless length. Looking down from a catwalk high overhead, rows of boxlike canisters loom beneath the dark-blue water. Two men consulting about machinery on a bridge over the radioactive liquid seem the cavern's only human occupants. The only sound is a mechanized hum, and the constant, echoing blip, blip of a security system whose silence or changing rhythm and tone would indicate something amiss.

Inside the canisters are spent fuel rods from Britain's aging Magnox reactors, waiting to be stripped of their magnesium casings and reprocessed into usable fuel.

Depending on the feelings and knowledge one takes inside, the building is filled with a sense of menace or of man's mechanical ingenuity.

Although British Nuclear Fuels, Ltd. (BNFL), the government company that owns and operates Sellafield, maintains that it always has been willing to arrange tours for journalists and the public, a new, post-Chernobyl eagerness is evident. Public visits inside still require a two-month wait for security checks. But interest has been so high lately that BNFL will start hourly bus tours of the grounds for anyone who shows up at the gate.

"The vast majority" of visitors, says Jake Kelly, head of information at Sellafield, "go away saying, 'What's all the fuss about?' "

Until recently, BNFL tended to respond to public concerns with a curt "We're right and you're wrong," said Richard Staples, in charge of the environment for the Copeland Borough Council, whose territory includes Sellafield.

"The attitude of the company was, 'We're completely safe.' "

The installation began in 1947 as a top-secret nuclear-bomb factory. Although the government still does not allow inspectors from the European Community agency Euratom to inspect part of Sellafield on grounds of joint military-civilian use, it has insisted for decades that the site's residual security-sensitive functions are minimal.

But officials here now admit that the site's old imperviousness to the outside world encouraged what the Financial Times newspaper recently called "secrecy, suppression, denial, evasion, subterfuge, arrogance and bland reassurance."

The record stretches back to the Oct. 10, 1957, fire. The day after it occurred, a government statement said that no one had suffered injury. "It is untrue to say that a large amount of radioactivity was released," the statement said. "The amount released was not hazardous to the public and what there was, was in fact carried by the wind out to sea."

The next day, one worker was reported to have been contaminated in the incident. Four days later, that figure went up to "a few." After two days, milk within a 14-mile radius was labeled dangerous and ordered destroyed. After five days, the contaminated area was expanded to 200 miles.

Most of Sellafield's current problems with the public began in November 1983, when a television program entitled "Windscale -- The Nuclear Laundry" documented sea and air discharges and an incidence of childhood cancers ten times higher than the national average. Three weeks later came the beach contamination, for which a British court ultimately found BNFL guilty of two violations of its own code.

This year, it seemed like scarcely a week passed without an incident involving Sellafield. In January, nearly a ton of uranium was inadvertently dumped into the Irish Sea. A week later, a pump malfunctioned in the reprocessing building, releasing a mist of plutonium nitrate vapor that contaminated 11 workers. Shortly thereafter, a leak was found in its fuel storage pond. And, in the coup de grace, newspapers reported that a previous study of district cancer deaths had used figures underestimating the level of radioactive discharges in the early 1950s by a factor of 40.

Not too long ago, BNFL's method of dealing with such news was to ignore it. Even today, there is a tendency within the company to answer questions about nuclear safety with testy citations of casualty figures in other industries -- 399 deaths in mining pits during the last 10 years, compared with nine in the nuclear industry, none of them from radiation.

BNFL responded to reports of the January and February incidents with a broadside against what its chairman called media "ignorance" and desire to "spread propaganda" in writing about them. The chairman at the time, Con Allday, wrote in The Times that journalists "will swallow any scare story fed to them by anti-nuclear campaigners."

A new management team was brought in on April 1. Adopting a markedly different tone, the new chairman, Christopher Harding, wrote last month in The Observer that "we shall certainly gain no credit from the public by complaining about the unfairness of it all. We shall do better instead to admit that, if people view us with suspicion, then it is almost certainly our own fault and we have to do something about it."

One of Harding's first moves, headlined in the BNFL in-house newspaper, was to order industry staff to "cut out the jargon" in explaining their work to the public and press.

Eventually, the company has provided relatively full explanations of every radiation accident here. Other than the 1957 fire and the 1983 emissions, they have been demonstrably minor and of little consequence. None of them has come near exposing workers or public to dosages of radiation judged dangerous by international standards.

As for the cancers -- acute leukemias and myeloma (bone-marrow cancer) -- the company and government reports have shown that similarly high levels exist in other "pockets" around Britain where there are no nuclear installations, showing, they say, that no certain connection can be made. Even the radiation effects of Chernobyl fallout in Britain, a company spokesman said in illustration, are no more than the dose equivalent of four weeks spent in Devon, the county in southwest England where natural radiation is the highest in the country.

At the same time, however, BNFL has moved to reduce emissions from Sellafield, particularly those pouring into the Irish Sea. Substantial sums of money have been spent and even more is earmarked for emission safety procedures far beyond what are required in many other countries.

But "in an industry whose continued development must depend on public acceptance and support," Harding acknowledged, "the public's perception of the facts can count for more than the facts themselves."

West Cumbrians say they are most concerned that the controversy surrounding Sellafield has frightened off other potential investment, leaving them ever more dependent upon the nuclear industry. Now that the Chernobyl disaster has focused even more attention on them, their greatest fear seems to be that Sellafield will be closed, leaving them with nothing.

Known for their self-described taciturnity, they say they do not believe that the site is a danger to them or their children. Besides, said Copeland Council member James Johnston, "what's the difference between being killed by a flying atom or having a ton of coal fall on you? It sounds callous, but I can't put it any other way. Death is death is death."

Debate over nuclear power and reprocessing, Cumbrians feel, is for people who have the time and don't need the work. "The farther away you get from here," Johnston said, "the more divisions there are" over the issue. "The people who worry most about Sellafield are in London."