When my visit to Forsmark nuclear power station was cancelled at the end of April 1986, I was impressed by the candour of my Swedish hosts. I was with a group of journalists who were about to enter the plant, 50 miles north-east of Uppsala, when we were told that a nuclear leak had set off an alarm.
Later that day, the Swedish Nuclear Power Inspectorate told us that the pattern of radioactive isotopes in the leak was nothing to do with Forsmark -- in fact, it was typical of a Soviet reactor design.
It would take Moscow almost three days to admit that, on April 26, there had been an accident at the Chernobyl nuclear facility in the Ukraine, 1,000 miles away. Staff at the number four reactor had been running tests at low power to find out how well they could cope with a temporary shutdown of the cooling system. Things went badly wrong: the Russians tried to shut the plant, but instead the reaction accelerated with disastrous consequences.
Since then, Chernobyl has been the benchmark by which all other nuclear accidents have been judged -- especially the one at the Fukushima in northern Japan, which suffered major damage from the earthquake and subsequent tsunami on March 11.
Luckily, the reactors in Fukushima were much safer designs. They shut down as soon as they felt the earthquake. By contrast, Chernobyl was running -- albeit at low power -- at the time of the accident.
Crucially, Chernobyl had a "positive void coefficient'': the formation of steam bubbles cut the ability of the liquid water coolant to absorb neutrons, which in turn increased the reactor's power output. This caused yet more water to flash into steam, increasing the power up to 1,000-fold. The result was an explosion inside the reactor itself.
In the Ukraine, 1.5 tonnes of highly radioactive material were blasted many thousands of feet into the air. The damaged unit burned for 10 days, spewing out more radioactive material and a plume that could be detected around the globe. Even though Fukushima has been categorised as a Level 7 accident, the highest on the scale, the amount of radioactive material that has escaped is far smaller.
But what will be the long-term impact? In the Ukraine, even today, a 19-mile exclusion zone remains around the plant, which includes the ghost town of Pripyat. Altogether, 350,400 people were evacuated and resettled from the most severely contaminated areas. Yet the health consequences are far harder to calculate.
Radiation poses two distinct threats. One is from the huge doses encountered by firefighters and plant workers that cause burns and radiation sickness: in Chernobyl, these included the 600,000 so-called liquidators brought in to clean up after the accident and the 3,500 "biorobots", soldiers and reservists who had to dart -- with primitive protection -- to the edge of the burning reactor to clear debris, in searingly radioactive conditions that disabled real robots. Even after working for less than a minute, many of the men complained of nosebleeds, exhaustion and a metallic taste.
The second threat is the long-term impact, in the shape of additional cancers.
In the Ukraine, two people died immediately after the blast and another 29 died in hospital during the next few days, but the longer-term impact was far harder to measure. Hot spots were dotted around where rainfall washed radioactivity out of the plume of fallout: even today, Welsh sheep farmers are still dealing with the problem. And studies of the impact were hindered by the reluctance of the Soviet Union to release details of the calamity: it was only in 1989 that official contamination maps were released.
Two decades ago, Professor John Gittus of the Royal Academy of Engineering advised the British government that there could be around 10,000 fatalities. Today, some environmental groups put the toll well into six figures.
There is also growing evidence that the effects of radiation can be passed to future generations. Studies of mice by Prof Yuri Dubrova, of the University of Leicester, reveal that large doses of radiation can make the genetic code more likely to suffer mutations, and that this propensity can be passed down the generations in the "germ line", the genes contained in sperm and eggs.
The long-term cost is not just medical, however. The Tokyo Electric Power Company hopes to shut down the Fukushima reactors completely within nine months, but experience at Chernobyl suggests that the subsequent cleanup could take decades.
There, efforts are still under way to raise just over a billion dollars to complete the construction of a huge long-term shelter to replace the so-called sarcophagus, a crumbling protective structure that was hastily thrown up over the Chernobyl reactor in the months after the accident.
It will be the largest of its kind in the world, more than 100 metres high, 250 metres wide and 160 metres long. The great hangar will serve as an enduring monument to the world's worst civil nuclear accident, and to our remarkable state of ignorance about the impact of nuclear fallout. (© Daily Telegraph, London)