Chernobyl 25 years on: learning the lessons of nuclear disaster

Twenty-five years ago, the Chernobyl nuclear plant in the Ukraine suffered a catastrophic explosion. The real lessons are still being learned, says Roger Highfield.

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A Communist-era statue of Lenin stands in the grounds of the deserted Chernobyl nuclear reactor. Photo: AP

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 Scandinavian 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.

We felt a mix of surprise, excitement and unease when, 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, known as an RBMK. 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 down, 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. The working reactors in the plant automatically shut down as soon as they felt the earthquake, by inserting control rods, so the reactions in their cores began slowing within seconds. By contrast, Chernobyl was actually 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 (unlike Fukushima, where hydrogen gas generated as the fuel melted detonated outside the reactors).

In the Ukraine, 1.5 tons 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 categorized as a Level 7 accident, the highest on the scale, the amount of radioactive material that has escaped is far smaller: Japan’s Nuclear and Industrial Safety Agency estimates that the plant has spilled between 370,000 and 630,000 terabecquerels of radioactivity into the environment, while Chernobyl released a total of 5,200,000.

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 of such disasters are far harder to calculate. Five years after Chernobyl, a 1,500-page report, drawing on the work of 200 health experts from 25 countries, attempted to assess the effect on 825,000 people in the most contaminated areas, Byelorussia, Russia and the Ukraine.

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 – such as the estimated 1,900 extra cancers seen over the years in those exposed to the blasts at Hiroshima and Nagasaki, some of whom received significantly more radiation than those around Chernobyl.

In the Ukraine, two people died immediately after the blast and another 29 died in hospital over 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, Prof John Gittus of the Royal Academy of Engineering advised the Government that there could be around 10,000 fatalities. Today, some environmental groups put the toll well into six figures. However, Professor Wade Allison of Oxford University is adamant that “the only deaths that have been firmly established, either individually or statistically, are the 28 victims of Acute Radiation Syndrome and 15 cases of fatal child thyroid cancer”.

The mainstream view lies between these extremes. Dr Jim Smith of the University of Portsmouth prefers to cite a 2006 study by Dr Elisabeth Cardis of the International Agency for Research on Cancer, which predicted that by 2065, about 16,000 cases of thyroid cancer and 25,000 cases of other cancers may be expected, compared with several hundred million cases from other causes. Thyroid cancer in particular is treatable, so the latest estimate of the death toll from the UN’s Chernobyl Forum puts the death toll among the most exposed populations at around 4,000.

When it comes to Fukishima, the earthquake and tsunami killed two workers and injured many more. There were further casualties from the hydrogen explosions, and at least one worker was sent to hospital after an overdose of radiation. But as Prof Richard Wakeford of the University of Manchester explains, there will be difficulty getting reliable estimates of radiation doses, which are crucial for predictions of excess cancers.

As far as the workers are concerned, a dose at the emergency limit of 250 mSv would produce a lifetime risk of serious cancer of around 1 or 2 per cent. Therefore, if 100 workers received this dose then one or two additional serious cancers would be expected, in addition to the 20-25 which would develop in the normal course of things. For those further away, the risk would be negligible: “if the dose to the thyroids of young children was successfully limited by the various countermeasures,” says Prof Wakeford, “I doubt whether any effect could be detected”.

An editorial published in Lancet Oncology to mark the anniversary argues that the accident at Fukushima will at the very least offer a much better opportunity than Chernobyl to study the health consequences of a major nuclear accident – not just from radiation, but from psychological factors such as stress.

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.

Although there have been claims that these effects have been seen in the families of Chernobyl clean-up workers, the jury is still out on whether people react in the same way – after all, such effects were not observed in the A-bomb survivors.

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 which was hastily thrown up over the reactor in the months after the accident. The replacement will be the largest of its kind in the world, more than 100 meters high, 250m wide and 160m long. The great hanger 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.

Date: 2015-09-02; view: 396; Нарушение авторских прав