Forty years on from the world’s worst nuclear disaster, Chornobyl is still contaminated with almost half the caesium-137 that exploded from the Unit 4 reactor in 1986, as well as much longer-lived hazards such as plutonium, tritium and americium. But according to some experts, the long-term affects on nature may be less than if the area had been left to humans, resulting in unexpected consequences in an environment left to its own devices.

The reminder of the protracted fallout from Chornobyl was made ahead of Sunday’s anniversary, which coincides with renewed lobbying for nuclear power and a rise in fears about atomic brinkmanship due to the oil crisis and wars in the Middle East and Ukraine.

The latter conflict continues to threaten Chornobyl and make the contamination worse. It was revealed last month that the giant containment structure around the most radioactive area inside the defunct plant will need €500m (£434m) worth of repairs after a strike by a Russian drone.

Inside the containment structure is an estimated four tonnes of radioactive dust, fuel pellets and other debris from the disaster on 26 April 1986 that resulted in the largest release of radioactivity in the history of nuclear energy and contributed to the fall of the Soviet Union.

More than 300,000 people were evacuated from the Chornobyl plant and the surrounding 4,200 sq km area of land in Ukraine and Belorussia (now Belarus). Radionuclides spread across most of western Europe, raising fears of crop contamination as far away as the Lake District, Scotland and Ireland. But the main focus was on the health risks to people in the immediate area, not least because the Soviet Union tried to cover up the consequences. The official death toll was 134 people, mostly firefighters and plant workers, though foreign analysts warned the fallout would lead to fatal cancer for tens of thousands of others.

The National Academy of Medical Sciences of Ukraine is to publish an assessment of the impact of the Chornobyl disaster this week. Its last update in 2022 recognised 41,000 fatalities. A study by outside experts in 2006 estimated 4,000 to 16,000 deaths.

Experts remain divided about the long-term affects of radiation on the environment of Chornobyl, but there is broader agreement about the benefits to wildlife and ecosystems of the accidental rewilding project that has emerged from the exclusion of most of its former human residents.

The Chornobyl exclusion zone (2,800 sq km) and the neighbouring Polesskiy radioecological reserve (2,170 sq km) in Belarus form one of Europe’s biggest unplanned nature sanctuaries, albeit one that is in the middle of a war zone.

“Wolf populations are seven times higher than they were before the accident because there is less human pressure,” said Jim Smith, an environmental scientist at the University of Portsmouth, who has been studying the region for more than 30 years. Elk, roe, deer and rabbit populations are also reportedly flourishing.

“The ecosystem in the exclusion zone is much better than it was before the accident,” Smith said. “It’s been a very powerful demonstration of the relative impact of the world’s worst nuclear accident, which is not so big, and the impact of human habitation, which is devastating.”

Similar conclusions have been reached in other no-go areas such as Fukushima, where wild boar, Japanese macaques and raccoons have become more abundant in places evacuated after the meltdown of a reactor in 2011, and the Korean peninsular’s demilitarized zone, where North-South tensions and exclusions of most humans have resulted in a sanctuary that is home to 38% of South Korea’s endangered species, including white-naped cranes, Siberian musk deer, Asiatic black bears and Korean gorals.

Ukraine is now experimenting with a resumption of agriculture in some of the less contaminated areas of land around Chornobyl. Smith co-authored a paper last year on how to assess radionuclide concentrations in wheat, maize, leafy vegetables and other potential crops.

Smith said he had started out as an opponent of nuclear power, but has become a cautious supporter because it posed lower risks to human health and the climate than fossil fuels. He acknowledged that radiation damages DNA and estimated there have been about 15,000 extra cancer deaths in Europe as a result of the Chornobyl accident, but said this was likely dwarfed by mortality caused by air pollution or as a result of the atmospheric nuclear bomb tests by the US and Russia in the 1950s and 60s.

“Since the 1990s, many scientists have been frustrated about how we failed to get the message across about what the important thing at Chornobyl is,” Smith said, adding that evacuations had also had a psychological and economic cost.

The long-term impacts of the disaster on wildlife are contentious. Several journal papers report long-term genetic damage to some mammal, bird and plant species, particularly in areas with the highest levels of contamination. A paper last year noted that barn swallows and great tits were among those suffering lower reproductive success due to “sperm abnormalities, oxidative stress and reduced antioxidant levels”.

Gennady Laptev of the Ukrainian Hydrometeorological Centre, who has conducted many years of research in Chornobyl, said he had not seen any visual evidence of mutations, but it was difficult to assert conclusively that the ecosystem was better than it was before the accident. “This is a complex question. In my opinion, if the wild animals are in abundance, it means it is OK with them,” he said.

The political implications of this debate are immense. The Trump administration is trying to weaken safety regulations to allow the construction of nuclear power plants in suburban areas, partly to alleviate the extra demand for energy from datacentres.

Meanwhile, the European Commission president, Ursula von der Leyen, as a result of the US-Israeli attack on Iran, which has inflated oil prices, said Europe’s turn away from nuclear power had been a “strategic mistake” because it had made countries more dependent on expensive and volatile imports. Twenty countries recently attended a summit in France on civil nuclear power’s potential to become “the sector of the future”. To have any chance, supporters need to persuade the world that nuclear is safe and affordable.

But anti-nuclear campaigners say that will be difficult while Russia deliberately targets Chornobyl and Japan discharges radioactive water from Fukushima into the Pacific Ocean.

Shaun Burnie, of Greenpeace Ukraine, said any attempt to revive the sector was a dangerous distraction by a nuclear industry fighting for its existence. “In contrast to the hype and misinformation, the probability of another severe accident remains. Unlike those in the Kremlin and the White House, who together promote nuclear power, these risks remain too great to ignore – while nuclear power remains massively uncompetitive financially.”

Burnie is working with scientists and engineers in Chornobyl, where he has encountered wild elk on the roads, heard Russian drones flying overhead on their way to targets in Ukraine, and made three visits inside “the new safe confinement” where radiation levels remain high.

“The nuclear industry will grab on to anything – like the Middle East crisis – to try to revive its fortunes, but the future in terms of energy security and decarbonisation is renewables,” he said. “After more than eight decades of massive subsidies and multiple nuclear disasters, including Chornobyl, nuclear power produces less than 10% of global electricity and 4% of global energy. That is not a track record to be proud of. What it remains very good at is what it was originally designed for: producing plutonium for nuclear weapons.”