* At least seven blasts in 1970s and 1980s - regional ministry
* Moscow says field is safe, environmentalists see risks
* No radiation leaks reported at field
* Russia building closer energy ties with China
MOSCOW, Oct 25 (Reuters) - A Siberian oilfield that Russia and China plan to develop together was the site of Soviet nuclear blasts in the 1970s and 1980s, Russian officials said on Friday.
The government and state oil firm Rosneft said the field was safe, rejecting environmentalists' concerns that oil extracted from it could be contaminated with radiation.
But the revelation raises questions for a strategic joint venture announced a week ago in which Russia, the world's top energy producer, ceded a share of its oil wealth to China, the leading consumer.
At least seven "peaceful" nuclear detonations were performed at the Srednebotuobinskoye oilfield, according to a report published by the environment ministry of the Republic of Sakha, a remote region in Eastern Siberia also known as Yakutia.
"Yes, indeed, there were nuclear explosions performed at the site," a ministry spokeswoman told Reuters from the city of Yakutsk. No radiation leaks were reported, she said.
Blasts at the field were intended to increase flows from oil-bearing rock and, in one case, create a storage reservoir.
Rosneft said in comments emailed to Reuters that it regularly monitors radiation at the blast sites - now mothballed - in areas where it holds production licences.
"Radiological examination of the deposits and the production extracted from them shows that no radionuclides have reached the surface - including in the oil," it added.
The company last week signed a joint venture memorandum with China National Petroleum Corp (CNPC) to develop the 1-billion-barrel oilfield. CEO Igor Sechin signed an agreement this week that would raise Rosneft's exports to China to more than 1 million barrels per day.
The deals reflect President Vladimir Putin's drive to pivot Russia's energy export strategy eastward, away from Europe's stagnating market and to the more dynamic Asia-Pacific region.
Rosneft did not say whether it had told CNPC about the blasts. CNPC could not provide immediate comment on the issue.
Environmental experts expressed concern that oil extracted from the field could contain radioactive elements.
"Any nuclear explosion resembles what happens in a reactor - and the blasts at Chernobyl and Fukushima," said Vladimir Chuprov, a nuclear expert at Greenpeace Russia, referring to the reactor disasters in Ukraine and Japan.
"The results are the same: the emission of radionuclides, including strontium-90 and caesium. There is a risk that the oil will be contaminated."
The Ministry of Natural Resources in Moscow, which issues licences to develop mineral resources, ruled out any danger.
"We analyse all the risks, including radioactive ones. If a field has been allocated for development, that means we consider there to be no risks," spokesman Nikolai Gudkov said.
Nuclear explosions for industrial purposes were not unusual in the Soviet era, but the practice ended after incidents in which hazardous nuclides escaped. In the days before hydraulic fracturing or 'fracking', the United States also carried out such explosions, including Operation Plowshare in the 1960s, to unlock resources of natural gas and oil.
As part of its atomic weapons programme, the Soviet Union separately performed above-ground nuclear tests from 1949 at Semipalatinsk in Kazakhstan, exposing hundreds of thousands of people to dangerous levels of radiation.
China carried out underground tests at Lop Nor, in its northwestern Xinjiang region that borders Kazakhstan and Mongolia, conducting the last in 1996 before setting a moratorium.
Some products of nuclear fission, like strontium and caesium, are unstable and give off harmful radiation until they decay to a stable state. Strontium-90 has a half-life of 28.8 years, while that of caesium-137 is 30 years, meaning that half of the radioactive nuclides created in a nuclear explosion would remain after those periods.
"Of course, there is danger from such deposits. The nuclides last for a long time after blasts and may leak to the surface," Alexei Yablokov, a member of the Russian Academy of Sciences and an environmental activist, told Reuters.
The Srednebotuobinskoye field, located north-west of the town of Lensk in Yakutia, was discovered in 1970, and more than 100 exploration wells have been drilled at the site.
The report of the regional ministry, dated November 2011, did not say whether developing the field would be dangerous. But some scientists have concerns.
"Humankind has little experience with deposits where nuclear explosions were carried out," said Viktor Repin of the St Petersburg Institute of Radioactive Hygiene, which has monitored the radioactive situation in Yakutia.
Eight peaceful explosions were carried out in Yakutia, of which two, at diamond deposits, "got out of control", he said. Accounts of the actual number of blasts vary.
"In one explosion, radioactive materials leaked out," he said, adding the site was covered with earth to make it safe.
The Srednebotuobinskoye field holds oil and gas condensate reserves of more than 134 million tonnes and over 155 billion cubic metres of gas. Output from the field started this month.
It is expected to pump 20,000 barrels per day of oil in 2014, rising to more than 100,000 bpd in 2017.