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* At least seven blasts in 1970s and 1980s - regionalministry
* Moscow says field is safe, environmentalists see risks
* No radiation leaks reported at field
* Russia building closer energy ties with China
By Vladimir Soldatkin
MOSCOW, Oct 25 (Reuters) - A series of underground nuclearblasts was carried out in the 1970s and 1980s at an oilfield inSiberia that state firm Rosneft has agreed to developwith China.
Officials told Reuters the field was safe, but environmentalexperts expressed doubts. The revelation raises questions overthe growing energy alliance between Russia, the world's largestenergy producer, and China, the leading consumer.
At least seven "peaceful" nuclear detonations were performedat the Srednebotuobinskoye oilfield, according to a reportpublished 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 thesite," a ministry spokeswoman told Reuters from the city ofYakutsk. No radiation leaks were reported at the site, she said.
Blasts at the field were intended to increase flows fromoil-bearing rock and, in one case, create a storage reservoir.
Rosneft recently ceded some of its oil riches by signing amemorandum to create a joint venture with China NationalPetroleum Corp (CNPC) to develop the 1-billion-barrel oilfield.
CEO Igor Sechin signed a new supply agreement this week thatwould raise Rosneft's exports to China to more than 1 millionbarrels per day.
The deals reflect President Vladimir Putin's drive to pivotRussia's energy export strategy eastward, away from Europe'sstagnating market and to the more dynamic Asia-Pacific region.
The Ministry of Natural Resources in Moscow, which issueslicences to develop mineral resources, ruled out any danger.
"We analyse all the risks, including radioactive ones. If afield has been allocated for development, that means we considerthere to be no risks," spokesman Nikolai Gudkov said.
But environmental experts expressed concern that oilextracted from the field would contain radioactive elements.
"Any nuclear explosion resembles what happens in a reactor -and the blasts at Chernobyl and Fukushima," said VladimirChuprov, 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 oilwill be contaminated."
It was not clear whether Rosneft was aware of the blasts orhad told CNPC about them. Neither Rosneft nor CNPC could provideimmediate comment on the issue.
BLAST FROM THE PAST
Nuclear explosions for industrial purposes were not unusualduring the Soviet era, but the practice ended after incidents inwhich hazardous nuclides escaped. The United States also carriedout such explosions, including Operation Plowshare, launched inthe 1960s, to unlock resources of natural gas and oil.
As part of its atomic weapons programme, the Soviet Unionseparately conducted above-ground nuclear tests from 1949onwards at Semipalatinsk in Kazakhstan, exposing hundreds ofthousands of people to dangerous levels of radiation.
China carried out underground tests at Lop Nor, in itsnorthwestern Xinjiang region that borders Kazakhstan andMongolia, doing the last in 1996 before setting a moratorium.
A nuclide is an atomic nucleus. Some created as a result ofnuclear fission, such as strontium and caesium, are unstable andemit 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 nuclidescreated in a nuclear explosion would remain after those periods.
"Of course, there is danger from such deposits. The nuclideslast for a long time after blasts and may leak to the surface,"Alexei Yablokov, a member of the Russian Academy of Sciences andan environmental activist, told Reuters.
UNKNOWN RISKS
The Srednebotuobinskoye field, located north-west of thetown of Lensk in Yakutia, was discovered in 1970, and more than100 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.
However, scientists say that there may be danger lurkinginside the wells.
"Humankind has little experience with deposits where nuclearexplosions were carried out," said Viktor Repin of the StPetersburg Institute of Radioactive Hygiene, which has monitoredthe radioactive situation in Yakutia.
Eight peaceful explosions were carried out in Yakutia, ofwhich 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," hesaid, adding the site was covered with earth to make it safe.
The Srednebotuobinskoye field holds oil and gas condensatereserves of more than 134 million tonnes and over 155 billioncubic metres of gas. Output from the field started this month.
It is expected to pump 20,000 barrels per day of oil in2014, rising to more than 100,000 bpd in 2017.
