* Ukraine has 15 nuclear power reactors operating
* Kiev says it is stepping up security of the plants
By Fredrik Dahl
VIENNA, March 5 Russia accused Ukraine of "hallucinating" on Wednesday after Kiev warned a U.N. atomic agency meeting of risks to the safety of its nuclear power plants in case of a Russian invasion, diplomats said.
Envoys of the two neighbours engaged in a fiery verbal exchange during a session of the 35-nation governing board of the International Atomic Energy Agency (IAEA), a U.N. body, diplomats who attended the closed-door discussions said.
In the wider dispute, Russia rebuffed Western demands to withdraw forces in Ukraine's Crimea region to their bases amid a day of high-stakes diplomacy in Paris aimed at easing tensions over Ukraine and averting any war.
At the IAEA meeting in Vienna, which also discussed Iran and other issues, Ukraine spoke of the security of its nuclear power plants, highlighting possible environmental consequences if they were caught up in a military conflict.
Ukraine's envoy to the IAEA, Ihor Prokopchuk, said the "physical protection" of its 15 nuclear power reactors "can be endangered," according to one diplomat who was in the room.
"Potential consequences of a military invasion would be a threat of radiation contamination on the territory of Ukraine and the territory of neighbouring states," Prokopchuk said.
"In addition, a significant amount of the spent nuclear fuel, which is stored on the territory of the nuclear power plants, would pose potential high risks," he said.
Russia's representative on the board, Grigory Berdennikov, reacted angrily, saying the remarks were "nothing but a provocation" and that Ukraine was conducting "malicious slander".
Ukraine is "hallucinating about phantom dangers from outside," the diplomat quoted the Russian envoy as saying.
Another diplomat confirmed the "hallucinating" quote.
Neither Russian nor Ukrainian diplomats were immediately available for comment.
In the same debate, the U.S. envoy to the IAEA said his country was "very much aware of the potential consequences should sabotage or an attack occur at a nuclear power plant".
"We reiterate the importance that all member states in their actions contribute to a strong nuclear security environment worldwide and avoid actions to destabilise it," U.S. Ambassador Joseph Macmanus said, according to a copy of his speech.
Ukraine's 15 nuclear power reactors accounted for nearly 44 percent of its electricity production in 2013.
In a letter to IAEA Director General Yukiya Amano on Tuesday, Prokopchuk said Ukraine was reinforcing the protection of its nuclear plants because the Russian military posed "a grave threat to the security" of his country.
On Sunday, Ukraine's parliament called for international monitors to help protect its nuclear power plants. (Editing by Alistair Lyon)