KYIV, Nov 27 (Reuters) - The head of Ukraine's state-run nuclear energy firm said on Sunday there were signs that Russian forces might be preparing to leave the vast Zaporizhzhia nuclear power plant which they seized in March soon after their invasion.Such a move would be a major battlefield change in the partially-occupied southeastern Zaporizhzhia region where the front line has hardly shifted for months.Russia and Ukraine, which was the site of the world's worst nuclear accident in Chornobyl in 1986, have for months repeatedly accused each other of shelling the Zaporizhzhia reactor complex, which is no longer generating energy.Russia's RIA news agency quoted Deputy Foreign Minister Sergey Ryabkov a day after the meeting as saying a decision on a protection zone should be taken "fairly quickly".On Friday, the U.N. nuclear watchdog said Ukraine's three nuclear plants on government-held territory had been reconnected to the grid, two days after a Russian missile barrage forced them to shut for the first time in 40 years."