Ukraine has had to temporarily shut down four nuclear reactors due to a Russian airstrike, the Ukrainian government said in a letter to the International Atomic Energy Agency (IAEA) in Vienna. The IAEA published the letter as requested by Ukraine, but has not yet commented on the matter. During heavy Russian airstrikes on Monday this week, reactors 1, 3 and 4 at the Rivne nuclear power plant had to be taken off the grid first thing in the morning, it said. The output of the South Ukraine nuclear power plant had been increased as a result. Due to voltage fluctuations in the grid, the third reac…