Key points:- Ukraine's nuclear energy operator has accused Russia of "preparing a terrorist act" at Zaporizhzhia Power Plant- In recent days, Russia has made an unsubstantiated allegation Ukraine is planning a dirty bomb- A dirty bomb uses explosives to scatter radioactive wasteOn Tuesday Ukraine's nuclear energy operator said Russian forces had carried out secret construction work at the occupied Zaporizhzhia Nuclear Power Plant.Just days before, Russian Defence Minister Sergei Shoigu made an unsubstantiated allegation Ukraine was preparing to launch a dirty bomb, which uses explosives to scatter radioactive waste.Ukraine said it was an attempt by Moscow to distract from the Kremlin's own alleged plans to detonate a dirty bomb.Kremlin spokesman Dmitry Peskov said Western nations' dismissal of Moscow's unsubstantiated dirty bomb allegation was "unacceptable in view of the seriousness of the danger that we have talked about"."In the past, the Russians have, on occasion, blamed others for things that they were planning to do" John Kirby, a spokesman for the National Security Council, said."