During the commissioning and the gradual increase of the reactor output, the safety vents used to release steam from the steam generators are being tested these days. This is not a non-standard operation, it is repeated regularly also during the commissioning of other nuclear units in Slovakia, always after the completion of planned shutdowns. These are carried out at least once a year for refuelling and maintenance.

Similar tests are also carried out in conventional power plants, because each pressure vessel has a safety device which by law must be checked at regular intervals. The tests are usually accompanied by a loud sound effect.

More about the power start-up of Mochovice Unit 3 here.