On 10th April 2003 during the annual refueling outage of unit the fuel assemblies cleaning were in progress. At 22:30 normal dosimetry systems of the exhaust stack showed a sudden increase in released noble gas activity (max. 4,14*10e13 Bq/10 min), at the same time, radiation monitors for noble gas activity in the reactor hall indicated alarm level. As a result of conducted technical actions the releases were reduced below the autorized limit. On 16th April 2003 at 20:00 after removal of the cleaning container head the video inspection revealed that most of fuel assemlies (perhaps all of them) inside the container had been damaged. After analysing of situation further actions were undertaken in order to maintain the safe cooling of the fuel assemblies. After container head removal no changes in noble gas release happened.