During normal power operation after checking the resin content of the K-filter and connection of the water cleaning system with the primary circuit, a leakage of the seal of the fill-in nozzle at the filter occurred. The nozzle is situated in the reactor hall out of the pressure room system. The personnel violated several operating instructions but the reactor was not shut down as prescribed in such situations. BASIS FOR RATING: No off-site impact, no on-site impact. Defence-in-depth (A-1R) 0/1. Violation of operating instructions, human error +1, with deficiency in safety culture = 2. DIFFICULTIES IN RATING: Judgement of the design deficiency by the containment aperture for the filter nozzle in the reactor hall and air ventilation system in the hall without filtration.