On 31 January during the investigation of a minor event the staff of Trillo NPP detected a design error in the level instrumentation in the annulus outside the containment sphere (between the sphere and the containment building, 3 out of 4 sumps). This error would have affected the actuation of Residual Heat Removal System (RHR) during a small LOCA, in the recirculation phase. Because of the particular design of Trillo (KWU) this deficiency in 3 sumps would have resulted in inadequate safety function of RHR during a small LOCA. This deficiency has been present since the start-up of the plant. The error has been corrected and the nuclear safety council is investigating the causes and actions of the Trillo staff. The plant is operating at full power. Press release is attached.