Possibility of such failures is considered in the design, i.e. the plant is equipped with means, protections and interlocks, and the Technical Specification document specifies the failure indicators and personnel response actions.
The plant management made a decision to transfer the unit to annual refuelling and maintenance mode, which was planned initially for June 10th, 2003. A 0.5 cm hole in one of the pipes became the cause of the coolant leak from the first circuit. Specialists found out that 6 cubic meters of water leaked to the second circuit, where the coolant goes to the turbines. An out-of-schedule inspection of radiation situation was performed within monitoring area and at the plant site. No deviations from usual radiation background were observed. The event was rated by level 1 according to the International Nuclear Event Scale as one of the plant radiation barriers had failed.