Development and evaluation of additional shutdown system at the Ignalina NPP by employing RELAP5 code Original Research Article

The anticipated transients without scram (ATWS) study in the ‘in-depth safety assessment of Ignalina NPP’ (SAR) showed that some ATWS scenarios can lead to unacceptable consequences. The apparent lack of effective inherent safety features for RBMK reactors leads to one high priority recommendation: that a second fast acting, independent and diverse reactor shutdown system needs to be installed. Another compensatory measure—the additional shutdown system (DAZ)—which has the potential to reduce the overall risk has been recommended for implementation at Ignalina NPP until the second shutdown system is installed. An analysis that demonstrated the suitability of the DAZ system—selected input process parameters and setpoints values—has been performed. The thermal-hydraulic analysis of loss of preferred electrical power and loss of both turbines with failure of the existing control and protection system (CPS), but with activation of the DAZ system, showed that the reactor is adequately protected by the DAZ system in these cases and acceptance criteria are not violated.