The primary safety functions are supported by many other systems which have the following basic functions: supply electric power or compressed air to equipment and instrumentation, provide raw and processed information to the operator, provide alternative sources of cooling water for fuel under post-accident conditions, provide cooling water to equipment, provide ventilation, etc. The results of the system analysis for following support systems are discussed in this Section:
A detailed Engineering Assessment of the Electric Power Supply System has been carried out by SAR and in particular in-depth demonstration of compliance with the corresponding IAEA safety guide . The high degree of redundancy in the Electric Power Supply System provides assurance with respect to reliability of the power supply. One exception from sound trainwise design was found in the automatic transfer of Instrumentation and Control and other loads between redundant uninterruptible power sources. The resulting dependency between buses must be assessed and eliminated, wherever possible. RSR recommended that Ignalina NPP perform an evaluation of the effect of persistent degraded voltage or AC-frequency to determine minimum voltage/frequency levels required to close breakers and operate equipment. The results should be used to determine if the 50 % nominal voltage or 46 Hz AC-frequency setpoints provide adequate protection of vital equipment.
The Service Water System represents a vital and common support system for a number of operational and safety systems. The SAR indicates there is the lack of segregation of the service water distribution within the whole plant. The Service Water System is built in two trains. The system function is not proven to be single failure tolerant, if an initiating event starts within the Service Water System, which may cause shut-down due to loss of operational function or flooding, but which simultaneously degrades the status of the SWS for the support of safety functions such as ECCS, ACS or AFWS. The SAR gives no evidence that the plant could be cooled down without Service Water System under the existing components specification, system configuration and operational procedures in force. The ECCS, AFWS and PCS rely directly or indirectly via IC on Service Water System. There is a potential for the plant to survive the losses of Service Water System and Intermediate Circuit. The credibility of the mutual support of the neighboring units has still to be demonstrated, with regard to tolerable downtime/periods of loss of service water for different front line systems. The loss of Service Water System can be caused by fire and seismic events. Improvements made during the last two years are not considered or assessed homogeneously within the SAR. The system analyses have to be adapted to the recent plant configuration.
The Intermediate Circuit consists of two separate circuits for different cooling functions. Redundancy is built in for pumps and heat exchangers to the Service Water System. Both circuits have single trains of piping and connections to the corresponding components to be cooled. IC-1 provides cooling to the Purification and Cooling System among others and has safety functions for low pressure residual heat removal. IC-2 is used for cooling non-safety components, but also for safety related cooling of ECCS and AFWS pumps. The dependence of ECCS on the availability of the ICC-2 is a high order critical issue. A failure of ECCS consequential to loss of IC-2 has to be postulated forming part of the design basis. No evidence is given in the SAR that the cooldown of the plant can be achieved without cooling of bearings and seals of the multistage ECCS pumps.
Besides the operational functions such as cleaning up and purging of primary coolant, the following characteristics and safety related functions are recognized to be the Purification and Cooling System: extension of the pressure boundary outside he ACS and low pressure residual heat removal. The PCS under normal operation receives water from the pressure headers of both loops to clean up to the required water quality. In case of breaks of the pipelines of the PCS system, it has to be isolated to limit the amount of primary water escaping from the pressure boundary. During reactor cooldown the system is actuated to perform its residual heat removal function. According to Engineering Assessment of the system, pipe ruptures in the PCS are mitigated by relieving to the ACS and by subsequent automatic closure of the isolating valves. However, the consequences of pipe breaks in the PCS and the design requirements of the mitigative systems as well as residual heat removal function have not be assessed in the SAR system analysis.
The SAR has shown that there are more than 250 ventilation sub-systems for various purposes at the power unit. The safety task of the ventilation systems are to prevent contamination of indoor and ambient air by radiological and explosive substances, to provide airflow towards more contaminated premises only and to provide conditions for the operation of the plant safety functions. The ventilation systems were installed before there were requirement for such systems to operate despite natural phenomena such as high wind, earthquake and flood. It therefore is not proven to withstand such natural phenomena. An obvious weakness in ventilation is related to the availability of reliable ventilation and thus habitability of the main and emergency control rooms in case of external events. However, the SAR assessment has not demonstrated that ventilation will fulfill its safety functions following an accident when additional equipment will be operated and escaping steam will tend to increase both temperature and humidity in the reactor building.
Summarizing the assessment of the support systems it is necessary to mentione that system analysis identified few high priority non-compliance’s related to vulnerability due to lack of physical separation or inadequate fire protection, lack of redundancy and failure of passive piping. Individually, these deficiencies do not in themselves represent major safety problems. However, the large number of low priority findings indicates a reduction in defense in depth provided by automatic systems, and potentially leads to an increased reliance on the operator.