Primary circuit and associated systems PC

ISSUE TITLE Overpressure protection of the primary circuit and connected systems (PC 1) 

There have been numerous reported incidents of pressure transients in PWRs where Technical Specification pressure and temperature limits of the RCS have been exceeded. The majority of these events occurred while the reactor was in startup or shutdown conditions and at low reactor vessel temperatures. The issue is the reliability of the cold overpressure protection system and especially the safety and relief valves situated either on the pressurizer or RHR systems. The protection systems in US plants used to mitigate and reduce the potential for these events are termed low temperature overpressure protection (LTOP) systems. 

More precisely safety and relief valves, when used to perform safety functions such as mitigation of steam generator tube rupture accident or cold overpressure protection, may have to fulfill their function in water flow conditions. However, such conditions were not always anticipated at the original design stage. Moreover, using safety and relief valves not qualified for water flow to limit the RPV pressure in water solid conditions could lead to a loss of coolant accident. 

The unavailability of overpressure protection system may result in a beyond design condition of the reactor coolant system including connected systems. 

At Kozloduy NPP, at the present time, there is no device to protect the primary circuit against cold overpressure for all the causes, particularly when the primary is in the water solid state. In addition to the administrative measures being taken, only a specific automatic protection device is installed for the makeup pumps (TK system) to disconnect them at low temperature if the primary pressure increases. 

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