Summary Report for Conduct of Kozloduy NPP Stress Tests

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“Kozloduy NPP” PLC SUMMARY REPORT for Conduct of Kozloduy NPP Stress Tests 5.3.3 Loss of ultimate heat sink 5.3.3.1 Design bases to remove heat to main ultimate heat sink Decay heat removal from spent fuel in SFP-3,4 is provided from SFP cooling systems. Through the heat exchangers of these systems, heat is transferred to service water system for cooling of safety system consumers. After cooling, due to steaming in the spray pools, water is delivered back to the cold channel. 5.3.3.2 Loss of main ultimate heat sink Alternative way to remove heat from SFP is usage of – as the ultimate heat sink – cooling water from the inlet chamber before CPS, as supplied by the diesel-pumps, located in FPS-2, to the heat exchangers for SFP cooling. If it is impossible to remove heat via normal scheme, this may be provided due to steaming from water surface. To replenish for loss due to steaming, two possibilities are envisioned: with SFP filling pump – from the tank-sump (EMT) or other tanks and with diesel-pumps of FPS-2 from the cold channel. There are many alternative ways to remove heat from SFP (described in emergency procedures), which are ensured with independent power supply (electric or with diesel motor) and with diverse water sources. Due to that at loss of main ultimate heat sink, loss of safety function „Heat removal from the spent fuel stored in the reactor building but outside to the reactor” is not expected. 5.3.3.3 Loss of ultimate heat sink and loss of alternative heat sink Times to occurrence of main events at complete loss of all the possibilities for cooling and filling of SFP-3,4 before uncoverage of fuel assemblies stored in the pools are the same, as at complete loss of AC sources. 5.3.3.4 Loss of ultimate heat sink, in combination with station blackout (i.e. loss of off-site and emergency power supply at the site) Times to occurrence of the main events are the same, as at complete loss of AC sources. 5.3.4 Measures to improve robustness to loss of ultimate heat sink In relation with SFP-3 and SFP-4 no measures are required to enhance robustness to loss of ultimate heat sink. 5.4 SPENT FUEL STORAGE (SFS) 157/202
“Kozloduy NPP” PLC SUMMARY REPORT for Conduct of Kozloduy NPP Stress Tests 5.4.1 Loss of power supply 5.4.1.1 Loss of off-site power supply 5.4.1.1.1 Design schemes at loss of off-site power supply, sources, capacity and availability for operation of emergency power supply Standby power supply is provided from the DG of the SFS and from UPS device. The standby DG energizes on priority the systems for radiation monitoring, pool filling and make-up system and leakage return system, ventilation systems. Later other consumers may be powered. SFS may be energized also from the mobile DG. 5.4.1.1.2 Independence of emergency power supply sources and measures to prolong operational time Total fuel reserve ensures operation of DG of the SFS during 72 hours (3 days). 5.4.1.2 Loss of off-site power supply and loss of emergency power Analysis of the postulated initiating events due to loss of power supply to SFS shows, that uncoverage is not reached for the spent fuel, located in storage pool, in the containers or storage tubes. Assessments done for water heating and steaming in the most loaded compartment of the pool show, that start of assembly uncoverage is after more than 29 days. 5.4.1.3 Loss of off-site power supply, emergency power supply and additional alternate current power supply 5.4.2 Measures to improve robustness to loss of power supply It is reasonable to analyze the possibility of installation of independent water cooling system to the SFS compartments, with independent power supply. 5.4.3 Loss of ultimate heat sink Connection of SFS with ultimate heat sink of the Danube River is provided by service water supply system. SFS is provided with service water via two independent lines of these systems of units 3 and 4. At complete loss of the main and alternative heat sinks, due to low value of decay heat release, heat may be removed to ambient air due to steaming. Effect of absence of possibility to replenish for losses is the same as at loss of the power supply. Times to occurrence of main events, assessed for the case of complete loss of alternate current power supply, should be increased with the time of reduction of emergency water reserve in the intake chamber of CPS-2 below elevation 158/202
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Summary Report for Conduct of Kozloduy NPP Stress Tests

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