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 Threshold effect on the progress of the process is provided by discharge of the accumulator batteries (after about 10 hours). After that the operator does not dispose information on unit parameters and shall perform accident management actions blindly. If an event occurs at power operations or in cold condition with pressurized primary circuit, threshold effect is provided by exhaust of water reserve in the tanks of SG emergency make-up system. In the first case this effect is expected after about 28 hours, and in the second case – after 50 hours. In case of depressurized reactor the threshold is start of core heating up which occurs relatively early – after less than 2 hours. During this period the operator shall prepare and start drainage of the hydro accumulators (preferable first to the cold part of the reactor). Drainage of the three hydro accumulators delays start of core heatup for 4 hours. 5.1.1.4 Loss of off-site power supply, emergency diesel-generators, additional dieselgenerator and mobile diesel-generator 5.1.1.4.1 Capacity of the batteries, operational time period and capabilities to recharge them in this situation See par. 5.1.1.2.2. 5.1.1.4.2 Actions envisioned to provide mobile on-site or off-site power supply sources See par.5.1.1.2.2. 5.1.1.4.3 Time period required providing alternate current power supply and restoration of core cooling before fuel damage occurs. Threshold values in plant behaviour Assessments were performed for conditions of the units, described in section of Ch.15 [7],[8]. For each of the modes operator actions considered in the analyses, are specified, as aimed at prolongation of time period before fuel damage occurs. Table 5.1-3: Assessment of time period to fuel damage at loss of normal and standby power supply, failure to all emergency DGs, failure to additional DG, failure to mobile DG. Time to fuel Option damage (h:min) 1 Power operations (Considered: passive make-up of SG from the high pressure deaerator) 2 Shut down reactor, planned cooldown at pressurized primary circuit and drained SG (Considered: drainage of ECCS three hydro accumulators.) 3 Shut down reactor, planned cooldown at pressurized primary circuit and non-drained SG (Considered: drainage of ECCS three hydro accumulators.) 16:15 16:44 19:44 145/202
“Kozloduy NPP” PLC SUMMARY REPORT for Conduct of Kozloduy NPP Stress Tests Option 4 Shut down reactor, planned cooldown at depressurized primary circuit (Considered: drainage of ECCS three hydro accumulators.) 7:28 Time to fuel damage (h:min) 5 Shut down reactor, outage cooldown (Considered: drainage of ECCS three hydro accumulators.) 3 days after shutdown 9:15 18 days after shutdown 16:53 Threshold effect on the progress of the process is provided by discharge of the accumulator batteries (after about 10 hours). After that the operator does not dispose information on unit parameters and shall perform accident management actions blindly. If an event occurs at power operations or in cold condition with the pressurized primary circuit and non-drained SG, threshold effect is drainage of the SG. In order to prevent rapid increase of fuel temperature it is required within 6 to 8 (up to 9 in cold condition) hours to ensure water supply to the SG. In case of depressurized reactor, threshold is start of core heating up, which occurs relatively early – after less than 2 hours. During this period the operator shall prepare and start drainage of the hydro accumulators (preferable first to the cold part of the reactor). Drainage of three hydro accumulators delays start of core heatup for additional 4 hours. 5.1.2 Measures to improve robustness at loss of power supply 1. 2 new mobile DGs will be delivered, and the existing one will remain standby for the remaining structures at the NPP area; 2. Power supply is provided for charging of the accumulator batteries of the safety systems from mobile DG; 5.1.3 Loss of ultimate heat sink 5.1.3.1 Design bases to remove heat to main ultimate sink at different operational conditions Decay heat sources at power units 5 and 6 are as follows: · Core decay heat; · Fuel pool decay heat. The main ultimate heat sink for the nuclear installations at the Kozloduy NPP is Danube River. Connection to the ultimate heat sink is provided via direct scheme from the service water 146/202
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“Kozloduy NPP” PLC Figure 1.2-2
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Summary Report for Conduct of Kozloduy NPP Stress Tests

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