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 change in the accident sequences progress) and the change in safety functions performance shall be defined. Consistent review of each range is used to determine the ability of nuclear facilities to withstand impacts of various degrees of severity. This systematic approach allows to achieve the main purpose of the reassessment of safety, namely to define the limiting values of the accelerations that the unit can withstand without a serious damage to the fuel and release of radioactive substances into the environment. 2.10.2 Evaluation of safety margins against earthquake for units 3 and 4 According to the result provided in [41] it follows that Units 3 and 4 margin is 0,16g or 80% of RLE (PGA=0,2g), i.e. the units can bear without fuel damage earthquake 1,8 times greater than the DBE reassessed and valid as for 30.06.2011. 2.10.2.1 Assessment of the seismic impact leading to severe fuel damage Based on analysis [44] it can be concluded that the fuel damage cannot be prevented at PGA over 0,36g, i.e. at acceleration for which liquefaction of sands under fire protection station-2 and CPS-2 is anticipated. Till this level of impact the unit performs reliable cooling of fuel in the SFP (or reactor). The upper part of the range 0,26
“Kozloduy NPP” PLC SUMMARY REPORT for Conduct of Kozloduy NPP Stress Tests available to allow for adequately arranging and effectively implementing all the required measures for recovery of access to switchgear 0,4 kV. For more detail for the time available see item 5.3.1.2 of the report. In the early phase of accident process, ASSGEM can be used for performance of any one of its functions. After the water level reaches Kozloduy NPP site, ASSGEM can be used only as an alternative source of supplying pool cooling pumps due to flooding of the basement of ASSGEM. The power supply from ASSGEM is guarantees for 72 hours as each EDG has at least 72 hours of fuel inventory [45]. As the ultimate heat sink from the heat exchanger for SFP cooling, at simultaneous loss of off-site power and emergency power supply from the diesel-generators in DGS-2, only the cooling water supplied from the diesel pumps to the fire pump station 2 will remain. Obviously, the SFP decay heat removal function can be ensured at selection of a scenario for combination of the external events. At this scenario the only significant consequence is associated with the seismic impact itself, i.e. with possible falling of covering elements in the SFP. As stated in [41], at mechanical damage of the fuel, release of fission products from the gas gap of the fuel elements can be anticipated. This activity will be released into the environment, as the Central hall does not have the status of a confined compartment. It can be concluded that the degree of redundancy of SFP fuel cooling function was reduced to a greater extent than at independent effect of each external event included in the scenarios. In the event that before the earthquake and flooding, the fuel is placed in the reactor according to the conditions of [46], [65] and [66], then due to the loss of the ПР system and loss of ASSGEM pumps, heat removal can be provided only by fire trucks and SG flow rate can be ensured by opening of SG SVs. [154],[155]. Regardless of low energy release of the fuel, the effectiveness of application of such cooling logic should be analytically reviewed. At the assessment of applicability of the cooling logic with fire trucks the following complicating circumstances shall be considered: · the fire trucks owned by the Kozloduy NPP are situated opposite the entrance to units 5 and 6. This means that their access to points of connection will not be hampered by the water level. Building structures and facilities of the fire service are designed according to the national seismic standards. This means that they are guaranteed for seismic impacts with Кс=0.10, assuming non-linear deformities and partial damages. This means that at an earthquake within the range of 0,26 < PGA ≤ 0,36g, the buildings under fire will be severely damaged or destroyed, thus, hindering release of the vehicle from the garages. Therefore, 94/202
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Summary Report for Conduct of Kozloduy NPP Stress Tests

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