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 containment integrity is subject to significant hazard of melting from the penetrations of the main slab at scenario with possibility of cooling outside the reactor vessel. The MCR and SCR of VVER-1000 are located at the lower level in the reactor building, i.e. under the structure of the containment. Main slab of the containment which might be affected by the corium in scenario with melting outside of the reactor vessel, is moved, and there is a set of premises below that which are outside the containment and in which safety related pumps and equipment are located, and after that these premises may become inaccessible, including the MCR. Melting penetration through the main slab of the containment is not the immediate hazard of the first day (days) after such hypothetical accident, due to structural characteristics of the shaft (wall thickness of the basis is 3.194 m, thickness of the main slab of the shaft is 3.6 m). There is more probable scenario, at which melt outside the reactor vessel can bypass the main slab as well as leak through the penetrations of ionizing chamber channels in the main slab in premises under it, but at present design is executed for Units 5 and 6 to eliminate this option [152]. The localizing systems were executed in the design of the unit which ensure meeting the set criteria for localization of releases of the radioactive substances to the environment. To execute the localizing functions, systems and means for controlling containment environmental parameters are installed in the containment, for isolating the containment and for reduction of concentration of radioactive substances of fission, hydrogen, other substances which may release to the containment environment during and after design and severe accidents. For performing of these safety functions the following systems were installed: · Containment system (CS); · Spray system; · Filtering pressure reduction system; · Hydrogen recombination system. Within modernization program the following measures were executed (section 12 of [7], [8]), related to CS: · Improvement of containment testing procedure; · Qualification of cable penetrations and planning of their replacement; · Installation of filtering ventilation; · Development and introduction of severe accident radiation monitoring systems. 6.3.1.2 Operational means Organizational and technical means at radioactive releases after loss of containment integrity of Units 5 and 6 are described in the Emergency Response Plan of the Kozloduy NPP. This includes operations of the shift personnel of the MCR on SBEOP and SAMG in parallel with coordination actions on external-to-plant National emergency plan (National emergency plan) [75]. 191/202
“Kozloduy NPP” PLC SUMMARY REPORT for Conduct of Kozloduy NPP Stress Tests Off-site forces and means, involved in providing support to «Kozloduy NPP» plc are defined in National emergency plan for conducting of rescue and urgent emergency-recovery activities at occurrence of disasters, accidents and collapses. 6.3.2 Accident management after uncoverage of the upper part of fuel in the SFP of Units 3 and 4 Modernization of units 3 and 4 includes execution of the correspondent measures and development of full strategy of severe accident management. These is modernization of level measuring in the reactor and the SFP, filtering ventilation system in the compartment, installation of the hydrogen recombination system, jet-vortex condenser (JVC), etc. Execution of the part of these measures is directly dependent upon the envisioned strategy of accident management. Applicability of the strategies on severe accident management at long-term storage of the SNF in SFP considering the design and actual condition. · Inertization of the SG box compartment (SGBC) is not envisioned at SNF storage in the SFP, since it is considered that: - emergency filtering and ventilation system (EFVS) won’t start. Event at assumption of EFVS actuation, its efficiency will be minimum (zero), as the SFP is directly connected to the central hall and hydrogen release to the SGBC will be minimum; - steam won’t be supplied to the SGBC; - availability of the spray system is not required for this condition. · Condition and integrity of the SGBC is not considered, as the SFP (with SNF in it) is located outside to it in the central hall (i.e. to the atmosphere) for the entire period of accident, and the jet vortex condenser (JVC) is not efficient for localizing radioactive substances within the reactor compartment boundaries; · Phenomena of HPME (high pressure emergency ejection) and DCH (direct containment wall heating) are not applicable for all the scenarios; · Operator actions of on pressure reduction in the reactor are not applicable; · The hydrogen recombination system in the SGBC is not efficient due to its location. 6.3.2.1 Control of hydrogen Measures for hydrogen control at long-term storage of SNF in the SFP (for more than 4 years after removing the fuel from the reactor and its low energy release rate), are not envisioned in the design. 6.3.2.2 Ensuring adequate protection against radiation Executing of protection measures, envisioned in the emergency response plan, see par.6.1.3.1.4. of [28], ensure adequate protection against radiation. 192/202
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Summary Report for Conduct of Kozloduy NPP Stress Tests

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