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 6.2 MAINTAINING INTEGRITY OF THE CONTAINMENT OF UNITS 5 AND 6 AFTER SIGNIFICANT FUEL DAMAGE (BEFORE CORE MELTING STARTS) 6.2.1 Prevention of possibility of damage/melting of fuel at high pressure 6.2.1.1 Design means Strategy was developed for pressure reduction in the reactor vessel [148] within the severe accident management guidelines. The main technical means for pressure reduction at accident progress before severe phase is the safety valves of the primary circuit and the system for emergency gas removal from the primary circuit. As additional possibility to reduce pressure in the reactor vessel, valves can be used on lines of sealing water drainage [150] of the main circulation pumps. For ensuring feasible possibility for use of the system for emergency gas removal from the primary circuit in conditions of severe progress of the accident, modification was performed on the power supply to system valves, and redundancy provided for power supply of the correspondent valves from the accumulator batteries. 6.2.1.2 Operational means At occurrence of emergency conditions the operators of Units 5 and 6 initiate performing SBEOP. This should happen several hours before the accident reaches severe phase with fuel damage. In SBEOP strategies are envisioned for accident management – restoration of the CSF- Heat removal and CSF- Core cooling, Restoration of electric power supply. Successful executing of actions envisioned in these strategies at the end will prevent fuel damage, as well as will decrease pressure in the reactor vessel. 6.2.2 Prevention of overpressure in the containment due to hydrogen explosion 6.2.2.1 Design means Units 5 and 6 are provided with Hydrogen reduction system in the containment which consists of 8 passive autocatalytic recombiners (PAR), located within the containment structure and intended to burn hydrogen generated during the accident. The system is designed with capacity to handle hydrogen generated at design-basis accident with maximum leak of the primary circuit. Additional analyses of the NPP show that the hydrogen reduction system is able to reduce hydrogen generated during in-vessel phase of severe accident to admissible levels, therefore explosive concentrations won’t be reached [150]. At the out-of-vessel phase of severe accident operation of the system is not efficient due to lack of oxygen in gas environment of the containment, as required for the process of fixation of hydrogen in the PAR. Though during out-of-vessel phase of severe accident within the containment significant amounts of hydrogen are generated, these do not impose direct hazard of explosion, since there is no oxygen. 6.2.2.2 Operational means 187/202
“Kozloduy NPP” PLC SUMMARY REPORT for Conduct of Kozloduy NPP Stress Tests SAMG and EP procedures envision actions for monitoring, assessment and predicting of hydrogen concentration within of containment premises. Actions are described for control of Spray systems depending upon concentration of hydrogen. 6.2.3 Prevention of overpressure in the containment 6.2.3.1 Design means The design of Units 5 and 6 envisions the Spray systems for heat removal from the containment and for fixing iodine in steam-air mixture at loss-of-coolant accidents. As a result of operation of these systems pressure reduction is reached in the containment to safe values. Spray systems are qualified as safety systems. They actuate automatically at pressure within the containment increases above safe operation limits. To control pressure in conditions of severe accident at units 5 and 6 additional filtering systems are installed for pressure reduction, operating on passive principle. At pressure increase in the containment to design level rupture to membrane occurs which is connected to the pipeline before the capacity filled with filtration solution. Gas environment from the containment passes through the filtering solution and released through the throttling system to ventilation stacks of units 5 and 6. 6.2.3.2 Operational and organizational means During normal operations pressurization is maintained in the containment. Means for protection against overpressurization are maintained ready. At emergency conditions active protection means actuate automatically depending upon actuation setpoints and are controlled in accordance with directives of Emergency instructions. 6.2.4 Prevention of resubcriticality 6.2.4.1 Design means In VVER reactors, at unaffected geometry of the core, water-uranium relation is close to optimal in relation with the reactivity. This means that at occurrence of significant changes to core geometry, the reactivity of the core reduces. Due to this reason repeated criticality in the core during severe accident is of low probability [151]. 6.2.4.2 Operational means All systems used for the reactor make-up in conditions of severe accident use borated water and don’t bear risk of repeated subcriticality [151]. 6.2.5 Prevention of melting on the foundation slab 6.2.5.1 Possible design means to maintain the corium in the reactor vessel Possibility was analyzed for external-to-vessel cooling of the reactor. It is defined that in some options of severe accident progress damage to vessel cannot be avoided. The SAMG envision actions for restoration of coolant supply to the reactor. If during invessel phase of severe accident the operators manage to deliver sufficient coolant to the core, there 188/202
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Summary Report for Conduct of Kozloduy NPP Stress Tests

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