Summary Report for Conduct of Kozloduy NPP Stress Tests

“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
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