Nuclear fission

7.List of Tables

8.List of Figures

9.APPENDICES

9.1 Practical containment analysis during shutdown states for French PWR (EDF)

This appendix is an illustration of a practical containment analysis, which could be used as a “checklist” by a PSA analyst to address correct containment analysis in shutdown states for a PWR.

To perform a L2 PSA analysis regarding containment performance during shutdown states, two phases are necessary: identification and quantification.

9.1.1 IDENTIFICATION

During shutdown states there are penetrations that are not normally open during power operation, in order to carry out maintenance outage operations in the reactor building. The following penetrations are often used for PWR maintenance:

• 
  Equipment access hatch,
  
• 
  Fuel transfer tube between reactor building and fuel building
  
• 
  Personnel access hatch,
  
• 
  Ventilation systems, in order to provide fresh and decontaminated air for workers inside the reactor building.
  

Additionally some connections between systems (that are not normally connected during power operation) can be operated, in order to anticipate potential failure. For example in PWR reactor, a connection between RHRS (Residual Heat Removal System) and FPCS (Fuel Pool Cooling System) can be pre-lined, in order to provide a quick mutual assistance between these two systems; this connection can also be open for additional refilling or draining during shutdown state.

9.1.2QUANTIFICATION

Once identified, penetration impact on containment performance has to be evaluated. The following points should be analyzed:

How long is the penetration open during each phase of the different shutdown states? (OPEX needed)

Is the penetration automatically closed by a sensor? (for example, reactor building ventilation system on EDF PWRs is automatically closed if activity inside the reactor building is detected)

Is the penetration closure available from the control room, or is it necessary to have an operator close it locally?

Is the penetration closure required and described in a formal procedure?

Does the penetration need power supply (air or electricity) to be closed, or is it automatically closed? (for example, reactor building ventilation valves in PWRs can be automatically closed by a spring, without external power supply)

If penetration is closed manually, do operators have time to close it before reactor building atmosphere becomes unacceptable for workers? (for example the equipment access hatch usually need few hours to be closed on PWRs : this delay might be too long for some accidents with a fast core melt progression)

Once analyzed, a quantification of containment failure and potential human mitigation will be performed for level 2 PSA model in shutdown states.

9.2 LEVEL 1 SHUTDOWN STATES PSA

Internationally, the number of safety significant events during outages and in low power states is found to be high. Because of the growing number of incidents during shutdown, some of them leading to substantial loss of reactor coolant through draining, plants and regulators began to focus attention on the significant risks during shutdown conditions.
The first full-scope LPSD PSA studies were French studies for 900 MW and 1300 MW series. The results of those studies were widely published and have shown that actual risk during shutdown is in the same order of magnitude as the risk during the full-power operation of NPPs.
This risk is not related to the plant design. It is rather related to the unavailability of equipment due to maintenance activities undertaken during an outage, presence of additional (contractor) personnel who may not be fully aware of the safety issues, presence of additional heavy loads and flammable materials etc. All of these aspects increase the risk during plant outage.
Adequate planning and preparation of activities during outages can reduce both the probability and the consequences of possible events. In other words, there is a lot of scope for safety improvements in low power and shutdown operating modes.
Performance of PSA for shutdown and low power operating modes, can support the enhancement of the safety during plant outage, and may as well contribute to reduction of the outage duration. Shutdown PSA can provide following useful insights and feedback [39]:

Risk level and licensing (showing that the risk is below a certain level);

Risk monitoring and risk follow-up;

Outage planning;

Training, procedures and emergency planning for outages;

Shutdown technical specifications;

Outage management practices;

Hardware modifications;

Contribution to a more economical plant operation.
External Hazard PSA Methodology

External hazards analysis methodology for LPSD PSA is generally adapted from the full power PSA External hazards [41] like seismic, aircraft crash etc., are assumed to occur randomly in time and hence the frequency of an external event of a given intensity for a particular shutdown POS can be obtained by multiplying the frequency used in full power PSA with the POS fraction of the corresponding POS. Seasonal variation factors may need to be taken into account for hazards which may occur in particular season only. For instance snow storms are unlikely during summer.

Following assumptions are generally made during the analysis of the external events:

Seismic events, extreme winds, lightning and aircraft crash events are conservatively assumed to result in non-recoverable LOOP scenarios.

External flooding, rain and ice formation events are generally assumed to result in loss of heat sink.

All the modules of LPSD level 1 PSA i.e., internal events, internal hazards and external hazards model should be integrated into a single PSA model to evaluate the overall Core damage frequency of the plant.