Nuclear fission

2.3Spain

2.3.1L2 PSA regulatory framework

In Spain, the Nuclear Safety Regulator (CSN) requires the utility to develop the L2 PSA by Safety Instruction IS-25 (June 2010) [24]. This instruction establishes that the scope of PSA must be Level 1 and Level 2 and must consider internal and external events in all modes of operation, considering also other radioactivity sources.

This instruction and Spanish Safety Guide 1.15 [25] are being updated and we know that the new requirements will be to submit:

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  L2 PSA at full power for internal events every 5 years. Each plant must also submit a report every 9 months after the end of the refueling outage with the analysis and reasons to prove that it is not necessary to submit a new revision of the L2 PSA.
  
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  L2 PSA at full power for internal flooding and fire events or L2 PSA at low power and shutdown for all events every 10 years.
  

Uncertainty/sensitivity analyses are also required for some issues.

This instruction includes applications of PSA related to modifications in design, specifications or procedures, including severe accident management guidelines, to prove that the risk of the plant remains within the acceptable level. The modification will be acceptable if it involves a risk reduction or if the rise of the risk is below the acceptable threshold indicated in the Spanish Safety Guide 1.14 [26]. The rise of the large early release frequency must be lower than 10^-6 and the rise of the large releases frequency must be lower than 2.10^-6. After the acceptance of an application, the PSA must be updated to represent the new situation.

On the other hand, Safety Instruction IS-36 (January 2015) [27] establishes the requirements that nuclear power plants must meet regarding the Emergency Procedure Guidelines (EPG) and Severe Accident Guidelines (SAG). This instruction requires the utility to develop SAG with the objective of mitigating the consequences of severe accidents in case that measures implemented by EPG did not have success in preventing core damage. This SAG must be developed for all modes of operation. For sites with more than one unit, SAG must be developed to be applied in case there was accident in several units simultaneously.

This instruction requires having instrumentation that could be used in severe accident conditions.

It also requires capabilities to protect containment for a selected group of accidents exceeding the design bases, in terms of:

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  Containment isolation,
  
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  Containment tightness,
  
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  Containment pressure and temperature control,
  
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  Combustible gases control,
  
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  Overpressure protection,
  
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  Reduction of high pressure melt ejection probability,
  
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  Reduction of containment degradation by molten core-concrete interaction.
  

The selected group of accidents exceeding the design bases must consider deterministic and probabilistic analysis and expert judgement.

The utility must also have portable equipment for a long term SBO.

IS-36 also specifies that verification and validation of SAMG must be done in the same way they are going to be used. When a modification is done in SAMG, the scope of verification and/or validation must be done according to the importance.

It indicates that the utility must have a plan for maintaining and updating SAMG based on the current situation of the systems in the plant, the operating experience, the plants owners group considerations and the newest recommendations of the supplier of the equipment.

2.3.2Role of L2 PSA

The role of L2 PSA has evolved over time as their development has been consolidated. Initially, identifying vulnerabilities of the containment to the severe accident, determining a quantitative safety criterion and providing input into the development of the accident management guidance and strategies and finally, developing risk informed applications and decision making support incorporated into the rulemaking more recently. This new application field demands a quality and a higher level of development that guaranty an effective use on it:

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  a sufficient level of detail, taking into account the most significant dependencies (operational and functional) and the procedures;
  
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  the impact of the conservatism should be measurable;
  
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  the detail of the models should permit changes related with the modifications to be analysed.
  

The Spanish regulation enables the use of the RG 1.200 [31] to determine the L2 PSA quality for risk-informed application and decision making support.

2.3.3SAM Objectives to be reached

For BWR, the operational emergency response is divided into Emergency Procedures Guidelines (EPGs) and Severe Accident Guidelines (SAGs). The EPGs define strategies for responding to emergencies and events that may degrade into emergencies up until it is determined that the core cannot be adequately cooled. The SAGs define strategies applicable after it is determined that the core cannot be adequately cooled. The SAGs strategies are grouped into two guidelines:

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  RPV and Primary Containment Flooding: which objectives are to cool the core and core debris, shutdown the reactor and depressurize the RPV and prevent it from re-pressurizing.
  
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  Containment and Radiological Release Control: which objectives are to protect equipment in the primary and secondary containments, maintain primary and secondary containment integrity and limit radioactivity release into areas outside the primary and secondary containments.
  

2.3.4SAM in L2 PSA

Severe accident management strategies can be included into the L2 PSA if they are adequately treated into the operational guidelines and all the interfaces related with a successfully implementation are covered, i.e. equipment and instrumentation survivability and impact of the environmental and stress conditions on the human actions. The uncertainty about all these aspects is significant higher; at any case the analysis is necessary not only to identify the relative importance but also to know the risk by indirect effects or due to an incorrect implementation or failure on it. In this sense, any SAM new strategy implemented in a NPP has to be analysed with the L2 PSA criteria for a risk evaluation.

The Stress Test evaluation on the Spanish NPPs, after Fukushima Dai-ichi accident, identified some SAM improvement to be implemented mainly based in the European experience and the state of the art: Passive Autocatalytic Recombiners and Filter Containment Vent System are the most significant new capacities installed on them. These systems are sized based on the severe accident specific impact for each plant, covering all potential phases of a severe accident progression and maximizing the risk to be mitigated. The mainly passive design of these systems reduces also a possible human error.