What kind of on-site accident management measures are under consideration or already in place to prevent severe accidents or mitigate their consequences (e.g. containment venting, bleed and feed, additional emergency power supply)?

Regarding the measures taken to handle severe accidents, NPP Krsko is in process of implementing plant specific Severe Accident Management Guidelines (SAMG) prepared based on Westinghouse Owners Group generic documents. Procedures are prepared for Main Control Room operators (interface documents between Emergency Operating Procedures and SAMG) and Technical Support Centre acting as a central point for handling the emergency conditions including severe accidents. 

Symptom oriented Emergency Operating Procedures were introduced in NPP Krsko in 1988. They already included feed-and-bleed recovery procedure.

In the past several hardware improvements were implemented:

• in respect to ATWS Rule AMSAC system was installed
  

• Station Blackout Rule resulted in several changes (upgrade of both trains of batteries for higher capacity, installation of compressed air tanks for the supply of air to PORVs on the steam generators
  

• since 1991 additional 400 V, 250 kW diesel generator is available on the site for emergency power supply

In response to the results of the Krsko PSA study several modifications were performed:

• change of dry reactor cavity design to wet (conceptual design completed, installation in two years)

• implementation of modifications in the fire protection systems (additional smoke detectors, additional emergency lighting, additional sprinklers, improved plant communication, replacement of fire doors, modification of local shut down panels by isolation of critical circuits from the control room circuits, 3-hour cable wrapping for engineered safeguards equipment)

TMI related items include installation of Post Accident Sampling System, Reactor Vessel Head Vent, Inadequate Core Cooling Monitoring, Reactor Vessel Level Instrumentation and High Range Containment Monitoring.

The report only indicates that US codes and standards were applied. Although the basic principles (defence-in-depth, barriers) are in compliance with the obligations under Article 18, it is difficult to appreciate this compliance without additional details. In particular, what are the evolutions since the initial design (severe accidents management, allowance for human factors, lessons learned from experience feedback)?

Since initial design numerous safety improvements based on NPP Krsko specific experience feedback have been implemented (modifications to the systems - IA,...) as well as numerous improvements based on US regulation requirements and on the plant specific severe accidents analysis performed in the scope of PSA study: action related to post TMI programme - NUREG 737, ATWS regulation 10CFR50.62- AMSAC system was added, SBO rule - 10CFR50.63 - improvements in DC supply.

(See also answer to the above question)

Is there a policy by SNSA for addressing severe accident response by Krsko?

SNSA priorities in nuclear safety for the NPP Krsko in the past 10 years mainly addressed operational safety items: completion of open TMI issues, Fire Hazard Analysis, Pressure Vessel Embritlement Study, new StandardTechnical Specifications, Station Blackout Study, revision of the Site Emergency Plan, etc. SNSA has formally requested in 1991 the initiation of the study of severe accidents in NPP Krsko in connection with the PSA Study.

Action plan addressing severe accident response has been drafted from the NPP Krsko PSA study which included several sensitivity calculations (dry versus wet reactor cavity, containment filtered venting, etc). SNSA is supporting all the activities aimed at increasing the plant preparedness to cope with severe accidents. 

Which are the licensing requirements for proven design and environmental qualification and which steps are taken e.g. in plant modifications to meet these requirements?

One of the main licensing requirements is that equipment or modification should be of the proven design already successfully used and licensed in other NPPs.

Modification process in NPP Krsko is covered by set of Engineering Procedures giving an uniform approach how to prepare Design Change Documentation (see the list at the end of this Section). During the preparation of Design Change Documentation each Responsible Engineer (RE) should prepare a design input document, which defines the design inputs, bases and other considerations for particular Design Modification. Procedures give detailed instructions for preparing this document (Design Inputs, DI). After the review of DI and approval, RE can proceed with the design of plant modification in accordance with approved DI. In addition to DI, RE completes a Design Impact Evaluation Form to determine whether the plant design modification affects or is affected by key plant design analyses or regulatory programme requirements. If any design documents or drawings are affected and require revision as a result of the Design Impact Evaluation the RE shall identify and revise the drawings and documents. Equipment Qualification (EQ) Programme in NPP Krsko is currently in development phase, however Design Impact Evaluation is performed also regarding the EQ concern. New installed equipment is screened in accordance with the requirements from the 10 CFR 50.49. If new or replaced equipment is recognised as a EQ equipment, it will be purchased and installed in accordance with all EQ requirements.

Which are the design requirements and the measures taken to handle severe accidents?

(See response to the question from Germany above)