Nuclear fission

2.10Bulgaria

In Bulgaria, for the Kozloduy NPP, units 5 and 6, in order to ensure a high level of safety throughout the plant’s operating lifetime, a systematic safety reassessment, termed Periodic Safety Review (PSR) is carried out at regular intervals to assess the cumulative effects of plant ageing and modifications, operating experience, technical developments and new regulatory aspects, as is defined at [18]. It is noted that the PSA is one of the main subjects of the PSR.

2.10.1L2 PSA regulatory framework

In Bulgaria, the Nuclear Regulatory Agency (NRA) requests the development of a L2 PSA to support the periodic safety review of the Kozloduy NPP. The activities for development and use of PSA for the Kozloduy NPP, units 5 and 6 are planned and carried out in accordance with the requirements of the “Regulation on Ensuring the Safety of Nuclear Power Plants” [18].

2.10.2Link with legal requirement

The regulation [18] contains quantitative criteria for severe accidents - limits of a large release requiring immediate protective measures. Furthermore, these criteria are specified for new and for existing NPPs.

According to the regulation, Art.10:

“(4) The frequency of a large radioactive release into the environment that requires undertaking of immediate protective measures for the population shall not exceed 1.10^-6 events per NPP per year.”

According to the “Transitional and final provision” from the current regulation regarding the existing NPPs:

“3. The frequency of large radioactive release into the environment that require implementation of urgent protective measures for the public shall be lower than 10^-5 events per NPP per year.”

Furthermore, regarding to the probabilistic safety analysis, the regulations contains in the Art.21, p.6, the next requirements:

“(1) Probabilistic safety analysis shall be carried out with the objective to:

4. assess the frequencies of severe core damage and large radioactive releases to the environment;

5. evaluate the frequencies and the consequences of the external events specific to the site;

6. identify SSCs that require design improvements or changes in operational procedures, leading to decrease of severe accident frequency or mitigation of their consequences;

7. assess the emergency operating instructions.

(4) Probabilistic safety analyses shall be used to support the deterministic assessments in the decision making for plant design and operation, for assessment of necessary changes of SSCs, operational limits and conditions, operating and emergency operating procedures and training programs of the operating personnel."

2.10.3Role of L2 PSA

The Section 7 “Emergency procedures and severe accidents management” from the Safety Guide “Use of PSA to Support the Safety Management of NPPs” [19], describes the PSA use in the development of the emergency procedures and the measures for severe accidents management, including in the assessment of their modification:

“7.9. The effectiveness of the existing alternative or additional systems, equipment and measures should be assessed in the procedures for management of severe accidents with the help of PSA.

7.10. In the case of operator actions to manage accidents, PSA should clearly present the operator actions referring to specific emergency instructions and accident management procedures. To support such applications the method of analysis of human reliability used in PSA should be able to predict the impact of procedure changes.

7.11. The reflection of the operator actions in PSA level 1 supports the improvement of the procedures for accident management for these actions to prevent a severe damage of the core. PSA Level 2 with limited scope reviews the strategies for mitigation of the consequences from severe accidents.

7.12. The mitigation of the consequences from severe accidents should include identification and categorization of emergency sequences based on PSA, together with descriptions of the NPP behavior and weaknesses.

7.13. PSA supports the understanding of accident development, the identification of successful ways to manage and the strategies related to it, as well as the prioritization of safety characteristics to reduce risk.”

The integral demonstration of NPP behavior with the PSA methodology supports the reduction of potential negative effects from certain measures.

The current L2 PSA for the Kozloduy NPP, units 5 and 6 is based on the L1 PSA and represents the status of the units up to 2007 year concerning the systems and procedures included in L1 PSA, and status up to 2011 for the systems and procedures (e.g. SAMG) related to containment and severe accident aspects [9].
Presently, the L2 PSA study includes full power operation modes, low power and shutdown modes, and Spent Fuel Pool (SFP). The analyses are performed by using RiskSpectrum program code. The L2 PSA study is comprised of two parts: interface PSA and CET. Two types of models for CET have been developed: one for conditional probabilities calculations and another for the integrated model – a set of simplified CET’s for each PDS group. The purpose of the first model is to be able to perform quick calculations and for sensitivity analyses as well. The simplified CET’s are used for integral calculation of the model [9]. The supporting calculations are performed with MELCOR code.

On the basis of PSA, a Risk Monitor was developed, which is used in everyday operation of Kozloduy NPP. Furthermore, other PSA applications are also developed or are under development - risk-informed testing, risk-informed maintenance, risk-informed Technical Specifications [28]. Also, based on the present PSA results, the effectiveness of the SAMGs are assessed qualitatively [9].

2.10.4SAM objectives to be reached

The main objective of SAM strategies is to prevent and mitigate possible core damages as well as further radioactive releases beside the system of NPP physical barriers. The other main role of SAM is to return the critical safety functions in normal state as well as to ensure long term NPP safe state.
For the Kozloduy NPP, units 5 and 6, the purpose of the SAMG is to define beforehand specific actions to perform in order to [20] :

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  avoid or limit radioactive releases;
  
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  avoid or delay the possible loss of containment integrity in order to give more time to activate the Emergency Plan for public protection;
  
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  bring the unit back to a controllable state. Keeping the corium under water inside the pressure vessel is a key objective in order to regain control of the situation.
  

Actions recommended in the SAMG aim at limiting the risk of radiologically significant radioactive releases in the short- and mid-term (a few hours to a few days).
According to the Updated National Action Plan of Bulgaria [29], the development and implementation of significant severe accident management measures at Kozloduy NPP was initiated before the Fukushima accident, in the frame of a Modernization Program of units 5 and 6 (in the period 2000-2007). The most substantial severe accident management provisions, implemented before Fukushima Dai-ichi accident, include installation of containment filtered venting system (scrubber type), installation of hydrogen recombiners, development of L2 PSA and SAMGs.
For the Kozloduy NPP, units 5 and 6, symptom-based emergency operating procedures (SB EOP) were developed on the bases of protection of fundamental safety functions with approach similar to the Westinghouse one, as step-by-step procedures with description of main and alternative operator actions in two-column format. SBEOP are controlled and maintained adequate to existing powers through strict on-site rules on verification and validation which envision multiple checks before these are introduced into operation. Analytic validation is conducted on the original technique provided by Department of Energy (DOE USA) [28].

Criteria for transfer from SB EOP to SAMG were defined.

SAMGs for units 5 and 6 were developed on the basis of system analysis of the processes and phenomena during severe accidents (PHARE Project BG 01.10.01, “Phenomena investigation and development of SAMG”). The purpose of system analysis is to define the basis for knowledge of processes and phenomena, occurring at progress of severe accidents, all the technical means (equipment and systems), which allow for reaching of the set purposes in the course of severe accident management (SAM). SAMGs were developed which follow the format of SB EOP. Strategies defined for severe accident management are the following [29]:

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  pressure reduction in the primary circuit;
  
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  pressure reduction in the secondary circuit;
  
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  water injection to the primary circuit;
  
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  water injection to the secondary circuit;
  
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  pressure reduction in the containment.
  

Following the stress tests, additional severe accident management actions have been implemented, e.g. the project for plugging of the ionization chamber channels in the walls reactor vessel cavity to prevent basement melt-through and early containment by-pass; the project for installation of wide-range temperature sensors to monitor the reactor vessel temperature and installation of additional recombiners. The scope of the EOPs has been extended to cover reactor shutdown states and spent fuel pool accidents. Also, an update of the L2 PSA was done taking into account L1 PSA results from 2010 [9]. The SAMGs have been developed to involve two sets of documents – SAMGs for Technical Support Team and SAMGs for MCR/ECR [29].