Safety Analysis Report for Decommissioning of KNPP Units 1 and 2 A. Ivanova, I. Ovcharova ( ENPRO CONSULT LTD ). International Nuclear Forum BULGARIAN NUCLEAR ENERGY – NATIONAL, REGIONAL AND WORLD SAFETY BULATOM , M ay 28-30, 2008 , Varna, Bulgaria.
International Nuclear Forum
BULGARIAN NUCLEAR ENERGY – NATIONAL, REGIONAL AND WORLD SAFETY
BULATOM, May 28-30, 2008, Varna, Bulgaria
Following the Contract for joining the European Union, the Council of Ministers of the Republic of Bulgaria took a decision and shut down the first and second units at Kozloduy NPP (KNPP) on 31.12.2002in accordance with Decision № 848 of 19.12.2002.
At present units 1 and 2 possess a license for operation in mode „Е” – storage of the spent fuel in the spent fuel pools. According to the order of BNSA, from the beginning of December 2006 the licenses for operation of KNPP units 1 and 2 have been modified; this does not change the current status – operation of the units in mode „Е”, but allows for dismantling of the systems and equipment, which are not important for safety and do not contain radioactive substances above the free release levels.
The next step is obtaining a licence for Stage 1 decommissioning activities – Safe enclosure (SE) and dismantling in Turbine hall. The development of Decommissioning Safety Analysis Report (DSAR) is one of the documents needed for this license.
2 Specific features of units 1&2, related to the decommissioning
Kozloduy NPP Units 1 and 2, type WWER 440, model B 230 have some design and structural peculiarities, which impact the SE. These are:
During the preparation for decommissioning measures have been foreseen and implemented for all specific features, having an impact on decommissioning. A number of infrastructure projects have been realized, including the physical separation of Units 1&2 from Units 3&4, erection of buildings, etc.
3 The Option for KNPP Units 1 and 2 Decommissioning
Kozloduy NPP approved an Updated Decommissioning Strategy for Units 1÷4 in June 2006. According to this “Continuous Dismantling Strategy”, the decommissioning of Units 1 and 2 will be done in two stages.
The generalized schedule for continuous dismantling is the following:
Safe enclosure zone of units 1 ÷ 4
SE of Units 1&2
Rest of SE
1 – Reactor building 1 (Units 1 & 2) 2 Reactor building II (Units 3 & 4)
3 – Auxiliary building 1 4 – Auxiliary building 2
5 – Sanitary and laboratory building 1 6 – Sanitary and laboratory building 2
7 – Vent tube (stack) 1 8 – Vent tube (stack) 2
9 – Turbine hall
4 The scope and the structure of DSAR
The content of the DSAR for decommissioning of Unit 1 is prepared in compliance with the documents:
This document is elaborated on the basis of the:
The DSAR document is prepared for Stage 1 of the Decommissioning, which is planned for 2011-2018 and includes:
DSAR consists of 13 chapters:
5 Design basis of the decommissioning
5.1 Safety objectives and safety principles
The main safety objective, as defined in the IAEA standard SF-1 “Fundamental safety principles”, is to protect people and the environment from the harmful effects of ionizing radiation.
To ensure that facilities are operated and activities conducted so as to achieve the highest standards of safety that can reasonably be achieved, measures have to be taken:
The fundamental safety objective applies for all facilities and activities and for all stages over the life time including decommissioning.
The same Standard SF-1 formulates ten safety principles, on the basis of which are developed the safety requirements and the safety measures to be implemented in order to achieve the fundamental safety objective.These principles are listed bellow:
Principle 1: Responsibility for safety
The prime responsibility for safety must rest with the person or organization responsible for facilities and activities that give rise to radiation risks.
This aspect of the decommissioning process is addresses in Chapter 8 “Management of safety”.
Principle 2: Role of government
An effective legal and governmental framework for safety, including an independent regulatory body, must be established and sustained.
In Bulgaria these matters are settled by Chapter II “Government regulation” of the Act on the safe use of nuclear energy. This chapter of the law defines the functions, structure and financing of the Nuclear Regulatory Agency.
Principle 3: Leadership and management for safety
Effective leadership and management for safety must be established and sustained in organizations concerned with, and facilities and activities that give rise to, radiation risks.
The problems of organization of safety, safety culture, ageing management etc. are addressed in Chapter 8 “Management of safety” of DSAR.
Chapter 6 “Safety analysis” includes the analysis of hazards arising during decommissioning.
Principle 4: Justification of facilities and activities
Facilities and activities that give rise to radiation risks must yield an overall benefit.
In many cases, decisions relating to benefit and risk are taken at the highest levels of government, such as a decision by a State to embark on a nuclear power programme. This evaluation has been performed at the stage of decision making for the construction of Kozloduy NPP. At the stage of decommissioning it has to be demonstrated that the risk is not higher than in the stage of operation of the nuclear facility. The evaluation of the radiation risk is presented in Chapter 10 "Radiation protection” and Chapter 6 “Safety analysis”.
Principle 5: Optimization of protection
Protection must be optimized to provide the highest level of safety that can reasonably be achieved.
The problems of optimization of protection are addressed in Chapter 3 “Site evaluation”, Chapter 6 “Safety analysis” and Chapter 10 “Radiation protection”.
Chapters 4 and 5 present the physical protection and the fire safety of decommissioning.
Principle 6: Limitation of risks to individuals
Measures for controlling radiation risks must ensure that no individual bears an unacceptable risk of harm.
Chapter 10 “Radiation protection” presents the application of the ALARA principle to the radiation risk during decommissioning. The application of this principle shall provide the highest possible margin to the fulfilment of the dose criteria for the workers and for the population, as they are defined in the Regulation for the basic norms of radiation protection.
Principle 7: Protection of present and future generations
People and the environment, present and future, must be protected against radiation risks.
Radioactive waste management is a major component of the process of decommissioning. Chapter 12 “Radioactive waste management” addresses the topics of RAW treatment, minimization, etc.
Principle 8: Prevention of accidents
All practical efforts must be made to prevent and mitigate nuclear or radiation accidents.
The most adverse consequences during decommissioning may arise as a consequence of loss of radioactive source or loss of control over a radioactive source. The principle of defence in depths is incorporated in the plant design. The assessment of the efficiency of the barriers against uncontrolled release of radioactivity to the environment is addressed in Chapter 10 "Radiation protection” and Chapter 6 “Accident analysis”.
Principle 9: Emergency preparedness and response
Arrangements must be made for emergency preparedness and response for nuclear or radiation incidents.
Emergency preparedness is addressed in Chapter 11 of DSAR.
Principle 10: Protective actions to reduce existing or unregulated radiation risks
Protective actions to reduce existing or unregulated radiation risks must be justified and optimized.
Chapter 7 “Decommissioning activities” gives a complete review of all activities at the given stage of decommissioning. Both the nuclear and the industrial aspect of each activity is assessed.
5.2 Classification of structures, components and systems
5.2.1 Safety functions and related structures, components and systems
The safety functions and the associated structures, systems and components (SSC) are defined both for the planned decommissioning activities, and for accident conditions. The safety functions required during decommissioning comprise a combination of safety functions that were needed during facility operation and additional functions that need to be delivered as a result of the specific decommissioning activities.During the decommissioning stage, which is a subject of DSAR, the following fundamental safety function shall be ensured (article 32 (2) of Regulation for Providing the Safety of Nuclear Power Plants):
„Confinement of the radioactive products within the prescribed boundaries”.
In Section 5 of DSAR, the safety functions, specific for the different phases of Stage 1 of the decommissioning, are discussed in detail. According to the Regulation for Providing the Safety of Nuclear Power Plants and to the IAEA standard NS-R-1, the systems which remain in operation during both phases - preparation for safe enclosure and safe enclosure - of Units 1 and 2 ensure the fulfilment of the following safety functions:
Since the nuclear fuel is removed from the safe enclosure area and is transported to the spent fuel storage at the beginning of Stage 1, all safety functions related to control of reactivity, maintaining the reactor in a safe sub-critical state and decay heat removal are not relevant to the process of decommissioning.
The importance of a given safety function with respect to ensuring of safety defines the safety class of this function. The higher the class of a safety function is, the higher are the requirements to the SSCs that support it. The class of a safety function helps to define the level and type of testing, checks and maintenance of the respective SSCs.
The class of the safety function of a given SSC has an impact on the assumptions with respect to this SSC in the safety assessment.
5.2.2 Classification of SSCs
The classification of SSCs is presented in details in Chapter 9 of the Decommissioning plan for units 1 and 2 . All existing systems are classified there in three categories:
After the completion of the preparation stage the systems ensuring nuclear safety are not needed, therefore their category changes from I to III.
The use of the existing systems during the SE phase, as well as the use of the systems for the dismantling phase during and after the SE, was evaluated very prudently. This was done by answering to the following questions on all the systems:
A differentiation is made between categories I and II, since the requirements for the systems are different with respect to their functions. The systems in category III can be dismantled (or isolated, or conserved) after separation from the remaining systems.
Based on the above-mentioned questions, a second classification is adopted, where the systems from the three groups (I, II and III) were classified according to four additional criteria:
6 The safety assessment approach
A graded approach has been used for the safety assessment. According to this approach, the scope and the level of the detail in the analysis are relevant to type of the hazards.
6.1 The safety objectives for decommissioning
The nuclear safety requirements are not a subject of this document, because the stage after the removal of all the nuclear fuel is treated and the requirements are not applicable.
The requirements for the industrial safety of the personnel do not change in connection with the decommissioning and these are also not treated in details in DSAR.
The radiation safety is the subject of Chapter 10 “Radiation Protection” under normal operational conditions and of Chapter 6 “Safety Analyses” in case of deviation from the normal operation and in case of accidents.
A description of the existing Fire Protection system is presented in Chapter 4 “Design Basis of the Decommissioning” and in Chapter 5 “Description and justification of processes and systems”.
6.2 Safety requirements and criteria
The safety of nuclear power plants (NPP) is defined as a capability the impact of the plant to the population and the environment to be maintained within the frame of the established limits. This capability is based on sequentially positioned physical barriers that avoid the discharge of radioactive materials as well as on a great number of means that protect these barriers from damages.
Safety requirements for NPP are a set of plant specific indicators and respective acceptance criteria, allowing for an objective safety assessment of the plant.
Safety assessment of NPP is ananalysis of all the available data by the use of modern methods, aiming to confirm that the respective safety requirements have been met.
The main safety requirements and criteria during the considered decommissioning stage are related to the radiation protection.
The objective of the safety analysis is to evaluate the radiological impact of the planned activities and potential accidents on the workers, public and environment.
The objective of the safety analysis is:
The hazards for the population, workers and environment, related to decommissioning, are:
Combined and additive effects of these hazards are considered to the extent to which they could give rise to radiological consequences to workers, public and environment.
The safety criteria, applied for all decommissioning activities, are derived from the regulatory documents in force in Bulgaria. They define the limits for:
The acceptance criteria for the radiological consequences are defined for normal operation and for anticipated operational occurrences.
The annual effective dose per an individual of the public, resulting from the discharge of liquid and gaseous material into the environment for all operational states and for all nuclear facility at the site shall be lower than 0.15 mSv.
The main objective of the safety analysis is to demonstrate that the acceptance criteria are met for all credible initiating events.
6.3 Methodology for the analysis
The methodology, which is applied for the analysis of the IEs includes the following steps:
The goal of the process of hazards identification is to identify all locations within the facility where radioactive material exists (intentional and inadvertent accumulations of radioactive materials and radioactive waste, surface contamination, contaminated ground, sources, activated components and ventilation system filters). The identification process considers also the potential new sources of radiological exposure of the workers as a result of the planned decommissioning activities:
The radiological consequences for the workers and for the public will be calculated for the identified bounding scenarios.
7 Regulatory frame and other referred documents
The DSAR document is developed based on the following Bulgarian regulations: