ELEMENT OF DID CONCEPT STRENGTHENING
3. BDBAMG DEVELOPMENT – AS “STRESS TEST” FOR UNIT DESIGN
3.1. Preparation for development (connection with Levels 1 and 2)
The effectiveness of personnel activity per guidance is formed during the phase of preparation for its direct development. Preparation contains two main components:
(1) evaluation of safety substantiation calculations fulfilled, including PSA;
(2) investigation and consideration of NPP Unit capabilities and the site of its location.
3.1.1. Evaluation of safety substantiation calculations presented in SAR and PSA 3.1.1.1. Evaluation of representative BDBA scenarios selection
a. The substantiation of representative BDBA scenarios selection for AM measures analysis and definition and development of plans arrangements for personnel and population protection is attained by postulating of NPP unit levels of severity state.
The possible combinations of protective barriers states, each of which identifies the corresponding unit’s levels of severity state, are generated with the help of combinatorial analysis. The physical realizability of each barrier damage rate is identified with revealing a mechanism of such damage, as well as reasons which may cause it (e.g. extreme IE). The following barriers structure should be considered for NPP units with VVER-1000, which mainly corresponds to those identified in [1]:
• fuel matrix and fuel cladding (combined in term nuclear fuel);
• first circuit boundary (excluding reactor vessel) ;
• reactor vessel;
• hermetic enclosure (for VVER-440) or containment (for VVER-1000).
b. Application of PSA results should be evaluated for vulnerability places identification through minimal cross sections of event/failure tree, as it is necessary for identification of BDBA initiation reasons, it progression and transition to severe phase. The PSA results should be obtained for the complete list of initiating events which are personnel errors or causing NPP components failures, on site EI - fires and flooding, external impacts of nature and anthropogenic character. All EIs should be considered, if they are physically not excluded at the place of unit location. If some scenario is of very low probability or even not considered in PSA, it should not be a reasons for exclusion from BDBAs list. Severe accidents management measures should be developed irrespective for calculated hazards frequency, taking into consideration uncertainties in severe accidents scenarios (it.2.12. [7]).
c. The analysis of selected BDBA scenarios and their consequences should be presented in SAR [5]. Those scenarios should be selected, which provide coverage of all physically possible severe states of a unit and meet following representativeness criteria:
• maximal dose rate for personnel and /or population;
• maximal intensity of radionuclide’s release;
• maximal integral radionuclide’s release;
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• maximal scale of systems and components damage on the plant;
• maximal input in cumulative core damage frequency for given group;
• more quick progression of accident events (minimal float in personnel disposal to undertake accident management measures);
• the worst conditions for personnel and equipment performance.
The evaluation of calculation scenarios selection may result in necessity of its changes.
3.1.1.2. Representative scenarios calculations for strategies and instructions substantiation a. Realistic (best estimate) method. 3-D codes application: BDBAs calculation analysis
for BDBAMG strategies and instructions substantiation (as for any accident instructions) should be performed in frame of so called realistic (best estimate) method which provides AM on the basis of realistic symptoms of unit state and evaluates available resources of existing unit’s systems and equipment serviceability and efficiency as well as personnel capabilities for control of such equipment. Based on calculations results design basis safety margins (e.g. departure from the nucleate boiling ratio criterion) in the frame of which operation with deviations is still possible, are checked. 3-D codes application is necessary when local effects are significant and processes in the core are asymmetrical.
b. Consideration of uncertainties: calculation analysis results should be accompanied by demonstration of uncertainties analysis. Consideration should be given to uncertainties of calculation methods, equipment characteristics, instrumentation sensitivity and other uncertainties, taken into account in the evaluation of the result.
c. Results of calculations and analyses usage: on the basis of calculations and analyses the following is identified, then to be used:
• specific symptoms of levels of severity;
• criteria of transition to severe phase;
• timing of accident scenario progression;
• parameters for identification of success/not success of personnel actions;
• radiation consequences in compartments and on the site.
d. Elaboration of auxiliary calculation means: results of calculations and analyses, if necessary, may be used for elaboration on their basis auxiliary calculation means, which may be used for indirect parameters assessment, in case of unavailability of technical means of measurements, and also for checking the authenticity of obtained information.
3.1.2. Investigation of NPP Unit capabilities and site location 3.1.2.1. Means and methods of information acquisition for AM
The following is to be checked and assessed:
a. Design control and instrumentation devices with respect to their applicability for acquisition of information required for accident management [8], including:
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• capabilities of control room and reserve control room, information calculated system/safety panel (if exist), local control desks, means of measurements in reactor shaft, in containment;
• correspondence of their measurement ranges to AM aims, gauges and cable lines operability in SA conditions at places of their location, the required processing speed;
• the capability of measurement systems to provide minimal required information (parameters) content necessary for tracking unit state signs and identification of accident level of severity, control of AM actions success;
• provision of required technical systems control.
b. Possibilities to compensate for failures of design measurements channels, e.g. by information acquisition through indirect methods, such as: noise, steaming, position of regulatory devices pillar position of locking devices etc. which may defined at places of location, in case radiation conditions permit. The possibility to use non-stationary (hand-held, transportable) measurement means should be considered.
c. Methods for indirect assessment of missing parameters by carrying out operational calculations or by auxiliary calculated means which forecast required parameters value with the help of physical processes mathematical models;
d. Methods of identification and screening of false information coming from measurement channels, which may be damaged in BDBA conditions.
Such investigation may result in recommendations on modernization of measurement channels, e.g. installation of additional gauges or replacement of existing ones.
3.1.2.2. Technological equipment of the unit
In relation to technological equipment (including equipment of SS):
a. Equipment should be revealed, which can be used for AM and also beyond the scope of its design dedication and/or qualification limits, it is check if it will be operable in these conditions and how long, what will be its performance beyond the design range of operation;
b. It should be identified if adverse conditions of environment and external and internal impacts (including mechanical) of severe BDBA will influence the equipment operability;
c. Effect of auxiliary and supporting systems failures should be assessed;
d. Alternative equipment needed for realization of actions in the frames of defined strategy should be identified, as well time required for putting it in operation;
e. Assessment of both the necessity and possibility of main and alternative equipment to operate jointly2 should be performed.
2 The assessment is performed by additional calculations; on results of which it should be defined whether it is necessary to continue recovery of main equipment operability and for what time.
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3.1.2.3. Ensuring electricity supply 3
The reasons of complete electricity supply failure (station “black-out”, i.e. diesel generators are not operable) should be defined and in case of all sources of alternating-current loss the relevant measures of it restoration should be developed. Such means of restoration as delivery of portable generators, increasing of battery energy storage capacity, installation of alternative alternating-current sources, connecting up to external nets, should be studied.
3.1.2.4. Containment4
The strength of containment in BDBA conditions is assessed and real non-tightness of hermetic enclosure. Penetrations, hatches, doors, locks and their embedded fittings as well insulating devices are investigated for the results to be considered in BDBAMG. Special attention should be given to systems for containment depressurization and for algorithms of their actuation (in particular of sprinkler system in presence of hydrogen in containment atmosphere).
Containments of NPPs with VVER reactors are equipped with passive H2 recombiners.
Their sufficiency and places of location in design of unit should be assessed.
As a part of long term accident management, the means for water feeding in containment should be provided with pressure equal maximum permissible containment pressure and with capacity sufficient for residual heat remove [9].
It should be checked the necessity and admissibility of direct discharge of radioactive substances from containment through special filtered venting for severe accidents.
3.1.2.5. Utilization of site and neighbouring unit’s technical means
The possibility to utilize technical means (materials and equipment) from other units should be considered for multiunit site with conditions, that it is not hazardous for their operation safety ensuring. It should be considered if it will be necessary or not shut down neighbouring unit (or units).
3.1.2.6. Conditions of possibility technical means using
The investigation should be carried out in respect to possibility of and accessibility to technical means for personnel actions execution considering conditions which may be in compartment and on the site as a result of initial event or in the course of accident progression (radiation consequences, fire, possible blockages, building construction’s destruction, flooding, steaming).
Such investigation may result in auxiliary measures directed on the risk reduction and improvements of physical accessibility. The necessity of some plant system or areas of work modification (e.g. through addition of protections) should be studied.
3 Electricity supplyprovision is a top-priority task of accident management since without electricity supplyany actions are impossible.
4 The problem of “containment bypass” due to leak from prime circuit to secondary and unclosing of relief valve (BRU-A) is not considered in the section.
123 3.2. Development of strategies and actions (connection with Levels 3 and 5)
3.2.1. Transition to BDBAM actions (connection with Levels 3)
In case of unsuccessful prevention with help safety systems and relevant accident procedures transition of initial events in design basis accidents and design basis accidents to beyond design basis accidents, it is necessary to start with BDBAMG execution (Level 4).
The criteria (parameters, conditions) giving evidence, that Level 3 is surmounted (or there is a hazard of it surmount) should be defined in Level 3 instructions and in BDBAMG. Fact of BDBA beginnings is identifying on noncompliance of appeared symptoms with symptoms of design basis accidents and/or on deviations from design accident progression and systems and equipment behaviour anticipated according instructions for design basis accidents elimination on the unit, or from other instructions for prevention of progressing and for elimination of normal operation violation.
Actions in frames of BDBAMG for safety functions (SF) state analysis may be initiated in parallel with Level 3 instructions execution, e.g. after scram.
3.2.1.1. Immediate actions
The immediate actions should be stipulated after transition to BDBAMG, which relating to: “Control of reactivity and ensuring of reactor sub criticality”, i.e. reactor should be shut down and keeping in sub critical condition. Next on priority is SF “Core cooling (heat removal from reactor)”.
3.2.1.2. Strategy and actions on prevention (of severe accident) phase
Strategy on prevention phase is directed at SF restoration. It should provide identification of state of the unit (levels of accident severity) and correspondent to them SF, based on directly observe or indirectly assessed parameters (symptoms), and also combine all developed actions for recovery and ensuring of SF.
The sequence of actions for SF protection and recovery should be identified in strategy with consideration of this function prioritization. The necessity of simultaneous actions implementation of different SFs should be defined on the basis accidents analysis.
3.2.1.3. Strategies and actions on “mitigation of consequences” stage
The criterion of BDBA transition to severe phase and subsequent transition to strategies on stage of severe accident management should be defined. Strategies may be derived from
‘candidate high level actions’ [7], which should provide protection of physical barriers (body of reactor vessel, steam generator pipes, containment) as well restoration of core cooling to the maximum possible extent or fragments of core (derrises) cooling.
The limited number of parameters to be used for accident management and for identification of format and priorities of diagnostics should be defined on the base of severe accidents investigations.
The following should be envisaged in the development of personnel actions:
• Possible negative consequences of actions and also cliff edge effects;
• Limitations for implementation of actions coursed by doze, physical, psycho- emotional loads on personnel and those who are involved in AM;
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• Actions, resulting from insufficient5 number of personnel to carry management actions.
3.2.2. Radiation situation and start of emergency planning of personnel and population protection (connection with Level 5)
The procedure ordering introduction of Emergency planning of personnel and population protection should be specified in BDBAMG in case of BDBA occurrence (it. 4.11.
[4]), i.e. transition on Level 5. Numerical values of effective dose rates and (or) I131 volumetric activity in compartments air, on NPP’s site, in sanitary protective zone and zone of radiation tracking, which corresponds to conditions of “Emergency preparedness” and/or
“Emergency situation” [10] are criteria for the introduction.
3.2.3. Organizational structure of accident management.
3.2.3.1. Administration and NPP operations staff
Analysis of the existing on NPP organizational structure for normal operation should be carried out for its maximal application in BDBAM implementation. Allocation of rights, duties and responsibilities of individuals from operations staff should be defined for participation in BDBAM. The procedure should be developed to provide continuation of accident management in case if a new shift of NPP personnel and involved persons are not able to come on the NPP, in particular in long term of accident progression, which should identify the method of shift exchange.
3.2.3.2. The means of communication and warning The following is checked:
• sufficiency of existing communication means, including duplication ones for organization of NPP management and systems of warning in conditions of normal operation, design basis and beyond design basis accidents (it. 1.2.23 [3]);
• means for communication with external and internal accident centers for NPP management in conditions of BDBAs for assessment of situation and decision making (it. 2.4.28.[4]).
3.2.3.3. Involvement of external organizations for BDBAM and elimination of consequences Tasks distribution between personnel and involved organizations defined in BDBAMG for realization of measures directed at accident consequences mitigation, chain fission reaction termination and nuclear fuel cooling in reactor. In particular, NPP administration cooperation with Team for emergency assistance to nuclear plants (OPAS) is defined by requirements in Annex 20 (obligatory) to [10].
The issues of personnel and public protection in case of accidents at nuclear plants are settled in Russia in the frames of acting Unified National System for Prevention and Mitigation of Emergencies, which, in details described in [11]. Overview of these issues is not the subject of this paper.
5 As a result of injury, overexposure, death, desertion, etc.
125 3.3. Checking (verification6) of applicability: use of simulator
The final stage of BDBAMG development is verification of its applicability to confirm that BDBAMG is technically correct and provides proper consideration of human factor.
3.3.1. Verification tasks
Verification means resolving of two main tasks:
• to check usability, that means from one hand sufficient specification level, and from another simplicity of understanding of instructions and other provisions in Guidance;
• to check correctness (precision) of instructions and provisions in Guidance, which should confirm it compatibility with technical means of the particular unit and also compatibility with personnel capabilities.
3.3.2. Usage of full-scale simulator
Different methods and means may be used for BDBAMG checking. Method of modelling on full-scale simulator (method MFSS) is mostly preferable from them due to that MFSS is based on software enabling modelling initial state of the unit and also accident progression, including relevant personnel actions [12].
However, it should be considered, that in SA phase there are some challenges for MFSS applications, connected with limitations and uncertainties in severe accidents phenomenology knowledge, as well with the level of software verification in relation to some separate effects (e.g. steam explosion possibility, repeated criticality, hydrogen generation and distribution, including possibility of it detonation or deflagration, thermo shock). Moreover, the software of simulator should be able to model severe accident phase progression and NPP response in scale of real time mode. This may result in full-scale simulator software reprocessing.
Verification results may have feedback with BDBAMG, resulting in correspondent changes in it. For instance, if cliff-edge effects possibilities had not been investigated during preparatory phase, additional sensitivity analyses may be an input for this investigation.
Possibilities of cliff-edge effects should be investigated as a result of possible degradation of equipment performance and of alternative mitigation strategies for a common plant symptom and containment challenge [13]. Supplemental analyses should also include a small set of cases examining damage plant states resulting from low frequency, high-consequence scenarios.
4. PERSONNEL TRAINING TO BDBAMG ACTIONS (CONNECTION WITH LEVEL 1)