«Item 7b Severe Accidents Related Issues Preliminary Monitoring Report Report to the Federal Ministry of Agriculture, Forestry, Environment and Water ...»
3.3.2 Preventive Maintenance and Surveillance of AM-Related Equipment VLI No. VLI title / description 4.2.1 Is there a documented plant programme for preventive maintenance and surveillance?
Is the implementation of this programme properly supported by plant procedures?
4.2.2 Is the extent and frequency of preventive maintenance optimized with consideration given to safety significance of the equipment (risk informed approach)?
4.2.3 Are the provisions for minimizing human errors during maintenance defined?
4.3.4 Is all the relevant equipment subject to regular functional testing?
What are the schedules for testing and/or inspections?
4.3.5 Is the inspectability of safety equipment adequate?
State-of-the-art requirements and practices Appropriate preventive maintenance and surveillance of safety-related equipment is one of the relevant aspects that have impact on the performance of equipment during accident conditions. The assessment of plant status with regard to this aspect is beyond the scope of this project. Therefoe, only some general aspects that are relatively easy to verify are addressed in the related VLIs.
The existence of a documented plant programme for preventive maintenance and surveillance at the plant is essential [IAEA 03]. This programme should be properly supported by plant procedures. It is considered a good practice where the extent and frequency of preventive maintenance have been optimized with consideration given to safety significance of the equipment (risk informed approach). Provisions for minimizing human errors during maintenance should be defined and addressed in the related procedures.
All the relevant safety equipment should be subject to regular functional testing. The schedules for testing and/or inspections of this equipment should be comparable to the practices of nuclear industry elsewhere. Adequate inspectability of safety equipment is a relevant issue related to this aspect. It should be noted that not all equipment that is credited in AM is classified as safety or safety-related and subject to regular testing/inspection.
Current plant status General provisions for preventive maintenance and surveillance of safety-related equipment are similar to those implemented in other NPPs in Europe. It is known that Czech utilities are interested in the application of risk informed strategy to optimize the preventive maintenance and testing of safety related equipment. No information is available regarding plant specific situation in this regard.
Evaluation There is not enough information available on preventive maintenance and surveillance of accident management-related equipment upon which to base an evaluation. The plant licensing processcovers preventive maintenance and surveillance of safety-related equipment, however it is recognized that accident management-related equipment includes additional components and systems beyond those designated as ’safety-related‘.
ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 59 The Pn7 Specialist Workshop focused more on gaining an understanding of the Temelín accident management strategies (and the underlying accident progression analyses), and obtaining information about plant modifications to support those strategies (modifications which were first identified at the Specialist Workshop). Preventive maintenance and surveillance of accident management-related equipment could therefore be recommended to be a subject upon which to receive a presentation from the appropriate Czech experts at one of the next Czech-Austria nuclear issues bilateral meetings.
3.3.3 Equipment Performance in Severe Accident Conditions
VLI No. VLI title / description 4.3.1 Have the environmental conditions under severe accidents been determined and documented?
4.3.2 Has the equipment performance under accident conditions been evaluated and verified?
Has this process been conducted also for non-safety equipment which is considered in SAMGs?
4.3.3 Have ‘as-built’ characteristics of systems/equipment been determined and documented?
4.3.4 Is the development/implementation of SAMP (e.g. determination of limits, operator training, etc.) based on ‘as built’ characteristics?
4.3.5 Have feasible design changes of equipment been considered / implemented?
4.3.6 Have needs for special equipment to mitigate severe accidents (e.g. venting, H2 recombiners, corium catcher, etc.) been evaluated?
State-of-the-art requirements and practices Severe accident may create conditions that might be adverse for the usability of equipment and systems. Additionally, during the implementation of some of the strategies for returning the core or containment to a controlled, stable state, a side effect of these actions may be the creation of undesirable environments for equipment and systems. Maintaining equipment availability refers to both equipment presently in-service and equipment, which is not being used. During a severe accident, the various pieces of equipment and/or systems, which are not presently in use or required, may be useful at a later time during a severe accident. To maximise the flexibility in use of plant systems and equipment, several conditions should be
Equipment must be able to survive the environmental conditions, Survivability of the equipment must be not be challenged by water, Power supply for the equipment must be maintained, and Capability to repair and maintain equipment should be ensured.
Submergence in water is a special severe accident concern that must be considered with respect to equipment survivability. In severe accident management, the possibility exists for containment water levels well above the design basis or flooding the lower levels of the auxiliary building as a result of the accident or as a result of severe accident management strategies. Another important factor in equipment availability is the power supply, particularly control power under degraded DC power conditions.
The capability to repair and maintain equipment following the onset of a severe accident is important from several aspects. When arriving at a severe accident condition, it is quite likely that some of the plant equipment is not operable. During a severe accident, the potential exists for malfunctions in equipment, which is being used during the recovery. Since equipment may be used in non-standard ways for severe accident response, local access to areas may be required for alignment of valves and/or equipment maintenance.
60 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues The habitability of certain plant areas may be compromised either due to the progression of severe accident or actions taken to recover from the severe accident conditions. This may result in conditions (particularly radiation levels) in which some equipment cannot be aligned, maintained or repaired. Severe accident management decisions should take into account the habitability of plant areas in which alignment, maintenance or repair of equipment would allow for the recovery capabilities.
Information needs to be gathered on the requirements for the equipment to perform as necessary under accident conditions. These requirements are established by analysing the plant response to beyond design basis accident and severe accidents. Comparison of the expected environmental conditions and the equipment capabilities provides basis for the assessment of the equipment’s capabilities to perform as required for success of individual strategies. Where possible, estimates of the operability margin of equipment beyond its design basis should be made and factored into the evaluation.
It is important that the assessment of equipment survivability is conducted based on ‘as-built’ characteristics of systems/equipment. This aspect is also relevant in relation to training of the staff involved in SAM. If available equipment is not able to prevent severe accident from occurring or to mitigate their consequences (either not capable of doing the required job, or not ‘fit’ to survive accident environment), additional equipment needs to be specified that would perform as required.
Conclusions from the assessment of equipment survivability may also lead to design changes that involve replacement of certain equipment with new one or to provide special equipment to mitigate severe accidents. There is no single approach to addressing the need for new equipment for AM. In general, whilst the implementation of an AMP may generate the requirements for limited upgrade, the requirements for major equipment changes will not be generated here. PSA (Level 1) offers a mean to decide on the need for equipment upgrades.
Current plant status Systematic review of plant capabilities with regard to SAM has been performed. The Czech experts have not identified any need for additional hardware in relation to the implementation of SAMGs [Sỷkora 03 a].
Capabilities of equipment considered in SAMGs have been evaluated. With regard to the survivability of equipment during severe accidents, the Czech side reported no specific problems. It should be noted that the safety related equipment currently installed at Temelín plant is qualified for harsh environmental conditions (pressures, temperatures and radiation environment consistent with the accident analysis for double ended guillotine break of the main RCS piping). These conditions are comparable to those resulting from accident involving extensive core melt in-vessel. The available SA analyses seem to confirm that.
Several minor plant design modifications have been considered in relation to SAM (as briefly described in Section 3.1.2). Preparations for implementation of these modifications are underway [Sỷkora 03 b].
Evaluation Based on the available information the equipment survivability aspect appears to have been addressed properly and in accordance with the current state-of-the-art. However, detailed evaluation of this aspect was not possible. This issue should be addressed within a specialized independent review (e.g. IAEA RAMP mission).
ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 61 Future actions by the operator/regulator, which are recommended on the technical level to be monitored jointly in the framework of the pertinent bilateral Agreement between Austria and
the Czech Republic:
• The Specialist’s Team would recommend the Austrian Governement the consideration to verify that an independent external review (e.g. IAEA RAMP mission) is performed for the area of SAMG development and implementation. Such review would also assess all aspects covered in Sections 3.2 – 3.6.
3.3.4 Means for Monitoring of Plant Status under Severe Accident Conditions
VLI No. VLI title / description 4.4.1 Has systematic analysis regarding the performance of instrumentation beyond the operational range been performed?
4.4.2 Has systematic analysis regarding the successful diversity of monitored plant physical variables been performed (e.g. identification of all available means for RCS pressure monitoring - RCS wide range, SI pump discharge pressure, RHR discharge pressure, etc.)?
4.4.3 Have any specific needs for extending the range of indications by instruments been identified and changes implemented?
4.4.4 Are there any recognised weaknesses regarding the protection of instruments against damage in severe accident conditions?
Have any enhancement measures been implemented?
4.4.5 Is all the important information needed for transition from EOP to SAMP available in the MCR and in the on-site emergency centre (TSC)?
4.4.6 What is the functionality of instrumentation in station blackout conditions?
4.4.7 Is the separation of normal and emergency instrumentation / monitoring adequate?
4.4.8 Is the format of information, which is being provided to AM team, user friendly?
4.4.9 Was any instrumentation dedicated especially to SAM additionally installed?
4.4.10 Was any of computational aid (to compensate for insufficient instrumentation) developed?
State-of-the-art requirements and practices Means for monitoring of plant status under accident conditions are required to perform the diagnosis of the severe accident progression and to select appropriate AM strategies. Usability of the existing instrumentation during a severe accident may be affected. Either the instrumentation may not survive under environmental conditions during a severe accident or the design operational range of measurements may appear to be inadequate. Severe accident may create conditions (temperature, pressure, radiation level, humidity, etc.), which might be adverse for the usability of instrumentation. Additionally, undesirable environments for instrumentation may develop during the implementation of some of the AM strategies as a side effect of these actions. Several factors determine the usability of instrumentation to
monitor and forecast the progression of a severe accident, including:
Environmental conditions, Presence of water, and Availability of AC and DC electrical power.
While instrumentation may survive under environmental conditions well beyond their design basis, it should be acknowledged that their survivability might become questionable under some severe accident conditions.
62 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues The evaluation of plant specific status relating to these aspects should concentrate on demonstrating the potential availability of and the limitations associated with all equipment and instrumentation that can be involved in the implementation of the strategies covered by the SAMG package. Relevant aspects include the identification of all alternative equipment and instrumentation, likelihood that they are available during a severe accident, clear identification of associated limitations in the guidelines, and guidance on prioritization of their usage.
Important aspect is the assessment of adequacy of instrumentation ranges.