«Item 7b Severe Accidents Related Issues Preliminary Monitoring Report Report to the Federal Ministry of Agriculture, Forestry, Environment and Water ...»
The calculation tools used for SA analysis are similar to those used worldwide for the purpose of SAM and the team that has been responsible for calculations is competent. The existing analyses provide a reasonable basis for understanding plant specific vulnerabilities to severe accidents and the identification of AM strategies. Some of the existing analyses are old and do not necessarily reflect the current plant status and state-of-the-art in the area of SA codes, modelling and simulation. The plant is planning to improve these analyses using more current codes.
The PSA study for Temelín NPP includes Level 1 and 2. An IAEA mission has reviewed the first version of PSA and the resulting recommendations are reported be incorporated into the upgraded study. However, the upgraded PSA is still not finalized. Generally, the PSA study was developed in compliance with the current state-of-the-art. PN7 team has observed some deficiencies, but they are not expected to have significant impact on the final conclusions with regard to SAM strategies. The existing results have been used in the development of SAMG strategies and setting up priorities in the execution of strategies.
ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 131 Westinghouse has developed a plant specific SAMG package in close co-operation with plant staff. The contents, structure, and format of plant specific SAMG, which were shown at the Prague Workshop, have been found to reflect the current state-of-the-art practice. This package is currently under internal review and translation into Czech language.
Organizational arrangements related to SAMG have not been finalized yet. Although the upgraded ERP Emergency Response Plan has been submitted to SUJB for approval, the updated version of the Emergency Operating Procedures including transition points to SAMGs need to be developed and implemented. Some concerns can be raised in the definition of responsibilities/authorities for determination and approval of an intentional release of radioactive material during a SA. The staffing of SAMG Evaluation Group within the Technical Support Centre is another issue that is not clear enough. The monitoring process by the Austrian Government should cover these aspects.
The plant properly considers all further steps of SAMG implementation including validation and training and plans for their execution are being developed. Based on the available knowledge all the related plant arrangements are considered adequate. Little is known also about the training and refreshing courses of SAM staff and the related schedules for implementation. However, the related activities should be subjected to monitoring by Austrian Government.
It should be noted that proper evaluation of the SAMG package including the supporting analyses would require detailed investigations that involve specialized expertise and considerable effort. Such evaluation was beyond the scope of PN7 project. Therefore, it would be very desirable to have detailed aspects of SAM development and implementation addressed by qualified independent external reviewers (e.g. IAEA RAMP mission). It is known that the plant management and SUJB seriously consider having independent review of SAM.
6.1.3 Technical measures available in Temelín for SA management.
One of the main areas of hazards due to severe accidents is that of primary to secondary circuit leakages, since such leakages involve loss of coolant accidents with the leak point situated outside the containment. In case of such accident all four barriers preventing radioactivity release to the environment can be lost simultaneously. Both contemporary regulatory guidance and industrial practice stress the necessity to avoid large PRISE events. In Temelín the hazards involved in primary to secondary leakage (PRISE) accidents are well recognized, the appropriate strategies developed, and the technical means provided to cope with PRISE events.
Another potential hazard is connected with long term complete loss of electric power, both from outside sources and from emergency diesel generators installed at the NPP. In such a case the means of heat removal from the reactor are lost, except for gradual evaporation of water, first in the secondary, then in the primary coolant circuit. If this situation persists for several hours the coolant in the core will evaporate, which leads to the core dry out, and damage.
The preventive measures at Temelín NPP correctly address the issue of station blackout.
The most important measure for mitigation of the effects of blackout and other transients involving loss of electric power consists in forced depressurization of the primary circuit. The calculations showed that the capacity for depressurization in Temelín is well comparable with that in other plants of similar vintage. Moreover, the measures taken to prevent a blackout seem to be satisfactory.
The measures available in the plant are sufficient for timely depressurization of RCS. The Temelín NPP has two lines of defence in this respect (PORV and EGRS), which is better than in many other NPPs of similar vintage. The WOG SAM strategies being implemented in the plant recognize the importance of depressurization. However, while the capacity of PORV is fully sufficient for plant depressurization, the efficiency of the EGRS is just at the limit. If PORV should fail, the question of exact evaluation of EGRS efficiency would be important.
132 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues In view of the long delays of core damage in case of blackout, the limited capacity of batteries in Temelín seems to be inappropriate. According to the design the period of time that the batteries are sufficient for plant control is shorter than the time that would pass before severe damage of the core. Thus the potential advantages of good thermal hydraulic properties of Temelín could not be used due to battery limitations. Temelín EOPs and SAM strategies include measures to extend battery power supply time by re-structuring the load profile much beyond the design period of 1 hour. Nevertheless, it would be desirable to exchange batteries or include into the system additional power sources providing electric power during station blackout.
An important safety advantage of Temelín NPP is the fact that it is provided with a large dry containment. This reduces considerably the challenges to containment integrity during severe accidents. Similarly as in other NPPs with large dry containment, the hazards of early containment failure due to DCH in Temelín NPP have been evaluated as negligible and the strategy of RCS depressurization included in SAM in Temelín further reduces such hazards.
The long-term pressurization hazards are reduced by the fact that the basemat concrete in place in Temelín does practically not contain any carbon, so there is no build-up of carbon monoxide and carbon dioxide due to molten corium-concrete interaction. This reduces the long-term quantities of non-condensable gases inside the containment. The calculations with the MELCOR code showed that the containment integrity is not threatened by long term increases of pressure due to gas generation and the presence of the containment spray system.
The monitoring orients itself on approaches used predominantly in some Western European countries. In the case of WOG SAMGs the basic work accomplished orients itself on the USNRC position as introduced at the Temelín NPP.
During the monitoring according to the Melk Process no clarification could be found which rules and regulations were applied to consistently address the severe accidents related hydrogen issue at Temelín NPP.
Hydrogen hazards in NPPs with large dry containment are considered to be negligible by US NRC and some regulatory bodies in EU countries, but many EU regulatory bodies require technical means for hydrogen depletion. In Temelín the release rates of hydrogen during the in-vessel phase of the accident are comparable with those in PWRs, and the volume of the containment is similar. The geometry of the steam generator boxes and the ducts there is different from that in PWRs and makes hydrogen mixing less effective, which in case of SB LOCA in this area can lead to local formation of sensitive clouds of hydrogen during the invessel accident phase.
The likelihood of such an event is very low for Temelín (3×10-7 [1/a] for Temelín for small LOCA sequences; 1×10-7 [1/a] for medium LOCA; and 3×10-8 [1/a] for large LOCA), in total about 3% of the core damage frequency. In the ex-vessel phase the presence of a large dry containment and early inerting of the containment by steam contribute to prevention of hydrogen hazards. In the long term the installed hydrogen recombination system designed for DBA conditions, but passively operating also under severe conditions, will contribute to containment inerting by reducing the hydrogen and oxygen content. However, this process is slow and for severe accidents it would be advantageous to have properly located PARs of higher capacity.
In any case, the distribution analyses of hydrogen and steam inside the containment system should be performed with a more useful modelling concept taking into account also the operation of the spray system to evaluate more realistically the periods where inerting by steam may exist. Combustion should not be coupled to the distribution analysis without good reason (e.g., the activation of a reliable ignition source when small hydrogen concentrations are present).
The Czech strategy consists of early hydrogen deflagration, which should help prevent formation of sensitive clouds during in-vessel phase, and long-term inerting of containment with steam during the ex-vessel phase. Both Czech and PN7 calculations showed that in the case ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 133 of unplanned actuation of the containment spray system at the moment when the contents of hydrogen is the highest the containment integrity could be lost, and Czech materials provide an evaluation of radiological consequences of such a scenario. However, the SAM strategy proposed for Temelín correctly addresses the issue of reduction of the hazards of late confinement failure due to hydrogen deflagration. In the case of ultimate necessity, Temelín can actuate the containment filtered venting system (normally used during pressure tests) to reduce containment pressure and hydrogen content. This issue seems to be still under development. As the heating due to fission product collection in filters can result in rising filter temperatures (with loss of filter efficiency) or in the worst case induce filter burning, the issues of filtered venting in Temelín should be further monitored.
The main hazard consists in the possibility of containment basemat penetration.
The measures planned be implemented in Temelín in case of RPV failure assure slowing down of the molten corium concrete interaction (MCCI) process. While these measures go in the right direction, it cannot be proved that they assure protection of the basemat against penetration by molten corium if RPV failure occurs. The likelihood of RPV failure is small, as shown by recent analysis, but it exists. According to the statements of Czech specialists, the measures planned in Temelín include corium spreading and water-cooling, which together should enable to stop the corium progression.
The calculations performed within PN7 project confirmed that corium spreading slows down the process and provides additional time margins. The effectiveness of water-cooling was not studied in PN7 due to the lack of access to the latest experimental OECD data. Recent information about the results of large scale tests on concrete penetration by molten corium conducted within OECD programme on “The Melt Coolability and Concrete Interaction“ indicates that in large scale test in the US enhanced cooling was obtained due to long term water cooling of the molten corium mass. The Czech Republic participates actively in this programme and has the actual information available.
As of now, the stopping of the corium ablation progress cannot be clearly demonstrated. Therefore, the Temelín staff considers additional measures aimed at improving leak tightness of rooms below the containment basemat. The hazards due to radioactive releases in case of basemat melt-through are much smaller than in the case of an early containment rupture. As shown in the TACIS programme, the mass of radioactive aerosols still suspended in the containment atmosphere at the time of basemat melt through is dramatically smaller than the mass released from the core to the containment and available for release in case of an early containment failure. Not considering re-volatilisation and emanation of deposited contaminants during late containment failure, the radiological hazards are almost correspondingly reduced.
The strategy under consideration of Temelín NPP includes prevention of re-volatilization of the fission products that have already been deposited on containment and piping surfaces, which could result in case of violent air turbulence. This includes prevention of massive hydrogen deflagration and detonation after basemat melt-through and mixing of hydrogen in the containment with the air from outside atmosphere. Moreover, it is planned to reduce the pressure in the containment before the basemat melt-through to avoid sudden air expulsion from the containment to the environment and carry-over of re-volatilized fission products. If the pressure in the containment is lowered in time, it will also be possible to minimize leakages through the rooms below the containment, between the basemat and the environment.
During the Prague meeting Czech specialists mentioned these issues, but no detailed information was obtained on the approach being followed.
The measures and strategies to reduce fission product releases are in keeping with the international practice. The open issues are mostly connected with the reduction of radiological releases in the case of basemat penetration by molten corium. Czech specialists consider it a problem for future consideration, while they see as the most urgent tasks those, which are related to prevention of the basemat melt-through.
134 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues
6.2 Recommendations for Further Monitoring