«Item 7b Severe Accidents Related Issues Preliminary Monitoring Report Report to the Federal Ministry of Agriculture, Forestry, Environment and Water ...»
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. While not comprehensive, summary presentations of calculations carried out by the Czech experts as well as calculations performed within the PN7 project indicate that the capability for depressurization in Temelín is comparable with that in other plants of similar vintage and is sufficient for timely depressurization of RCS. The Temelín NPP has two lines of defence in this respect (the primary relief valve, PORV, and the emergency gas removal system, EGRS).
The WOG SAM strategies being implemented in the plant recognize the importance of depressurization and the EGRS, although of limited capacity, can serve as an additional means of depressurization in the unlikely case of a severe accident with PORV failure. Moreover, the measures taken to prevent a blackout seem to be satisfactory.
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 direct containment heating (DCH) in Temelín NPP have been evaluated as negligible and the strategy of reactor coolant system (RCS) depressurization included in SAM in Temelín further reduces such hazards. The long-term pressurization hazETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 9 ards are reduced by the fact that the basemat concrete in place in Temelín practically does not contain any carbon, so there is no buildup of carbon monoxide and carbon dioxide due to molten corium-concrete interaction. This reduces the long term quantities of noncondensible 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.
Rather, the calculations show that basemat failure occurs long before overpressure failure becomes an issue.
Hydrogen hazards in NPPs with large dry containment are considered to be unimportant by US NRC and some regulatory bodies in EU countries, but most 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 in Temelion NPP is different from that in PWRs and makes hydrogen mixing less effective, which in case of small break loss-of-coolant accident (SB LOCA) can lead to local formation of sensitive clouds of hydrogen during the in-vessel accident phase. The mean frequency of the accident scenario is about 1,7×10-7 per year (given the accident sequence frequency, a high likelihood of ignition, and a 50% chance of a detonation given ignition; the assumption is conservatively made that the detonation leads directly to containment failure with a large source term). Even with an arbitrarily large source term, the mean consequence would be of the order of 50 000 person-Sv (calculated over a year's worth of weather conditions). The
product of the mean severe accident frequency and the mean consequence would approximately represent the overall risk to the public per year of operation (note that this is a conservative calculation which assumes a 50% chance of a detonation leading to a large containment leak):
(1,7×10-7 1 [1/a]) × (50 000 person-Sv) = 8,5×10-3 person-Sv/a 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.
The Czech strategy consists of:
a) early intentional hydrogen deflagration (through planned actuation of equipment to try to initiate a deflagration), which should help reduce formation of sensitive clouds during invessel phase;
b) reliance on the hydrogen recombiners (PARs) to gradually reduce the hydrogen source in the containment;
c) long term inerting of containment with steam during the ex-vessel phase with procedural controls on sprary actuation to prevent de-inerting burns; and
d) as necesary, venting the containment through a high pressure venting line through filters to the plant stack to release hydrogen from the containment.
Both Czech and PN7 calculations showed that in the case 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 addresses the issue of reduction of the hazards of late confinement failure due to hydrogen deflagration in line with the Westinghouse SAMG approach. In the case of ultimate necessity, Temelín can actuate as an option the containment pressure test filtered venting system to reduce containment pressure or 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 10 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 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 severe accident hazard consists in the possibility of containment basemat penetration.
The measures planned to be implemented in Temelín in case of RPV failure at low pressure 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 reactor pressure vessel (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 with the planned remote opening of the cavity door 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. Other experimental studies in Germany in this matter indicate some limitations for the reduction of the core melt attack by top cooling of released core melt with water. The Czech Republic participates actively in some programmes and has the actual information available on the OECD MCCI program.
As of now, the stopping of the corium erosion progress cannot be clearly demonstrated.
Therefore, the Temelín staff considers additional measures aimed at improving leaktightness 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 elsewhere, for releases due to basemat failure, the mass of radioactive aerosols still suspended in the containment atmosphere is dramatically reduced (orders of magnitude) compared with early containment failure. Not considering re-volatilisation and emanation of deposited contaminants during late containment failure, the offsite radiological hazards are correspondingly reduced.
During the Prague meeting, in response to questions the Czech specialists discussed the environment in the reactor building after melt-through of the containment basemat. The Czech experts discussed an evolving strategy of attempting to prevent re-volatilization of fission products that have already been deposited on surfaces in the containment; this could result from violent air turbulence in case the containment would depressurize when the basemat melts through. The evolving strategy also includes prevention of hydrogen combustion in the reactor building after basemat melt-through. The reason for this is to preserve reactor building integrity to allow both for natural aerosol attenuation mechanisms to lower the source term, and to allow the release of fractions of the gas content from the reactor building via the plant stack (via the reactor building ventilation system) to achieve greater dispersion and lower radiation doses offsite.
The strategy would involve depressurization of the containment – before basemat melt-through – via the venting system (high pressure duct work to the plant stack). This would also reduce the hydrogen concentration in the containment. 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.
ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 11
V. Recommendations for Further Monitoring
The monitoring process conducted so far within the framework of the “Brussels agreement” (Annex I) in the area of severe accidents helped to clarify a number of relevant issues. It was demonstrated that a comprehensive process directed towards accomplishing the comprehensive SAM and mitigation of SA consequences is in place at Temelín NPP. However, this process is still ongoing and the Specialist’s Team at the present can only follow a number of views and expectations on the SAMs final implementation as expressed by the Czech side.
The Specialist’s Team would recommend the Austrian Governement the consideration of revisiting the findings in the framework of the pertinent bilateral Agreement between Austria and the Czech Republic.
The following areas were identified as of interest:
• The supporting severe accident analysis and PSA as well as their use in the verification of SAM strategies and the related procedures,
• SAMG implementation activities including procedural framework, SAMG validation, and SAM related staff training,
• Identification of the permissible degree of non-uniformities in the hydrogen distribution in the atmosphere
• Implementation of plant changes to enhance the technical measures for SAM.
More detailed discussion of the proposed monitoring issues in these areas is provided below.
The Specialist’s Team would recommend the Austrian Governement the consideration of revisiting the calculations to be made by Temelín NPP using MELCOR 1.8.5 and other code
systems, and consider to obtain more detailed and verified information on:
• The capabilities of PORV, together with the effectiveness of the planned coolant system depressurization procedure,
• The regulatory framework for and effectiveness of hydrogen control, and/or additional use of filtered venting for mitigation of radioactive releases,
• Operational capabilities of the emergency gas removal system in SA conditions,
• Analyses of basemat meltthrough failure.
The Specialist’s Team would also recommend the Austrian Governement the consideration of revisiting the SAMG implementation activities at Temelín in order to confirm that the remaining steps of the implementation process are successfully completed. Important items that need further monitoring/verification include the revised procedural framework, SAMG validation, and staff training process. At the same time the recommendations from any independent review of SAM and their resolution should be paid due attention by the Austrian Government.
Technical measures needed for prevention and mitigation of risk significant scenarios should be monitored to demonstrate that appropriate plant arrangements are in place (both procedures and hardware measures). Due attention should be given to SA situations that are most relevant from safety point of view such as basemat penetration in case of molten corium release from the RPV and station blackout. Aspects, which are worth to be mentioned in this context include the measures for timely opening of the reactor cavity door before the RPV failure, protection of containment penetrations and the containment liner against MCCI, and increasing the capacity of batteries. Further analytical work conducted by the plant and the TSO staff on the MCCI hazards and mitigation of the related radiological consequences should also be monitored.