«Item 7b Severe Accidents Related Issues Preliminary Monitoring Report Report to the Federal Ministry of Agriculture, Forestry, Environment and Water ...»
Later severe accident progression calculations for the in-vessel phase of the SB LOCA accident performed with the SCDAP-RELAP mechanistic code package, performed after the GASFLOW calculations were completed, indicate a similar total quantity of hydrogen produced for the small LOCA sequence but in a shorter time frame. This gave rise to a higher release rate, exceeding 0,75 kg/s for periods of a few hundred seconds. A sensitivity calculation performed with GASFLOW for a synthetic hydrogen source of 0,8 kg/s injected into the steam generator box in a WWER 1000 configuration gave rise to a much larger sensitive cloud.
In the ex-vessel phase of the accident the rates of hydrogen release depend on accident scenario and are from 0,001 kg/s (TACIS programme) to 0,1 kg/s (Temelín, [Svab 03]), and from 0,07÷0,16 kg/s (PN7). NEA indicates that a significant part of hydrogen is produced during the early phase of MCCI, while Zr is being oxidized [NEA 2001-15]. This quantity is clearly dependent on the level of Zr oxidation during the in-vessel phase of the accident. For a typical PWR the overall amount of H2 released into the containment by complete oxidation of all Zr in-vessel and ex-vessel is in the order of 1000 kg. After depletion of Zr and its followon products, long term H2 release during MCCI is governed by Fe oxidation with typical release rates of 4 g/s, which would continue over several days. However, the issue of hydrogen production by zirconium oxidation has to be dealt with as a whole (in-vessel and exvessel phases) and the main area of uncertainty, which requires further analyses, is the oxidation during the in-vessel accident progression [NEA 2001-15].
The quasi–steady state reached after the accident is characterized by high fraction of steam in the containment (above 53%), and a very low fraction of oxygen (below 5%) so that the atmosphere is inert. Reduction of the amount of hydrogen and eventual achievement of stable steady state is possible by the use of filtered venting system [Sykora 03].
Evaluation In Temelín several factors contribute to the hydrogen safety: presence of a large dry containment, early inerting of the containment by steam and long term inerting by decrease of hydrogen and oxygen content due to the action of the installed hydrogen recombination system. The results of Czech calculations presented during the Workshop showed that the containment integrity would be kept even in case of unplanned actuation of containment spray system at the moment when the contents of hydrogen are the highest, causing hydrogen deflagration [Kujal 03]. Similar results were obtained in PN7 calculations.
On the other hand the note in Ref. [SONS 01] indicates that in the case of hydrogen detonation due to operator’s errors the integrity of containment can be lost and the corresponding radioactive releases are evaluated. SONS pointed out that the operators are thoroughly trained and unlikely to make mistakes in severe accident conditions. The plant considers that following the SAMGs practically excludes the hazard of containment failure.
Hydrogen management strategies are addressed in the Temelín SAMGs. Several strategies aid in limiting the threat to containment integrity posed by hydrogen combustion, including limiting the containment pressure, reliance on PARs to recombine hydrogen over the longer term, use of containment venting to reduce containment pressure and deplete the hydrogen source in the containment, and maintenance of steam inerted conditions in the containment when possible to suppress the possibility of hydrogen combustion.
ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 127 A limited potential for energetic hydrogen combustion has been identified by a state-of-theart CFD code calculation for a SB LOCA sequence for the WWER 1000 containment configuration (specifically involving hydrogen "trapping" in the SG boxes). In the context of Temelín however, the frequency of the accident analyzed has a low frequency of occurrence (3×10-7 per year) and the occurrence of a containment-threatening detonation event even under the conditions identified is by no means certain.
However, during monitoring the team has not been able to identify the technical means by which the Temelín NPP could achieve “mixed atmosphere” as required by e.g. US-NRC regulations, therefore the question of assuring mixed atmosphere remains open.
5.6 Reduction of Radioactive Releases
VLI No. VLI title / description 8.9.1 What are the measures to reduce radionuclide releases?
8.9.2 Is there an accident management procedure for the reduction of the volatile organic iodide in the containment atmosphere?
8.9.3 Is there a qualified system to keep pH of water basic in the containment?
8.9.4 Has the decomposition of hydrazine due to severe accident radiation fields been considered?
8.9.5 Following a leak or breach in the primary containment, or following a bypass failure of the primary containment (e.g., steam generator collector leak, steam generator tube rupture, interfacing systems LOCA), radioactivity may escape to other parts of plant buildings (e.g., the reactor building, the condenser hotwell, etc.).
How do the Temelín SAMGs attempt to mitigate leakages to and from these other buildings – what strategies are followed?
8.9.6 If the basemat fails due to molten corium concrete interactions (MCCI), and the reactor building retains its structural integrity, what is the capability of the reactor building ventilation system to reduce the source term resulting from basemat failure?
What is your evaluation of the likelihood of maintaining structural integrity of the reactor building following basemat failure (considering pressure release from the containment, release of combustible gases from the containment and continued evolution of gases due to MCCI in the reactor building)?
State-of-the-art requirements and practices
The magnitude of radiological releases after containment break depends on several factors:
1. The size of the break in containment
2. Whether or not the sprays are operating (enhanced aerosol deposition)
3. Whether or not the release path passes through a pool of water (aerosol scrubbing)
4. The time margin between the release from fuel and the release from containment.
Therefore, not all containment failures lead to large releases. SAM strategies and technical measures used for reduction of fission product release from the containment include the use of containment sprays and addition of chemicals to increase pH of water, from the steam generators – steam release of the defect SG to the condensers or to the feedwater tank and increasing the water level on the secondary side of SG to provide scrubbing of fission products before their release to the environment, and generally injection of water into any path of fission product release to increase scrubbing and fission product deposition on the internal surfaces.
128 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues There are measures addressing specifically the issue of fission product releases, especially of iodine and caesium. Since the volatile organic iodide is most difficult to retain, special strategy should be developed to reduce it in the containment atmosphere. High pH assures effective partitioning of iodine and keeps iodine in the CSS water, but keeping water in basic conditions requires special qualified system of chemistry control under severe accident conditions. WWER 1000 units have the advantage of effective containment spray system with chemical additions aimed to keep spray water basic, but some of these additions can decompose in high intensity radiation fields. Analyses published in the past, e.g. for Dukovany NPP, addressed this issue and showed that it can be resolved.
Current plant status SAM strategies in Temelín NPP include measures aimed at prevention of PRISE accidents and at reduction of radiological releases if PRISE accidents do occur, as discussed above in Section 5.1.
For all accident sequences with fission product releases to the containment atmosphere there is a containment spray system of high reliability. There is a qualified safety grade system to keep pH of water high. Additional sources of water are available to back up CSS operation. The Czech specialists claim that the late failure of containment does not result in high radiological releases because the operation of spray system will have removed fission products from the containment atmosphere before the containment (or basemat) failure occurs. This is in agreement with the findings of analyses performed for other NPPs and with the results of experimental studies performed within the EU [Morozov 03, Schoels 02].
The strategy of flooding the reactor cavity before RPV break assures that there would be layer of water above the molten corium, so that the fission products being released during MCCI would be retained in water in the process of gas purging. Although water may not be effective in stopping MCCI, it is certainly an effective means of reduction radiological hazards.
Containment venting is regarded as the ultimate means of containment protection, but it is unlikely to be used to prevent containment failure in view of the analyses showing that the pressure inside the containment would be below the containment design capacity until basemat failure occurs. If venting were really necessary, it will be done by means of a venting system provided with filters so that the releases of fission products will be appropriately reduced.
The results of calculations regarding the reduction of fission product inventory available for release in case of late basemat melt-through indicate high effectiveness of sprays and internal deposition of volatile fission products, but no details have been disclosed concerning possible fission product revolatilization due to hydrogen burning after sudden changes in containment atmosphere composition in case of basemat melt-through. Temelín NPP experts stated during the Prague workshop that additional measures could be implemented in the case of basemat penetration to reduce fission product releases to the environment through the rooms below the containment. No detail of such measures was given.
In the case of basemat failure due to MCCI, which could occur in the late phase of the accident, 3 or more days after the RPV rupture, the fission products would be deposited on inner surfaces of the containment. TACIS results indicate that if the spray system were available, the reduction of source terms would be by three orders of magnitude [Schoels 02].
Similar results are given in Czech studies [SONS 01], [Pazdera 03]. However, there is no analysis of possible effects of hydrogen burning or explosion, which is a potential threat in case of sudden mixing of outside air with the containment atmosphere after basemat meltthrough. In case of an explosion the pressure wave could result in re-volatilization of a large part of particulates that have been deposited on containment surfaces. Although such a possibility is remote, it should be analyzed and the means for reducing possible radiological consequences of such a sequence should be considered.
ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 129 Evaluation The measures and strategies to reduce fission product releases are in keeping with the international practice. The only open issues are the reduction of radiological releases in the case of basemat penetration by molten corium as well as the potential for hydrogen combustion in the reactor building after basemat penetration. The Specialist’s Team would recommend the Austrian Governement to consider monitoring of both the details of calculations and the means to reduce fission product releases in case of basemat melt-through.
130 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues
The PN7 project aimed to clarify whether the measures already implemented and planned in Temelín address plant vulnerabilities in the severe accident area and whether they will provide a safety level similar to NPPs presently in operation in the EU and US. The report presents a clear picture of the Temelín NPPs behaviour in case of severe accidents, determines open issues and formulates proposals for further monitoring.
6.1 Overall Conclusions 6.1.1 Regulatory approach and practice SUJB has required the plant to prepare and accomplish a program to deal with BDBAs, including estimation of plant vulnerabilities, proposed accident management procedures and the schedule of their implementation. The targets set for severe core damage frequency and for large off-site releases are to underscore 10-4 and 10-5 per reactor year, respectively, which is consistent with the INSAG targets for existing NPPs.
The development of SAMGs is performed by the utility. The regulatory body defines acceptance criteria and provides guidance to Temelín NPP, leaving enough flexibility for potential candidate actions to address specific challenges.
6.1.2 Temelín programme of severe accident management
The development and implementation of Temelín SAM programme has not been finalized, however, the whole process is well advanced.
The overall concept and approach to development/implementation of SAMG package was found to reflect the current good practice in the SAM area. The selection of plant specific SAM strategies has been based on the well-established generic approach developed by Westinghouse Owners Group. These generic strategies have been adapted to Temelín plant conditions based on a systematic process that reflects the current state-of-the-art in this area.
The programme is supported by severe accident analysis and plant specific PSA. However, there were some instances when the existing results of SA analysis were not properly incorporated into the PSA. It should be noted that also some SAM strategies, apparently the most recent, are not well supported by SA analysis. The interface between the PSA team and thermal hydraulic analysis team needs improvement.