«Item 7b Severe Accidents Related Issues Preliminary Monitoring Report Report to the Federal Ministry of Agriculture, Forestry, Environment and Water ...»
Liquid corium will spread on the floor of the reactor cavity. The results of spreading experiments with prototypic corium melts at Siempelkamp and Forschungszentrum Karlsruhe indicate average melt thicknesses of about 2÷4 cm [Spengler 02]. This shows a strong influence of cooling on the real spreading processes. The temperature of the molten corium is kept up by residual heating. In the course of molten corium penetration into the concrete the temperature of corium decreases, its viscosity increases and the rate of reaction with concrete goes down. The thickness of crust grows due to cooling and phase changes. Small-scale experiments showed that the formation of this crust decreases effectiveness of molten corium cooling from above with water, but in larger scale experiments the crust was cracking and letting water get into more intimate contact with molten corium. The rate of the MCCI reaction depends also on the generation of gases from heated concrete and their influence on the heat transfer from corium to the concrete.
Generally, the phenomena occurring during MCCI are complex and not fully understood.
Several experiments have been performed or are under way to gain better understanding of these processes. Large-scale tests are performed in Germany at Siempelkamp CARLA plant in the COMAS facility [Steinwarz 01] with the aim to model the phenomena related with the retardation of the flow, which may lead to the final arrest of the corium.
4.3.4 Long-Term Overpressure
In the long term the main reason for possible containment integrity failure would be inadequate heat removal from the containment, with temperature and pressure increase above the design values. As the containment is the main and final barrier against release of radioactive products to the environment, the protection of containment integrity is recognized as the ultimate objective of Severe Accident Management.
Energy and mass releases into the containment during a core melt accident result in pressure increase. If the means of heat removal from the containment should fail, the pressure in the containment would slowly increase and after several days – if there were still no heat removal – the containment could fail due to overpressure in several days time.
In case of MCCI large amounts of heat are generated. In parallel, corium-concrete reaction involves generation of non-condensable gases, which increase containment pressure. If the fraction of hydrogen in the containment is elevated and the containment is inerted due to high fraction of steam, actuation of containment spray system might lead to de-inerting the containment, increase of hydrogen volumetric concentration and hydrogen burn or even deETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 103 flagration to detonation phenomenon. Therefore, the actuation of CSS would not be allowed in the late stage of the accident, and the pressure in the containment would steadily grow till the containment failure.
To prevent it, filtered venting of the containment has been proposed. Various methods of filtered venting of containment have been implemented in Sweden, France and Germany, and are being introduced in other countries. In Germany pressure relief system includes deep bed fibre filters and molecular screens for elemental iodine or venturi scrubber with retention capacity for aerosols 99,99%, and for elemental iodine 99%. In France pressure relief systems are provided with sand beds, assuring effective retention of volatile fission products.
Some NPP designs include dedicated systems for containment cooling by natural convection, which can assure effective heat removal even during total blackout conditions and prevent pressure build-up inside the containment.
104 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues
5 EVALUATION OF ACCIDENT MANAGEMENT STRATEGIES5.1 Prevention and Mitigation of PRISE The problems of prevention and mitigation of PRISE sequences are given the highest attention in all NPP analyses, which also has been reflected in the number and detail of VLIs formulated in the PN7 project on the subject of PRISE accidents.
VLI No. VLI title / description Prevention 8.1.1 Have there been any cases of primary collector breaks in those SGs that were made using rolling technology of fastening of heat transfer tubes in the collectors?
8.1.2 Have there been any cases of deviations from secondary circuit chemistry observed during operation of Temelín so far?
8.1.3 Are the temperatures at water-steam interface kept outside the temperature range of phase transition for the steel from which the primary collectors are made?
8.1.4 Is the N16 system installed and observed on-line by the operator?
8.1.5 Are the criteria for tube plugging established?
Are they the same as used in Dukovany NPP?
8.1.6 Are the eddy current methods implemented for detection of developing tube cracks?
8.1.7 Have the analyses of possible cracking of SG tubes been made for the case of DBAs with EFWS operation and for cases of severe accidents with High Pressure scenarios?
8.1.8 Have the SAMs involving cold-water injection into SGs during severe accidents been analyzed from the standpoint of possible tube breaks?
8.1.9 Has the actuation point for BRUA forced opening for RCS cooling been analyzed from the standpoint of possible breaks in the SG tubes due to tubes uncovery?
Mitigation 8.2.1 Are the Fast Closing Main Steam Isolation Valves qualified for water hammer and steam-water or solid water flows?
8.2.2 Are the lines to BRUA and safety valves qualified for steam – water mixture and water flows?
8.2.3 Is the operator allowed to throttle HPIS pumps? When?
8.2.4 Are there any means for providing water to SGs in case of EFWS failure?
8.2.5 Is there in SAMGs the requirement to keep SG filled with water?
8.2.6 Is RCS pressure reduction through PRZ spray injection possible in case of LOOP?
8.2.7 Is PRZ spray injection protected against single failure?
8.2.8 Are there any means to provide firewater or other water supplies to the ECCS tanks?
8.2.9 Is forced cooling of RCS foreseen in case of PRISE in SAMGs?
What is the entry point?
8.2.10 Is high boron concentration in primary coolant required before intensive RCS cooling can be started? How and when can it be achieved?
8.2.11 Does depressurization of the RCS result in leakage stopping before loss of primary coolant and core uncovery? For what entry point and bleeding capacity?
ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 105 State-of-the-art requirements and practices There is a broad international consensus in OECD countries on the necessity to improve the design of NPPs in order to avoid containment bypass sequences. It has been observed that the Postulated Initiating Event (PIE) of Steam Generator Tube Rupture (SGTR) is for some designs still classified in a category of PIEs “not expected to occur”. Given that the operating experience has proven that the occurrence of this PIE is not so low, the TSO Group stated that the categorisation of this PIE has to be reconsidered [TSO 01]. According to the opinion of GPR/RSK and European utilities sequences with SGTR should be considered assuming simultaneously a failure to close of the main steam isolation valve or steam relief valve.
The importance of PRISE is highlighted by Westinghouse SAMGs, in which the highest priority have actions that aim at mitigating radiological releases after PRISE and preventing SGTR during high pressure accident sequences such as blackout or SB LOCA.
Primary collector rupture was not included among the Design Basis Accidents in the original design of WWER 1000 NPPs. After cracks were revealed in a number of steam generator collectors in various WWER 1000 NPPs, close attention was paid to this hazard. It was found that one of the main reasons of cracks was explosive bonding, which involved excessive stresses in the collector. Another factor influencing the process of collector cracking is the secondary side chemistry. Strict adherence to the requirements for secondary water chemistry is important for reducing the rate of crack development. However, although the requirements in this respect have been known for a long time, many NPPs in Russia and Ukraine observed cracks in their SG collectors due to deviations from the required chemistry.
The injection of cold water from EFWS on uncovered SG tubes can result in cracking of hot tubes. The issue is important in the implementation of SAM measures, especially in the case of high-pressure scenarios. Both EOPs and SAMGs should be analyzed and the actuation points for EFWS injection chosen in such a way as to minimize the danger of SGTR.
All valves exposed to possible steam-water mixture flows after PRISE should be qualified for such flows, or remain closed during the transient. If BRU-A is opened during the initial blowdown and fails open, then the leak can turn into a severe accident with potentially very large radioactive releases.
RCS pressure should be reduced in case of PRISE so as to reduce radioactive leaks from the primary side and – what is even more important – reduce losses of primary coolant inventory. One of standard measures for RCS pressure reduction is injection through PRZ spray system. However, in the case of LOOP and loss of RCPs the pressure head on the RCP delivery side disappears and the sprays are not functional. In some NPPs PRZ sprays can be provided with water from the RCS make up system supplied by Diesel generators. In some WWER NPPs it has been found, that even if make up system can inject into the PRZ spray line, that line is prone to single failure, because in one section between the make-up system and the PRZ there is a single valve which can fail closed. It is worth noting that in Loviisa the system was redesigned and parallel lines to PRZ sprays were installed to keep this mode of RCS depressurization in case of that valve failure.
The operator should be allowed to throttle ECCS injection if PRISE is identified. All sources of available water should be used in case of need to keep the core covered. Appropriate SAM strategies should be developed.
106 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues Current plant status Temelín NPP is fully aware of the hazards involved in PRISE leakages and various preventive measures have been implemented to limit the frequency of PRISE. These include the improved technological method of hydraulic expansion to fasten SG tubes in the primary collectors instead of explosive bonding. According to international WWER experience, there have been no cases of cracking collectors produced using this technology. On-line monitoring of secondary side chemistry has been implemented. The turbine condenser tubes produced originally from stainless steel were replaced with tubes made of titanium to ensure leak tightness and prevent the leakage of cooling water into the secondary circuit. Owing to the change of tube material it was possible to increase pH of feed water to above the value of 9. Each SG at Temelín NPP is equipped with chemical diagnostics that enables taking of samples from water volume on the secondary side and with automatic monitoring of blowdown water. The operator can observe on line the actual chemistry of water.
Advanced NDT methods are used in Temelín for ligament checking. To detect SG tube failures, which are still possible on line monitoring for SG integrity is realized by means of gamma scanning of SG blowdown water. An extensive programme of SG integrity assessment and maintenance has been implemented.
N16 detection system to detect early signs of leaks is installed and in operation. The system is not sufficient per se to protect against developing leakages, but it helps in early detection of leaks and it also serves for more accurate identification of the affected SG if the leak does develop. The criteria for tube preventive plugging before large leaks appear are established and eddy current methods are used for early detection of tube cracks. The analyses of possible cracking of SG tubes in high-pressure scenarios have been made and SAMG strategies include keeping water level in SG high enough to maintain SG tubes covered with water.
The design of the collector header cover in Temelín was modified to reduce consequences of header lifting due to connecting bolt rupture, reducing the break flow area from 100 mm equivalent diameter down to a value equivalent to 40 mm diameter leak. All these measures have reduced the frequency of PRISE initiating event in Temelín NPP, which is reflected by much reduced likelihood of core damage due to medium size PRISE, from 4,3E-5 to 3,09E-6.
The probability of PRISE induced in the course of an accident is reduced by proper strategies introduced in the plant EOPs and SAMGs. They include keeping the SG secondary side filled with water to maintain integrity of SG tubing. There are also developed emergency means of water delivery to the secondary side of SGs, which can be applied even if EFWS fails, such as providing firewater connections and a reserve diesel generator that can be connected to any Temelín unit.
There are several possibilities to mitigate PRISE if it does occur, starting with RCS pressure reduction by injection to the pressuriser. The injection is provided by make-up system, which is powered by diesel generator and designed against single failure criterion so that parallel control valves are provided in the injection lines. The SAM strategies allow operator to throttle ECCS injection if PRISE event is identified. This would be achieved by gradual switching off the individual HPIS trains and the use of make-up system, which has capabilities for throttling the make-up flow. To replenish the water inventory in the RCS the operator can use two large pools, which are usually empty, but can be filled with water for fuel reloading purposes.
During an accident these pools will be filled up with water and made available for refilling the RCS if the inventory of the RCS and ECCS is low.
The question of major importance is the qualification of BRU-A for water-steam mixture flow.