«Item 7b Severe Accidents Related Issues Preliminary Monitoring Report Report to the Federal Ministry of Agriculture, Forestry, Environment and Water ...»
RSK stressed, that the steam would provide inert atmosphere in the containment, but after steam condensation the hydrogen burning could occur under conditions of high hydrogen concentration. Therefore, the Commission recommended installing in NPPs with steel shelled containments passive catalytic recombiners, which can work in low hydrogen concentrations even in the presence of steam, support atmosphere mixing and provide long term hydrogen removal. RSK observed that it is not realistic to expect that recombiners would be able to prevent in short term high concentrations of hydrogen and discussed possible application of dual hydrogen removal concept, with igniters and recombiners. Acknowledging the complexity of this area, the RSK formulated recommendations that for beyond design basis accidents the NPPs should install recombiners, and the capacity of the recombiners should be such ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 123 that they would reduce the amount of produced hydrogen within several hours and thus contribute to reduction of containment failure risk. [RSK 94]. The operation of recombiners should be verified by tests conducted every year on randomly chosen catalytic elements.
For new NPPs the European TSO position states, that the containment volume and the mitigation means must be such as to prevent the possibility of global hydrogen detonation. The possibilities of local high hydrogen concentration must be prevented as far as achievable by the design of the internal structures of the containment, when it will not be possible to demonstrate that the hydrogen local concentration remains below 10%, specific provisions must be implemented such as inertisation or reinforced walls of corresponding compartments and of the containment. [Manuel 95] In the long-term ex-vessel phase, autocatalytic recombiners should be able to handle the hydrogen generated by MCCI and water radiolysis in the sump area.
In order to eliminate the risks of detonation, the efficiency of the mitigation means has to be such that with a hydrogen production corresponding at least to 100% fuel clad metal water reaction coupled with an appropriate kinetics of release, the local concentration of hydrogen in the containment should be lower than 10% per volume. It must be verified in addition that a global deflagration of a total amount of hydrogen corresponding to these criteria does not endanger the containment integrity [EUR 20163].
• Technical means of hydrogen control The main technical means to reduce the amount of H2 in a closed containment are igniters and recombiners. Besides that, especially in large dry containments, which can withstand hydrogen deflagration without losing their integrity, natural convection driven atmosphere mixing helps limit the formation of pockets of high hydrogen concentration. When the hydrogen concentration is high, containment atmosphere can be kept inert by avoiding steam condensation. Finally, hydrogen can be removed from the containment by venting, which is implemented in several EU countries. In the case of large dry containments however, the hazard of hydrogen burning is generally considered to be small.
Available igniters exist in the form of spark igniters, catalytic (i.e. passive) igniters and sparkplugs. They are designed to ignite gas mixtures as soon as these mixtures have reached the flammability limit (e.g. 5% hydrogen and 95% air, or e.g. 20% hydrogen, 30% air and 50% steam). The corresponding pressure increase will lie in the order of some 0,1 Bar depending of course on the hydrogen concentration at the moment of ignition.
With the exception of catalytic igniters, spark igniters and sparkplugs have to be initiated. The existing spark igniters are passively initiated when either the surrounding temperature or the pressure increase beyond a certain limit. Active initiation by the operator, or actuation after measurement of hydrogen concentration shows that a certain limit is exceeded, can be also envisaged.
Catalytic recombiners exist in various forms and sizes. The volume of the system is about 0,5÷1 m3; the flow area typically 0,5 m2 while the steady state flow velocity attains around 1 m/s, independent of hydrogen concentration.
The removal rates depend on the recombiner size, number and location. In most EU NPPs the total capacity of recombiners, even those designed for severe accident conditions, is much lower than the maximum hydrogen production rates. For example, in German NPP Biblis B the total maximum capacity of all recombiners is 0,05 kg/s, and similar rates of recombination are ensured in other German NPPs. This is in keeping with the recommendations of RSK, which indicated that the capacity of recombiners should be sufficient to control hydrogen concentration in long term, not at the moment of the maximum rate of hydrogen production. In French NPPs the total capacity of recombiners planned to be installed in 1300 MWe PWRs is about 216 kg/h, which corresponds to 0,06 kg/s. There are, however, 124 ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues also such plants as Doel in Belgium with much higher recombination capabilities, and Spanish (except Trillo) or US plants where no recombiners are deemed to be necessary.
Atmosphere inerting is an inherent feature of many scenarios with loss of containment spray system. In some instances the containment atmosphere steam inerting has been demonstrated to provide a hazard self-limiting feature so, that the excessive hydrogen production resulting in elevated hydrogen concentrations in the containment can be outweighed by the steam produced from coolant.
Since this behaviour is rather sensitive to a number of boundary conditions, including uniform hydrogen distribution, it cannot be considered a perfect solution. It is by no means an inherent safety feature as suggested in several instances, because it does not return the plant intrinsically into a safe state avoiding incident or accident conditions like inherently safe installations would do. The need for producing steam to overcome excessive hydrogen production can paradoxically lead in some cases to shifting the critical safety function “cool the fuel” to a quite different safety function “avoid internal hazards” in which the cooling procedures are changed according to the inerting needs.
Severe accident conditions can result in hydrogen concentrations of a few percent up to 12but as long as there is plenty of steam in the containment atmosphere there is no danger of hydrogen detonation. In the process of steam condensation the volumetric fraction of hydrogen increases, but before it can come to detonable concentrations, deflagration is expected to occur [Blanchat 97]. In the TACIS calculations for WWER 1000 units, significant hydrogen concentrations reaching up to 12% vol. were found in several scenarios. Keeping containment atmosphere inert was one of the principal measures to deal with such situations.
In order to maintain mixed atmosphere within the containment the main technical means consists in having containment fan units. This is an inherent feature of US hydrogen control strategy, which corresponds to the WOG-SAMGs being implemented at Temelín NPP.
Current plant status The strategy of NPP Temelín in dealing with hydrogen hazards consists in prevention of detonable hydrogen concentrations by frequent hydrogen deflagrations and reduction of the hydrogen concentration by means of recombiners. There are 22 catalytic hydrogen recombiners (PARs) of Siemens make, Model No FR90/1-150, distributed within the containment with due consideration to the expected distributions of hydrogen in the containment atmosphere for design basis accidents. The capacity of these recombiners has been chosen so as to control hydrogen concentration after Design Basis Accidents, and in the case of severe accidents the recombiners provide long-term reduction of hydrogen concentration and depletion of oxygen, thus contributing to inertization of the atmosphere.
The producer has extensively studied the capacity of PARs under accident conditions. The relationships defining PAR capacity in relation to hydrogen volumetric concentration and atmosphere pressure are proprietary and have not been made available to the Austrian side by Temelín NPP. In preliminary calculations of PN7 project the threshold of PAR operation was taken as 0,5 vol.% concentration of H2 and the PAR capacity was determined according to the available correlations. Recently, an update of PAR capacity curves was made in PN7 calculations, and the peak capacity of 1 PAR under LOFW or LB LOCA conditions was shown to be below 0,18 g/s for hydrogen and below 0,1 g/s for CO. For 22 available hydrogen recombiners at Temelín, the total nominal recombiner capacity is approximately 4 g/s.
The recombiners in this case were designed for design basis accident (DBA) conditions. The range of hydrogen recombination values for other European PWRs with large dry containments is from zero (for plants without recombiners) to appximately 50 g/s for plants with recombiners designed for severe accident conditions.
ETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues 125 The comparison of maximum rates of hydrogen releases during severe accidents with the maximum recombination rates provided by the existing PARs made within PN7 shows that the role of PARs in controlling short-term hydrogen concentration is small. The main impact of PARs is on long-term oxygen depletion and atmosphere inertization. However, their overall recombining capacity is small and thus the depletion is a very slow process. Under severe accident conditions, PARs of larger capacity would be desirable.
In Czech calculations made for Temelín for Medium Break LOCA with spray system in operation and without recombiners a large number of successive deflagrations (41 during 12 hours) were predicted [Kujal 03]. They resulted in maximum pressure peaks reaching 0,25 MPa, with mass of hydrogen burnt equal to 1150 kg. In the case with recombiners, deflagrations were also predicted to occur, but the mass of hydrogen burnt in deflagrations was less (630 kg) and the mass removed by recombination was 870 kg, so that after 12 hours the amount of H2 remaining in the containment atmosphere was only 200 kg out of 1700 kg released into the containment.
The Czech assumption regarding hydrogen deflagrations is not convergent with the state-ofthe-art, because there are neither igniters nor other technical means designed to initiate hydrogen burn. Temelín staff stated that it would be enough to actuate any mechanical devices with electrical drives to initiate hydrogen burning, and that the recombiners would initiate burning themselves. This is possible, but not certain. (Tests of the PARs installed at Temelín indicate that with sufficient oxygen and not too much steam, the PARs always initiate a deflagration once the hydrogen concentration in the PAR inlet reaches about 7%.) Moreover, the distribution analyses, when performed with the nodalisation concept as presented, does not allow to base the effects of combustion on a purely numerical assumption on automatic ignition, in case the concentrations “homogenised” within the control volumes reach flammability conditions.
In Czech calculations the case of superposition of hydrogen deflagration together with DCH was considered and shown to result in the maximum containment pressure of 0,45 MPa, so below the design strength of containment [Kujal 03]. This conclusion is confirmed by the calculations performed within PN7 for LB LOCA, which showed that even with a large amount of hydrogen in the containment atmosphere (of about 800 kg), an unplanned deflagration would not increase the containment pressure above the design strength value.
Inertization of containment by steam is the main factor preventing hydrogen burns. It is a natural phenomenon, because the release of hydrogen always occurs together with the release of steam, so it can be regarded as an inherent safety feature of the plant. The calculations conducted within PN7 indicate rapid increases of steam fraction in the containment, so that the concentration of steam providing inert atmosphere is reached before high average concentrations of hydrogen are possible. Within some periods of time hydrogen can burn (concentration above 4,1%) but its concentrations are far below deflagration to detonation transition (DDT) level.
According to the position of European Union Technical Support Organizations (TSO) there is no hazard of dry large containment failure if the average concentration of hydrogen in its rooms is below 10 vol.%. According to Czech calculations, this condition is fulfilled with a large safety margin in Temelín NPP. However, this is true only as far as average concentrations in the rooms are concerned. The modelling of 3-D hydrogen distribution made with GASFLOW provides more information on local hydrogen distribution indicates that local gas clouds with higher hydrogen concentrations are possible resulting from trapping in the steam generator compartments in case of small LOCAs. The calculations made with GASFLOW and a model including 52 080 calculation nodes for Temelín NPP showed that for the hydrogen release rate of 0,2 kg/s there are some volumes within steam generator room where hydrogen reaches mildly burnable concentration except for a small transient region with a more sensitive mixture. This region, within the steam generator box just above the hydrogen reETE Road Map - Preliminary Monitoring Report – Item 7b: Severe Accidents Related Issues lease point, does not last long, nor grow very large, and it is not therefore felt to pose a significant hazard. The hydrogen release value of 0,2 kg/s that was used for the initial GASFLOW calculation was the initial MELCOR data provided as input for GASFLOW calculations. It agrees well with the evaluation of NEA experts who wrote in the report on “In vessel and ex-vessel hydrogen sources” that “It is commonly agreed that the hydrogen source rate typically about 0,2 kg/s for a large size PWR of 1000 MWe is sufficiently accurate” as long as the core geometry remains intact [NEA 2001-15].