atw 2018-05v6

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atw Vol. 63 (2018) | Issue 5 ı May OPERATION AND NEW BUILD 302 and its both follow up phases (latest on THAI TH-27 test and THAI HR-49). The past activities in the severe accident for both Czech NPPs are were mostly focused on the enhancement of the safety using only existing systems on site, but out of their operational conditions. The Fukushima Daiichi accident significantly changed the approach to the solution of severe accidents at the Czech NPPs and the implementation of new, and only for severe accident conditions dedicated, systems was fully accepted as the necessary step forward in the safety enhancement. The proposals were not started from the scratch, but both NPP were evaluated within the Stress Tests, which reports were issued to the Czech Republic State Office for Nuclear Safety (SONS), and based on these reports the national Stress Test report [A] was prepared and evaluated within the evaluation process under the ENSREG leadership resulting in the forming of the National Action Plan [B]. Concerning the severe accident issues the eight main areas were defined (as a selection of those most important ones from much higher number of issues) • Increase of the capacity of the system for liquidation of emergency hydrogen (action 46 – Dukovany NPP, action 47 – Temelín NPP) – this title is undertaken from the official National Action Plan and it means increase of recombination power of emergency hydrogen as the term “liquidation” is unusual in this meaning • Cooling of the melt from the outside of RPV (action 48 – Dukovany NPP) • Recriticality (action 61 – both NPPs) • Control room habitability (actions 58, 31, and 51 – both NPPs) • The means for maintaining containment integrity due to overpressure (actions 46 – 50 – both NPs) • Corium in/ex vessel cooling ( actions 48, 49, 50 – Temelín NPP) • Extension of SAMGs for shutdown/ severe accident in SFP ( action 56 – for both NPPs) • System setup of training (drills), exercises and training for severe accident management according to SAMG, including possible solution of multi-unit severe accident The solutions of some topics listed above are independently described in following subchapters with the pointing out the contribution of the UJV to their solution. 3.1 Increase of capacity of system for emergency hydrogen removal The UJV performed a full analytical support for the increased design of the hydrogen removal system. The analytical program consisted from several steps. The first step contained the integral analysis of the severe accident progression with the aim to define mass and energy source into the containment for selected severe accident scenarios. The scenario selection was based on several conditions like – a location of the hydrogen source in the containment, its potential intensity, an operation of the containment spray system, or conditions of the molten corium concrete interaction. Generally three initiating events were selected and overall six integral analyses performed with the MELCOR 1.8.6 code. The sources to the containment were later used in the stand alone analyses of the containment response using the very detailed model of the containment again for the MELCOR 1.8.6 code. Two kinds of analyses were performed – first the hydrogen risk evaluation based on the Sigma and Lambda criteria, which were used for the first proposal of PAR design, second the optimization analyses with the aim to fullfil predefined succes criteria – an elimination of Lambda criterion in all parts of containment, an elimination of Sigma in practically all parts of containment (small individual spaces allowed for temporary occurrence), global and local concentration limits after recalculation of hydrogen concentration in dry air. The design of the PARs of the NIS vendor was succesfully implemented at both units of the Temelín NPP with finalization and starting of its full operation after outages in 2015. 3.2 Recriticality of degraded core due to reflooding with demi water The UJV is recently performing analytical investigation of this topic independently for each of Czech NPPs, because of their principal design differences. The methodological approach is identical and consists of the integral analyses of the severe accident progression using the MELCOR 2.2 code with an externaly defined boric acid to analyze the development of boric acid concentration. In parallel the most important configurations of the degraded core are defined to be analysed using the MCNP code to identify the minimum concentration of boric acid leading to the recriticality. The evolution of boric acid concentrations vs. the minimum value will allow to define conditions for the applicability or the restriction of application of demi water for injecting into the degraded core under various stages of severe accident course. 3.3 Control room habitability This study was performed again independently for each of Czech NPPs and the UJV performed these analyses. The methodology consisted of two main analytical steps – the first one covered the integral analyses with the MELCOR 1.8.6 code for the identification of fission product distribution and releases via different leak paths. The second step included the analysis of dose rates in the control room based on the precalculated distribution of fission products and shielding of control room walls or window (if presents). 3.4 Corium localization for Temelín NPP The issue of the corium localization is very complex and determines the solution of others activities, like a solution of long-term containment pressure control. Before the Fukushima Dai-ichi event the analytical activities were focused only on the ex-vessel corium cooling (ExVC) strategy, because of a restriction on an implementation of any new equipment for severe accidents. The request from the NAcP opened a way for the solution of the in-vessel corium retention (IVR) strategy as an alternative one, which applicability has to be evaluated. The utility opened at the beginning two preparatory projects, which enabled to prepare the fisrt analytical models for the corium behaviour in the lower head and cooling conditions of rector pressure vessel from outside. As the outcomes from the analytical work identified some potentials, the complex project on the IVR solution was initiated in 2015 with expected duration up to 5 years. The project is not focused only on analytical work, but also on the experimental confirmation of the VVER-1000 RPV coolability under the IVR strategy. The project is focused on three types of activities – analytical investigation with proposing of additional systems for strategy solutions, designing of new systems required for the implementation of strategies, and the experimental program for the RPV Operation and New Build Continuous Process of Safety Enhancement in Operation of Czech VVER Units ı J. Duspiva, E. Hofmann, J. Holy, P. Kral and M. Patrik
atw Vol. 63 (2018) | Issue 5 ı May coolability. As the activities have to produce a reasonable contribution to the safety enhancement, the following conditions were defined • Effectiveness, it means a quantified benefit to safety (prevention of early FP releases and minimize FP releases) plus confirmed physical fruitfulness with sufficient margin • Reasonable technical feasibility • No negative impact to reactor operation (including all procedures/ activities during outages) • Simplicity – applicability under SA condition (limited personal capacity, limited accessibility …) • At least partial independence of functionality assurance in comparison with existing emergency systems • Consistency of approaches with other utilities operating VVER-1000 (or reactors of similar power) and VVER-1000 designers Taking into account above defined conditions the activities were defined for following six main topics which have very well defined structure and relations each to other • Primary circuit depressurization under SA conditions • Corium cooling with water injection into RPV (new acronym IVR-IN is used) • In-vessel retention with external reactor vessel cooling strategy (originally used IVR, but newly IVR-EX acronym is used) • Ex-vessel corium cooling strategy (ExVC) • Containment response to severe accident and long term issues • Severe accident initiated in the spent fuel pool Short description of the effort done up to know is included in following sub-chapters excluding the last topic, of which activities are not yet initiated, but some analyses were already performed in past. 3.4.1 Primary circuit depressurization under SA conditions The activities related to this topic were mostly focused on the conditions for the IVR-EX strategy as the primary pressure is strogly determining success of this strategy. The analytical investigation using the MELCOR 1.8.6 code was performed to identify if the depressurization initiated after the entry to SAMGs is sufficiently fast to meet predefined criterion – to reach the pressure reduction below 2 MPa-g till the time of starting relocation of corium into the lower plenum of RPV. The analytical studies, for variant opening of one or two safety valves or additionally the system for a removal of gases from the pressurizer, confirmed that the safety valves if can be kept open are sufficient, but the contribution of the gas removal system is negligible. The important condition is that the valves must be kept open, the modification of a control of safety valves is in the final phase of its preparation with an expectation to initiate an appropriate technical negotiation with the SONS. 3.4.2 Corium cooling with injection into RPV (IVR-IN strategy) The extensive analytical investigation was performed with the MELCOR 1.8.6 code to analyze possibility of a termination of the core degradation due to water injection into the RPV. The analyses varied with the assumed mass rate of the water injected (to alternate among the repaired LPI and HPI, and the new alternative system called TB50, which has the lowest mass rate of water injection and also relatively lower maximum pressure head). The second variations were in timing of the water injection activation, it means different configurations of core degradation at the time of water injection initiation. The last variation was in the initiating event as the most of analyses were performed for the large break LOCA initiated envent (plus loss of all active systems) and as an alternative the scenario initiated with postulated SBO was chosen with the early depressurization after the entry to SAMGs and also loss of all active systems. The conclusion from the analytical work is that the results are of course uncertain, but they show that the initiation of water supply before starting of the corium relocation into the lower plenum should prevent the loss of RPV integrity. More detailed, the analysis predicted un-failed RPV wall if the lower head is not dried out, but such answer seems to be too optimistic due to some modeling assumptions in the MELCOR code which favoured cooldown with even very small remaining mass of water in the lower head. 3.4.3 Strategy IVR-EX The greatest effort was done for this topic during last two years and the activity will continue with the experimental program on the RPV coolability. The first issue related to the IVR-EX strategy is the flooding of reactor cavity and long term water supply to the cavity. The design of the VVER-1000/320 containment is absolutely un-favorable for this IVR-Ex strategy, because the recirculation sump is located one floor below the cavity and the water from the containment is drained to this sump. So it is absolutely impossible to close the water circulation inside of the containment, like it is in case of the AP1000 or VVER-440/213 reactors. The water for flooding of the cavity must be injected from sources outside of the containment. The feasibility study was performed to investigate all necessary modifications to be done for a possibility to cool the RPV from outside. The preliminary project solution was prepared for new systems on fast cavity flooding (based on pressurized tanks located on roof of the auxiliary building) with the second part for feeding water from the second pair of pressurized tanks. This solution enabled to cover the first 6 hours by this passive system and thus to have a way sufficient time for an activation of the new active system which is capable to feed water from own tanks till 72 hours of the accident. The injection of the water is not the only condition, but the second one is the protection of water releases from the cavity. The injection of water is assumed via. the venting system channels and those must be equipped with new valves to prevent water overflow to the containment. Generally it means to install 9 new valves which must all of them be surely closed before starting of injecting water to the cavity. This is the most critical part of the solution, because of possible failure, which would cause the loss of the IVR-EX strategy. The second critical point is the control of water level for the second set of pressurized tanks. The control is proposed to prevent loss of this water due to overflow, but this control can be tested and tuned up only at the experimental facility, so its credibility is not too high. The second issue is the release of steam produced during the cooling of RPV by boiling. The design drawings of the Temelín NPP showed that the gap between the RPV itself and its supporting ring should be sufficient (including thermal expansion of RPV), but the data from measurement after the vessel installation were not available. So at least part of this gap was measured during the outage at the unit 2 in 2016 and it confirmed sufficient gap. But the gaps among the boxes of thermal and biological shielding seem OPERATION AND NEW BUILD 303 Operation and New Build Continuous Process of Safety Enhancement in Operation of Czech VVER Units ı J. Duspiva, E. Hofmann, J. Holy, P. Kral and M. Patrik
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