Containment Hydrogen Control and Filtered Venting. Design and Implementation

Research into the hypothetical event of core melt accidents has continued and new accident mitigation technologies have been developed. Decisions have been taken to implement these new mitigation measures in operating nuclear power plants to mitigate severe accidents. In order to prevent loss of containment integrity as a result of over-pressurization, nuclear power plants in the Federal Republic of Germany as well as in most other European countries have been or will be back-fitted with systems for filtering the containment atmosphere and systems for H2-control. Similar discussions starts for the WWERs. For these tasks it has been necessary to develop means for containment atmosphere control which are capable of handling high H2 production rates as well as cleaning and measuring high contaminated atmosphere. In addition, the systems should feature a simple design, incorporate passive equipment to as large an extent as possible, be cost-effective and be easy to integrate into various types of plants. The advantages and drawbacks of various hydrogen countermeasures to mitigate severe accidents, for example of igniters, passive auto-catalytic recombiners (PARs), post inertization, etc. have been investigated and ultimately the PAR system selected as the H2 mitigation technique to be implemented or already have been back-fitted to Western PWR plants and Soviet-designed VVER nuclear power plants. A PAR system consists of a large number of passive individual auto-catalytic recombiners - for example anywhere from 30 to 60 individual PARs - distributed inside the containment to accommodate a wide range of release scenarios. The effectiveness of such a PAR system has been demonstrated by analysis comparing a representative severe accident sequence with and without the presence of PARs. Initial Framatome-ANP PAR system installations were carried out in Belgian nuclear power plants in 1995. To date more than 25 PAR systems have been installed in nuclear power plants. In total more than 1000 PAR units have been ordered or back-fitted to Western PWR plants and Soviet-designed VVER nuclear power plants. The hydrogen control system is based on the Passive Auto-catalytic Recombiner (PAR) technology. There is no need of any operator actions because of the self-starting feature of the catalyst if H2 is released. In addition the hydrogen concentration values of the containment atmosphere will be available for the plant operators in most PAR applications, using the measuring results of the hydrogen sensor system or PASS systems. During venting system process design particular importance is attached to the requirements regarding, for example, high aerosol loading capacity, provisions for decay heat removal from the scrubber unit, the aerosol spectrum to be retrained and entirely passive function of the retention unit. To meet above - mentioned requirement, a combined venturi scrubber unit was developed which comprises a venturi section and metal fibre section and could be operated in the sliding pressure mode and the atmospheric pressure mode. This scrubber systems was tested using a full-scale model and has now been installed in 16 PWR and BWR plants in Europe. Detail design work of venting systems for WWERs application, e.g. for Kozloduy 5 / 6 and in TARCIS project No. R2.08/95, for the reference NPP Balakovo, have been completed. The activation of the venting system shall be performed to avoid over-pressurization of the containment and will be decided from the on-site crisis board and needs: - the permission of the local state government, - and will be initiated in close contact with Emergency Technical Support Centre of the plant. Additional information before and during operation of the venting system will be received from the In-Situ PASS and Emission Control System. (authors)