High speed sliding pressure venting^plus (AREVA’s FCVS with enhanced organic iodine retention) design principles and qualification of severe accident equipment

AREVA NP GmbH, Germany

Abstract

During venting system process design particular importance shall be attached to the requirements regarding, for example, high aerosol loading capacity, provisions for decay heat removal from the retention unit, the enveloping aerosol spectrum to be retrained, minimization of re-volatilisation and re-entrainment effects, long term operation of e.g. one week or even up to some months, and entirely passive function of the retention unit. Newer requirements request in addition high CH3I retention (organic iodine), a hard to retain gaseous component. Based upon the observation of the PHEBUS test organic iodine is produced in a greater extent during core melt accidents as assumed before [1]. Although the expected total amount of CH3I remains small, the radiological impact of unfiltered organic iodine is significant when high decontamination factors (DF) are reached for aerosols and elemental iodine.

After the Chernobyl accident the discussion about Filtered Containment Venting Systems (FCVS) started worldwide. Different process designs of FCVS have been suggested and were tested in laboratory tests, occasionally followed by large size test facilities. Finally it has been decided to carry out an international large size filtered venting test in order to investigate upon the advantages and drawbacks of the different filtered venting technologies. In the frame of this so called international ACE program the community discussed under participation of authorities and various research institutes and filter experts the relevant requirements and finally formulated the enveloping standardized test conditions. These international tests could be considered as the agreed venting qualification standard requirement and completes the venting process qualification. The results showed significant features in retention ability, loading capacities und resuspension effects of the different dry and wet filter devices. Dry filter systems were found to be sensitive in case of large filter loading. Wet filter systems demonstrated high loading capacity, but for fine aerosols medium retention efficiency. Combined scrubber systems showed high loading capacity and high fine and large aerosols retention efficiencies.

To meet above mentioned requirement, AREVA finally favorites a combined venturi scrubber unit which comprises a high speed venturi section and metal fiber filter section and could be operated in the sliding pressure mode and in atmospheric pressure mode. This scrubber system was tested using a full-scale model under sliding pressure operation up to 10 bar and finally in the frame of the international ACE filter test program with excellent results. Retention results of the international filter tests will be presented, reaching retention for

- large aerosols > 99,999%

- fine aerosol > 99,99%

- Iodine > 99,9 %

- Low resuspension / re-volatilization

Taking into account the reevaluation of organic iodine AREVA started to develop, design and qualify an additional retention stage for organic iodine retention – the FCVS PLUS. Based upon the current data gained during the FCVS PLUS qualification it can be stated with safety margin that the new development features an additional retention of

- Org iodine > 98 %