Containment venting through the off-gas system: a way of upgrading the accident manageability of the plant

The technical feasibility of the OGTS (Off-Gas Treatment System), installed in BWR's, as means to mitigate the consequences of severe accidents has been investigated in the past. The studies are still in progress. In particular are under investigation problems associated with the pressure drop through the off-gas line, the adoption and optimization of a water basin, useful to condense the first part of steam leaving the containment in case of an accident. Also fire problems on the charcoal beds were investigated as consequence of the heating induced on the charcoal by the decay power associated with the fission products collected on it. A short presentation of the main findings up to now is given in this paper for each item. Another point of interest is to quantify the benefits of using the removal capability, associated with the big mass of charcoal contained in the system, after determining the new margins of operability of the off-gas system. As generally understood the effectiveness of a mitigative system against radioactive contamination is ultimately connected with the delay allowed for the release of contamination into the environment. In a situation of continuous flow through the beds the delay can be of the order of ten hours (for the most volatiles components), but this activity get bottled in the charcoal almost indefinitely if it is assumed that the flow is stopped after a certain period. The venting of the containment can then be routed through an hardened stand by gas treatment system, if still necessary. The residual, less volatile, activity still present in the containment is released after removal through the filter trains. Using this procedure a decontamination factor of more than 100 for the most volatiles radionuclides can be attained. In conclusion: The conditions under which the feasibility of a venting through the OGTS could be assured has been evidenced in the report. The demonstration of the benefit of this feature, as part of a containment venting system has been given in terms of further reduction in the noble gases releases (factors of 50 for Kr, 300 for Xe). In addition it is reduced the uncertainty connected with the effectiveness in removing the organic forms of iodine. Those benefits make possible a greater flexibility in the operation of the system, which otherwise should be limited with regard to the time of actuation