Indirect assessment of 135Cs activity in the ventilation system of the Ignalina NPP RBMK-1500 reactor

The accumulation of long-lived 135Cs isotope in the ventilation system components of the Ignalina NPP Unit 2 was investigated by spectrometric measurements and mathematical modeling. Volumetric activities of fission noble gas and other short-lived isotopes (41Ar, 85mKr, 87Kr, 88Kr, 88Rb, 133Xe, 133mXe, 135Xe, 138Xe, 138Cs) have been measured by gamma spectrometric technique. Modeling of radionuclide transport in the ventilation system provides possibility of determining essential transport parameters: effective gas flow, mean gas retention time, deposition rate of aerosols. Estimated parameters were used for indirect evaluation of difficult to measure 135Cs isotope activity in the ventilation system components: a delay chamber and aerosol filters. The results show that the major part of 135Cs activity is accumulated in aerosol filters, whereas the total surface activity of the delay chamber is considerably lower. Specific activities of the ventilation system components of the Ignalina NPP Unit 2 are below the clearance levels for 135Cs.

► Short-lived gaseous fission products in the RBMK-1500 reactor off-gas.
► Measurement and modeling of gaseous activity in the reactor ventilation system.
► Determination of difficult to measure 135Cs in the reactor ventilation system.