Analysis of dose rates in the containment during the Fukushima accident

Dose rates that were measured during the Fukushima accident using the CAMS (Containment Atmosphere Monitoring System) were analyzed. Distributions of Kr, Xe, I and Cs in the containment drywell and wetwell, calculated with the MELCOR code, were converted to dose rates using conversion factors published by Uchida et al. The calculated dose rates were then compared with the measurements. While the wetwell dose rates were reproduced reasonably well, significant discrepancies were observed in the drywell dose rates, particularly for units 2 and 3. The calculated peak drywell dose rates occurred much too early, and the long-term dose rates were significantly underestimated. Several possible reasons for these discrepancies were identified, including missing models of iodine chemistry, wash-off of deposited aerosols by condensed water, desorption of chemisorbed CsOH, complex structures and piping in the drywell, threedimensional effects, debris spreading from the pedestal to the drywell, and absence of conversion factors for certain nuclides.