Tepco has revised its estimates of the extent of fuel damage in units 1, 2 and 3 of the Fukushima Daiichi nuclear power plant. It said that mistakes made in data measured by the gamma detector-based Containment Atmospheric Monitoring System (CAMS) in each unit’s drywell and wetwell has thrown off the estimates.

As a result of the correction, its estimates of the unit 1 core damage ratio have decreased from 70% to 55%. The change is due to a corrected CAMS radiation measurement in the unit 1 wetwell that was one-third of the original value.

Its estimates of the unit 2 core damage ratio have increased from 30% to 35%. The change is due to a reinterpretation of a prior CAMS radiation wet well measurement, which has not changed. Core damage measurements are estimated based on a radiation-versus-time logarithmic chart – contour lines on the chart mark damage at intervals 5%, 10%, 25%, 50%, 75% and 100%. Tepco estimates the extent of core damage by plotting the measurements and interpolating the distance between contour lines. It has reassessed its earlier estimation.

Its estimates of the unit 3 core damage ratio have also increased, from 25% to 30%. The change is due to plotting different data on the graph. Tepco used radiation measurements from 62 hours after reactor scram for the unit 3 drywell, but did not plot 62-hour data for the wetwell, for reasons that are unexplained. The data has now been plotted.

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