Reply #6: The adjustement is explained in the Yomiuri Shimbun excerpt

[caraher]

Iodine-131 and cesium-137 were released into the atmosphere that day at the estimated rates of 0.69 terabecquerel per hour and 0.14 terabecquerel per hour, respectively, the NSC said.

Emissions are converted into iodine-131 equivalents for assessment on the international nuclear event scale (INES), to arrive at the total 154 terabecquerels per day, the nuclear safety watchdog said.

What does this mean?

The original claim of "less than 1 TBq per hour" was absolutely correct.

But not all radioactive isotopes are created equal. In 80 days 1 TBq of I-131 becomes 1 GBq of I-131 - in 10 half-lives the radioactivity is 1/1000 of what you started with (technically, 1/1024). But in that same time, 1 TBq of Cs-137 is still about 1 TBq of Cs-137, because its half life is around 30 years.

For use with the INES scale, the standard practice is evidently to scale releases to some exposure equivalent to a given release of I-131. This means that you have to multiply the actual Cs-137 by a big number to reflect that fact that 1 TBq of Cs-137 stays radioactive much longer than 1 TBq of I-131 (and thus represents many more decays). There is also probably further mathematical weighting to reflect different biological half-lives, accumulation patterns in the body, and energy deposited per decay.

So it's not the case that the previous values were off by a factor of 154. They've simply re-standardized the emissions into new units that are not TBq/day but I-131-equivalent TBq/day. (It's just like converting tons of CH4 into tons of CO2 equivalent in a greenhouse gas calculation - you still have a ton of CH4, but its effect is equivalent to that of a greater amount of CO2.) Evidently the weighting factor is something like 154/0.14 or around 1000 - that is, the long-term exposure risk posed by 1 TBq of Cs-137 is estimated to be around 1000 times greater than that posed by 1 TBq of I-131.