Instead, the decision will be made using the air radiation levels and the water levels inside the nuclear reactors.
In other words, when (if) they do distribute the pills to citizens who may be affected by a nuclear accident, it may be too late. If the last year is any indication, such data will be released several weeks to several months after a nuclear accident happens.
As to the water levels inside the reactors, it's a joke. TEPCO and NISA insisted there was water inside the Reactor Pressure Vessels until mid May.
But no matter, that's what the Nuclear Safety Commission under Dr. Haruki "Detarame" Madarame has decided and is duly reported by NHK.
NHK conveniently omits the now-known fact that the SPEEDI simulation data was there from the beginning. The politicians and the bureaucrats, in their petty turf war, couldn't make any use of it. They even had three sets of simulation from three different entities - the Ministry of Education, the Nuclear and Industrial Safety Agency, and TEPCO, from the beginning of the accident. Everyone decided to drop the ball all at once.
How did they manage to do that? Asahi Shinbun's "Trap of Prometeus" has an excellent coverage on that (part 1 here, part 2 here).
From NHK News (1/4/2012):
In the Fukushima I Nuclear Power Plant accident, the system to forecast the dispersion of radioactive materials didn't function, causing the confusion as to what to do with the potassium iodide pills to avoid radiation exposure in the thyroid. So the Nuclear Safety Commission has decided not to use the system any more to instruct residents to take the pills; instead, the air radiation levels and other indicators will be used.
In a nuclear accident, in order to decide whether to take potassium iodide pills to prevent thyroid irradiation, the national government is to use the data from the simulation system to forecast the dispersion of radioactive materials, called SPEEDI, and to instruct the residents.
However, in the Fukushima I Nuclear Power Plant accident, this system did not function and the national government and the Fukushima prefectural government did not make a decision [on whether to instruct residents to take the pills], causing confusion in the municipalities affected on what to do with the potassium iodide pills. The working group at the Nuclear Safety Commission has determined that it is doubtful that SPEEDI data will be used for a quick response in the future and therefore the SPEEDI system won't be used in determining the need for the administration of the pills.
Instead, the Commission is considering using the data such as the air radiation levels and the water level in the reactor. Further, the levels to instruct the residents to take the pill will be revised based on the prevailing international standards. For 1 year-old, it will be an equivalent dose of 50 millisieverts at the thyroid, which is half of the current level. The working group will further consider the numbers and will make a suggestion to the government by March.
WHO's standard for 1-year old is 10 millisieverts.
(I sent a tweet to NHK Kabun (culture and science), telling them the article is false about SPEEDI. I was surprised when NHK Kabun marked my tweet as "favorite".)
Speaking of potassium iodide, there are tweets from the residents in Iwaki City, Fukushima Prefecture that they just received an envelope from the city that contained potassium iodide pills. They are wondering why, and wondering if Reactor 4 is as safe as TEPCO has said after the water level decrease in the Skimmer Surge Tank.