Fukushima watch 2011-05-01

The city of Koriyama in Fukushima prefecture, where top soil was removed from schools to reduce radiation exposure to children recently, is not only 50 km west of the Fukushima 1 nuclear power plant, it also lies in the heart of the central valley where most of the agricultural products of the otherwise mountainous region are grown.

Fukushima has already lost 90% of its fishing industry due to the tsunami. How much income will its farmers lose? Consumers can not be sure they will be safe if they eat crops produced in the region unless systematic testing takes place, but if high levels re found, farmers will demand compensation. How much will TEPCO have to pay and how much the government? In the 1950s nuclear power was sold to the public on the promise of being able to deliver electricity “too cheap to meter”. Now it could turn out as too expensive to measure.

Government advisor resigns in protest

Mr. Toshiso Kosako, a radiation export at the University of Tokyo has resigned as an advisor of the Japanese government, citing his opposition to raising permissible radiation exposure of school children. On April 19 the Ministry of Education and Scienced had announced a limit of 3.8 μSv/h (microsievert per hour), which is comparable to international maximum exposure rates of people working in nuclear power plants. Actually, 3.8 μSv/h times 24 hours times 365 days equals 33 mSv (millisievert), whereas in other countries the maximum permitted extra exposure on the job is 20 mSv, which comes on top of a natural radiation exposure of 2-3 mSv.

Radiation exposure of workers

Two workers at the Fukushima 1 nuclear power plant, employed by a sub-contractor of the power company, have been exposed to close to their maximum legal radiation dose:

The two workers have been exposed to 240.8 millisieverts and 226.6 millisieverts of radiation, respectively, when internal exposure is taken into account, among 21 workers exposed to over 100 millisieverts of external radiation since the crisis erupted following the magnitude 9 quake and tsunami, it said.

Under Japanese law, the Ministry of Health, Labor and Welfare has limited by an ordinance radiation exposure of each nuclear plant worker at 100 millisieverts a year in an emergency situation, but raised the limit to 250 millisieverts to cope with the Fukushima crisis on March 15.

The two were hospitalized for possible radiation burns to their feet after standing in water that contained radioactive materials 10,000 times the normal level while laying a cable underground at the troubled plant on March 24, the utility known as TEPCO said.

The limit for non-emergency work is 100 millisieverts over five years with no more than 50 per single year. These two workers received over 200 in a single month. The Ministry of Health and Welfare is planning to scrap the 50 mSv per year limit, leaving only the 100 mSv per five year limit in place for non-emergency workers. This is meant to make it possible to send workers from other reactors to Fukushima for cleanup work and still make it legal for them to absorb further radiation at regular work once they return.

Emergency power

TEPCO tested it’s mobile power generators at Higashidori nuclear power plant in Aomori prefecture on April 20 but revealed that the truck-mounted diesels had fuel for only 2 1/2 hours. If they have to provide power for longer they will need frequent refueling. In Fukushima 1 when the backup diesels failed it took about 9 days until grid power was restored inside the turbine halls to run electric pumps.

Radioactive contamination in unit 4 spent fuel pool

TEPCO has released updated numbers for radioactive contamination of the pool water in the spent fuel element pool of unit 4 of Fukushima 1. Here are the figures for 2011-04-29 (number for sample taken on 2011-04-13 in brackets):

  • Caesium 137 (half life: 30 years): 54,000 (was: 93,000 Bq/l)
  • Caesium 134 (half life: 2 years): 49,000 (was: 88,000 Bq/l)
  • Iodine 131 (half life: 8 days): 27,000 (was: 220,000 Bq/l)

Because 16 days have elapsed since the previous analysis, levels for iodine-131 should have halved twice (to about 55,000 Bq/l) while levels for both caesium isotopes should be virtually unchanged. Instead both caesium values dropped by more than 40% while Iodine is about 50% below the expected value. Where did the rest of the caesium and iodine go? There are several possible explanations.

Perhaps one of the analysises was simply wrong. That would not be out of the question, as TEPCO has had trouble with its figures before. More likely though, perhaps the water level was much lower on the first sample, meaning the current sample is diluted by a significant amount of added water (+72% water would explain the figures). That is to be hoped, because if the water level was in fact the same and the figures were correct then a lot of the pool water radioactivity would have had to escape into the atmosphere with steam or leaked with water to some other part of the reactor building.

Arnold Gundersen (YouTube, starting from 4:28) raised an interesting point about the isotope testing in the unit 4 pool: There’s too much iodine-131 for this to be from leaking fuel rods. The unit 4 reactor was shut down for maintenance on 2010-11-29, about 5 months ago. 151 days from 2010-11-29 to 2011-04-29 is close to 19 half lives for iodine-131, meaning only 2 millionths of the original iodine-131 levels should be left in the freshest nuclear waste in that pool, even less in older fuel rods. If 220,000 Bq/l came from atmospheric contamination (e.g. fallout from overheated unit 3 which is right next to it), then that is 220,000,000 Bq/m3 of water, left over after 4 half lives, so the original amount would have been 3,520,000,000. The pool is about 13m deep. To get it that radioactive, some 45,760,000,000 Bq/m2 had to land on the pool, which is a staggering amount.

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