Fukushima nuclear disaster: interpreting contamination reports; Japanese vs U.S. air/food/water standards

UPDATES:
NGOs call for better fallout monitoring and testing of U.S. food supply
Fukushima radionuclides in U.S. rainwater and milk; FDA not testing vegetables
EPA's RADnet radiation monitoring network (most of these volunteer-operated stations are back up)

Ongoing EURAD modeling of Cs-137 dispersion over the Northern Hemisphere
German estimates of I-131 and Cs-137 releases to the atmosphere in the first week are in the millions of Curies, approaching the magnitude of the Chernobyl releases. Attention is also focused on water-borne releases; there may be large long-term (months) release of cesium contaminated water; this will bioaccumulate in fish.

Early chronology as reported by the International Atomic Energy Agency:
(NB: the IAEA has a daily running update at: "http://www.iaea.org/newscenter/news/2011/fukushimafull.html")

Dose rates:
March 20, 2011
Dose rates reported to IAEA by Japanese officials at distances of 16 to 58 kilometers (10 to 35 miles) from the Fukushima station ranged from 2 to 160 microSieverts per hour (uSv/hr), or 0.2 to 16 millirems per hour; this is roughly 25 to 2000 times the unavoidable background dose rate of 0.0114 mrem per hour. The inadequate 20 km (12 miles) evacuation area was not extended, instead the Japanese government advised people in a ring 10 km further out to stay indoors.
See this NYT article: http://www.nytimes.com/2011/03/21/nyregion/21towns.html?src=twrhp
March 22, 2011, dose rates measured by IAEA and Japanese officials at distances of 35 to 68 kilometers from the Fukushima station ranged from 0.8 to 9.1 uSv/hr, or ~80 to 910 urem/hr; 10 to 114 times normal background.

Food, water and milk contamination:
March 21:
Spinach from Ibaraki Prefecture (just north of Tokyo) had Iodine-131 concentrations up to 55,000 Bq per kg (1kg = 2.2 pounds); this is equivalent to a very worrisome 0.68 microCuries per pound, in the more familiar English units. The Japanese limit for spinach is a quite weak 2000 Bq/kg; the U.S. FDA limit is 170 Bq/kg (or 2100 pCi/pound, or 0.0021 microCuries/pound). Normally there is no I-131 in food.
No reporting by IAEA on March 22, other than food from 5 cities south of Fukushima contained "high levels of contamination." Japanese Chief Cabinet Secretary Edano's comments that eating spinach for a year at this level of contamination posed "no risk" are reckless lies; health effects would be found and are quantifiable at the population level. High ground surface contamination (unidentified), up to 22.5 microCuries per square yard, was reported. No significant alpha radiation was detected, indicating no significant release of plutonium.
March 23:
Milk contamination is reported and safety assurances are given: without giving the actual dose Edano compares the dose from drinking such contaminated milk for a year to a medical CT scan (CT delivers the highest diagnostic radiation doses); this is intentionally misleading since CT scan doses vary widely and range from 150 mrem for head alone to 500 mrem for an abdominal CT to over 1300 mrem for a whole body scan.
[NB: The most recent report of the U.S. National Academy of Sciences' Committee on BEIR (Biological Effects of Ionizing Radiation) states that persons who have received whole body CT scans should be followed for increased cancer risk, and further, that there is no "safe" or threshhold ionizing radiation dose below which there are no health effects.]
Vegetable contamination is reported to be widespread. No reporting by IAEA of actual measurements.
Tokyo drinking water is surface sourced: 78% from the Tonegawa and Arakawa River systems,19% from the Tama River system. Tokyo drinking water showed levels of I-131 at 210 Bq/L; the Japanese limit for adults is 300 Bq/L, 100 Bq/L for infants. The U.S. EPA limit for drinking water is 4 Bq/L (108 pCi/L), normally none is present.
Seawater: 1-131 contamination reported up to 75 Bq/L (2000 pCi/L) 18 miles offshore; slightly lesser concentrations of Cs-137. This degree of contamination of ocean water at such a distance is indicative of large radioactive releases.
March 24:
Three contracted workers working on reactor 3 suffer acute radiation burns and bone marrow damage after exposure to radioactive water; dose 18 rems (~1.5 lifetimes of background exposure). Reactor 3 contains MOX (mixed oxide fuel, i.e. uranium mixed with plutonium from reprocessed spent fuel), which runs hotter than uranium fuel.

March 25:
"Today, Japanese officials told the population living up to 30 kilometers from the plant that they should consider leaving the area, expanding the previous 20-kilometer radius evacuation zone. But according to news reports, the advice stems from difficulties in supplying the region with food and water, not radiation levels.

Meanwhile, on Wednesday the Japanese science ministry began to report measurements of cesium-137 in upland soil around the plant. The levels are highest from two points northeast of the plant, ranging from 8690 becquerels/kilogram to a high of 163,000 Bq/kg measured on 20 March from a point in Iitate about 40 kilometers northwest of the Fukushima plant.

The soil measurements are more significant for evacuation purposes than radioactivity in the air, says nuclear engineer Shih-Yew Chen of Argonne National Laboratory in Illinois, because cesium dust stays underfoot while air is transient. Levels of cesium-137 are also more important than soil readings of iodine-131, which is short-lived and more of a concern in milk and vegetables. "It's the cesium that would prompt an evacuation," says Chen.

Based on a rough estimate, a person standing on soil with 163,000 Bq/kg of cesium-137 would receive about 150 millisieverts per year of radiation, says Chen. This is well above the U.S. Environmental Protection Agency standard of 50 millisieverts per year for an evacuation. (Per day, it's 0.41 millisieverts, which is equivalent to four chest x-rays.) But Chen adds, "one point [of data] doesn't mean that much."

The hot spot is similar to levels found in some areas affected by the 1986 Chernobyl nuclear reactor accident in the former Soviet Union. Assuming the radiation is no more than 2 centimeters deep, Chen calculates that 163,000 Bq/kg is roughly equivalent to 8 million Bq per square meter. The highest cesium-137 levels in some villages near Chernobyl were 5 million Bq/sq. m."
Quoted from Science Insider: see http://news.sciencemag.org/scienceinsider/2011/03/japan-soil-measurements-surprisingly.html
Note: In such a short weathering time the cesium contamination wouldn't be distributed much below the topmost surface layer, ie ~1 centimeter. So the 163,000 Bq/kg soil level reported by Japanese officials likely represents surface contamination closer to 16 million Bq/sq. m.
For those more familiar with Curies and millirems/rems:
The U.S. NRC's maximum allowable annual dose to a member of the public due to licensed, man-made sources (e.g., nuclear power stations) is 100 millirems, or 1 milliSievert.
The maximum allowable annual nuclear worker dose is 5,000 millirems, or 5 rems. 150 milliSieverts equals 15 rems.
163,000 Bq/kg Cs-137 is equivalent to a very high 4400 pCi/g Cs-137 (in the U.S., Cs-137 contaminated soils are usually cleaned up [i.e., removed to a licensed radioactive waste disposal facility] to a level below 10 pCi/g);
An I-131 maximum concentration of 1,170,000 Bq/kg was found in the same spot at Iitate village; 1,170,000 Bq/kg is equivalent to an astounding 31,590 pCi/g of 1-131.



Radioactivity units:
1 TeraBecquerel (TBq) = 1 million million Bq (1 x 1012) = 27 Curies (Ci)
1 Becquerel (Bq) = 1 atomic nucleus disintegration per second = 27 picoCuries (pCi)
1 pCi = 2.22 disintegrations per minute = 0.000000000001 Ci
1 Ci = 37 billion disintegrations per second

Radiation doses:
1 Sievert (Sv) = 100 rem
1 milliSievert (mSv) = 0.1 rem = 100 millirem (mrem)
1 microSievert (uSv) = 0.1 mrem = 100 microrems (urem)

Background dose rate components:
Background external radiation dose (composed of terrestial and cosmic ionizing radiation) = 60 mrem per year;
Internal radiation dose (mostly due to ingestion of naturally occurring K-40) = 40 mrem per year;
Outdoor radon (gas) exposure at ave. conc. of 0.4 pCi/L (this causes an absorbed internal dose) = 60 mrem;
Total unavoidable minimum background dose = 1.6 mSv per year = 160 mrem per year or 0.0183 mrem per hour, plus additional radon exposure from living in unmitigated dwellings (up to hundreds of additional mrems).
(Note: the U.S. Nuclear Regulatory Commission routinely reports a much higher annual average dose [600 mrem] because it assumes a much higher radon dose [indoor] and it also includes a relatively high medical dose [a chest x-ray, one of the lowest medical diagnostic x-ray doses, delivers <10 mrem, however a CT scan can deliver more than 2000 millirems]; see: http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/bio-effects-radiation.html

Normal background external dose rate in Tonawanda, NY is about 7 microrems per hour (61.3 mrem/yr).

The U.S. NRC has set an (additional) dose limit for individual public exposures from NRC licensees (nuclear power plants, etc.) at 100 mrem/year.