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Monday, February 20, 2012

Plutonium & A Fukushima Mother's Fears


Almost a year ago, three nuclear reactors at the Fukushima Daiichi Nuclear Power Station on the northeast coast of Japan incurred fuel core meltdowns in the wake of the Tohoku-Chihou Taiheiyo-Oki Earthquake and Tsunami, releasing massive concentrations of highly radioactive material into air, land and sea. A few weeks later, a Fukushima mother composed a moving letter to the world, sharing her fears that the toxic pollutants would pose hazards to the environment and human health in Japan as well as in the world at large, and she apologizes for the trouble this incident may cause for everyone. I re-published a translation of her letter on May 27, 2011.

Among the radionuclides released from the stricken power station, the gamma-radiation emitters iodine-131 and cesium-137 have been of foremost concern. Iodine-131 with an eight-day half-life posed the greatest public health thread in the first two months immediately after the reactor accidents and has decayed. The effects of iodine-131 accumulated in the thyroid of residents of all ages living in the vicinity of the power station remain to be seen. Japanese health officials are still trying to determine the exact effective radiation absorbed doses people may have received.

Furthermore, Japan has continuously been grappling with pervasive, ubiquitous cesium-137 contaminations. Cesium-137 with a half-life of 30 years has been accumulating on the ground in hot spots and has been concentrating in plants and animals, contaminating crops and lifestock (Aya Takada and Yasumasa Song's post with the title "Beef Contamination Spreads in Japan as Straw Tainted" published online on Bloomberg Jul. 15, 2011), as well as seafood (Mark Willacy's post with the title "Toxic caesium found in fish off Japan" published online on ABC News May 25, 2011). Wind, precipitation and human activity (see post on JAPANTODAY with the title "Radioactive crushed stone may have been used in over 80 buildings, METI says" published online Jan. 23, 2012) have spread cesium-137 across the country. Radioactive cesium contamination will pose risks to public health in Japan for decades to come.

The Fukushima mother, however, was most concerned with plutonium in her letter. The plutonium isotopes that occur in commercial nuclear power reactors comprise plutonium-238 (half-life: 88 years), plutonium-239 (half-life: 24,110 years), plutonium-240 (half-life: 6560 years) and plutonium-242 (half-life: 373,300 years). Decaying into uranium isotopes with the mass numbers 234, 235, 236 and 238, respectively, these isotopes predominantly emit high-energy alpha-radiation, that is helium nuclei that travel a short range because they readily interact with atoms and are rapidly absorbed in the materials they penetrate. Therefore, they pose the greatest threat to health when inhaled or ingested. Plutonium concentrates mainly in the lungs when inhaled and in bone-marrow and in the liver when ingested. Though no plutonium toxicity-caused fatalities have ever been reported, alpha-emitters are known to cause lung cancer, e.g. in uranium miners, and can lead to acute radiation syndrome, e.g. polonium poisoning.

Uranium oxide containing uranium-238 and, in small amounts, uranium-235 represents the most common fuel used in nuclear power reactors. Uranium-238 decay produces plutonium-239 which decays into plutonium-240. Plutonium-242 is the product of successive neutron capture of the other plutonium isotopes with rising mass number beginning with plutonium-239. Therefore, spent nuclear fuel contains a mixture of the four plutonium isotopes discussed.

Uranium and plutonium can be extracted from spent fuel, recombined into a mixed oxide fuel, or MOX fuel for short, and reused in commercial light water reactors with the idea of improving the total burn-up. According to Table IV-3-1 in the update of the Government of Japan to the International Atomic Energy Agency with the title "Report of Japanese Government to the IAEA Ministerial Conference on Nuclear Safety - The Accident at TEPCO’s Fukushima Nuclear Power Stations- submitted to the International Atomic Energy Agency Jun 7, 2011", 32 of the 548 fuel assemblies loaded in the reactor of Fukushima Daiichi Unit 3 and 52 unused assemblies stored in its spent fuel pool consisted of MOX fuel last March when the meltdown unfolded. On Mar. 14, 2011, Unit 3 incurred the most damaging hydrogen explosion of the three operating reactors. Therefore, it is possible that plutonium may be found outside the reactor.

The operator of the Fukushima Daiichi Nuclear Power Station, Tokyo Electric Power Company (TEPCO), has repeatedly tested dry soil samples for plutonium-238, plutonium-239 and plutonium-240 in various locations on the station premises, and found the isotopes to be present in minut concentrations close to the detection limit. The locations with positive identification are shown in the map below.
Sampling locations at Fukushima Daiichi Nuclear Power Station, where plutonium was detected (TEPCO).
The results must be compared to the concentrations that already existed before the accident. Traces of plutonium-239 and plutonium-240 occur naturally, produced by rare spontaneous uranium-238 fission. Atmospheric and underwater nuclear bomb tests as well as accidental releases added small amounts to these concentrations. As a rule of thumb, the historic ratio between the concentration of plutonium-238 and the sum of the concentrations of plutonium-239 and plutonium-240 has equaled roughly 0.026 in Japan. The Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) conducted a comprehensive study from 1987 to 2008, producing an average ratio of 0.033. Finding a value in excess suggests that extra shorter-lived plutonium isotope is present, possibly released from the damaged reactors.
The table is composed of data TEPCO collected at the three locations shown on the map above on Mar. 21, 2011, and on Feb. 6, 2012 (※: MEXT environmental radiation database 2008 - 1978).
The table above shows TEPCO's findings at the three sampling locations with positive results, that is the athletic field, the Forest of Wild Birds and near the industrial waste disposal site, surveyed Mar. 21, 2011, and Feb. 6, 2012. Each location is situated at a distance of about 500 meters from the stack between Units 1 and 2. The company found excessive isotope ratios at each location. On the athletic field, excess was observed on both sampling dates.

Though the amounts of reactor plutonium TEPCO detected so far may seem small, the results strongly suggest that the Fukushima mother's fears were not unfounded.

Acknowledgement

I wish to express my deep sympathies for the Fukushima mother who wrote her letter to the world. I am indebted to the contributors of SimplyInfo.org who are my sources in this matter.

Addenda
  • Accident-related increases in plutonium concentration have not been restricted to the premises of Fukushima Dai-ichi Nuclear Power Station. On July 13, 2011, a Fukushima Prefecture radiation survey team found a [Pu-238] / [Pu-239+240] ratio of 0.214 in a dry soil sample collected in Ottozawa, Okuma District, in the immediate vicinity of the station southwest of its main gates (reference line 4 in the second table of attachment 1 to the Prefecture's report with the title "Radiation monitoring of soil in Fukushima Prefecture (plutonium). Survey results (preliminary report)" released Nov. 29, 2011). On September 16, 2011, the team found the second highest ratio in their survey, that is 0.0532, in Fuju, Minamiaizu District, nestled in the mountains 68 miles west of the station (attachment 1 of the report; first table, row 33)(06/17/2012).
  • The government's plutonium finds made it into the Japanese press yesterday. According to a Jiji Press report with the title "Plutonium Detected at 10 Locations in Fukushima", plutonium attributable to the reactor accidents at Fukushima Dai-ichi Nuclear Power Station has been positively identified at 10 locations in Namie, Okuma, Iitate and Minamisoma municipalities of Fukushima Prefecture. The findings are contained in the Ministry of Education, Culture, Sports, Science and Technology report with the title "Nuclide Analysis for Plutonium-238, 239+240 and 240 (Second Investigation)" (08/22/2012).
  • NHKWorld reports in a post with the title "Town in Fukushima adopts 5-year no-return policy" published online today that most residents of Okuma directly adjacent to Fukushima Dai-ichi Nuclear Power Station will not be permitted to permanently return to their homes for five years because of persistently high doses of ionizing radiation. The the ordinance adopted by the municipal council based on government recommendations covers 95 percent of the town's area (09/21/2012). 

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