The world is finely attuned to nuclear disaster. In the past two weeks, global monitoring stations designed to detect the detonation of atomic bombs began alerting the world to what it already knew: a disaster was unfolding at Fukushima nuclear power plant, and radioactive particles had escaped. Radioactivity is a devilish thing to track: it’s easier to determine trace levels of radiation thousands of miles away in Massachusetts or Scotland than it is to determine the precise danger posed by contamination in the area surrounding Fukushima’s six stricken reactors. The threat must be assessed piecemeal, often by mobile measuring units navigating through a disaster zone. Even without any further leaks, it may take weeks before the full extent of radioactive release from Fukushima–and the implications for human health–are known. Health officials will be focusing on several different types of radiation.
* Environmental Radiation In the immediate aftermath of the Fukushima crisis, radiation escaped into the atmosphere through hydrogen explosions and steam deliberately vented from the reactor buildings. This is the only known source of radiation since the incident—reports of contaminated crops, sea and tap water are likely to have been caused when radioactive airborne particles from these early leaks fell to earth or water.
Environmental or background radiation remains high outside the 20 km (12.5 mile) “exclusion zone” surrounding the Fukushima plant. However, these “high” levels are not considered highly dangerous to human health. On March 28, the highest reading occurred approximately 20 km from the Fukushima plant at 78 microsieverts per hour. At that level, an individual would have to stand outdoors, unshielded, for 27 consecutive days in order to reach the maximum annual limit allowed for workers at a U.S. nuclear power plant.¹ It would require double that amount of time before that individual would face a (very slightly) elevated risk of cancer. Even so, high levels may be unacceptable to full-time inhabitants of the area, so eventually clean up crews will need to scrub streets and buildings to remove longer-lived radioactive particles. A decision will also need to be made about whether the area inside the exclusion zone can ever be inhabited again.
If no further radiation is released, public health will almost certainly not be threatened by radiation beyond the immediate vicinity of Fukushima, and certainly not as far away as the United States—that includes potential risks from contaminated water, milk and crops in the U.S. and elsewhere. On Tuesday, Massachusetts became the latest U.S. state to detect trace levels of radiation in collected rain water. But experts say that even in a worst-case scenario at Fukushima, radioactivity would be so diluted by the time it arrived on U.S. soil as to be rendered harmless.
However, a local health disaster could still occur in Japan. Greenpeace says that simulations of radioactive dispersion effects for a reactor similar to those found at Fukushima— the Biblis pressurized water reactor in Germany— found that following a total, “worst-case” release form a reactor core, even people staying indoors at a distance of 25 km (15.5 miles) could die of acute radiation sickness. Emergency workers continue to try to prevent such a scenario, but the Japanese Prime Minister said on March 28 that the the country remains on “maximum alert.”
*Contamination of the Ocean: In the latest challenge at Fukushima, emergency workers scrambled on March 28 to stop highly contaminated water from reaching the ocean. A series of underground tunnels containing radioactive water open only a few hundred yards from the sea, and workers brought in sand bags to try to prevent a leakage. Already, however, marine monitoring stations have picked up contamination as far off as 30 km (19 miles) from shore, probably after particles deposited in the sea following the explosions and venting in the immediate aftermath of the earthquake and tsunami. Radioactive elements in the ocean can enter the human food chain via fish and shellfish, but the IAEA website states “It is still too early to draw conclusions for expected concentrations in marine food.” Further testing is ongoing.
* Ground Contamination: So far, high levels of iodine-131 and cesium-137 have been detected in the ground around Fukushima. Iodine-131 decays quickly, so is of less concern than cesium, which can stay in the soil for hundreds of years. The IAEA reports that the highest level of “daily deposition” for cesium-137 was recorded at the Yamagata prefecture on March 26 at 1,200 becquerels per square meter. At that rate, it would take more than a year before Cesium-137 levels rose to the threshold of 555,000 bacquerels per square meter that triggered strict land-use controls after the Chernobyl incident. Even so, it’s difficult to ascertain the danger posed by ground contamination due to the way the IAEA records deposition levels. The Union of Concerned Scientists has complained that the IAEA has “failed to establish a consistent reporting framework so the public can assess whether radionuclide release rates are changing”–i.e. it’s difficult to know if the problem of ground contamination is getting worse, and why.
* Tap Water Contamination: Tokyo suffered a public health scare last week when officials reported that tap water contained more iodine-131 than was safe for infants. Most of the iodine-131 quickly decayed or was diluted, however, and the tap water in Tokyo and elsewhere in Japan is now safe. The highest levels of iodine-131 was recorded at 90 becquerel per litre, according to the IAEA. The Japanese limits for the ingestion of drinking water by infants—who are most sensitive to radiation exposure—is 100 becquerel per litre.
* Food Contamination: According to the IAEA, samples of fruits and vegetables in six prefectures around Fukushima reported abnormal levels of radioactive contamination. One sample of wasabi flower from Fukushima prefecture measured above the regulated values for iodine and cesium set by Japanese safety authorities. However in the remaining five prefectures, cesium-137 was not detected or the results were below regulation values. A delegation from the UN’s Food and Agriculture Organization met with local government authorities on March 27 to discuss a mitigation strategy to deal with contaminated foods.
According to the U.S. Food and Drug Administration., all milk and milk products and vegetables and fruits produced or manufactured from four Japanese prefectures around Fukushima–and locally-sourced seafood, too—will be detained upon entry into the United State and tested for radionuclide contamination. However, the heavy damage caused by the earthquake and tsunami to the region means few products are currently being exported from that region of Japan anyway. Food and dairy production in the U.S. will not be effected, according to the FDA.
¹ 50 millisievert is annual limit for U.S. nuclear workers. That equals 50000 microsieverts. 50000/78=641 hours=27 days.
