Sunday, March 20, 2011

Situation stabilizing at Fukushima

Current reports from TEPCO and NISA indicate that workers have restored off-site power at Units 1 and 2, with plans to restore off-site power to Units 3 and 4 tomorrow (March 21). This is a welcome development, indicating that the situation at the reactors is likely stabilizing. With the restoration of off-site power, cooling pumps can begin to circulate coolant in the reactors and spent fuel cooling pools once more, largely eliminating any further risk of fuel overheating and subsequent inadvertent release of radioactive materials. In addition, holes have been bored into the roofs of Units 5 and 6 as a preventative measure to vent any potential hydrogen buildup as to prevent further explosions of the kind that damaged Units 1 and 3. As off-site power is restored and the radiological levels decline, the damage can be more fully assessed and cleanup work can begin.

Meanwhile, news reports from Japan indicate that through the use of infrared sensors, the surface temperatures at each of the reactors have been verified to be well below 100 C (i.e., the boiling point of water), meaning that water is present at each of the reactor units. It would thus appear that the efforts of workers to restore water to the spent fuel pools and reactors has been successful.

XKCD recently posted a very interesting diagram putting the radiation levels observed in perspective - it's a very useful diagram for directly comparing some of the levels being discussed with levels encountered in everyday settings. This presentation by UCSB physics professor Benjamin Monreal also gives an excellent and concise summary of the likely radiological release consequences of Fukushima as well as past historical incidents (such as Three Mile Island and Chernobyl).

Reports of produce contamination

Now that the situation with the reactors has appeared to have calmed down, much of the media focus now has shifted to reports of radioactive contamination of milk and spinach from areas near the plants. It should be emphasized that the levels of radioactivity found, while above regulatory limits, do not pose any immediate threat to human health.

I will be posting a longer discussion of how radiological contaminaiton gets into the environement (and what we mean by "radiological contamination"), but in the meantime, it is useful to summarize what is going on in this case and the chief areas of concern.

Given the nature of the radioactive release, most the released products fall into the categories of a) Radioactive noble gases (such as krypton, xenon, etc.) and b) Radioactive, chemically active gases such as iodine-131, c) Radioactive isotopes of cesium and strontium.

Noble gases are of little concern, as they do not generally interact with the biosphere (i.e., they don't "stick" to anything, given that they are chemically inert). As a result, these gases will generally travel further into the atmosphere, becoming increasingly diluted (and thus of little concern for human health).

Chemically active species such as iodine are one of the more serious concerns for contamination, as iodine is readily absorbed by the human body (and rapidly taken into the thyroid); this is the basis for using potassium-iodine tablets as a "prophylactic" measure, flooding the thyroid with natural (non-radioactive) iodine such to prevent the uptake of the radioactive species. Iodine in fission products generally falls into two species - iodine-131, which has a half-life of 8 days, and iodine-129, which has a half-life of 15.7 million years. Only iodine-131 is of any serious contamination concern, given the extremely long half-life of iodine-129 (which means the activity from this species is extremely low). (Iodine-129, while being inconsequential to short-term dose, is an isotope of concern for geologic repositories, given its long lifetime and iodine's ease of movement in groundwater.)

Radiological half-life denotes the time in which half of a radioisotope species decays away into a different species. After one half-life, half of the original radioactive species remains; after two half-lives, only a fourth remain, and so on. In some cases, the "daughter" isotope is stable, meaning that no further decays occur. Other times (like with radon), the daughter products are also unstable, leading to a series of decays. (These isotopes can thus be of greater concern - like with radon.)

Iodine-131 decays into Xenon-131, which is stable. Thus, after 8 days, the radiological contamination due to iodine will decrease by approximately half. By three weeks' time, this level will have dropped to an eighth of the original level; by three months' time, the radioactivity from iodine will have virtually disappeared.

Other radioisotopes of concern are strontium-90 and cesium-137, which have half-lives of about 29 and 30 years, respectively. (These two isotopes are incidentally of concern from a waste management perspective, as the heat generated by this pair is a working constraint on repository capacity over the first hundred years.) Because the half-lives of these species are longer, their contributions to dose tends to be extremely small (much of what is ingested would be released by the body before it would decay). Strontium, being of the same element family as calcium, tends to chemically act like calcium in that it is accumulated in the bones; however, the dose levels from strontium would be quite small (far lower than that from natural sources).

Many have pointed to the iodine contamination in the environment (particularly in milk) following Chernobyl as a reason for concern from Fukushima, however the levels of contamination from Chernobyl were far, far greater than those from Fukushima. Unlike Chernobyl, comparatively little radioactive material escaped the reactor, nor was this material lofted high into the air through fires like those found in Chernobyl. As a result, the levels of contamination are far smaller and the effects far more localized. Evidence of this is found in the relative dose estimates based upon monitoring stations around Japan and the plant itself; doses at the plant boundary have been around a few millirem per hour and dropping (particularly as coolant has been restored to spent fuel pools); levels are far lower than this farther away from the plant. 

Beyond reconstruction of the heavily damaged area (both from the earthquake and tsunami), one of the major challenges appears to be in overcoming the stigma of radiological contamination, despite the extremely low levels present (particularly after a few weeks). While any actual long-term health risk from radioactive contamination will be quite minimal after a few weeks (with no immediate-term risk at all), overcoming the stigma of radioactive contamination will take some time. In particular, one of the most pressing challenges will be to further educate the public about both comparative levels of radiation found in nature and the (relative lack of) increased risk from low levels of additional radioactivity.

Update: Alan asked about the nature of emissions from these isotopes of concern and what their subsequent "weighting" factors would be. Cs, Sr, and I are all beta emitters; Cs-137 decays into Ba-137, which has a short-half life and gives off a gamma as it decays. Given this, the weighting factors for each is "1" - so in other words, the conversion from gray/rad is the same value in sievert/rem.

I also found some more resources on maximum recommended contamination levels - the World Health Organization generally recommends contamination levels expressed in units of activity per unit mass - i.e., Becquerels per kilogram (Bq/kg). One Becquerel is one decay per second - so 1000 Bq is 1000 decays per second. (While this sounds large, keep in mind just how many atoms are in even one gram of material - 1000 Bq is actually pretty small.)

The WHO recommends the following limits in foods:

Radioisotope Max activity (Bq/kg)
I-129, I-131 1000
Cs-137 100
Sr-90 100

This of course is not a comprehensive list (the list is found on page 33 of the above-linked document), however it covers the radioisotopes of concern here.

(For those curious - one can calculate an equivalent dose received by multiplying the following formula: Exposure = Activity [Bq/kg] * Mass consumed [kg] * Age-dependent ngestion coefficient [mSv/Bq]. These ingestion factors are in the above-linked document; I will cover this topic in more detail in a further post.)

CNN is reporting measurements of 965 Bq/kg in tap water in a village in Fukushima prefecture (where the Fukushima Daiichi plants are), compared to a national regulatory limit of 300 Bq/kg. As a result, authorities are advising residents to avoid drinking tap water for the time being.

Again, however, knowing the half-life of I-131 allows us to know when these levels will fall to the safe (conservative) limits; in this case, in about two weeks for an areas closest to the contamination source. While certainly undesirable, in this time frame there are likely other problems with tap water as well beyond radiation, including contamination from biological pathogens, particularly given the tsunami.