Why Cesium?

The goal of Kelp Watch 2014 is to measure the radioactive isotopes Cesium-134 and Cesium-137 in the tissue of kelp from the west coast of North America. These radioactive isotopes and others were released into the environment by the Fukushima Dai-ichi nuclear power plant accident in Japan. But what makes these two isotopes particularly interesting for our study?

Radiocesium was among the largest releases from Fukushima Dai-ichi

In March 2011, as a result of the Tōhoku earthquake and tsunami, the Fukushima Dai-ichi nuclear reactors suffered a major accident and released radioactive materials into the environment. These releases went directly into the atmosphere through stream and hydrogen explosions as well as into the ocean through rainout from the atmosphere and through leaks of radioactive water from the plant [NERH 2011]. Due to the nature of the releases, the major contaminants were isotopes of volatile elements: Iodine-131 (8 day half-life), Tellurium-132 (3 day half-life), Cesium-134 (2 year half-life), and Cesium-137 (30 year half-life) [Steinhauser et al. 2014].

Because of the relatively short half-lives of 131I and 132Te, by 2014 these isotopes have completely decayed away. This leaves 134Cs and 137Cs as the largest components of the initial radioactive releases that could also still be in the environment. In fact, these two isotopes have been measured in the ocean far from the Japanese coast [e.g., Buesseler et al. 2012, Kameník et al. 2013].

Cesium-134 and 137 are relatively easy to detect

In addition to being the most abundant isotopes remaining, 134Cs and 137Cs are relatively easy to detect using high resolution gamma-ray spectroscopy such as the high purity germanium (HPGe) detectors at the LBNL Low Background Facility. Both radioisotopes undergo beta decay with associated characteristic gamma rays that act as "fingerprints" for identifying and quantifying the isotope. The strongest characteristic gamma rays of 134Cs are 605, 796, and 569 keV, while 137Cs has a single gamma-ray line at 662 keV.

One complication: legacy 137Cs

It will be important to measure both Cesium isotopes, not just the longer lived 137Cs. The reason is that decades of above-ground nuclear weapons testing (mostly in the 1950s and 1960s) released significant amounts of 137Cs into the Pacific Ocean [UNSCEAR 2000]. Since 137Cs has a 30 year half-life, any 137Cs measured in the environment could be from these "legacy" sources and not from Fukushima Dai-ichi. In fact, 137Cs has been measured for decades in the Pacific Ocean, and the pre-Fukushima levels off the California coast are approximately 1–3 Becquerels per cubic meter (Bq/m3) [Hirose & Aoyama 2003]. (One Becquerel is one nuclear decay per second.) For comparison, models of the dispersion of 137Cs from Fukushima Dai-ichi predict a maximum of about 2–3 Bq/m3 on the West Coast in the next few years [Behrens et al. 2012, Povinec et al. 2013], although some models predict as high as 10–30 Bq/m3 [Rossi et al. 2013]. So how are we going to tell 137Cs from Fukushima Dai-ichi apart from "legacy" 137Cs?

The answer to this problem lies with 134Cs. With its 2 year half-life, any legacy 134Cs has decayed away to negligible amounts. This means that if we measure 134Cs in kelp tissue, then we know that it came from the Fukushima Dai-ichi reactors. The last piece of the puzzle is the ratio of 134Cs to 137Cs: it was approximately 0.9 in March 2011 [Kirchner et al. 2012]. So when we correct the measured activities for radioactive decay since March 2011, we will therefore be able to infer how much 137Cs is from Fukushima Dai-ichi as opposed to legacy sources.

In a preliminary measurement for Kelp Watch 2014, kelp tissue from Long Beach, CA was found to have small levels of 137Cs (0.40 Bq/kg) but no 134Cs (<0.04 Bq/kg). This means that we detected only legacy 137Cs and no cesium from Fukushima Dai-ichi, as expected.

Another complication: naturally occurring radiation

It is important to note that although 134Cs and 137Cs might become detectable and distinguishable from legacy 137Cs, it is not the only radioactivity expected in kelp tissue. In fact, the radioactivity of kelp tissue is dominated by the naturally occurring isotope Potassium-40. 40K is a primordial isotope present in anything containing the element potassium, which includes most organic matter and many minerals. In kelp tissue, the levels of 40K are around 4,000 Becquerels per kilogram (dry weight), whereas in our preliminary sampling we found existing 137Cs levels around 0.4 Bq/kg — ten thousand times lower than 40K. This is why we require the use of high resolution gamma-ray spectroscopy using high purity germanium (HPGe) detectors and simple Geiger counters or even scintillation detectors will not be good enough.

Why not radiostrontium?

Strontium-89 and Strontium-90 were also released from the Fukushima Dai-ichi reactors. With a half-life of 29 years, 90Sr can remain in the environment for a relatively long period of time. However, the Fukushima Dai-ichi reactors released orders of magnitude less 89Sr and 90Sr than 134Cs and 137Cs [Steinhauser et al. 2014]. Measurements of radiostrontium in the ocean off the coast of Japan in 2011 showed radiostrontium had less than 3% the radioactivity of 137Cs [Casacuberta et al. 2013].

Besides there being a much smaller amount of radiostrontium than radiocesium, there are two other issues affecting our ability to measure those isotopes. The first difficulty is that both strontium isotopes are pure beta emitters, and the lack of any identifying gamma-ray fingerprints makes any measurement of 89Sr or 90Sr tedious and time-consuming. Another problem is that, as with 137Cs, there is a significant amount of legacy 90Sr from nuclear weapons testing [UNSCEAR 2000], on the order of 1 Bq/m3 in the Pacific [Casacuberta et al. 2013]. Therefore the 90Sr from Fukushima would have to have some marker to distinguish it from legacy sources. The analogue of 134Cs in this situation is 89Sr, but 89Sr has a half-life of only 50 days and is also a pure beta emitter. These two facts make it practically impossible at this time to measure 89Sr and therefore tell how much Fukushima-derived 90Sr is in the ocean.

What about other isotopes?

There were many other radioactive elements released by the Fukushima Dai-ichi reactors, including Silver-110m, Barium-140, and Molybdenum-99 [Le Petit et al. 2012]. Some even have evidence that Plutonium isotopes were released in small quantities [Zheng et al. 2012]. Most of these isotopes are too short-lived to remain today, and all were released at levels orders of magnitude below radiocesium [Le Petit et al. 2012]. Nevertheless we should be able to set limits on those isotopes that are gamma-ray emitters.

Contributed by Mark Bandstra, Ph.D.; 2/18/2014; updated 3/15/2014