The purpose of this blog is to bring to the attention of interested readers a recent peer-reviewed, open-access study published in the Journal or Radiological Protection . The investigators describe the design and manufacture of a whole body sensor whose purpose is the detection of 137-Cs (half-life ~30 years) in children who were proximate to radionuclide releases after the triple meltdowns at the Fukushima-Daiichi nuclear power plant which began in March 2011. Health impacts of the disaster are likely to be most acute in Japan given that levels of radionuclides in air, soil and water resulting from the disaster were higher compared to levels measured and expected on the west coast of North America. The detector in question (called BABYSCAN) is demonstrated to have a detection limit of better than 50 Bq/body and has been installed in a hospital in Fukushima. Because children are most vulnerable to the impacts of ionizing radiation, 100 Fukushima children were scanned for the presence of 137-Cs and none were found to have detectable levels of the isotope in their bodies. Larger scale measurements of the population will be reported as the long term impacts of low levels of ionizing radiation present owing to the Fukushima disaster warrant further study.
As a result of the radionuclides released to the atmosphere after the meltdowns in March 2011 at the Fukushima-Daiichi nuclear power plant, Fukushima Prefecture was contaminated with radioactive cesium (134-Cs and 137-Cs) and other isotopes, which pose radiological health risks to the resident population. As would be expected for parents around the world, parents of small children in Fukushima Prefecture have great concern about the internal exposure experienced by their offspring.
The paper by Hayano and colleagues describes the design and manufacture of a whole body scanner designed specifically for the purpose of detecting gamma-emitting isotopes that have been internalized by children. The instrument was first installed at the Hirata Central Hospital in Fukushima Prefecture in December 2013. The design principles, implementation and the initial operating experience are reported in the paper.
The age distribution of the first 100 children who were selected for isotope counting are given in the figure below:
Radioactive 134-Cs and 137-Cs was not detected in any of the 100 subjects. Naturally occurring radioactive 40-Potassium (40-K) was detected in all subjects. Typical gamma-ray energy spectra are are shown in the figure below with black dots indicating data collected with a subject in the instrument (4 min of counting), and those shown in grey dots were taken without subject (blank measurement taken over 5 hours and normalized to 4 min).
Given the experimental conditions the minimum detectable activity (MDA) for 137-Cs (Bq/body), was calculated for each subject and plotted against weight for each child as shown below.
The detection limit of BABYSCAN for 137-Cs, one of the most significant isotopes with respect to radiological health risks released from Fukushima, is better than 50 Bq/body. Despite this low detection limit, 137-Cs was not detected in any of the first 100 children scanned from the most contaminated areas of Japan in Fukushima Prefecture. Ongoing analyses will be carried out on a larger-scale with BABYSCAN and reported in publications by the investigators in the future. I will report on data as it becomes available.
This diary is part of an ongoing series here that aims to report measurements of Fukushima derived radionuclides in the North Pacific Ocean to help determine the likely impact on ecosystem and public health in western North America. The purpose of this diary is to report the results of a recently published study by Kumamoto and colleagues in the open-access journal Scientific Reports. The study measured the activity of Fukushima derived cesium (Cs), a tracer for other radionuclides, in the upper 1000 meters of the western Pacific Ocean along the 149 degree E meridian as of winter 2012. These measurements indicate that 10-60% of the total Fukushima derived 134-Cs in the North Pacific has been transported to the south at a depth of ~300 m below the surface. This result is surprising as most models suggest that transport would be primarily to the east toward North America. The study demonstrates that the amount of Fukushima derived radionuclides being transported to the east towards North America is lower than predicted by previous models and provides important information on the circulation of the ocean.
The disaster at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), precipitated by the earthquake and tsunami on March 11, 2011, resulted in meltdowns at Units 1-3 and a massive release of radionuclides to the North Pacific Ocean by direct discharges from the plant and by deposition of radionuclides released to the atmosphere. While a suite of radionuclides were released, 134-Cs is a useful tracer of Fukushima impact. 134-Cs has a relatively short half-life (~2 years) that unequivocally fingerprints a Fukushima source. It was also released in large quantities and therefore poses a potential radiological threat to organisms. 134-Cs was released along with 137-Cs (half-life = ~30 years) in a 1:1 ratio from Fukushima.
Scientists use a variety of units to measure radioactivity. A commonly used unit is the Becquerel (Bq for short) which represents an amount of radioactive material where one atom decays per second and has units of inverse time (per second). Another unit commonly used is disintegrations per minute (dpm) where the number of atoms undergoing radioactive decay in one minute are counted (so 1 Bq = 60 dpm).
Estimates of direct release of Cs to the ocean were on the order of 11-15 PBq (10^15 Bq) while the deposition of Cs to the surface of the ocean were about 5.8-30 PBq. In 2012 the authors of the study occupied a series of stations along 149degree E as shown in the figure below:
In addition the surface plume of radionuclides that has been modeled and detected (by InFORM team member Dr. John Smith of DFO) in surface currents heading to the east toward North America depth distributions of 134-Cs in the western Pacific show that a concentrated plume of Fukushima derived radionuclides has been transported to the south at a depth of 300 meters:
Based on the integration of the activity of Cs over the depth the authors estimate that about 6 Pbq (10^15 Bq) are present in the subsurface feature being transported to the south. This represents on the order of 10-60% of the total radiocesium that was introduced to the Pacific by the disaster. This helps to explain the lower activities being measured in the eastern Pacific compared to what models predict and suggests that maximum activities on the west coast of North America will likely fall toward the lower end of model predictions that were in the range of 2-30 Bq/m^3. Simply stated more of the radioactive elements released from Fukushima to the Pacific Ocean are headed south rather than east to North America in the plume than previously thought.
More direct measurements of radioactive elements in the North Pacific Ocean through the InFORM project will help to determine what activities are likely on the west coast of North American as the plume arrives from 2013 onward. The measurements or radionuclides in seawater, combined with measurements of radioactive elements in marine organisms, will help to assess the risk of exposure of west coast residents to radionuclides from Fukushima.