Category Archives: NW Pacific

Plutonium in the Pacific Ocean From Fukushima

By Jay T. Cullen

Introduction

This post is part of an ongoing series that represents an effort to communicate peer-reviewed scientific studies of the impact of the Fukushima nuclear disaster on the North Pacific Ocean and residents of the west coast of North America. A frequently asked question of those involved in monitoring the health of the North Pacific is why more measurements of the long lived, alpha-emitting isotopes of plutonium (239-Pu half-life 24,100 years; 240-Pu 6,570 years) are not being made given the potential for these isotopes to pose radiological health risks. Measurements of air, soil and water indicate that Pu was released and broadcast into the environment as a result of the triple reactor meltdowns with estimates of the source on the order of 2.3×10^9 Bq of 239,240-Pu or 580 milligrams of the isotopes. Measurements of isotope composition and activity of Pu in seawater and sediments off the coast of Japan indicate that there was no detectable change resulting from the nuclear disaster (behind pay wall). Given that the Fukushima signal is not detectable in the ocean off Japan relative to legacy sources from atmospheric weapons testing in the 20th century there is likely little information in making the same measurements in the eastern Pacific off of North America.


Members of the public are concerned about the presence of the alpha-emitting isotopes of Pu and have been asking why measuring for these elements in seawater and marine biota is not a priority of the InFORM network. The purpose of this diary is to explain why such measurements are less likely to provide information about the plume and its impacts.

A recently published paper by Bu and colleagues in the peer-reviewed Journal of Chromotography A reports the development of a new method to determine Pu isotopes in small (20 – 60 liters) samples of seawater and measurements made of these isotopes off the coast of Japan from July 2011 to January 2013 until the present. Locations where samples were collected are shown in the figure below:

Map showing seawater collection stations from the western North Pacific and Tokyo Bay since the FDNPP accident.

For all the seawater samples analyzed by Bu and colleagues, the 239-,240-Pu activities and 240-Pu/239-Pu atom ratios where found to be 0.00043 to 0.0056 Bq m^-3 and from 0.227 to 0.284, respectively. The results are summarized in Table 4 of the paper and are shown below:

 

Before the Fukushima accident in March 2011, Pu isotopes were being monitored off the coast of Japan to assess the radiological impact of the nuclear plants on the marine environment. The 239-,240-Pu activities before the meltdowns were below 0.0083 Bq m^−3 and 0.022 Bq m^−3 respectively, with 240-Pu/239-Pu atom ratios between 0.173 and 0.322. These ratios represent the influence of the Pacific Proving Ground nuclear weapon test site, which was characterized by a high 240-Pu/239-Pu atom ratio (0.30–0.36). Results after the Fukushima disaster were typically in the background data range, suggesting no detectable Pu contamination from the accident in the marine environment ~30 km offshore of the Fukushima Dai-ichi reactor complex. This conclusion is consistent with findings from previous studies of Pu isotopes in marine sediments in the western North Pacific after the Fukushima accident.

Given the absence of isotopic and concentration anomalies thus far in the western Pacific resulting from the Fukushima meltdowns there is not very much information to be gained about the evolution of the contaminated seawater plume in time and space. Similarly, the impact of the Fukushima disaster on the health of marine ecosystem with respect to Pu isotopes will be difficult to quantify relative to weapons testing background levels that persist in the environment.

On the Methodology Used to Make the Measurements (If You are Interested, IYI)
The approach used by Bu and colleagues to measure Pu isotopes at such low concentrations and activities involves applying sector field high resolution inductively coupled mass spectrometry. The instrument is able to separate chemical species by their respective mass to charge ratios using a strong electromagnetic field downstream of the plasma ionization source. Great pains were taken maximize the instruments sensitivity to measure the isotopes of interest 238-U, 239-Pu, 240-Pu, and 242-Pu. To remove the seawater matrix (cations and anions that would reduce instrument sensitivity) and elements with mass to charge ratios that would interfere with Pu detection like 238-U the seawater samples were purified using ion selective resins held in columns by passing them through successive loading and elution steps. This process is summarized in the following flow diagram from the paper:

Flow chart of the analytical procedure for the determination of Pu isotopes in seawater by anion-exchange chromatography and SF-ICP-MS.

The preconcentration and sensitivity of SF-ICP-MS allows for the very low detection limits required to quantify Pu in relatively small (20 – 60 L) volumes of seawater.

 

Advertisements

More Measurements of Southward Transport of the Fukushima Contaminant Plume in the Western Pacific

by Jay T. Cullen

This post is part of an ongoing series that endeavors to report measurements of Fukushima derived radionuclides in the environment to help determine the likely impact on ecosystem and public health in western North America. One of the goals of the InFORM project is to provide quality measurements of Fukushima derived radionuclides in the North Pacific to help verify model predictions of ecosystem and public health impacts of the disaster. The purpose of this post is to summarize results of a recent peer reviewed study by Kaeriyama and colleagues published in Environmental Science & Technology who measured radioactive isotopes of cesium (137-Cs half life ~30 yr and 134-Cs half life ~ 2 yr) in the western North Pacific Ocean to help track the location and movement of the Fukushima contaminated seawater plume.

Continue reading More Measurements of Southward Transport of the Fukushima Contaminant Plume in the Western Pacific

Measurements to Verify Models of the Fukushima Plume: Significant Radioactivity Heading South in the Pacific

by Jay T. Cullen

Introduction

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:

Location of stations sampled for 134-Cs by Kumamoto et al. (2014) in the western Pacific south and east of Japan.

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:

Cross sectional views of 134Cs activity (Bq/m^3) with depth and latitude along 149 degree East

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.