The following is a brief post to describe how radionuclides are measured in seawater so that the methodology being applied to the monitoring of Fukushima derived radionuclides in the ocean can be better understood.
The primary goal of the post is to answer basic questions like:
Why are most up to date reports in the scientific literature talking about samples collected in 2012 or early 2013 when we know release of radionuclides from the accident site are ongoing?
Why are researchers only reporting values for isotopes like cesium-134 (Cs-134), cesium-137 (Cs-137) but rarely for potentially more harmful isotopes like strontium-90 (Sr-90)?
I will focus on how the Cs isotopes are measured and contrast this approach with the detection of Sr-90 in seawater. These elements are selected here because Cs isotopes were released in large amounts and can be used to trace the impact in the environment and the potential for Sr-90 to have significant impact on organisms makes it an isotope of interest. This focus on Cs and Sr should not be interpreted to imply that these are the only radionuclides to have been released from Fukushima.
To begin with the Cs isotopes are detected primarily through the release of gamma radiation while Sr-90 is detected by beta radiation released during decay.
Measurement of Cs-134 and Cs-137 in seawater
Example study using this methodology here
Large volume seawater samples (~20 – 100 liters or ~ 5 – 20 US gallons) are collected using pumps for surface waters or sampling bottles on a frame deployed from the ship on a hydrowire for sampling deeper waters. On return to the ship these samples are filtered through 1 micron (10^-6 meter) filter capsules to remove particulate material with >99% of the Cs passing through the filter as Cs is present primarily in the dissolved phase. A known amount of stable Cs is added to monitor the recovery of Cs in the chemical separation and concentration steps that lead up to counting the gamma radiation given off by Cs-134 and Cs-137. The seawater is allowed to flow across plastic beads that have been modified to be chemically “sticky” for Cs. At a rate of ~30 – 300 milliliter per minute each sample is passed through a cartridge containing the plastic beads and anywhere from 85 – 99% of Cs isotopes are retained in the cartridge. These cartridges are dried, placed in plastic containers and transferred to the gamma radiation spectrometer to determine Cs isotopes. Investigators generally use either coaxial or well-type high purity germanium detectors. These detectors are ~40% efficient with respect to measuring gamma decay events and have a resolution of ~1.5 kilo electron volts. Gamma spectra are integrated and quantified. Counting times for such samples are usually days to a week depending on the Cs activity with resulting uncertainties in Cs isotope activities between 5 – 12% largely from counting statistics.
Measurement of Sr-90 in seawater
Example of a study using this methodology.
The measurement of Sr-90 is a little more complicated given its lower levels in the environment and the presence of other beta radiation emitters that it must be separated from to obtain accurate data. For each measurement an additional 20 L of seawater is collected. This seawater is acidified and then stable Sr and yttrium (Y) are added to monitor yield of subsequent purification steps along with high concentrations of iron (Fe3+). Strontium-90 concentrations are actually determined by monitoring its daughter product Y-90 (half-life = 64 hours) using beta counting. The large volumes of amended seawater are left to equilibrate overnight and then the pH is adjusted up to >8 to precipitate Fe-oxides that the Y-90 sticks to and settles out of solution as a solid. This precipitate is filtered out onto a nitrocellulose filter and completely dissolved in 20 milliliters of nitric acid (8 mole per liter). The Y-90 is purified from this solution by a two step ion-exchange column process to separate Y-90 from other beta emitters. After the final purification step the Y-90 is recovered onto another filter and then is ready for beta counting. Recovery of yttrium ranges from 10 to 85% through this process. Counting is carried out over and initial period of 2 to 2.5 days. Samples are remeasured for a similar duration 1 week and again at 3 weeks to determine that the half-life of beta decay in the samples is the result of Y-90.
So for paired measurements of the Cs isotopes 134 and 137 and the determination of Sr-90 up to 120 L of seawater must be processed as described above. Counting of an individual sample can take on the order of a week for Cs and days for Sr with the Sr samples requiring multiple analysis over a period of at least 3 weeks to provide data with appropriate accuracy and precision.
The data are hard won and require significant amounts of skilled labor and expertise to carry out.