Extended testing of select 2016 salmon samples has identified the Fukushima-fingerprint isotope in one sample re-measured earlier this year. The maximum level of contamination observed in a sample (134Cs: 0.07 Bq kg-1, 137Cs: 0.51 Bq kg-1) is over 1,700 times lower than the Health Canada Action Level (1,000 Bq kg-1) and is not known to be a health risk for either humans or the environment.
The Integrated Fukushima Ocean Radionuclide Monitoring (InFORM) project is a network involving academic, governmental, and non-governmental organizations, as well as citizen scientists. InFORM is acquiring data to support a thorough radiological impact assessment for Canada’s west coast stemming from the Fukushima Dai-ichi nuclear power plant (FD-NPP) accident, and to effectively communicate these results to the public. The first measurements undertaken as part of the InFORM project to look for Fukushima derived radionculides were made on 19 fish in collaboration with the Nisga’a First Nation. Sockeye salmon (Oncorhynchus nerka) and Steelhead Trout (Oncorhynchus mykiss) were caught off the west coast of Canada in Summer 2014 as they were returning to the Nass River in northern BC.
Measurements of radioactive elements in these fish are reported in the data table below and are available for download through Health Canada:
Samples Combined (average)
Numbers with “<” indicate that levels were below the detection limit of the analysis and numbers inside parenthesis “( )” report the uncertainty associated with the measurement.
What are we measuring and why?
The triple meltdowns at the Fukushima Dai-ichi nuclear power plant (NPP) released many different radioisotopes to the environment, however only a very few of these are both measurable and unique to Fukushima. A reliable finger-print radioisotope for Fukushima is Cesium-134 (134-Cs half life ~ 2 years). This is because 134-Cs is only produced in nuclear reactors and it has a relatively short half-life, so that 134-Cs from other human sources, like the Chernobyl NPP disaster in 1986, are no longer present in the environment. Other isotopes such as Cesium-137 (137-Cs half life ~30 years) are not positive indicators of Fukushima since they were also a products of atmospheric testing in the 20th century and Chernobyl and are still present in the environment from these legacy sources.
How scientists talk about radioactivity in the environment
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). The measurements above represent that numbers of Bq detected in a kilogram of fish flesh.
Measurements of Sockeye Salmon and SteelheadTrout
We measured the activities of cesium radioisotopes 134-Cs and 137-Cs that were released in large quantities from the Fukushima Dai-ichi Nuclear Power Plant disaster in 2011. We also measured naturally occurring radioisotopes Potassium-40 (40-K) and Polonium-210 (210-Po) that always contribute doses of radiation to human consumers of marine fish. Samples of sockeye salmon and steelhead returning to the Nass River in northern BC, obtained from the Nisga’a First Nation, were analyzed and none were found to contain detectable levels of Fukushima derived radionuclides. By adding together the signals obtained for all of the samples we calculate an average activity concentration of 0.27 Bq/kg for 137-Cs. This represents residual 137-Cs in the North Pacific largely from atmospheric nuclear weapons testing in the last century. At present, Fukushima derived radionuclides cannot be detected. Nuclear weapons testing fallout (137-Cs) can be detected in BC fish at levels that represent a fraction of the radiation exposure owing to naturally occurring radionuclides neither of which represent a dangerous health risk to consumers in Canada. Results for a further ~80 fish collected this summer and fall from various returning runs up and down the BC coast will be reported as samples are analyzed.
19 fish samples, flesh only (9 Sockeye Salmon and 10 Steelhead Trout) were obtained from the Nisga’a First Nation
A sub-sample of ~125g of wet mass (average wet mass (measured) = 126.67 g) was taken and placed in a sample counting container
Gamma-radiation emitting isotopes were measured by gamma spectroscopy using a high purity germanium detector and each sample being counted for 6 hours
Planar BE5030 high purity germanium (HPGe) detector with relative counting efficiency of 46%
counting geometry = Parkway Jar (Polyethylene, active volume = 129 mL) placed in a polyacrylate sample holder (2.95 mm thickness)
counting efficiency was determined by applying an empirical efficiency curve, determined from a multi-nuclide (12) standard (Eckert and Ziegler Analytics, SRS: 79535-411) of similar density (1.15 g/cm^3, Parkway Jar format) spanning energies of 46.5 – 1836.1 KeV
true coincidence summing (where applicable) is accomplished by extracting detailed decay scheme data from the UniSampo-Shaman nuclide library
Spectral summation was done by adding all the spectrum into a single one and then reanalysed using UniSampo-Shaman gamma spectral analysis software from Baryon Oy, Ltd., Finland, with decay correction to the catch date of August 28, 2014