On March 11th 2011 a powerful earthquake shook the Pacific Ocean triggering a tsunami that travelled over eastern Japan with devastating consequences. The tsunami waves also covered the Fukushima Dai-ichi nuclear power plant (FDNPP) causing the cooling systems to fail. This led to the biggest nuclear accident since Chernobyl in 1986.
The partial melting of the fuel in the reactors caused the pressure and temperature to rise in the reactor pressure vessel, which led to major radioactive releases. The three most significant radionuclides were cesium isotopes (134Cs, 137Cs) and iodine isotope (131I). They all are highly volatile elements and that is why they were released in the early stage of the accident. Also lower amounts of intermediate volatile elements, such as strontium, were released. The release of refractory elements, such as plutonium, was negligible.
The radionuclides travelled over the Pacific Ocean all the way to Europe and Finland, where elevated levels of radioactivity were detected but the levels were 4-5 orders of magnitude lower than those after the Chernobyl accident. In Japan, the most radioactive contamination was found north-west from the FDNPP (IAEA, 2015). The most contaminated areas had to be evacuated and still today the surrounding areas of the power plant are uninhabitable.
The Fukushima accident has been widely studied over the past six years and the distribution of radionuclides and environmental effects are fairly well-known. Still more studies are needed to understand the long-term effects of the accident to the environment and humans. This study focuses on surface water samples collected on April 10th 2011, less than a month after the accident. The radionuclides studied in this research are isotopes of cesium, strontium, tritium and iodine. Both radiometric and mass spectrometric methods were used following sample pretreatment and chemical separation in determining the activity concentrations.
The highest activity concentrations for all nuclides were found in the samples collected closest to the FDNPP. Cesium isotopes had the highest activity concentrations among all the nuclides measured, which could be expected due to the fact that both isotopes, 134Cs, 137Cs, were released in high amounts. Tritium levels were also over 200 times over the pre-accident background values in the most active samples. Strontium, being an intermediate volatility element, was not released in high amount and the measured results support this fact. The activity concentrations of 90Sr were very low with high uncertainties. 129I activity concentration was also highest in the sample collected closest to the FDNPP. Unfortunately, not all samples were measured due to too high activities for the accelerator mass spectrometer.
Some observations could be made about the behavior of the radionuclides in the environment and especially their interactions with soil, since differences were detected in samples collected from different sites. Unfortunately, no further analysis could be perfomed to estimate the total areal contamination or distribution between soil and solution, because that would have required soil and vegetation samples in addition to the water samples.
