The effect of the accident of Fukushima Daiichi Nuclear Power Plants on Niigata city based on tritium concentration in precipitation

The maximum value of tritium concentration in precipitation was 200 Bq/kg in Niigata city when atmospheric nuclear-bomb tests were performed in 1960's. After that, tritium concentration continuously decreased and reached to environmental revel (0.5∼1.0 Bq/kg). However, after the accident of Fukushima daiichi nuclear power plants, the tritium concentration in precipitation increased in Niigata city. Therefore the observation of tritium concentration had to be carried out. In our laboratory, we have investigated the tritium concentration in precipitation and also investigated the relation between the tritium concentration and other ion (Na+, Mg+, K+, Ca2+, Cl-, NO3- or SO42-) concentration in precipitation in Niigata city. In this study, precipitation in Niigata city was gathered monthly and the evaluation of tritium concentration in precipitation was performed. In addition, we also collected the precipitation hourly (short precipitation). Each water sample thus obtained was distilled with sodium peroxide and potassium permanganate. Then the water sample thus distilled was enriched in SPE electronic enrichment apparatus, and the tritium concentration in the sample thus treated was measured in a liquid scintillation counter. On the other hand, each ion concentration in the sample was measured by ion chromatography or Atomic Absorption Spectrometry. From the above the mentioned, the following five matters can be found. (1) The tritium concentration in the samples in March and April 2011 were twice or three times higher than that in March and April in annual years. In other words, it is considered that the thus high level concentration of tritium leads to the evaluation of the effect of the accident of the Fukushima nuclear power plants on Niigata city. (2) As to the sample, the concentration of the non-sea salt Ca2+ (nssCa2+) is similar to that in March and April in annual years. (3) For each short precipitation sample collected on March 15, 2011, the tritium concentration of each sample gathered during 2∼5 PM was about 2 Bq/kg, but in during 5∼6 PM was about 12 Bq/kg. (4) For each short precipitation sample collected on April 19, 2011, the tritium concentration was about 6 Bq/kg. (5) The tritium level in precipitation after May was similar to annual level (0.5∼1.0 Bq/kg). From the above-mentioned, as to the precipitation on March 15 and April 19, 2011, adding the consideration of air mass and the direction of the wind, it was revealed that tritium concentration varied with the direction of the wind and an altitude. From these results, it is thought that the effect of the accident of Fukushima daiichi nuclear power plant on Niigata city is so small. (authors)