Concentration of Strontium-90 at Selected Hot Spots in Japan

FROM: PLOS ONE

Introduction

In the course of the Fukushima nuclear accident, large amounts of volatile radionuclides were released into the environment. In particular, the radioisotopes of xenon, krypton, iodine (especially 131I), cesium (especially 134Cs and 137Cs), and tellurium are regarded as the most relevant ones, which caused partly significant contamination of the Japanese land surface[1]–[3] and the Pacific Ocean [4]–[9]. These radionuclides were monitored globally in various environmental media, see e.g. [10]–[16]. Few studies also indicated the release of low amounts of less volatile radionuclides, such as 59Fe, 95Nb, 140Ba, 140La, 239Np and many others [17].

What all the above mentioned radionuclides have in common is that they are γ-emitters, which allows their straightforward detection and quantification using γ-spectrometry. The analysis of pure β− -emitters, in contrast, requires greater efforts. This also refers to radiostrontium, in particular 90Sr (T1/2 = 28.90 yr). Due to its chemical similarity to calcium, 90Sr is accumulated in the bone and may cause leukemia or skeletal cancer. Its presence in the environment, therefore, causes much concern as it is often dictating risk of contaminated sites over longer periods of time and calls for the monitoring of this inconvenient radionuclide [18]. This is of great importance especially for ensuring food safety.

Apart from sea water [19], data base publications [20], [21] and governmental and/or industrial analyses [22]–[24], hardly any data for 90Sr released during the Fukushima nuclear accident were published in peer-reviewed literature. Measurements of airborne radiostrontium have been conducted by European networks but did not reveal detectable activities that could be attributed unambiguously to the releases of the Fukushima nuclear accident (e.g. via the presence of short-lived 89Sr; T1/2 = 50.5 d) [12]. In some cases detectable levels of 90Sr were reported for seawater around the Korean Peninsula [25]. The present study is one of the first ones published in peer-reviewed English literature dedicated to the 90Sr contamination levels on the Japanese land surface.

(...)

Conclusions

Several hot spots in Japan were investigated with respect to the activity concentrations of β− -emitting 90Sr and β−/γ-emitting 134Cs and 137Cs in soil and vegetation samples. Although the137Cs activity levels were partly as high as in the kBq⋅g−1 range, the 90Sr contamination levels of any sample did not exceed the Bq⋅g−1 range. The radiocesium contamination could be clearly attributed to the Fukushima nuclear accident via its activity ratio fingerprint (134Cs/137Cs). Since short-lived 89Sr could no longer be determined, the source of the 90Sr theoretically could, in part, also be fallout from the nuclear explosions of the 20th century or previous nuclear acidents. In any case, it is likely that releases from the Fukushima nuclear accident contributed much of the 90Sr that was measured at the hot spots.

The low contamination levels confirmed previous simulations by Schwantes et al. [18], who predicted that most of the radiostrontium was retained inside the reactors. In fact, the 90Sr activity concentrations were partly four orders of magnitude lower than the respective 137Cs activity concentrations.

The data set (though limited in terms of sample numbers) suggests an intrinsic coexistence of137Cs and 90Sr in the contaminations caused by the Fukushima nuclear accident. This observation is of great importance for the current food monitoring campaigns, which currently rely on the assumption that the activity concentrations of β−-emitting 90Sr (which is relatively laborious to determine) is not higher than 10% of the level of γ-emitting 137Cs (which can be measured quickly). This assumption could be confirmed for the samples investigated herein.

(LINK TO ABSTRACT OF RESULTS)