Assessment of the calibration of gamma spectrometry systems in forest environments.

A Monte Carlo simulation was used to develop a model of the response of a portable gamma spectrometry system in forest environments. This model was used to evaluate any corrections needed to measurements of 137Cs activity per unit area calibrated assuming an open field geometry. These were shown to be less than 20% for most forest environments. The model was also used to assess the impact of activity in the canopy on ground level measurements. For similar activity per unit area in the lower parts of the canopy as on the ground, 10-25% of the ground based measurement would be due to activity in the canopy, depending on the depth profile in the soil. The model verifies that an optional collimator cap can assess activity in the canopy by repeat survey.

Radiocarbon Releases from the 2011 Fukushima Nuclear Accident.

Radiocarbon activities were measured in annual tree rings for the years 2009 to 2015 from Japanese cedar trees (Cryptomeria japonica) collected at six sites ranging from 2.5-38 km northwest and north of the Fukushima Dai-ichi nuclear power plant. The 14C specific activity varied from 280.4 Bq kg-1 C in 2010 to 226.0 Bq kg-1 C in 2015. The elevated 14C activities in the 2009 and 2010 rings confirmed 14C discharges during routine reactor operations, whereas those activities that were indistinguishable from background in 2012-2015 coincided with the permanent shutdown of the reactors after the accident in 2011. High-resolution 14C analysis of the 2011 ring indicated 14C releases during the Fukushima accident. The resulted 14C activity decreased with increasing distance from the plant. The maximum 14C activity released during the period of the accident was measured 42.4 Bq kg-1 C above the natural ambient 14C background. Our findings indicate that, unlike other Fukushima-derived radionuclides, the 14C released during the accident is indistinguishable from ambient background beyond the local environment (~30 km from the plant). Furthermore, the resulting dose to the local population from the excess 14C activities is negligible compared to the dose from natural/nuclear weapons sources.

(14)C levels in the vicinity of the Fukushima Dai-ichi Nuclear Power Plant prior to the 2011 accident.

A 50-year-old Japanese cedar (Cryptomeria japonica) from Okuma, ∼1 km southwest of the Fukushima Dai-ichi Nuclear Power Plant, was cored and each annual ring was analysed for (14)C. The (14)C specific activity values varied from 330.4 Bq kg(-1) C in the tree ring formed in 1971 to 231.2 Bq kg(-1) C in the 2014 ring. During the periods 1971-1976 and 2011-2014, the (14)C specific activities are indistinguishable from the ambient background values. However, compared with the ambient atmospheric levels, the (14)C specific activities between 1977 and 2010 are significantly elevated, clearly indicating (14)C discharges from the reactors during their normal operations. In addition, the specific activities are positively correlated with the annual electricity generation values. The excess (14)C specific activities were <36 Bq kg(-1) C, corresponding to an additional annual effective dose of <2 µSv via the food ingestion pathway in the study location. The primary wind direction is east-southeast/southeast with a frequency of ∼30%, in comparison to ∼20% frequency for the direction of the site under study (north-northeast/northeast). This would tend to indicate a similar magnitude of additional effective dose and consequently no significant radiological impact of atmospheric (14)C discharges from the FDNPP during the entire period of normal operations. Additionally, no (14)C pulse in activity can be observed in the year 2011 ring. This might be caused by a limited (14)C release from the damaged reactors during the accident or that the prevailing wind during the short period of release (11th-25th March 2011) was not in the direction of Okuma.

Iodine isotopes in precipitation: Four-year time series variations before and after 2011 Fukushima nuclear accident.

Rainwater samples were collected monthly from Fukushima, Japan, in 2012-2014 and analysed for (127)I and (129)I. These are combined with previously reported data to investigate atmospheric levels and behaviour of Fukushima-derived (129)I before and after the 2011 nuclear accident. In the new datasets, (127)I and (129)I concentrations between October 2012 and October 2014 varied from 0.5 to 10 µg/L and from 1.2 × 10(8) to 6.9 × 10(9) atoms/L respectively, resulting in (129)I/(127)I atomic ratio ranges from 3 × 10(-8) to 2 × 10(-7). The (127)I concentrations were in good agreement with those in the previous period from March 2011 to September 2012, whereas the (129)I concentrations and (129)I/(127)I ratios followed declining trends since the accident. Although (129)I concentrations in five samples during the period of 2013-2014 have approached the pre-accident levels, (129)I concentrations in most samples remained higher values in winter and spring-summer. The high (129)I levels in winter and spring-summer are most likely attributed to local resuspension of the Fukushima-derived radionuclide-bearing fine soil particles deposited on land surfaces, and re-emission through vegetation taking up (129)I from contaminated soil and water, respectively. Long-term declining rate suggests that contribution of the Fukushima-derived (129)I to the atmosphere would become less since 2014.

Radiocarbon concentration in modern tree rings from Fukushima, Japan.

A 30-year-old Japanese cedar (Cryptomeria japonica), collected from Iwaki, Fukushima in 2014, was analyzed for the long-lived radionuclide (14)C. Values of Δ(14)C varied from 211.7‰ in 1984 to 16.9‰ in 2013. The temporal Δ(14)C variation can be described as an exponential decline, indistinguishable from the general Northern Hemisphere Zone 2 (NH Zone 2) values in the atmosphere, until at least 1994. Values of Δ(14)C for 1999 and 2004 are slightly depleted compared with NH Zone 2 values, while from 1999 to 2013 the data suggest a clear depletion with a 2-8 ppmV additional CO2 contribution from a (14)C-free (i.e. fossil carbon) source. This change coincides with local traffic increases since two nearby expressways were opened in the 1990's. In addition, the small but visible (14)C pulse observed in the 2011 tree-ring might be caused by release from the damaged reactors during the Fukushima nuclear accident.

Photostimulated luminescence detection of irradiated shellfish: international interlaboratory trial.

An interlaboratory trial was conducted to validate photostimulated luminescence (PSL) detection of irradiated shellfish. Five species of shellfish (Nephrops norvegicus, mussels, black tiger prawns, brown shrimps, and king scallops) were presented blind as nonirradiated and irradiated to 0.5 and 2.5 kGy. Precharacterization analysis of each product and treatment was performed on both whole (including shell) and intestinal samples. The results for whole samples (including shell) confirmed that the method was able to distinguish between nonirradiated and irradiated samples, regardless of dose. Intestinal data have identified that the method is dependent on the quantity and sensitivity of grits present within the intestinal tract, which can be assessed using calibration by normalization to 1 kGy. Five laboratories returned both initial screening and calibrated data and sample classification. All laboratories correctly identified all irradiated products using the screening criteria. There were no false positives. The results confirm the validity of the PSL method for shellfish, which has been adopted as a European standard method and by the Codex Alimentarius Commission. Calibration is required where only intestinal material is available. For whole samples with shell, screening alone is sufficient.

Thermoluminescence detection of irradiated fruits and vegetables: international interlaboratory trial.

An international interlaboratory trial was conducted to validate thermoluminescence methods for detecting irradiated fruits and vegetables. Five products were used in this study. This paper presents the results from prestudy material, homogeneity testing, details of sample preparation, and participants' results. Prestudy results provided a basis for cross comparison of instruments in different laboratories. A wide range of sensitivities, reproducibilities, and signal-to-background ratios were observed. Homogeneity testing showed that the method can distinguish between nonirradiated and irradiated products, including those bleached with 100 J/cm2 artificial daylight, provided that sensitivity rejection criteria are rigorously applied. Blind results were returned by 9 participants in the form of first and second glow integrals and glow ratios for all samples and a qualitative classification for each product. Of the 387 results reported, 327 valid results were obtained from participants. Where valid data were obtained, correct qualitative identifications were made by participants in all cases. Participants' results and homogeneity testing both confirm the validity of the thermoluminescence method for detecting irradiated fruits and vegetables.

Photostimulated luminescence detection of irradiated herbs, spices, and seasonings: international interlaboratory trial.

An interlaboratory trial was conducted to validate photostimulated luminescence (PSL) methods for herbs, spices, and seasonings. Forty products (11 herbs, 17 spices, and 12 seasonings) were purchased from a local commercial source, and randomly selected samples were irradiated with 10 kGy. Four blended products were prepared at Scottish Universities Research and Reactor Centre, mixing varying proportions of irradiated material with the untreated product. Precharacterization against a predefined threshold identified low sensitivity products (black and white peppers) and products with high natural signals (thyme, sage, parsley, and mixed herbs), both of which might be susceptible to misclassification. Precharacterization also revealed whether calibration was likely to resolve overlap between classification categories. Eight sets of screening data and 5 sets of calibrated data were returned by participants. Of the 840 samples sent, 1593 screening measurements and 788 calibrated measurements were received from 662 samples. In screening mode, participants reached definitive conclusions in 87% of cases, 99% of which were correct. Of the remaining 13%, calibration to identify low-sensitivity resolved 60% of cases. Overall, 94% of samples were correctly identified by either screening alone, or screening plus calibration; 6% remained unclassified and therefore required further investigation by thermoluminescence. The results confirm the validity of the PSL method for herbs, spices, seasonings, and blends, and emphasize the need for calibration to identify low-sensitivity samples. This method has now been adopted by the European Committee for Standardization (CEN) and the Codex Alimentarius Commission.

Thermoluminescence detection of irradiated shellfish: international interlaboratory trial.

An international interlaboratory trial was conducted using thermoluminescence for the detection of irradiated shellfish, aimed at validating the method for routine use. Nephrops norvegicus, mussels, brown shrimps, black tiger prawns, and king scallops were presented as nonirradiated and irradiated to 0.5 and 2.5 kGy. The protocol called for the use of 3 preparation methods: extraction of silicates from whole shellfish by acid hydrolysis and physical separation, and of carbonates from powdered shells. Homogeneity was tested on each product and each treatment. Results verified that all methods were able to distinguish between nonirradiated and irradiated samples regardless of dose. Silicate methods produced better discrimination than powdered shell, and acid hydrolysis showed some evidence of better separation between the 2 doses than the physical method. Participants received each product in each treatment category for blind analysis. Six participants returned results for acid hydrolysis, 7 for physical separation, and 5 for the powdered shell method. Their results confirmed the homogeneity testing. Qualitative results gave 100% correct classification for both silicate methods and 85.3% for powdered shell. Silicate methods are therefore preferable unless only shell is available. Overall, the results confirmed the case for validation.