Long-term variation of 14C concentration in atmospheric CO2 in Japan from 1991 to 2000

Prolonged production of 14C during the ~660 BCE solar proton event from Japanese tree rings

Annual rings record the intensity of cosmic rays (CRs) that had entered into the Earth’s atmosphere. Several rapid ¹⁴C increases in the past, such as the 775 CE and 994CE ¹⁴C spikes, have been reported to originate from extreme solar proton events (SPEs). Another rapid ¹⁴C increase, also known as the ca. 660 BCE event in German oak tree rings as well as increases of ¹⁰Be and ³⁶Cl in ice cores, was presumed similar to the 775 CE event; however, as the ¹⁴C increase of approximately 10‰ in 660 BCE had taken a rather longer rise time of 3–4 years as compared to that of the 775 CE event, the occurrence could not be simply associated to an extreme SPE. In this study, to elucidate the rapid increase in ¹⁴C concentrations in tree rings around 660 BCE, we have precisely measured the ¹⁴C concentrations of earlywoods and latewoods inside the annual rings of Japanese cedar for the period 669–633 BCE. Based on the feature of ¹⁴C production rate calculated from the fine measured profile of the ¹⁴C concentrations, we found that the ¹⁴C rapid increase occurred within 665–663.5 BCE, and that duration of ¹⁴C production describing the event is distributed from one month to 41 months. The possibility of occurrence of consecutive SPEs over up to three years is offered.

Local variance of atmospheric 14C concentrations around Fukushima Dai-ichi Nuclear Power Plant from 2010 to 2012

Radiocarbon (¹⁴C) has been measured in single tree ring samples collected from the southwest of the Fukushima Dai-ichi Nuclear Power Plant. Our data indicate south-westwards dispersion of radiocarbon and the highest ¹⁴C activity observed so far in the local environment during the 2011 accident. The abnormally high ¹⁴C activity in the late wood of 2011 ring may imply an unknown source of radiocarbon nearby after the accident. The influence of ¹⁴C shrank from 30 km during normal reactor operation to 14 km for the accident in the northwest of FDNPP, but remains unclear in the southwest.

(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.

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.

Radiocarbon Concentrations in Environmental Samples Collected Near the Spent Nuclear Fuel Reprocessing Plant at Rokkasho, Aomori, Japan, During Test Operation Using Spent Nuclear Fuel

The contribution of atmospheric discharged C to local C concentrations was investigated by analysis of C in environmental samples collected around the spent nuclear fuel reprocessing plant in Rokkasho, Aomori, Japan. From June 2006 to October 2011, the range of monthly averaged specific activities in atmospheric CO2 collected 2.6 km from the plant was 0.226-0.279 Bq g C; at several sampling times, the observed concentrations were higher than the background value. Specific activities of atmospheric C simulated with an atmospheric dispersion model coupled with a mesoscale weather model reproduced the measured values fairly well, supporting the idea that the higher measured specific activities were due to C discharged from the reprocessing plant. For investigation of the C distribution around the facility, samples of Cyperus microiria, wild annual sedge, were collected from nine locations. Plant samples collected east and west of the facility in 2008 showed slightly higher specific activities than samples collected at the other locations, reflecting the prevailing wind direction. The measured C specific activities in vegetable, polished rice, pasture, and milk samples collected around the facility did not differ significantly from the background value (except for one polished rice sample, which had a value slightly higher than background). The annual committed effective radiation dose to humans resulting from consumption of the polished rice was estimated as 3.6 × 10 mSv, which is negligibly small compared with 1 mSv, an index of the dose limit for the general public.