Abstract
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.
Highlights
Operation of nuclear reactors for the generation of electrical power can produce various radionuclides by fission within the fuel or by neutron activation in the structural materials and component systems of the reactor. 14C is mainly produced in reactors by 14N(n, p)14C reactions with nitrogen in the fuel, fuel cladding, coolant and moderator as a primary impurity, by 17O(n, α)14C reactions in oxide fuel, coolant and moderator, and by 13C(n, γ)14C reactions in graphite moderators
14C emissions from the coolant and moderator are expected during normal operation of a nuclear reactor, whereas accumulated 14C in the fuel and fuel cladding is expected to be emitted at the time of melt-down of the reactor
Within the context of this background, the current work (1) investigates 14C levels before and after the accident to identify any release from the accident by measuring 14C in annual tree rings of Japanese cedar trees from sites 2.5–38 km in a north and northwest direction from the FDNPP, (2) compares these values with the global and local atmospheric 14CO2 datasets, and (3) briefly discusses the difference in the magnitude of 14C released during the Fukushima accident compared to that at the Chernobyl NPP in 1986
Summary
Operation of nuclear reactors for the generation of electrical power can produce various radionuclides by fission within the fuel or by neutron activation in the structural materials and component systems of the reactor. 14C is mainly produced in reactors by 14N(n, p)14C reactions with nitrogen in the fuel, fuel cladding, coolant and moderator as a primary impurity, by 17O(n, α)14C reactions in oxide fuel, coolant and moderator, and by 13C(n, γ)14C reactions in graphite moderators. Within the context of this background, the current work (1) investigates 14C levels before and after the accident to identify any release from the accident by measuring 14C in annual tree rings of Japanese cedar trees from sites 2.5–38 km in a north and northwest direction from the FDNPP, (2) compares these values with the global and local atmospheric 14CO2 datasets, and (3) briefly discusses the difference in the magnitude of 14C released during the Fukushima accident compared to that at the Chernobyl NPP in 1986. Detailed discussion of this was made in our previous study[9]
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