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Cs falloutOta, Masakazu; Koarashi, Jun
Science of the Total Environment, 816, p.151587_1 - 151587_21, 2022/04
Times Cited Count：4 Percentile：72.85(Environmental Sciences)In forests affected by the Fukushima Daiichi Nuclear Power Plant accident, trees became contaminated with Cs. However, Cs transfer processes determining tree contamination (particularly for stem wood, which is a prominent commercial resource in Fukushima) remain insufficiently understood. This study proposes a model for simulating the dynamic behavior of Cs in a forest tree-litter-soil system and applied it to two contaminated forests (cedar plantation and natural oak stand) in Fukushima. The model-calculated results and inter-comparison of the results with measurements elucidated the relative impact of distinct Cs transfer processes determining tree contamination. The transfer of Cs to trees occurred mostly (
99%) through surface uptake of Cs directly trapped by leaves or needles and bark during the fallout. By contrast, root uptake of Cs from the soil was unsubstantial and several orders of magnitude lower than the surface uptake over a 50-year period following the accident. As a result, the internal contamination of the trees proceeded through an enduring recycling (translocation) of Cs absorbed on the tree surface at the time of the accident. A significant surface uptake of Cs at the bark was identified, contributing 100% (leafless oak tree) and 30% (foliated cedar tree; the remaining surface uptake occurred at the needles) of the total Cs uptake by trees. It was suggested that the trees growing at the study sites are currently (as of 2021) in a decontamination phase; the activity concentration of Cs in the stem wood decreases by 3% per year, mainly through radioactive decay of Cs and partly through a dilution effect from tree growth.
Okano, Yasushi; Yamano, Hidemasa
Proceedings of 10th Japan-Korea Symposium on Nuclear Thermal Hydraulics and Safety (NTHAS-10) (USB Flash Drive), 9 Pages, 2016/11
This paper deals with an event sequence by forest fire heat effect on a decay heat removal function of a sodium-cooled fast reactor. Related to the potential vulnerability, an event scenario was developed using conservative assumptions. An event tree was developed with an initiating event of the loss of off-site power, and the headings are related to "external diesel fuel tanks", "emergency diesel generator and its auxiliary system", "alternative cooling system and its power source", and "decay heat air cooler". A failure probability on each heading was given from a fragility curve as a function of reaction intensity or by assumptions based on conservative models. A core damage frequency, under the conditional of the loss of off-site power, was conservatively evaluated around 10
/year. A key heading in the event tree with large effect on the frequency is the intactness of the external diesel fuel tanks.
Okano, Yasushi; Yamano, Hidemasa
Journal of Nuclear Science and Technology, 53(8), p.1224 - 1234, 2016/08
Times Cited Count：3 Percentile：29.07(Nuclear Science & Technology)A method to obtain a hazard curve of a forest fire was developed. The method has four steps: a logic tree formulation, a response surface evaluation, a Monte Carlo simulation, and an annual exceedance frequency calculation. The logic tree consists domains of forest fire breakout and spread conditions, weather conditions, vegetation conditions, and forest fire simulation conditions. The new method was applied to evaluate hazard curves of a reaction intensity and a fireline intensity for a typical location around a sodium-cooled fast reactor in Japan.
