Effective half-life of radiocesium in kiwifruit in the evacuation zone after the Fukushima Dai-ichi nuclear power plant accident

Abstract

Radiocesium contamination of agricultural products persists in areas affected by the Fukushima Daiichi Nuclear Power Plant accident, particularly within evacuation zones where decontamination has not been conducted. This study examined the temporal variation and effective half-life of radiocesium in kiwifruit (Actinidia deliciosa) collected from the Omaru district of Namie town, Fukushima Prefecture, an area characterized by high initial fallout and complex topography. Kiwifruits were harvested from a single tree over five consecutive years (2021–2025), more than ten years after the accident. A total of 533 kiwifruits were analyzed using high-purity germanium gamma spectrometry. ¹³⁷Cs was detected in all samples while ¹³⁴Cs was not quantifiable due to its short physical half-life and low residual activity levels. Although no decontamination has occurred at the study site, no kiwifruit has exceeded the Japanese regulatory limit of 100 Bq kg⁻¹ since 2023. Statistical analyses confirmed the annual distributions of ¹³⁷Cs activity concentrations were approximately normal. The ecological half-life of ¹³⁷Cs in kiwifruit was estimated as 10.7 years using weighted regression of annual mean values. Combining this with the physical half-life of ¹³⁷Cs (30.2 years), the effective half-life in kiwifruit was calculated to be 7.9 years. This value is markedly longer than those reported for orchard fruits outside evacuation zones, indicating sustained root uptake and secondary deposition under minimally disturbed conditions. These findings provide important insights into long-term radiocesium dynamics relevant to agricultural recovery and evacuation policy.