Sources of Pu Isotopes in Marine Sediments in the Western North Pacific off Japan
Influence of the FDNPP Accident on Pu Distribution
FDNPP-derived Pu has been characterized by high atom ratios of 240Pu/239Pu (0.303–0.330) and 241Pu/239Pu (0.103–0.135) . Although the 240Pu/239Pu atom ratios in the sediments after the FDNPP accident were higher than the global fallout value, these values cannot be simply explained as the influence of the FDNPP accident because higher 240Pu/239Pu atom ratios were observed in a wide range of sea areas in the Western North Pacific before the accident as the result of the influence of PPG close-in fallout. Comparison of 240Pu/239Pu atom ratio and 239+240Pu activity for the marine sediments in the Western North Pacific before and after the accident is shown in Fig. 9.4. All the data observed after the FDNPP accident were in the
Fig. 9.4 Comparison of 239+240Pu activities (mBq g−1) and 240Pu/239Pu atom ratios in the marine sediments off Japan before and after the FDNPP accident. (Data for Pu isotopes in the marine sediments after the FDNPP accident cited from references [8, 9])
background range before the accident. In the deeper layer sediments, higher 240Pu/239Pu atom ratios (>0.25) were observed, which could be explained as the earlier deposition of PPG close-in fallout Pu, considering the sediment depth. Therefore, it can be concluded that no extra Pu injection from the FDNPP accident has been detected in the marine sediments in the Western North Pacific 30 km off the FDNPP site after the nuclear accident.
The data on the distribution of 241Pu provide further support to this conclusion. The 241Pu/239Pu atom ratios in the Fukushima surface sediments were two orders of magnitudes lower than the values derived from the FDNPP accident. The 241Pu/239Pu atom ratios were between the global fallout value and the PPG close-in fallout value, thus further implicating global fallout and PPG close-in fallout source contributions to the investigated regions.
Resolving Global Fallout and PPG Close-In Fallout Pu
Because two sources (global fallout and PPG close-in fallout) of Pu contamination in the marine sediments off Fukushima are identified, a simple two-end-member mixing model was used to calculate the relative contributions:
where (Pu) is the 239+240Pu activity, R represents the 240Pu/239Pu atom ratio, and the subscripts P, G, and S refer to PPG close-in fallout, global fallout, and measured sediment samples, respectively. The inventory-weighted percentages of the PPG close-in fallout in each sediment core can be calculated by Eq. (9.1).
The results showed that Pu in the sediment cores from the PPG contributed 16–43 % to the total Pu contamination . The contribution of PPG close-in fallout Pu at ES4 station is 16 %, lower than the values (38–43 %) for the other stations. As discussed before, ES4 is located at the pathway of the Oyashio Current, which brought a large amount of global fallout Pu to that region. The contribution of PPG close-in fallout Pu in other stations is almost the same as the reported value (41 %) in CM-03 at the same sea area before the accident , suggesting there was no significant variability for the distribution of Pu isotopes in the marine sediments off the FDNPP site before and after the nuclear accident.