Activities and transfers of Pu and Am in rice samples from Camargue, France.
C. Duffa, P. Renaud, F. Goutelard
Journal of Radioanalytical and Nuclear Chemistry, vol. 252, n° 2, (2002), 247-248
In Camargue, the use of Rhone water does not cause any significant increase in additional Pu and Am contamination to rice. Ultra-trace quantities of these radionuclides in rice grains are mainly due to root transfer. Estimated transfer factors for Am and Pu isotopes are higher than mean values proposed in international report however, are consistent with the 95% confidence intervals that are recommended.
Little information about the concentration of actinides in agricultural products has been reported in the literature. Some laboratory experiments have been carried out but field experiments are scarce, due to the difficulty in detecting these alpha-emitting radionuclides at very low concentrations in biological samples.
Rice is one of the most consumed products worldwide. In France, the Camargue area is the main rice producer with about 20,000 has cultivated. The Rhône River that carries transuranic elements from the Marcoule Nuclear Reprocessing Plant located 60 km upstream is used to irrigate this area and the rice paddies. In order to estimate the Pu and Am contamination through Rhône River floodwater, rice grains, stalks, roots and associated soil were collected in three different rice paddies. The relative importance of soil and water to the transfer of these radionuclides to the rice grain can be considered using activity ratio. The possibility of a direct contamination from floodwater to rice through the stalk is studied. This process has been described for other radionuclides that are known to translocate from water to rice such as cesium. The Am/Pu activity ratio in soil and water is one of the tools that can be used as a tracer to differentiate the source of contamination: 0.34 for global fallout and 2.8 for the Marcoule installation release from April to September 1998 (irrigation period).
The sampled rice belongs to Koral rice, the mainvariety in Camargue. Biological samples (grains, roots and stalks) of rice were collected during the harvest period. To eliminate organic matters, several kilograms of fresh product was grinded and calcinated at 500°C leading to 50 g of ashes.
Associated soils were sampled at 20 cm deep, where Pu and Am activities are homogeneous due to the plowing. Each sample analyzed consisted of 8 elementary soil samples, which was homogenized and sifted at 2 mm. Then, 200 g of the calcinated samples were processed.
The activities of actinides in these samples were measured by radiochemical separations and alpha counting. The radiochemical yields were measured by using 242Pu and 243Am tracers. Stable elements and natural alpha-emitting isotopes were removed from Pu and Am by combination of coprecipitation and extraction chromatography steps. Plutonium and Am were electro-deposited and counted for ten days.