Radioactive particles are aggregates of radioactive atoms formed by, e.g., condensation or precipitation of radionuclides or breakdown of larger radioactive materials, and can contain significant radioactivity. They have been released into the environment from nuclear weapons tests, and from accidents and effluents within the fuel nuclear cycle.
Aquatic filter feeders can be expected to take up and potentially retain radioactive particles, which could then provide concentrated localised doses to nearby tissues. Despite the high potential for accumulation and the potency of radioactive exposure, studies of the retention of radioactive particles in filter feeders are scarce. This study experimentally investigated the retention and effects of radioactive particles in the blue mussel, Mytilus edulis.
The spent fuel particles, collected in the field, comprised of a U and Al alloy containing fission products such as 137Cs and 90Sr/90Y. The particles were introduced into mussels in suspension with plankton food or through implantation under the mantle tissue. Induced effects of the particle exposure were measured using micronucleus and Comet assays on haemocytes. Of those particles introduced with food, 37.5 % were retained for 70 h, and were found in the siphons and gills, with the notable exception of one particle that was ingested, found in the stomach. Those not retained seemed to have been actively rejected by the mussels. In several cases where particles were retained or implanted, white marks suggesting necrosis were seen in the tissues near the particles; these are thought to be caused by radiation and physical irritation. The largest and most radioactive particle (estimated dose rate 3.18 ±0.06 Gy.h-1) caused the largest such mark in the mantle tissue; in this case, increased micronucleus frequency and Comet tail DNA % was also observed in the haemolymph collected from the muscle, implying that non-targeted effects of radiation were induced by the high dose particle.
The results showed that radioactive particles can potentially be retained by blue mussels and retained high activity particles can potentially induce negative effects, particularly in tissues close to such particles. Thus, current methods which are used for risk assessment that calculate “no-effect dose” estimates and are based upon the absorbed dose equivalent limit are inadequate for radioactive particle exposures. In addition, knowledge is lacking about the ecological implications of radioactive particles, for example potential recycling within a population, or trophic transfer in the food chain.