Mathematical Modeling for Estimation of Ingestion Dose via Grass-Cow-Milk-Human Pathway during Postulated Accident in NPPs

摘要

The licensing practices necessitate the estimation of offsite radiological consequences following postulated accident in the nuclear power plants (NPPs). There are four pathways by which doses to a hypothetical individual residing at or near plant boundary could be delivered. Plume dose from radionuclides released into the atmosphere continuously at an elevation will cause exposure at the ground level even if the plume or cloud does not touch the ground. The radionuclides present in air at the ground level will lead to internal exposure through inhalation. During plume transient, the radionuclides are also deposited onto the ground surface resulting in external exposure from beta and gamma radiations to the person standing on the ground. Atmospheric radioactivity deposited onto the grass is consumed by bovine animals. A fraction of this activity reflects in the lactated milk of the animal. The consumption of the milk or milk products will lead to internal exposure and it is treated separately as dose from grass-cow-milk route. The history of NPP mishaps and its consequences, like Chernobyl accident, has shown severity of the ingestion of milk from the contaminated cow. It is found to be the most crucial internal exposure pathway as the thyroid gland (responsible for growth) of young children, for whom milk is the prime food, is adversely affected due to radioactive iodine. A mathematical model is developed to estimate the dose exposure due to the ingestion of radioactive nuclides through Grass→Cow→Milk→Human pathway during postulated accident in NPPs. The model can estimate dose at any accident initiation time. Time dependent radionuclide transfer factors are considered for grass to cow's milk transfer. The radionuclide concentration in milk with time is analyzed for continuous intake of the contaminated grass. The model gives dose rate (per day) and cumulative dose with time. The postulated loss of coolant accident with simultaneous failure of emergency core cooling system in a typical Indian PHWR at Kakrapar is considered in the present paper. Radioactive isotopes of iodine, cesium, strontium and tellurium are considered for the dose exposure calculation. The long term ingestion dose depends on the accident initiation time. It is found that the daytime accident is more crucial case because the contaminated milk from cow may be available as soon as 1 hour after the accident. The maximum total dose is found when accident occurs at 05:00 hr. and earliestdose exposure happens if accident occurs at 17:00 hr.