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# Answer to Question #13901 Submitted to "Ask the Experts"

Category: Radiation Fundamentals — Doses and Dose Calculations

The following question was answered by an expert in the appropriate field:

Q

Following Chernobyl, the Soviets instituted an emergency limit of 1 × 10-6  Curies (Ci) per kg for meat products, or 37,000 becquerel (Bq) kg-1. I used an online calculator to estimate a dose rate for such a piece of meat, assuming cesium-137 (137Cs) and found a value of around 24 microSievert per hour (uSv hr-1). This seems difficult to reconcile with the finding that internal doses to the population were generally several times lower than external doses, and the fact that a fairly large proportion of ingested 137Cs is absorbed by the body.

How exactly does 137Cs get absorbed by the body? Does a larger proportion get excreted immediately if it is ingested in solid foods? It seems like otherwise eating a piece of meat containing 37,000 Bq would result in a dose of several dozen milliSv (mSv).

A

Unfortunately, I do not know which online calculator you used. The free versions I find available, such as Rad Pro Calculator, allow gamma point source, beta point source and planar source calculations but do not appear to support calculations for volume sources. I am assuming the value of 24 µSv h-1 that you determined represents an estimate of the external dose rate at the surface of the piece of meat in question. Since I don't know how you calculated this I cannot comment on the value. I did do a simple calculation using the principle of charged particle energy spatial equilibrium (ESE) for the conversion electrons and beta radiation produced in the decay of 137Cs to barium-137m (137mBa) and obtained a surface dose rate of 2.6 µSv h-1. The gamma radiation would add some to this, but it’s difficult to estimate without specifying the geometry of the meat. In a very large piece of meat, we could estimate the maximum surface dose rate from gamma radiation using the same ESE principle; this would yield a surface dose rate from the gamma radiation of about 6.0 µSv h-1. The maximum possible surface dose rate from all radiations would then be about 8.6 µSv h-1.

This external dose rate at the surface of the meat has little significance as far as internal dose is concerned since when such meat is eaten it gets broken down by digestive processes in the body. If a very hungry adult person were to ingest an entire kilogram of the contaminated meat, the estimated total internal dose that the individual might experience following intake would be about 480 µSv, based on an ingestion intake conversion factor of 1.3 x 10-8 Sv Bq-1 cited by ICRP Publication 72.

This dose estimation takes into account the expected distribution of cesium in the body and the metabolic model for how the cesium is retained and excreted from the body. Doses are calculated to all the significantly irradiated tissues in order to evaluate the effective dose. Cesium is an alkali metal and behaves biochemically in a similar fashion to sodium, being easily absorbed into the bloodstream and distributed among many soft tissues. Virtually all of the ingested cesium, after digestion of the meat in the stomach and small intestine, is absorbed out of the small intestine into the bloodstream, after which it distributes to various soft tissues. Ninety percent of this cesium is excreted with a half-life of 110 days, and the remaining 10% with a two-day half-life, meaning that within a year, 90% of the total internal dose would have been delivered. Eighty percent of the cesium excretion is expectedly via urine and 20% via feces.