The rate of radioactive disintegration, e. g., curies, is only a crude measure of the importance of individual fission products in irradiated fuel and in radioactive wastes. A more meaningful measure of potential biological hazard must also include the sensitivity of humans to inhalation
or ingestion of these radionuclides. For this purpose we use the radioactivity concentration limit C, which is the concentration of radioactivity (curies) of a given radionuclide in air or water such that an individual who obtains his or her total intake of air or water from this source will receive a radiation dose from this radionuclide at the rate of 0.5 rem/year.+ Values of the public-exposure radioactivity concentration limit C for selected radionuclides are listed in App. D. A more complete listing appears in the Federal Regulations 10 CFR 20 [F2]. Assuming that the biological hazard to an individual exposed to low levels of radiation is proportional to the accumulated radiation dose, then the potential biological hazard from inhalation or ingestion of a mixture of radionuclides is proportional to the toxicity index, defined as

V Mi

V cik

l

where X( = radioactive decay constant for nuclide і Nt = number of atoms of nuclide і

Сік = radioactivity concentration limit for nuclide і in medium к (i. e., air or water)

The toxicity index is the volume of air or water with which the mixture of radionuclides must be diluted so that breathing the air or drinking the water will result in accumulation of radiation dose at a rate no greater than 0.5 rem/year. However, the toxicity index still does not measure ultimate hazards and risk, because it does not take into account the mechanisms by which the radionuclides could be released to air or water and transported to humans.

The inhalation-toxicity indices of the fission products in the fuel discharged yearly from the 1000-MWe uranium-fueled LWR are shown in Fig. 8.3 as a function of storage time. Ingestion toxicity indices for the same fission products are shown in Fig. 8.4. If Fig. 8.4 is compared with the activity plot of Fig. 8.1, it is apparent that the relatively high toxicity, i. e., low C, of bone-seeking 90Sr makes this nuclide more important than any other fission product in terms of potential inhalation or ingestion toxicity during the first few hundred years after discharge from the reactor. Thereafter, the long-lived thyroid-seeking 1291 is potentially the most important of the fission products, even though only about 1 Ci of 129I is produced yearly in a 1000-MWe reactor.

1.2 Effects of Fuel-Cycle Alternatives on Fission Products in Irradiated Fuel

Because the nuclides 232 Th, 233U, 23SU, 23®U, 239Pu, and 241 Pu yield different amounts of individual fission products, different fuel cycles such as uranium fueling without recycle, uranium-plutonium fueling, and thorium-uranium fueling will result in different amounts of fission products in the discharge fuel. Calculated yearly production and composition of some of the principal fission products for some of the alternative fuel cycles described in Chap. 3 are listed in Table 8.3.