Safety of radioactive waste from two-component nuclear energy system disposed in a deep geological repository for permanent storage: radiological migration equivalence

The article presents analysis of safety of long-lived high level radioactive waste (RW) disposed for permanent storage in a deep geological repository. According to the study protocol it is necessary to estimate the human and environmental health safety of RW, generated in different nuclear fuel cycles, during their long-term stay in the repository. To estimate biological effects, RW composition from thermal reactors in open nuclear fuel cycle (“TR”) and from simultaneously operated thermal and fast reactors in closed nuclear cycle (“TR+FR”) in different time after their disposal have been evaluated. It is suggested that RW to be safe to humans and the environment if committed effective doses from RW components including minor actinides and fission products and from natural uranium are equivalent (radiation equivalence) or lifetime attributable risks of cancer mortality from RW and natural uranium are equivalent (radiological equivalence). For this purpose it is important to evaluate the time of achievement of radiation or radiological equivalence. To evaluate health effect of RW containing radio-nuclides on the public and the environment their migration activity has been evaluated with the use of distribution coefficient and retardation factor. Probable annual effective doses to the public from exposure to the radionuclides released from the repository to the biosphere at different time after the RW disposal have been estimated. Radiation doses to the public following the consumption of water from the well installed on the repository, and lifetime attributable risk (LAR) of cancer mortality associated with the exposure to radionuclides contained in “TR ” and ”TR+FR” generated waste have been evaluated. It turned out that LARs of cancer mortality from “TR+FR” radionuclides is lower than LAR from natural uranium, it means that radiological migration equivalence after its achievement will exists until the repository conditions allow, about 1 billion years. We have found that LARs from “TR+FR” waste is about 100 times lower than the negligible level, 10-6, established in the Russian National Radiation Safety Standards (NRB-99/2009). Estimated LAR of cancer mortality from “TR”-generated waste is 360 times higher than LAR from “TR+FR”-generated waste. As evaluated for the waste from TR radiological equivalence will be achieved not earlier than 1 million years after permanent disposal. If RW is generated in the closed cycle estimated LAR value from consumption of well water does not exceed 10-5 year-1, if RW is generated in open fuel cycle, the LAR value is 10-3 year-1, this is socially unacceptable risk. In the first 10 thousand years after the disposal the major dose- and risk-forming radionuclides in the well water will be 241Am, 239Pu и 240Pu. Estimates of radiation detriment from ”TR” generated waste put to the permanent deep geological disposal are given in the article.