Direct Investigations Of The Immobilization Of Radionuclides In The Alteration Phases Of Spent Nuclear Fuel

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The safe disposal of the nation's nuclear waste in a geologic repository is one of the most significant and difficult scientific endeavors of the twenty-first century. Unique scientific challenges are posed by the very long-lived radioactivity of nuclear waste. Many radionuclides of vastly different chemical character must be retained by the repository for several thousand years. Some with longer half-lives, such as Pu-239 and Tc-99, need to be isolated for periods approaching a million years. In order to ensure the safety of a geologic repository, a detailed understanding of the mobility of radionuclides in complex natural systems is essential. Most of the radioactivity in a geological repository will be associated with spent nuclear fuel. In the United States spent fuel is derived from several sources. The majority is UO2 (LWR) spent fuel from commercial reactors. About 30,000 metric tons of spent fuel was in storage at commercial reactors by 1995, with the expectation that this quantity will more than double by 2010 (Integrated Data Report 1995). All spent fuel derived from commercial reactors is intended for eventual disposal in a geological repository. In addition, the DOE is the custodian of about 8000 metric tons of spent fuel, most of which is also intended for disposal in a geological repository. Although there are more than 250 types of spent fuel in the DOE inventory, the fuels may be broadly classified into (1) uranium metal fuel, (2) aluminum-based fuel, (3) mixed oxide (MOX) fuel containing substantial plutonium, and (4) graphite fuel (Colleen Shelton-Davis, personal communications, January 2000). Disposal of spent fuel in a geological repository requires detailed knowledge of the longterm behavior of the waste forms under repository conditions, as well as the fate of radionuclides released from the waste packages as containers are breached. The proposed Yucca Mountain repository is intended to hold 70,000 metric tons of high-level nuclear waste. Nine radionuclides considered in the TSPA-VA (Total System Performance Analysis - Viability Assessment) Base Case Performance Analysis (CRWMS, 1998, cf. Table 3-14 in section 3.5.1) are of special concern because of their long half-lives, radiological toxicities, and potential mobilities under repository conditions. These are five actinide isotopes, Np-237, Pu-239, Pu-242, U-234, and Pa- 231, and four fission products, Tc-99, I-129, Se-79, and C-14. In addition, Am-241 is important because it is a parent of Np-237. An understanding of the behaviors of these elements under repository-relevant conditions is essential to safe disposal. Natural analogue studies of the mineral uraninite, UO2+x (an analogue for UO2 in spent fuel), as well as several laboratory-scale simulations, confirm that spent fuel is unstable under the moist, oxidizing conditions expected in the proposed repository at Yucca Mountain. Once containers are breached, alteration of the spent fuel m ay be rapid, with the most abundant alteration products being uranyl (U6+) phases. Shortly after groundwater or condensed water vapor contacts spent fuel in the proposed repository, uranyl phases are likely to be abundant in the vicinity of the spent fuel. Most of the uranyl phases that will form in the repository are already known as minerals from natural systems. Many of these uranyl phases can persist for thousands of years, as demonstrated by studies of natural analogues (Finch et al. 1996). It is likely that uranyl phases forming due to the alteration of spent fuel will incorporate many of the radionuclides contained in the spent fuel (Burns et al. 1997), thus having a profound impact upon the mobility of the radionuclides. Our ongoing research is leading to an understanding of the impact of incorporation of radionuclides into uranyl phases. Such information is essential to an understanding of the long-term performance of the geological repository. Knowledge of the crystal structures, chemistries, stabilities and paragenesis of uranyl minerals lag far behind most other mineral groups, owing in large part to the occurrence of these minerals as complex intergrowths of multiple phases, making routine analysis very difficult.