Laboratory experiments designed to provide limits on the radionuclide source term for the NNWSI Project

Abstract

The Nevada Nuclear Waste Storage Investigations Project is investigating the suitability of the tuffaceous rocks at Yucca Mountain Nevada for potential use as a high level nuclear waste repository. The horizon under investigation lies above the water table, and therefore offers a setting that differs substantially from other potential repository sites. The unsaturated zone environment allows a simple, but effective, waste package design. The source term for radionuclide release from the waste package will be based on laboratory experiments that determine the corrosion rates and mechanisms for the metal container and the dissolution rate of the waste form under expected long term conditions. This paper describes the present status of laboratory results and outlines the approach to be used in combining the data to develop a realistic source term for release of radionuclides from the waste package. RESUME "The Nevada Nuclear Waste Storage Investigations Project" est en train d'etudier la pertinence des rochers en tuf dans la montagne Yucca en Nevada pour son utilisation possible pour 1e stockage definitif des dechets radioactives. Le terrain, sous investigation,se situe au-dessus de la nappe aquifere et offre done un emplacement qui differe notablement d'autres lieux de stockages possibles. Ce'cte zone d'environnement non-s.ature nous permettrait de designer un conteneur simple et a' la fois effectif. La "source term" pour remission de radionuclides du conteneur des dechets se sera basee sur ties experiences de laboratoire qui determineront le taux de corrosion et les mecanismes necessaires au conteneur metallique, ainsj que le taux de dissolution des dechets dans des conditions previsibles a long termes. Ce papier decrit le statut present des resultats de laboratoire et decrit l'jpproche qu'on utilisera en combinant toutes les donnees pour developper une "source term" realiste aux fins de minimiser la relache de rMionucleides du conteneur de* dechets. BlIMlMIJM OF THIS DOCUMENT IS UNLIMirEB/,

Progress report on the results of testing advanced conceptual design metal barrier materials under relevant environmental conditions for a tuff repository

“…A value of 0.2 um/yr would appear to be adequate to describe conservatively the rate data so far acquired (Oversby and McCright, 1984). Extrapolation of these rates…”

“…It appears that the molybdenum and nitrogen additions impede the diffusion of carbon atoms in austenitic stainless steels; therefore, the growth rates of carbide nuclei are slowed, and materials containing these alloy additions are considerably more resistant to sensitization both above and below 500°C. Figure 5 shows an extrapolation of the LTS data from the BWR work and superposition of this on a typical time-temperature (plotted as log time vs. 1/T) profile for a spent fuel waste package in the thermal environment of a tuff repository (Oversby and McCright, 1984). The prediction from the superposition is that even the heavily cold-worked AISI304L would not develop a sensitized microstructure {from the Briant et al, 1982 work cited above).…”

Repository seals requirement study

“…For 14C, 0.3 percent of the radionuclide inventory in a stored canister of spent fuel is released as a gas (Section 3.6) (Van Koynenburg et al 1984). For l291[, the radionuclide inventory in spent fuel may be concentrated in the pellet-cladding gap and/or on the grain boundaries of the fuel (Oversby and McCright 1985). The percentage of the total inventory is assumed to be 0.5 percent (Oversby and Wilson 1985).…”

Repository seals requirements study

“…For 14C, 0.3 percent of the radionuclide inventory in a stored canister of spent fuel is released as a gas (Section 3.6) (Van Koynenburg et al 1984). For 12()1, the radionuclide inventory in spent fuel may be concentrated in the pellet-cladding gap and/or on the grain boundaries of the fuel (Oversby and McCright 1985). The percentage of the total inventory is assumed to be 0.5 percent (Oversby and Wilson 1985).…”