Radiolytic Specie Generation from Internal Waste Package Criticality

The effects of radiation on the corrosion of various metals and alloys, particularly with respect to in-reactor processes, has been discussed by a number of authors (Shoesmith and King 1998, p. 2). Shoesmith and King (1998) additionally discuss the effects of radiation on the proposed Monitored Geologic Repository (MGR) Waste Package (WP) materials. Radiation effects on the corrosion of metals and alloys include, among other things, radiolysis of the local gaseous and aqueous environment to produce both oxidizing and reducing radicals. In particular, radiolysis processes in moist air environments lead to the fixation of nitrogen as NO, N02, and especially HN03 (Reed and Van Konynenburg 1988, pp. 393-404). Nitric acid is assumed to be the principal corrosive radiolytic chemical specie and is produced in an irradiated air-water vapor system when the hydroxyl radicals generated from the water vapor convert nitrogen dioxides, that are formed by the radiolytic reaction between nitrogen and oxygen, to nitric acids. Chemical species produced by radiolysis have been identified in the Disposal Criticality Analysis Methodology Topical Report (DCTR) (YMP 2000, p. 2-2) as a mechanism for accelerating corrosion of the MGR engineered barrier system (EBS). Radiolytic sources of corrosion have also been considered in the screening of processes and issues in the drip shield and WP degradation (Civilian Radioactive Waste Management System [CRWMS] Management and Operating Contractor [M&O] 2001a, Section 6.2.27), Yucca Mountain Project (YMP) features, events, and processes (FEP) No. 2.1.13.01.00. The latter reference dealt specifically with radiolytic effects of gamma radiation on the WP and drip shield, excluding them from further consideration because of low consequence. The potential for chemical interactions within the WP from radiolytic effects was considered insignificant in the Waste Form Degradation Process Model Report (CRWMS M&O 2000c, p. 3-21) and therefore neglected except as a possible perforation mechanism for the Zircaloy cladding (CRWMS M&O 2000c, p. 3-40).