Zircaloy Corrosion in a Repository Environment
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engineered barriers consisting of an outer corrosion-allowance material of carbon steel and an inner Alloy C-22 corrosion-resistant material. It will not be penetrated until thousands of years after emplacement. Approximately five percent of waste packages are estimated to eventually be dripped on by water, under the current climate. If the climate changes to a wetter long-term average, about 30 percent of the packages could experience dripping water [1]. For the base case analysis, approximately one percent of those waste packages that are dripped on are estimated to be breached by at least one pit perforation at 10,000 years [2]. For the no drip case, no waste package failure will occur until 700,000 years [2]. A sensitivity study of the effect of cladding failure fraction on the modeled dose rate as a function of time from the Yucca Mountain repository shows that little effect is seen until after 250,000 years [3]. The time of peak dose rate is approximately 320,000 years. Therefore, in order for cladding integrity to have a significant impact on doses from the repository, it will have to substantially protect the fuel from water for a considerable time beyond 250,000 years. One million years is the maximum time modeled. Based on current cladding failure modeling, the range of dose rate after 250,000 years for the maximum expected (95th percentile) cladding failure of 47 percent exposed fuel area at one million years relative to that for the minimum expected (5 percentile) value of 1 percent exposed fuel area is approximately 40 at 320,000 years to 29 at 1,000,000 years [3]. Important inputs to the evaluations about the corrosive attack on the Zircaloy cladding include the chemistry of the water contacting the cladding, fuel temperature, radiolytic effects on 935 Mat. Res. Soc. Symp. Proc. Vol. 556 © 1999 Materials Research Society
the water, surface conditions of the fuel (e.g., crevices), etc. Then, technical assessments can be made based on experimental and modeling data that are pertinent to the conditions. If insufficient information is available, a good determination of the repository environmental conditions will enable definition of experimental approaches to testing the cladding material. A difficult and important task is to properly estimate the repository chemical conditions. The starting point has generally been the composition of J-13 well water. It is then modified according to postulated scenarios. While one can postulate a probable chemical condition, the uncertainties have generally led to corrosion testing of waste package materials over a range of conditions, especially of pH. Only limited testing of Zircaloy related to direct repository conditions have been made. However, there is sufficient information to make some reasonable judgements and limited predictions regarding its performance. Gaps in knowledge will be identified to suggest further testing. REPOSITORY CONDITIONS
Water Composition The Viability Assessment of a Repository at Yucca Mountain gives the pH, carbonate concentration, an
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