High Level Nuclear Waste Repository Thermal Loading Analysis

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Title: High Level Nuclear Waste Repository Thermal Loading Analysis
Author: Stahala, Mike Peter
Advisors: Man-Sung Yom, Committee Chair
David McNelis, Committee Co-Chair
Morton Barlaz, Committee Member
Abstract: A spent nuclear fuel (SNF) decay heat model was developed by revising an existing analytical decay heat model to factor for enrichment, irradiation time, and burnup levels beyond 37,000 MWd/MTU. Because many radioactive decay processes occur during the decay of SNF, the 10,000 year time range of interest was broken down into time regions when a specific process (or group of processes) dominates in the contribution of decay heat in SNF. Some regions were further subdivided based on processes occurring within the time region. Each time region was also divided into burnup greater than and less than 10,000 MWd/MTU. A multivariable regression analysis tool was used to fit a polynomial equation, as a function of burnup, enrichment, and irradiation time, for each time and burnup region to develop a SNF decay heat model. A MATLAB program implementing the decay heat model was developed that could quickly calculate the decay heat of a dataset of SNF. Comparing the results of this model to SNF data generated in ORIGEN-ARP yielded maximum errors less than 7% for 120 time points with varying burnup, enrichment, and irradiation time. This was an improvement to the unrevised model, which had a maximum error of 48.1% for the same 120 points. The MATLAB SNF decay heat model was used to determine the decay heat of nation's inventory of SNF up to the year 2002 and SNF projected to be discharged until 2010. The SNF information was provided by the Department of Energy (DOE) Energy Information Administration (EIA). It was determined that applying APD (or linear thermal loading) values to the calculated decay heat would not be suitable because a derived APD assumes a decay characteristic specific to the fuel used for the APD calculation. Because areal power density and linear thermal loading could not be applied to the calculated decay heat inventory, an alternative method for repository thermal analysis was required. The computer code TEMPERATURE was used to calculate the temperature at (1) the SNF cladding, (2) the drift wall, and (3) the mid-drift. For the default decay heat values used by the TEMPERATURE code, none of the temperature limits were exceeded. Applying the decay heat calculated using the actual SNF data provided by the EIA to the TEMPERATURE code resulted in temperatures exceeding the mid-drift temperature limit. The minimum drift distance for the EIA applied data to meet repository temperature limits was 89 meters. When 81 meter drift spacing was utilized, a cooling time of 64 years was required to meeting repository temperature limits. An analysis on the impact of high burnup SNF was performed based on the total energy produced by low and high burnup values. The results showed that for high burnup SNF the first temperature limit that would be met was the drift wall temperature. If interim storage, on the order of approximately 30 to 40 years, was implemented for high burnup SNF, the drift wall temperature limit would no longer be exceeded and drifts could be moved closer together to minimize the repository footprint. When a 36 year cooling period was applied to the 58,000 MWd/MTU burnup case, the repository foot print was 865 acres.
Date: 2006-04-13
Degree: MS
Discipline: Nuclear Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/2309

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