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Product Abstract

Data Needs for Long-Term Dry Storage of LWR Fuel

Product ID:TR-108757
Date Published:30-Apr-1998
File size:4.86 MB
Sector Name:Nuclear
Document Type:Technical Report
FileType:Adobe PDF (.pdf)
Price:No Charge

This Product is publicly available.

Abstract
The NRC approved dry storage of spent fuel in an inert environment for a period of 20 years pursuant to 10CFR72. However, at-reactor dry storage of spent LWR fuel may need to be implemented for periods of time significantly longer than the NRC's original 20-year license period, due to uncertainties in the time frame for U.S. DOE acceptance of spent fuel. This study shows that the lower radiation fields and temperatures after 20 years of dry storage promote acceptable fuel behavior and the extension of storage for up to 100 years. Potential changes in the properties of dry storage system components, other than spent-fuel assemblies, must still be evaluated.

Background

To meet NRC standards, dry storage must 1) maintain subcriticality, 2) prevent release of radioactive material above acceptable limits, 3) ensure that radiation rates and doses do not exceed acceptable limits, and 4) maintain retrievability of the stored radioactive material. The technologies developed for storing spent-fuel assemblies in a dry, inert environment outside the reactor storage pools typically use metal cask- or concrete canister-based systems. Uncertainty as to the date the DOE will begin accepting commercial spent fuel for disposal or storage in an interim centralized storage facility has prompted utilities to plan not only for life-of-plant spent-fuel storage during reactor operation but also for the contingency of a lengthy post-shutdown storage.

Objective

To evaluate the potential for storing spent LWR fuel for up to 100 years; to identify major uncertainties and the data required to eliminate them.

Approach

The project team reviewed environmental conditions such as temperature, radiation fields, and moisture level that would be expected over 100 years for normal, off-normal, and accident events. They also reviewed information generated from past and ongoing national and international dry storage programs. Next, they analyzed the behavior of the fuel and cladding for normal and off-normal conditions. In particular, they evaluated fuel oxidation due to a mistaken backfill of air or a small cask leak, cladding oxidation under the same conditions, potential cladding rupture, and hardware performance. Because utilities are pushing fuel assemblies to higher burnups, they considered characteristics important to dry storage of spent-fuel assemblies with burnups higher than approximately 50,000 MWd/MTU. They also evaluated the interactions of zinc and its compounds (used to coat the interior of the cask) with the fuel and cladding. Finally, they developed a list of data needs and potential research priorities.

Results

Based on this evaluation of available data, a number of conclusions can be drawn regarding the feasibility of extending dry storage of spent LWR fuel to 100 years. They are as follows:

o Other than the NRC's position that diffusion-controlled cavity growth (DCCG) be considered the primary mechanism of cladding breach, the existing criteria for dry storage appear to remain valid and consistent.

o During normal storage, the lower radiation fields and estimated temperatures of 100-125 degrees Celsius after 20 years favor acceptable fuel behavior for extended storage.

o The potential for off-normal and accident events would be the same during any part of the storage cycle. Fuel or cladding oxidation degradation would take place early in storage life, when the temperatures are sufficiently high.

o Both domestic and international experience with dry storage provides evidence that would encourage utilities to extend dry storage of spent LWR fuel beyond 20 years.

EPRI Perspective

The aim of the analysis performed in this project was to determine the potential behavior of LWR spent-fuel elements in dry storage for up to 100 years. The results obtained so far lead to the view that any concerns about long-term dry storage, in all likelihood, do not lie with the behavior of the spent-fuel assemblies -- at least for those with burnups lower than approximately 50,000 MWd/MTU. Chief concerns in terms of the highest priority research needs focus on the following: 1) evaluating the changes in properties of the cask system components, with emphasis on long-term deterioration of polymer neutron shields and seals, 2) determining temperature limits, cladding degradation mechanisms, and postirradiation mechanical properties of the newer high-burnup claddings and fuels, and 3) identifying whether DCCG constitutes an appropriate technical basis for Zircaloy cladding breach. EPRI plans future research in these areas.

Program
Program 41.03.01  Used Fuel and High-Level Waste Management (QA)
Keywords
  • Radioactive Waste Disposal
  • Radioactive Waste Management
  • Spent Fuel Storage
  • Spent Fuels
  • Uranium dioxide
  • Zirconium Alloys
Report
TR-108757
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