By John Kessler, Program Manager, Electric Power Research Institute
|Workers put a nuclear waste container in place on a dry cast storage pad. Photo courtesy of the U.S. Department of Energy.|
In April, the U.S. Department of Energy (DOE) announced a $16 million, five-year award to a team led by the Electric Power Research Institute (EPRI) to conduct a “High Burn-up Fuel Cask Research and Development Project.” The EPRI team includes AREVA Federal Services, Transnuclear International (TN), Dominion Virginia Power, AREVA Fuels, and Westinghouse Fuels. The objective of the demonstration is to observe and confirm the long-term characteristics and behaviors of high burn-up fuel under real conditions in a full-scale dry storage system.
The term “burn-up” refers to the amount of power extracted from the fuel, typically stated in gigawatt-days per metric ton of uranium (GWd/MTU). Nuclear plants have been shifting from lower burn-up (less than approximately 45 GWd/MTU) to higher burn-up fuels (above 45 GWd/MTU) in recent years, and continued research is needed to better understand the impacts, if any, of high burn-up fuels on storage, transportation, and disposal. The demonstration project will load a Transnuclear TN-32 bolted lid used fuel dry storage cask with high burn-up fuel at Dominion Virginia Power’s North Anna nuclear plant.
The characteristics of the high burn-up fuel to be loaded into the demonstration cask will be measured prior to initiation of the demonstration so that fuel collected after the cask is re-opened can be compared to the initial fuel conditions. Fuel cladding properties to be measured include: zirconium hydride concentration and orientation, cladding metal and oxide thickness, internal gas pressure, and ductility, including the amount of creep that the cladding will experience prior to rupture. These properties relate to the ability of the fuel cladding to withstand potential structural challenges during storage, transportation, and disposal.
The cask will be specially instrumented to collect detailed temperature data from inside the cask during the initial cask drying and subsequent storage period. The cask also will be designed so samples of gas inside the cask cavity can be taken to determine if fuel has failed during drying or storage, if residual water after the drying process is present, if any of the helium backfill gas has escaped, and if oxygen is present. At the end of the multi-year storage period, detailed fuel property data will be collected again on the high burn-up fuel to determine if any property changes occurred during storage.
This demonstration will be similar in concept to those conducted at the Idaho National Laboratory (INL) in the mid 1980s through the early 1990s to assess degradation of lower burn-up fuels. Because fuel burn-ups into the 1990s were primarily “low,” less than approximately 45 GWd/MTU, the cask demonstrations at INL involved low burn-up fuel. After about 14 years of storage at INL, one of the low burn-up fuel storage casks was reopened to determine if there was degradation of the fuel or the cask internals during storage. No degradation was found.
The data collected during this cask reopening project provided a significant amount of the technical bases to support the low burn-up dry storage system license renewals that have been granted nuclear plant operators to date.
Since the 1990s, almost all used fuel being removed from the reactors have burn-ups in excess of 45 to 50 GWd/MTU. This is considerably higher than the burn-up of the PWR assemblies in the initial demos at INL. The higher burn-ups have generated regulatory interest regarding the ability of used fuel that has undergone prolonged storage to remain intact during transportation; there are some concerns that high burn-up fuel could become critical after a transportation accident. The U.S. Nuclear Regulatory Commission (NRC) has asked for more long-term, high burn-up fuel property data to support the license renewal requests for high burn-up storage.
NRC and regulators in other countries also are investigating whether dry storage of higher burn-up used fuel beyond even 20 years would require evaluation prior to receiving a certificate of compliance (CoC) extension. A CoC is the document granting permission to proceed with dry storage subject to limitations listed in the plant’s accompanying technical specifications (e.g., cask/canister type, fuel types). To date, NRC has granted few transportation CoC’s for the dual-purpose casks (designed for both storage and transportation) containing higher burn-up used fuel. Moreover, NRC has yet to grant a storage CoC extension beyond 20 years for higher burn-up used fuel dry storage systems.
Therefore, if very long-term dry storage and transportation of nuclear fuel – both low and high burn-up – are required, it will be necessary to establish technical bases for: wet and dry storage of lower burn-up used fuel beyond 60 years; wet and dry storage of higher burn-up used fuel beyond 20 years, preferably beyond 60 years; transportation of lower burn-up used fuel after very long-term storage; and transportation of higher burn-up used fuel at all time periods.
The primary focus of the current demonstration co-funded by DOE is to initiate a long-term dry storage demonstration using high burn-up fuel to provide data on the fuel and cladding behavior of high burn-up fuel assemblies in dry storage. The five-year project will culminate in the loading of the TN-32 storage cask with three different kinds of high burn-up fuel, and initial collection of data.
|Workers inspect spent nuclear fuel in dry cask storage. Photo courtesy of the U.S. Nuclear Regulatory Commission.|
Different fuel types display somewhat different mechanical properties after high burn-up and subsequent drying prior to storage. One of the primary reasons why EPRI proposed the demonstration to be conducted at the North Anna site was that there are three different kinds of high burn-up fuel in the North Anna spent fuel pool: Westinghouse Fuels’ Zircaloy-4 and Zirlo, and AREVA Fuels’ M5. The demonstration will serve the same role in forming the technical bases for regulatory decisions of high burn-up fuel that the previous confirmatory studies at INL (reference 2) served for low burn-up fuel: providing the data to evaluate the fuel’s ability to comply with various regulations governing confinement, retrievability, and physical form (including 10 CFR 72.122(h) and (l), and 10 CFR 71.33 (b)(3).
In addition to collecting fundamental property data on the high burn-up fuel in the demonstration cask, other data will be collected to support “cross-cutting” extended storage data needs. The collection of data to improve thermal modeling of the cask internals during drying and storage is the highest priority of the cross-cutting data needs. Improved thermal models would lead to more efficient use of existing and future dry storage systems while still providing confidence that regulatory thermal limits will be met.
Another cross-cutting issue of interest is the adequacy of existing drying methods to ensure sufficient water is removed from the cask/canister internals to avoid significant degradation of the fuel or cask internals. Thus, cask cavity gas samples will be collected in the hours after completion of the standard drying process and measured for the presence of residual water. This will provide information to develop models to better quantify the amount of residual water remaining after a normal drying cycle.
In a manner similar to the earlier low burn-up dry storage cask demonstrations at INL, the high burn-up demonstration cask will include a specially modified lid with penetrations for insertion of thermocouples to measure the axial and radial distribution of temperatures inside the cask during drying and subsequent transportation. An additional lid penetration will be designed to obtain period cask cavity gas samples.
Additional work required after the initial five-year period will include periodic temperature and gas sample measurements, and reopening of the cask to examine the cask and fuel for signs of degradation. While no specific storage time has been set, a storage time on the order of a decade may be appropriate to determine the evolution of high burn-up spent fuel properties. After that time, the demonstration cask would be reopened and the cask internals and high burn-up fuel would be examined to look for degradation during drying and storage.
|Nuclear fuel assembly. Photo courtesy of EPRI.|
The target date for loading fuel into the instrumented cask is early 2017. This year, EPRI will be developing a draft test plan, scheduled for release for public comment by DOE by September. Following an approximately month-long public comment period, a final test plan will be completed by the end of the year. Activities in 2014 through 2016 will focus on designing the instrumented lid, obtaining a license for the modified lid, identifying the fuel rods to be included in the test program, procuring the cask, and conducting a dry run.
Thus, the proposed demonstration, in conjunction with separate effects testing and predictive models, will provide the complete technical bases for understanding the behavior of high burn-up fuel during storage and subsequent transportation. All of these data will be used in the development of effective aging management programs to ensure safe, long-term management of low- and high-burn-up used fuel.
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