Brady D. Hanson

Corrosion of commercial spent nuclear fuel. 1. Formation of studtite and metastudtite

Summary The contact of commercial spent nuclear fuel (CSNF) with water over a 2-year period led to an unexpected corrosion phase and morphology. At short hydration times, crystallites of metaschoepite [(UO2)8O2(OH)12](H2O)10 were observed on the hydrated CSNF particles. Over the 2-year contact period, all evidence of metaschoepite disappeared, and the fuel particles were coated by a new alteration phase. Additionally, films of the reacted fuel were observed at the sample air-water interface of each sample. The corrosion phases on fuel powders and on the suspended films were examined by scanning electron microscopy, energy-dispersive X-ray fluorescence, and X-ray diffraction and were identified as studtite [(UO2)(O2)(H2O)2](H2O)2 and metastudtite (UO4·2H2O), respectively. The reason for the partitioning of the latter phase to the sample air-water interface is unclear at this time but may be due to structural differences between the two phases. Scanning electron micrographs of the CSNF powders indicated surface corrosion along grain boundaries and fragmentation of the primary solid. The occurrence of studtite and metastudtite on CSNF could have implications for the potential attenuation of released radionuclides during oxidative corrosion of CSNF in a geologic repository.

Microscale characterization of uranium(VI) silicate solids and associated neptunium(V)

Summary The uranium(VI) silicate phases uranophane, Ca[(UO2)(SiO3OH)]2·5H2O, and sodium boltwoodite, Na[(UO2)(SiO3OH)]·1.5H2O, were synthesized in the presence of small, variable quantities (0.5–2.0 mol % relative to U) of pentavalent neptunium (Np(V), as NpO2+), to investigate the nature of its association with these U(VI) solid phases. Solids were characterized by X-ray powder diffraction (XRD), gamma spectrometry (GS), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM) with electron energy-loss spectroscopy (EELS). Neptunium concentration was determined in the bulk solid phases by GS and was found to range from 780–15800 μg/g. In some cases, Np distributions between the aqueous and solid phases were monitored, and 78–97% of the initial Np was associated with the isolated solid. Characterization of individual crystallites by TEM/EELS suggests the Np is associated with the U(VI) phase. No discrete Np phases, such as Np oxides, were observed. Because the U(VI) silicates are believed to be important solubility-controlling solids on a geologic timescale, these results suggest that the partitioning of the minor actinides to these solids must be considered when assessing the performance of a waste repository for spent nuclear fuel.

Corrosion of commercial spent nuclear fuel. 2. Radiochemical analyses of metastudtite and leachates

Summary Immersing commercial spent nuclear fuel (CSNF) in deionized water produced two corrosion products after a 2-year contact period. Suspensions of aggregates were observed to form at the air–water interface for each of five samples. These suspended aggregates were characterized by X-ray diffraction (XRD) to be metastudtite (UO4·2H2O), while the corrosion present on the surface of the fuel itself was determined to be studtite [(UO2)(O2)(H2O)2](H2O)2]. The presence of unreacted UO2 matrix was below the limits of detection by XRD for the three samples examined. The result prompted a radiochemical analysis of the solids collected from the sample air–water interface. The analysis indicated that high concentrations of 90Sr, 137Cs, and 99Tc, relative to the fuel inventory, had concentrated at the air–water interface along with the aggregates of metastudtite. Concentrations of 241Am were at least two orders of magnitude lower than expected in these solids, and retention of 237Np and 239Pu into the corrosion product was observed. The combined radiochemical analyses of the air–water interface aggregates and leachate samples are a rare example of radionuclide partitioning to an alteration phase and may provide preliminary evidence for mechanisms that give rise to such noticeable departures from fuel-inventory values. The leachate radiochemical data are compared to existing data from hydration of the same CSNF.

Observation of Studtite and Metastudtite on Spent Fuel

We have characterized significant quantities of uranyl peroxide phases on commercial spent nuclear fuel (CSNF) samples formed under immersion conditions over a two-year-period. Milligrams of corroded fuel aggregates were observed at the air water interface in each sample. The bulk fuel and the suspended material were examined by SEM, EDX, and XRD and were found to contain studtite and metastudtite, respectively. The reason for the partitioning of the two phases is unclear at this time. SEM micrographs of the bulk powders indicate extensive corrosion. Indeed, under the conditions that developed in the sample containers, dissolution of the fuel was in some cases as severe as a purposeful etching of the surface with concentrated nitric acid. Radiochemical analyses of the leachates and the aggregate materials indicate that dissolution of the fuel surface by hydrogen peroxide may have resulted in rapid release and increased solubility of radiocontaminants in the fuel matrix.

Pretreatment Engineering Platform Phase 1 Final Test Report

Pacific Northwest National Laboratory (PNNL) was tasked by Bechtel National Inc. (BNI) on the River Protection Project, Hanford Tank Waste Treatment and Immobilization Plant (RPP-WTP) project to conduct testing to demonstrate the performance of the WTP Pretreatment Facility (PTF) leaching and ultrafiltration processes at an engineering-scale. In addition to the demonstration, the testing was to address specific technical issues identified in Issue Response Plan for Implementation of External Flowsheet Review Team (EFRT) Recommendations - M12, Undemonstrated Leaching Processes.( ) Testing was conducted in a 1/4.5-scale mock-up of the PTF ultrafiltration system, the Pretreatment Engineering Platform (PEP). Parallel laboratory testing was conducted in various PNNL laboratories to allow direct comparison of process performance at an engineering-scale and a laboratory-scale. This report presents and discusses the results of those tests.

MAKING THE CASE FOR SAFE STORAGE OF USED NUCLEAR FUEL FOR EXTENDED PERIODS OF TIME: COMBINING NEAR-TERM EXPERIMENTS AND ANALYSES WITH LONGER-TERM CONFIRMATORY DEMONSTRATIONS

The need for extended storage of used nuclear fuel is increasing globally as disposition schedules for used fuel are pushed further into the future. This is creating a situation where dry storage of used fuel may need to be extended beyond normal regulatory licensing periods. While it is generally accepted that used fuel in dry storage will remain in a safe condition, there is little data that demonstrate used fuel performance in dry storage environments for long periods of time. This is especially true for high burnup used fuel. This paper discusses a technical approach that defines a process that develops the technical basis for demonstrating the safety of used fuel over extended periods of time.

EFRT M-12 Issue Resolution: Comparison of Filter Performance at PEP and CUF Scale

Pacific Northwest National Laboratory (PNNL) has been tasked by Bechtel National Inc. (BNI) on the River Protection Project-Hanford Tank Waste Treatment and Immobilization Plant (RPP-WTP) project to perform research and development activities to resolve technical issues identified for the Pretreatment Facility (PTF). The Pretreatment Engineering Platform (PEP) was designed, constructed, and operated as part of a plan to respond to issue M12, “Undemonstrated Leaching Processes” of the External Flowsheet Review Team (EFRT) issue response plan.(a) The PEP is a 1/4.5-scale test platform designed to simulate the WTP pretreatment caustic leaching, oxidative leaching, ultrafiltration solids concentration, and slurry washing processes. The PEP replicates the WTP leaching processes using prototypic equipment and control strategies. The PEP also includes non-prototypic ancillary equipment to support the core processing.

The geochemical behaviour of Tc, Np and Pu in spent nuclear fuel in an oxidizing environment

Abstract Spent fuel from commercial nuclear reactors consists mainly of uranium oxide. However, the changes that occur during reactor operations have a profound effect on chemical and physical properties of this material. Heat build-up in the fuel pellet during reactor operations can cause redistribution of fission products. The fission products may aggregate in one of three types of precipitates; gaseous, metallic, or oxide, depending on the burn-up and in-core treatment. Radiation damage and variations in fission and neutron capture yields across the fuel pellets lead to Pu enrichment and increased porosity with increasing burn-up. A more porous surface may make the fuel more susceptible to oxidative dissolution. As the level of actinides and fission products increases, the fuel may become more resistant to oxidation. These changes may limit the usefulness of natural uraninite (UO2) analogues for predicting the geological behaviour of spent fuel disposed in a high-level waste (HLW) repository. In this Chapter, an overview of spent fuel microstructure, radiolytic effects, and alteration processes is presented. Evidence for Np incorporation into U6+ phases, the nature of Pu surface precipitates on spent fuel, and evidence for the preferential removal of 4d-metals from ε-particles in corroded spent fuel is discussed. Understanding the potential mechanisms of radionuclide attenuation through sorption and/or incorporation requires techniques with both high spatial resolution and excellent elemental sensitivity.

Possible Incorporation of Neptunium in Uranyl (VI) Alteration Phases

This study examines existing data on Np behavior from both spent fuel and borosilicate glass tests in effort to resolve issues concerning the selection of possible solubility limiting phases for neptunium and the methods for detecting neptunium at low levels in spent fuel. These issues were raised in a recent report by Finch and Fortner (2002) that argues that the Np analysis with Electron Energy-Loss Spectroscopy (EELS) reported by Buck et al., (1998) is incorrect and that based on a series of experiments with Np-doped U3O8, NpO2 should be adopted as the solubility controlling phase for Np, in the Yucca Mountain performance assessment model. In this report, we will refute the claim that EELS is unable to detect Np and will suggest that the use of NpO2 as the Np solubility controlling phase is not supported by available scientific data from both spent fuel and borosilicate glass.

Np Behavior in Synthesized Uranyl Phases: Results of Initial Tests

Initial tests were completed at Pacific Northwest National Laboratory for developing a potential mechanism to retard the mobility of neptunium at the Yucca Mountain repository. Neptunium is of concern because of its mobility in the environment and long half life, contributing a large percentage of the potential dose over extended times at the perimeter of the site. The mobility of neptunium could be retarded by associating with uranium mineral phases. The following four uranium mineral phases were examined and are potential secondary phases expected to form as a result of interactions of spent nuclear fuel with the local environment: meta-schoepite, studtite, uranophane, and sodium boltwoodite. The fate of the neptunium was examined in these synthetic experiments.