Tammy L. Trowbridge

Development and testing of an advanced neutron-absorbing gadolinium alloy for spent nuclear fuel storage

The U.S. Department of Energy requires nuclear criticality control measures for storage of its highly enriched spent nuclear fuel. A new alloy based on the Ni-Cr-Mo alloy system with a gadolinium addition has been developed. Gadolinium has been chosen as the neutron absorption alloying element because of its high thermal neutron absorption cross section. The metallurgical development, mechanical and physical properties, thermal neutron absorption properties, and accelerated corrosion-testing performance of this Ni-Cr-Mo-Gd alloy is described. A brief comparison is also included of the corrosion performance of this alloy as compared to borated stainless steel, which is commonly used as a neutron-absorbing, structural alloy.

Studies of the Corrosion Properties of Ni-Cr-Mo-Gd Neutron-Absorbing Alloys

Abstract The corrosion properties of Ni-Cr-Mo-Gd alloys that are being developed for use as a neutron-absorbing structural material were examined. The corrosion work was part of a larger alloy development program. The corrosion properties were examined by both electrochemical and longer-term immersion testing in standard test solutions and in simulated solutions used in corrosion testing by the Yucca Mountain Project. The addition of Gd to a Ni-Cr-Mo alloy results in the formation of a gadolinide (Ni5Gd) secondary phase. This phase was observed to preferentially dissolve under electrochemical testing at anodic potentials. The reaction appears to be mostly limited to the secondary phase exposed to the surface. A brief comparison with another neutron-absorbing alloy, borated stainless steel, is made.

Electrochemical Corrosion Testing of Neutron Absorber Materials

This report summarizes the results of crevice-corrosion tests for six alloys in solutions representative of ionic compositions inside the Yucca Mountain waste package should a breech occur. The alloys in these tests are Neutronit A978a (ingot metallurgy, hot rolled), Neutrosorb Plus 304B4 Grade Ab (powder metallurgy, hot rolled), Neutrosorb Plus 304B5 Grade Ab (powder metallurgy, hot rolled), Neutrosorb Plus 304B6 Grade Ab (powder metallurgy, hot rolled), Ni-Cr-Mo-Gd alloy2 (ingot metallurgy, hot rolled), and Alloy 22 (ingot metallurgy, hot rolled).

Sample Preparation Techniques for Grain Boundary Characterization of Annealed TRISO-Coated Particles

Abstract Crystallographic information about layers of silicon carbide (SiC) deposited by chemical vapor deposition is essential to understanding layer performance, especially when the the layers are in nonplanar geometries (e.g., spherical). Electron backscatter diffraction (EBSD) was used to analyze spherical SiC layers using a different sampling approach that applied focused ion beam (FIB) milling to avoid the negative impacts of traditional sample polishing and address the need for very small samples of irradiated materials for analysis. The mechanical and chemical grinding and polishing of sample surfaces can introduce lattice strain and result in the unequal removal of SiC and the surrounding layers of different materials due to the hardness differences among these materials. The nature of layer interfaces is thought to play a key role in the performance of SiC; therefore, the analysis of representative samples at these interfacial areas is crucial. In the work reported herein, a FIB was employed in a novel manner to prepare a more representative sample for EBSD analysis from tristructural-isotropic layers that are free of effects introduced by mechanical and chemical preparation methods. In addition, the difficulty of handling neutron-irradiated microscopic samples (such as those analyzed in this work) has been simplified using pretilted mounting stages. The results showed that while the average grain sizes of samples may be similar, the grain boundary characteristics can differ significantly. Furthermore, low-angle grain boundaries comprised 25% of all boundaries in the FIB-prepared sample compared to only 1% to 2% in the polished sample from the same particle. This study demonstrated that the characterization results from FIB-prepared samples provide more repeatable results due to the elimination of the effects of sample preparation.

Fission Products Distribution in TRISO Coated Fuel Particles Irradiated to 3.22 X 1021 n/cm2 Fast Fluence at 1092°C

Mechanisms by which fission products (especially silver [Ag]) migrate across the coating layers of tristructural isotropic (TRISO) coated fuel particles designed for next generation nuclear reactors have been the subject of a variety of research activities due to the complex nature of the migration mechanisms. This paper presents results obtained from the electron microscopic examination of selected irradiated TRISO coated particles from fuel compact 1-3-1 irradiated in the first Advanced Gas Reactor experiment (AGR-1) that was performed as part of the Next Generation Nuclear Plant (NGNP) project. It is of specific interest to study particles of this compact as they were fabricated using a different carrier gas composition ratio for the SiC layer deposition compared with the baseline coated fuel particles reported on previously. Basic scanning electron microscopy (SEM) and SEM montage investigations of the particles indicate a correlation between the distribution of fission product precipitates and the proximity of the inner pyrolytic carbon (IPyC)-silicon carbide (SiC) interface to the fuel kernel. Transmission electron microscopy (TEM) samples were sectioned by focused ion beam (FIB) technique from the IPyC layer, the SiC layer and the IPyC-SiC interlayer of the coated fuel particle. Detailed TEM and scanning transmission electron microscopy (STEM) coupled with energy dispersive X-ray spectroscopy (EDS) were performed to identify fission products and characterize their distribution across the IPyC and SiC layers in the areas examined. Results indicate the presence of palladium-silicon-uranium (Pd-Si-U), Pd-Si, Pd-U, Pd, U, U-Si precipitates in the SiC layer and the presence of Pd-Si-U, Pd-Si, U-Si, U precipitates in the IPyC layer. No Ag-containing precipitates are evident in the IPyC or SiC layers. With increased distance from the IPyC-SiC interface, there are less U-containing precipitates, however, such precipitates are present across nearly the entire SiC layer.Copyright