Date of Award

Summer 2024

Language

English

Embargo Period

7-28-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

College/School/Department

Department of Nanoscale Science and Engineering

Program

Nanoscale Engineering

First Advisor

Kathleen A Dunn

Second Advisor

Matthew Armstrong

Third Advisor

Nathaniel Cady

Committee Members

Vidya Kaushik

Keywords

LWR, hydrothermal synthesis, ferrite, FA-SMT (Ferritic Alloy-Sandvik Material Technology); ATF (accident tolerant fuel); PM-C26M; hydrothermal corrosion; FeCrAl.

Abstract

The primary corrosion products in Light Water Reactors (LWRs) are nickel ferrites (nominal stoichiometry NiFe2O4) having the spinel crystal structure. These products, commonly called CRUD (Chalk River Unidentified Deposits) are challenging to study in situ, yet understanding their properties is key to improving reactor performance and reducing worker’s radiological exposure risk. In this thesis, a hydrothermal synthesis technique was used to produce nickel ferrite particles from goethite and nickel nitrate hexahydrate in the presence of NaOH. X-ray diffraction was used for phase identification, with scanning electron microscopy used for particle shape and size analysis. By varying the [Ni]:[Fe] ratio of the precursors and synthesis temperature between 100-250 °C, a phase diagram was developed to determine the stability field in both composition and temperature for obtaining a single phase, non-stoichiometric nickel ferrite products. The compositional boundaries of the single phase region of the diagram are a function of temperature, consistent with the increased solubility and reaction rates at temperatures above 125°C. However, even at the highest temperature studied, the end members ([Ni]:[Fe]=0 and [Ni]:[Fe]=0.5) did not produce a single phase product.

Usually, LWRs operate under either Normal Water Chemistry (NWC) which provides an oxidizing environment or Hydrogen Water Chemistry (HWC) which provides a reducing environment. However, the hydrogen overpressure achievable in the laboratory-scale hydrothermal reactor were insufficient to replicate the HWC. Instead, a chemical reducing agent, Ethylenediamine (EDA), was introduced during hydrothermal synthesis. This provided the needed reduction of Fe ions to synthesize the full range of Ni-poor ferrites that were not achieved in the first study. The iso-electric (IEP) point of all synthesized particles was found to depend on composition, but the use of EDA changed the nature of the compositional dependence. In particular, Ni-poor particles synthesized with EDA had a lower IEP than particles of the same [Ni]:[Fe] ratio synthesized without EDA, and in the extreme case, particles synthesized with no Ni approached the IEP of magnetite (Fe3O4) in the presence of EDA rather than hematite (Fe2O3).These findings suggest that re-deposition of liberated particles downstream in the coolant loop (and strategies for mitigating same) will need to take water chemistry into account as well as particle composition.

Along with understanding the behavior of corrosion products, the latter part of this thesis focused on the corrosion of current and future fuel cladding materials and the growth of CRUD products on these cladding tubes, motivated by Fukushima disaster wherein an explosion was directly related to steam oxidation of the Zircaloy fuel cladding. Iron–chromium–aluminum (FeCrAl) alloys are one of the leading contenders in this race. Several alloy compositions were corroded in a test loop at General Electric Vernova in Niskayuna, NY over the course of 6 months. The results implied that water chemistry along with alloy chemistry has a profound effect on the corrosion rate of FeCrAl alloys. Finally, CRUD deposition on latest generation of FeCrAl cladding materials, cladding rods were submerged and heated, followed by CRUD ion injection. The resulting oxide and CRUD on FeCrAl claddings was more difficult to remove than those growing on standard Zircaloy, which can be removed by simple ultrasonication. This apparently stronger adhesion between FeCrAl claddings, oxide layers, and CRUD on these cladding rods could prove to be an impediment to the implementation of next-generation accident-tolerant fuel materials.

License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.

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