Fabrication and Characterization of a Silicon Carbide Alpha Detector for Molten Salt Application PDF Download

Author: Joshua Taylor Jarrell
Publisher:
ISBN:
Category : Nuclear engineering
Languages : en
Pages : 195

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Book Description
There exists a need for monitoring the actinide concentrations in elevated temperature molten salt environments. Reprocessing of used nuclear fuel through pyroprocessing is being investigated as a viable method to manage the growing stockpile of used nuclear fuel. Idaho National Laboratory has demonstrated the ability to reprocess both breeder and blanket fuel from the Experimental Breeder Reactor II using an electrorefining system. This system uses a molten eutectic salt mixture of lithium chloride and potassium chloride. This electrorefining system can produce high purity uranium ingots and mixed uranium-plutonium ingots. The fundamental electrochemistry used for this process precludes the separation of high purity plutonium when operated within the suggested process limits. However, special nuclear material may be diverted by operating outside of the normal process. Complete draw down of the uranium dissolved into the molten salt would allow for the subsequent removal of high purity plutonium. Monitoring of the operational history of the electrorefiner is therefore essential to address these non-proliferation and safeguard concerns. There is thus a need to monitor the concentrations of individual elements and isotopes present in the electrorefiner salt. Currently, such assays require time on the order of weeks to provide an accurate description of isotopic concentrations within the salt. Thus, a near real-time measurement system for the actinide isotopic concentrations within the salt is needed. All actinide isotopes of interest to non-proliferation and safeguards interests emit characteristic alpha particles. Semiconductor radiation detectors have been shown to provide a compact, high energy resolution solution to spectroscopic measurement needs. Silicon carbide, a wide band-gap semiconductor, provides elevated temperature operation capability and corrosion resistance in the molten salt environment that is superior to silicon. As a result, for this work, alpha radiation detectors comprised of 4H-SiC with Schottky barrier contacts have been fabricated and shown to operate above 500oC. Detector contact compositions of nickel-platinum was explored as possible Schottky contact structures. The electrical and diode characteristics of the detectors were measured. Alpha spectra from multiple source isotopes and source geometries were obtained in vacuum with the detector heated from 20oC to 500oC. The resulting detector behavior including alpha spectrum centroid position and detector energy resolution were measured. To avoid energy attenuation in the molten salt, a repeatable method for depositing actinides to the surface of the detector was devised that allows for repeated spectroscopic measurements by a single detector. The resilience of detector performance to submersion in a molten salt was investigated as well as energy resolution during elevated temperature operation. Detectors were characterized prior to being submerged in a 500oC molten LiCl-KCl eutectic salt for increasing time intervals. After submersion, the detectors were again characterized to identify any degradation. Detector packaging capable of withstanding the corrosive 500oC molten salt environment was developed which allows for electrical connections between the detector and spectrometry equipment. The packaging was designed to allow for actinide deposition on the active area of the detector, allowing for accurate calculations of the actinide mass deposited by a known current. Additionally, nuclear forensic applications of 4H-SiC alpha detectors in conjunction with electrodeposited source fabrication were explored. A method was determined to calculate the 235U enrichment in the product stream of an enrichment facility through measurement of the 234U and 235U enrichments in an electrodeposited source fabricated from depleted uranium.