U-Pu Mixed Oxide Particle Analysis by NAUTILUS and Implications for Next-Generation Verification Challenges.

Area of Science:

• Nuclear Chemistry
• Analytical Chemistry
• Mass Spectrometry

Background:

• Accurate measurement of uranium (U) and plutonium (Pu) isotopes in micrometer-sized particles presents verification challenges for international safeguards.
• U and Pu isotopic composition in mixed oxide (MOX) fuels offers insights into fuel production, processing, and intended use.

Purpose of the Study:

• To demonstrate the capability of the Naval Ultra-Trace Isotope Laboratory's Universal Spectrometer (NAUTILUS) for direct measurement of U and Pu isotopes in MOX fuel particles.
• To assess the accuracy and precision of the NAUTILUS for U-Pu elemental and isotopic ratio determination in MOX reference materials.

Main Methods:

• Utilized reactor-grade MOX reference materials (UKMOX10, UKMOX100) with known isotopic compositions.
• Employed standard sample preparation methods for large geometry-secondary ion mass spectrometry (LG-SIMS).
• Measured U and Pu isotopes directly from MOX particles using the NAUTILUS instrument.

Main Results:

• The NAUTILUS successfully discriminated ²³⁸U¹H⁺ from ²³⁹Pu⁺, enabling accurate ²⁴⁰Pu/²³⁹Pu ratio measurements.
• Accurate U-Pu elemental ratios (3:1 to 300:1) were achieved within 2σ of certificate values, significantly improving upon previous LG-SIMS limitations (<15:1).
• All measured U and Pu isotopic ratios (e.g., ²³⁴U/²³⁸U, ²⁴⁰Pu/²³⁹Pu) for reference materials were determined accurately within 2σ without correction.

Conclusions:

• The NAUTILUS provides a direct and accurate method for determining U and Pu isotopic and elemental compositions in MOX particles.
• This capability addresses emerging verification challenges in international safeguards, particularly for environmental sample analysis.
• The NAUTILUS enhances the precision and accuracy of isotopic measurements for nuclear fuel cycle applications.