Overview
ISO 16793:2018 provides guidelines for the ceramographic preparation of UO2 sintered pellets used in nuclear fuel technology. The standard defines procedures for preparing pellet cross-sections for qualitative and quantitative microstructure examination, including steps for sectioning, mounting, grinding, polishing, etching (chemical or thermal) and optional dismounting. It targets reliable observation of fissures, pores, inclusions and accurate measurement of pore and grain size and their distributions.
Key Topics and Requirements
- Preparation workflows: Three accepted procedures are described - unmounted samples, mounted samples not requiring dismounting, and mounted samples that will be dismounted for further processing (e.g., thermal etching).
- Sectioning and mounting: Use of a metallographic sectioning machine with a diamond wheel and appropriate water cooling; choice of resin (epoxy, polyester, acrylic) depends on whether dismounting is required.
- Grinding and polishing: Multi-stage grinding (progressive abrasive grits) followed by polishing with diamond suspensions (0.25–15 µm) and fine aqueous suspensions (alumina or silica, 0.05–1 µm). Polished surfaces must be scratch-free and inspected microscopically.
- Etching methods:
- Chemical etching: Typical reagents include water, hydrogen peroxide, sulfuric acid, hydrofluoric acid and chromium(VI) oxide. Example solution: water 20 ml, H2O2 2 ml, H2SO4 1 ml; etch time ~3–6 minutes (laboratory-dependent).
- Thermal etching: Conducted in an oxidizing atmosphere (e.g., CO2) at elevated temperatures (1 300–1 450 °C) to uniformly reveal grain boundaries - recommended when using image analysis for grain-size distributions.
- Measurement techniques: Average grain size can be measured by classical counting methods (intercept procedure, grid comparison or reference photographs per ISO 2624 or ASTM E112). Pore- and grain-size distributions are typically measured using automatic image analysers.
- Apparatus and safety: Key equipment includes polishing machines, ultrasonic baths, vacuum impregnation chambers and thermal furnaces. The standard highlights hazards of reagents (H2O2, concentrated H2SO4, HF, Cr(VI)) and the need for appropriate PPE and ventilation.
Applications and Users
ISO 16793:2018 is intended for:
- Nuclear fuel manufacturers and quality control laboratories
- Materials characterization and R&D teams studying UO2 microstructure
- Inspection and regulatory organizations verifying pellet integrity
- Failure analysis groups investigating fissures, pores or inclusions
Benefits include standardized preparation to improve reproducibility of microstructural observations and quantitative image analysis across facilities.
Related Standards
- ISO 2624 - methods referenced for grain-size measurement (classical counting/intercept procedures)
- ASTM E112 - standard test methods for determining average grain size
- ISO 3696 - demineralized water quality (referenced for cleaning)
Keywords: ISO 16793:2018, ceramographic preparation, UO2 sintered pellets, microstructure examination, chemical etching, thermal etching, grain size, pore size, nuclear fuel technology.