Appendix 2 - Important fuel quality characteristics

An Investigation into the Falsification of Pellet Diameter Data in the MOX Demonstration Facility at the BNFL Sellafield Site and the Effect of this on the Safety of MOX Fuel in Use

The following sections give details of the most important fuel quality characteristics, how BNFL measures them and assures the customer that the required standards are met. (NB Details of the pellet diameter specification are given elsewhere in the report. Some of the others have already been mentioned in the descriptions of the MOX fuel production process, Section 2.2, and the factors affecting fuel safety in use, Section 4.1 of this report).

Chemical Composition

Pu/U isotopes, Pu enrichment, Metal content ratio, Oxygen/Metal ratio, impurities, gas content and pellet solubility.

Measurements are carried out by Sellafield Analytical Services and Springfields Chemical and Metallurgical Services (NAMAS accredited). The measurements are manually entered onto a computer.

Visual Inspection

Visual inspection of the pellets identifies chips, cracks, surface defects, major shape abnormalities.

Every pellet is visually inspected by operators - QC Inspectors randomly sample up to three pellet trays per lot (~300 pellets/tray). If one defective pellet is found in these inspections the lot is rejected. Results are manually recorded on Lot record sheet.

Additional visual inspection of pellets, chosen at random is performed by the customer.

Pellet Length

Random sample of 20 pellets per lot - length measured using a micrometer with precision 1 µm. Length must be within specified range with 95/95 confidence level.

Results are computer logged and manually recorded on Lot record sheet.

Geometric Density

Random sample of 20 pellets per lot - outer diameter and length are measured using micrometer (see above) and weight is measured using electronic balance - density calculation is computerised and results are stored on computer.

Geometric density must be within specification range a 95/95 confidence level.

Resinter Behaviour

This test is to demonstrate that material is thermally stable. Ten pellets samples are taken at regular intervals agreed with the customer. All pellets must meet specification limit on geometric density following high temperature, extended sintering. Data is computer logged and manually recorded on resinter record sheet.

End Squareness

Random sampling of 20 pellets is taken at any point in production for sampling after press tooling changes, when end features may alter. End squareness is measured using a gauge with a precision of 1 µm. Measurement must be less than specified limit. Results are computer logged and manually recorded on Lot record sheet.

Dish and chamfer

A random sampling of six pellets (12 faces)/Lot are taken at the same time for chamfer height and chamfer length measurement using image processing with precision 0.01 mm, dish depth is measured using depth gauge with a precision of 1 µm. Each feature must be within the specified limits. Results are computer logged and manually recorded.

Surface roughness

A random sample of five pellets is taken at regular intervals, measured using a proprietary gauge with precision 0.02 µm. Roughness must be within the specified limit. Results are computer logged and manually recorded on Lot record sheet.

Pu spots - Pu homogeneity

PuO₂ particle size is measured using colour alpha autoradiography of ceramography samples - the measurements are made on two pellets sampled at regular intervals.

Pu spot size and Pu concentration data are computer logged and manually recorded. Autoradiographs are retained on colour film.

Grain size

Grain size is measured on the same samples as Pu Spot size. Measurements performed on ceramography samples according to ASTM-E112 are recorded in ceramography report.

FUEL ROD

Visual inspection

Visual inspections of all rods are carried out for surface finish and weld appearance (100% inspection). There should be no clad defects greater than the specified limit or discoloration of welds. Additional overinspection of rods sampled at random is carried out by the customer. Results of inspection are manually recorded.

X-ray inspection of rods

Full length x-rays are taken of every rod manufactured in MDF (100% inspection). The full length x-ray allows inspection for the presence of the correct components, the absence of gaps in the fuel column, the absence of unintended materials and the absence of significant fuel faults.

The x-radiographs are also used to check plenum length using a ruler and the results are manually recorded for every rod. The plenum length must be within specified limits.

Additional high resolution X-radiography of top girth and seal weld zones is also undertaken on every rod - this is used to ensure the welds are sound and meet specified limits. Image quality indicators are included in each radiograph to demonstrate the required definition has been achieved.

Results are recorded on radiographs.

Weld metallography

Examination of one metallographic test sample is carried out each day for cracks and penetration depth. Results are manually recorded on metallographic report and control document.

Helium leak detection

All rods are tested for leaks using a mass spectrometer. All rods must meet the specified requirements.

A pass/fail result is manually recorded for each rod on the control document.

Rod surface contamination (smear and fixed)

All rods are checked for both fixed and loose contamination to allow out of glovebox working. Results are manually recorded.

Rod length

The length of each rod is measured on a calibrated traNDucer. The length must be within the specified limits. Results are manually recorded on computer and the control document (100% inspection).

Rod straightness

Each rod is inspected on a table for straightness (100% inspection). The lift-off from the table must be less than the specification limit. Results are manually recorded on the control document.

Weld region - diameter check

All top girth welds are tested using a ring gauge for weld diameter (100% inspection). A pass/fail result is manually recorded for each rod on the control document.

Helium pressure test

Pressure test equipment automatically displays the internal pressure while end cap seal weld is completed. The value is manually recorded on the control document.

End Plug Seal Corrosion Resistance

Autoclave test according to ASTM-G-2 of one test sample is carried out per week followed by visual examination of the sample. Results are manually recorded on control document.

Wrong Enrichment Detection

Each rod is inspected for pellets of the wrong enrichment (100% inspection). Results are automatically recorded on the scan output.

FUEL ASSEMBLIES

The following checks are carried out on all assemblies:

Visual, length, bow, tilt, end squareness, fuel rod gaps, fuel nozzle gaps, grid spacer locations, the control rod fit. Automatic recording of bow, tilt and rod to rod gaps is provided. Other gauging manually recorded.

Added to HSE Website 18th February 2000