Fuel failure experiences related to fabrication defects and QMS

There were many fuel failures related to fabrication defects in the 1970s. The main causes for BWR fuel failures were local hydrides and PCI, while PWR failures were caused by collapse and rod bowing. There were some cases in which failure resulted from defects in cladding and end plug materials.

Design and quality control improvements have been effective in mitigating and preventing these fuel failures.

The ratio of fabrication defects causing fuel failure has been decreasing during last 30 years, nevertheless, some fuel rods still fail today due to fabrication defects.

(1) Pellet chips

To prevent PCI failure caused by pellet chips, as is presented in Section 5, FANP has undertaken corrective actions, including improving the manufacturing process [8.52].

Manufacturing process changes involved improvement in the overall quality of fuel pellet surface conditions, in the effectiveness of the pellet visual inspection process and inspection acceptance standards and, finally, in the technology employed to fabricate fuel rods.

FIG. 8.3. Failure rate versus design and process improvements.

The following activities were directed at improving pellet surface conditions within rods:

— Active and passive scanner upgrades — improved sensitivity to pellet gaps and enrichment transitions;

— Pellet grinder wheel increase in diameter and setup, regulator wheel dressing improvements, and more frequent in-process verifications — reduces pellet end chipping at the grinders;

— Co-milling of pore former and improved roll compactor equipment — better control of pellet pore size and distribution;

— Qualification of silica addition process and optimized additive controls for all powder lots to control powder activity and grain size — better control of pellet behaviour characteristics;

— Application of press tooling wear monitoring via Con-Tracer hardware to measure the inside surface of pellet press dies for wear — reduces weakening of pellets due to worn dies;

— Elimination of an in-line ring gauge diameter check — removes the potential for pellet chipping.

Pellet fabrication is now producing fewer pellet surface condition challenges for the pellet inspection process.

The effectiveness of the pellet inspection process was enhanced by: 1) transferring pellet surface condition inspection to an off-line inspection queue which de-coupled the time available to inspect pellets from production through-put requirements; 2) upgrading environmental lighting, defect sample presentation and inspector training/qualifications; 3) introducing special pellet sheets and a tray flipping device which supported 100% surface examination, and; 4) applying a pellet inspection OverSEER system with inspection process feedback to both inspectors and pellet grinder operators. In addition, the allowable missing pellet surface defect size was significantly reduced from the previous standard. The new standard screens at 0.050 inches (1.27 mm) width and 0.025 inches (0.64 mm) depth.

New technology to fabricate fuel rods has been introduced to address the potential for pellet insertion into cladding tubes causing either pellet surface damage or clad liner upsets. Previous technology employed rod segment pushers to slide a column of pellets into a closed tube (lower end cap welded on the tubing). As successive segments were loaded into a rod, loading forces required to seat the fuel column increased. On occasion, the pellet column would buckle or wedge into the cladding during loading. This had the potential to damage pellets or gouge the inner surface of the cladding. Pellet or liner damage was difficult to reliably detect in these circumstances. As part of the integration between Siemens and Framatome worldwide fuel fabrication facilities under AREVA’s joint venture, international best practice manufacturing technologies were selected to standardize operations. With respect to rod fabrication, vibratory rod loading using low amplitude, high frequency excitation was chosen to insert pellets into rods. In June 2004, FANP qualified and placed into service the vibratory rod loading system. This system has eliminated the hard loading conditions which could have damaged pellets or upset the inner surface of tubing.

The Exelon failures were a significant challenge to FANP BWR fuel reliability performance. Following extensive evaluation, several contributing factors to failures were identified. FANP introduced a comprehensive set of corrective actions to address implications of the failure mechanism and identified contributing factors. These corrective actions have produced the expected results; no additional FANP ATRIUM-9B failures have occurred in previously delivered fuel reloads and the ATRIUM-10 product continues to perform very well in a wide range of operating environments.

Another example of improvements to reduce pellet chips are those undertaken by Westinghouse [8.53].

Westinghouse has initiated company wide efforts to enhance the overall quality of UO₂ pellets by enhancing pellet pressing processes, examining pellet handling during various stages of pellet fabrication and enhancing the inspection process with a primary focus on reducing pellets with side chips. Pellet press improvements are being pursued to improve press reliability, efficiency and consistency. By using tools such as cause and effect diagrams, statistical evaluations, gage reproducibility and repeatability etc., Westinghouse has performed in-depth examinations of pellet quality after every process step. Based on these evaluations, process and inspection improvements are being pursued to modify and upgrade the process steps provoking the most pellet defects. New and improved methods and equipment have been and will continue to be implemented as a result of this work.

Different sites, however, use different conversion and pelletizing processes, thus site specific improvements are being made where they will have the most impact.

Comparisons of pellet manufacturing within the BNFL group [8.53]

A benchmarking/best practices review has been conducted among all Westinghouse and BNFL sites to minimize pellet chipping, and enhance overall pellet quality and inspection processes. In addition to pellet handling, which is one of several aspects benchmarked, pellet quality variation sources were considered.

— Pellet design — chamfer angle as well as pellet L/D ratios were considered important;

— Pellet microstructure — uniform microstructure with relatively small pores was generally considered optimum for pellet strength;

— Green density — may have an impact on green pellet strength and thus on pellet chipping;

— Powder morphology — may have an impact on pellet properties. An increasing number of dendrites may improve pelletizing properties. These conclusions were applicable to wet or dry route processes;

— Add-back material — the addition of U₃O₈ to UO₂ at levels of up to 20% could strengthen green pellets. U₃O₈ particle structure and morphology was considered important;

— Press tool maintenance — the wear rate should be monitored to set alarm points before worn tooling can lead to a bad product;

— Pellet die design — the pellet ‘taper’ (the difference between top diameter and bottom diameter due to uneven pressing forces and relaxation) was believed to have an effect on pellet chipping.

These general areas of interest led to further studies, but actual areas of improvement varied between manufacturing sites since both conversion (IDR, ADU and AUC) and pellet manufacturing processes are different.

Automatic visual inspection

An alternate end chip criteria which allows inference of an end chip from the side view has been developed to facilitate rapid development of automated inspection. In preparing for automation studies, Westinghouse has identified 16 different types of defects among manufactured pellet designs UO₂, Gadolinia, Erbia and Integrated Fuel Burnable Absorber (IFBA). Comprehensive visual standards have been prepared for all defects and studies have been undertaken in both Columbia and Västeras on scanning of these pellets using different techniques, including 3D-laser, direct light reflection and diffuse lighting. Options for a complete automatic system were reviewed and a final design chosen.

(2) End plug welding

A review of fuel bundle manufacturing and the quality control plan adopted at the Nuclear Fuel Complex (NFC) was undertaken [8.54]. Fresh fuel in stock at each reactor site and the NFC was investigated for He leaks from fuel bundles to identify any manufacturing defects. Investigations indicated that defects could be due to incomplete end closure welds of elements. End cap sheath junctions are normally under great stress in PHWR fuel, where practically no gas plenum exists. This, accompanied by a manufacturing deficiency, especially in an end closure weld, can also cause fuel failure. Based on a systematic study, the quality control plan and welding procedures were revised.

The TIG method has been employed for plug to tube welding of a fuel rod, and X ray radiography was formerly applied as a non-destructive testing (NDT) means in order to verify the weld integrity of every fuel rod. As X ray radiography had limited capabilities in areas such as shooting time and direction, and inspection of fuel rod weld integrity is a key characteristic of regulatory inspection according to the law, JNF has developed and applied a more reliable and effective probe rotation type ultrasonic method [8.55].

RMD, BARC and NFC have successively developed a UT method to test sheath to end cap welds [8.54].

These modifications not only reduced fuel failures, but also decreased the element reject rate in helium leak tests from above 0.1% to around 0.001%.