Phase 2 — Fuel performance qualification

Fuel performance qualification testing is limited to testing performed to demonstrate that the product, in the configuration to be used as a driver fuel in one or more reactors, meets specified requirements. This does not include testing to develop or improve characteristics of the product — a Phase-1 activity — or testing performed during or after production to ensure that the product meets quality requirements — a Phase-3 activity. Where the fuel to be qualified has been developed for a generic class of reactors, it is important that qualification irradiations be conducted under the most stringent conditions applicable to that class of reactors.

Usually, the qualification test FA is prototypic of fuel assemblies used in at least one of the higher performance reactors of the same class. It may be irradiated in the reactor in which it is meant to be used, or better yet, in a test reactor capable of creating conditions exceeding those reachable by the target reactor . There may be cases in which a full FA cannot be irradiated for lack of a suitable irradiation facility. In such a case, it is acceptable to use a partial FA, one having fewer FEs, in order to achieve desired irradiation conditions. An essential requirement is that the FA and irradiation conditions be prototypic enough to convince the regulatory body in charge to issue a license for use of the fuel in the proposed reactor.

Prior to fuel performance qualification, detailed design and technical specifications for the test FA should be prepared by the competent cognizant organization; for completeness, these activities are included below. In the following sections, the term ‘prototype FA’ can mean either a full or partial FA.

5.2.1. Detailed design

A detailed design should be developed for a prototype test FA based on design inputs, design requirements, design description (starting from conceptual design), information obtained from Phase-1 results (such as statistics on manufacturing and irradiation test results), and previous experience.

— Design Requirements:

• Performance requirements (fuel meat temperature, fission rate, burnup, cladding temperature);

• Design constraints (fuel stability, chemical compatibility);

— Design Description:

• Material selection and specification;

• Application of appropriate codes and standards;

• Preparation of design drawings.

5.2.2. Technical specification

Technical specification documents should be prepared based on detailed design and requirements to procure one or more prototype FAs for qualification testing.

5.2.3. Prototype manufacturing assessment and development

An assessment may be required to identify whether technical and industrial challenges remain in the manufacturing of FAs with the proposed new fuel. This assessment should provide information about technical and industrial changes required in the manufacturing of the new fuel prototypic FEs (with respect to the process for manufacturing final irradiation test samples during Phase 1), if any. It is important that any changes to the manufacturing process made during Phase 2 are not detrimental to fuel performance, such as an unfavourable affect on the acceptable irradiation behaviour demonstrated during Phase 1. The following issues should be addressed:

— Manufacturing scale up issues for FEs;

— Maximum fuel loading limits;

— Fabrication porosity (both inside fuel particles and elsewhere in the fuel meat);

— Fuel meat uniformity (meat thickness and uranium density);

— Dimensional tolerances;

— Fuel element manufacturing;

— Fuel assembly manufacturing;

— Before manufacturing the LEU FAs for irradiation tests, it may be necessary or desirable to have one complete dummy FA or more (FAs containing either aluminium FEs or FEs with natural or depleted uranium in the fuel meat) manufactured to verify the:

• Manufacturing process;

• Design dimensions and tolerances, possibly demonstrated by insertion of the dummy assembly into an actual core position;

• Hydraulic characteristics.

If any unfavourable characteristics are noted, either the design or the manufacturing process or both must be modified.

5.2.4. Prototype FE and FA manufacturing

Prototype FEs and FAs are to be manufactured with a new LEU fuel on a scale suitable to demonstrate feasibility of industrial manufacturing, according to technical specifications and procurement documents. The number of prototypes to be manufactured and the number of those which should be irradiated may need to be determined through interaction with the local regulator.

5.2.5. Qualification test planning

Before commencing the qualification test irradiation, a document should be produced for both out of pile and in-pile tests which gives:

— Objectives and requirements of the tests;

— List of tests to be performed, including any required PIEs;

— Test conditions (at a minimum, prototypic of the expected application);

— Acceptance criteria;

— Reports required;

— Other essential information.

In particular, the irradiation tests should demonstrate acceptable performance of the FA with respect to the design considerations discussed in Section 5.1.1, such as:

— Operating requirements (‘Performance expectations’ of Section 5.1.1), including:

• Temperature range of cladding and fuel meat;

• Fission rate range;

• Expected EOL burnup;

— Functional requirements (‘Design requirements’ of Section 5.1.1), such as:

• Stability of irradiation swelling;

• Mechanical demands, such as stresses and strains generated by thermal expansion and irradiation effects (creep, etc.) and by hydraulic forces;

• Thermal–hydraulic requirements (thermal conductivity, heat capacities, etc.);

• Chemical compatibility of involved materials (fuel–matrix, fuel–cladding, and fuel–coolant interactions);

— Safety considerations (‘Safety expectations’ of Section 5.1.1), such as:

• Radiological safety considerations, such as no fission product release.

5.2.6. Prototype full size FA irradiation testing, including post-irradiation examinations

The objective of these tests is to demonstrate that an FA containing a newly developed fuel and manufactured under industrial conditions performs satisfactorily and that fuel performance is consistent with that established during Phase 1. It is useful to irradiate at least two FAs under prototypic conditions — one to the normal burnup planned for the FA (expected EOL burnup) and one to a higher burnup to demonstrate a margin of safety. The post-irradiation examination (PIE) can be much less extensive than that performed during Phase 1. The activities include (see also Section 4.5):

— Irradiation — Irradiation should be carried out as required by the test plan to demonstrate acceptable performance, as noted in the previous section;

— PIE and analysis — A PIE should be performed to provide information on the following items:

• Geometrical deformations (such as warping or twisting of the FEs or FA);

• Fuel element (meat) swelling;

• Irradiated fuel particle microstructure (fission gas bubbles, phases);

• Blister threshold temperature;

• Integrity of cladding (fission product release);

• Cladding corrosion;

— Final report on irradiation and PIE — The report should include:

• Irradiation conditions, based on measurement or calculation:

• Reactor power history;

• Coolant temperature history;

• Fuel assembly power history;

• Fuel element power history;

• Fuel element heat flux history;

• Discussion of any operational problems or abnormalities observed;

• PIE results and analysis:

• Processed data from PIE measurements;

• Analysis of the processed data;

• Comparison to expected results.

5.2.7. Fuel qualification report

A fuel performance qualification report should be prepared demonstrating that the newly developed fuel can be safely used within the realm of its intended applications. The report should discuss all evidence of appropriate and safe behaviour obtained during fuel development (Phase 1) and fuel qualification (Phase 2) activities. The items discussed should include:

— Fuel description;

— Material specification;

— Manufacturing process and manufacturer;

— Irradiation test conditions;

— Swelling versus burnup;

— Limiting irradiation conditions;

— Corrosion behaviour under irradiation;

— Fuel disposition options.

5.2.8. Fuel licensing

Proof that a fuel is qualified with respect to its irradiation performance characteristics lies in approval by a regulatory body for use of the fuel in at least one of its licensed research reactors. The reactor operating organization applying for a license to use a new fuel should attach a fuel performance qualification report to its application to cover all issues related to the fuel.