Procurement | Fuel review
Fuel vendor audits6 October 2010
The current goal of zero fuel failures requires meticulous quality control (QC) and quality assurance (QA) of fuel design, fabrication and fuel operation. Although fuel vendors are responsible for the fuel’s design and fabrication, utilities have the added responsibility of exercising QA over the design and fabrication of the fuel at the vendor’s facilities. By Alfred Strasser
Within the timeframe available, it is usually neither feasible nor necessary for a utility to review and audit every aspect of the fuel design and fabrication; it must focus on items that are most likely to affect fuel performance. The approach and conduct of such audits is the subject of this article. The details of auditing the many complex aspects of a fuel assembly design cannot be condensed here; they are, however, available in a publication by ANT International of Sweden .
Obvious examples of items with a high priority for audits are incidents of failed components. The audits must identify the conditions that caused the failure and assure that their causes have been eliminated for their design. The definition of failure must go beyond the commonly-used ‘breach of fuel rod cladding’ concept and include any issue that keeps a fuel assembly or assembly component from fulfilling its design or safety function without repair or removal. Failures are usually due to multiple causes, and their sources can be the result of any combination of design, fabrication or operation of the fuel. Examples of the relationships of recent failures in these three areas are shown in Figure 1. There is a need for QC and QA in all three areas in order to assure that current problems are fixed and potential future problems are prevented.
For the purposes of this paper, the definition of ‘design audit’ is the verification that the design meets the established specifications, and design limits within the approved vendor methodologies and QA plan. The definition of ‘design review’ is meant to be the evaluation process of the design criteria, design bases and design methodologies, as defined by the vendor, to determine whether they are adequate to meet the fuel performance goals and limits. For the sake of simplification, I will use ‘design audits’ to include ‘design reviews’. Design limits apply to design and operational performance limits as well as licensing safety limits.
The design and operating conditions for a new fuel reload are rarely identical to the previous reload, and any changes, no matter how small, that could harm the fuel performance or create a loss in design margin deserve a review and audit. Performance problems that may have occurred in an identical prior reload require audits as well. The types of changes have been grouped into three categories. The scope, timing and schedule of the audits will be established accordingly.
1. Changes in the mechanical, materials, thermal-hydraulic and nuclear design:
In one or several components of an otherwise standard assembly
A completely new assembly and reload
A lead test assembly (LTA)
Comparison of different designs in a bid evaluation
2. Changes in design methodology and/or data input:
Modified design models and calculation methods
New design data input from in- or ex-reactor experience
New licensing regulations
Modified vendor design QA procedures
3. Changes in operating conditions:
Modified performance goals, such as cycle length or burnup
Modified fuel management method
Reactor power uprate
Modified water chemistry
The broad objectives of the design audits are:
- Comparison of the design to the contractual, operating and licensing requirements
- Confirmation that the design provides the best balance between operating flexibility, reliability, licensing and economic goals
- Determination that the core can be operated for the intended cycle with adequate margins to provide sufficient flexibility and manoeuvrability while also being compatible with the existing fuel in the core
- Assurance that the causes for any past problems have been eliminated.
Prior to the detailed technical audits the auditors must be familiar with the vendor’s detailed design QA plan, since the intent of the audit will be to determine compliance with this plan. A review and evaluation of this plan and its adherence to the contracted standards should be made in the early stage of the fuel supply, if not already during the evaluation of the bids. The basic standards are the US government’s 10CFR50, Appendix B and the extensive Q9000 Series published jointly by the ISO, ASTM and ANSI. The functions of the quality management systems, as an example, are defined by Q9001, ‘QM Systems–Requirements’. The QA requirements placed on the vendor’s subcontractors must be audited as well.
An overview of the drawings at the initiation of fuel supply is also highly desirable. The detailed review of the drawings should be done later during the audit of the mechanical and materials design of the components. However, the early overview should assure that the drawings represent the fuel under contract, that they are complete, that they represent the latest revisions and that they are approved. Changes from the previous reload and the reasons for the changes should be evaluated at this point.
The audit will then move on to the mechanical/materials design, the thermal-hydraulic design and the nuclear design. The general topics to be covered will include:
- Design input, design methodology and design tools
- Design bases, criteria and limits
- Experience base and testing programmes that provided the design bases
Prior to carrying out an effective audit at the vendor’s design facility, the auditor has to do a significant amount of homework to be sufficiently knowledgeable. The pre-audit preparation includes review of the fuel and fuel cycle performance requirements specified in the contract, topical and reload design reports by the vendor, applicable licensing documentation, and material and process specifications.
The choice of specific items to audit will depend to a large extent on the design and its performance experience, a topic too extensive to cover here. A detailed guidebook has been published which provides a prioritized list of what, why and how to audit .
The extent of margin of error from the design limits to the performance or licensing limits is one of the most important criteria for audit. The relationship between different types of margins is shown in Figure 2. The margin is intended to cover the uncertainties that exist in the design database, the design tools, and the numerous operating conditions the fuel will be exposed to. Large margins are desirable, as they provide a high confidence level that the fuel performance will be within regulatory limits; they also provide a high level of operating flexibility. By contrast, small margins are less desirable in that they increase the probability that the specified limits could be exceeded, potentially violating a safety limit and/or resulting in failure. Hence, design features with the smallest margins between design and failure limits should get a high priority for a design review.
Since the margins are intended to cover uncertainties, the determination of the level of uncertainties is an important aspect of the review. Uncertainties in computer code calculations need to be considered to ensure that limits are not exceeded. In the example of a fuel performance code, these uncertainties apply to: 1) the models in the code and, 2) the fuel and cladding geometry of the fresh fuel rod, and 3) the in-reactor performance of the fuel and the cladding.
The treatment of uncertainties can be performed in essentially three different ways: 1) deterministic, 2) analytic statistical and 3) probabilistic methodology, that is, Monte Carlo analysis, which is the most representative, since it includes all the key parameters statistically.
In the deterministic methodology the following input data are used: the most unfavourable fuel and cladding geometry of the fresh fuel, consistent with fabrication specifications and unfavourable fuel response during irradiation.
When these data are calculated on a fuel rod with the severest rod power history, the result of the code calculation is a worst-case prediction; the result provides the bounding (worst possible) performance of for example rod internal pressure buildup rate as a function of exposure.
There are two major drawbacks with the deterministic methodology: it is over-conservative, and it is impossible to assess the probability that the calculated response will occur. In the case of rod internal pressure, the deterministic approach will provide a very conservative value of maximum rod internal pressure, but it is not possible to determine the probability that the actual end-of-life rod internal pressure will exceed the value calculated by the deterministic methodology.
In the analytical statistical methodology, the actual distribution of input parameters related to fuel rod geometry and materials and analysis code models are used in conjunction with the severest rod power history. A prerequisite to be able to use this methodology is that the input parameters must have normal (or approximately normal) distributions and they must be statistically independent (or the statistical dependency must be definable). In most cases, the curve of increase in fuel rod internal pressure with irradiation provided by the vendor is a so-called 95/95 curve. Ninety-five percent of the rods will meet the maximum internal pressure limit with a 95% probability.
In probabilistic statistical methodology, the actual distributions of input parameters related to fuel rod geometry and materials and analysis code models are used in conjunction with the projected power histories for all rods in the core based upon fuel cycle analysis results. The distributions of input parameters are not limited to normal and may have any form. Variations in the rod power histories are addressed by assigning uncertainties to the projected rod histories. The distributions and rod histories are used to perform the statistical analysis using the Monte Carlo method. In this case a 95/95 curve can also be plotted. The drawback to the probabilistic statistical methodology is that a large number of code calculations must be done as compared to the deterministic and analytical statistical methodologies where only a small number of code calculations are needed. The advantages of the probabilistic methodology is that it reduces conservatisms inherent in the other methodologies, particularly those due to using bounding rod power histories.
Mechanical and materials design
Initial steps in the audit should identify the performance functions of the components to be audited, their mechanical design bases and their past performance experience.
The detailed audit of the mechanical and materials design of a new assembly should start with the review of the individual components, such as the fuel pellet and cladding, then go on to the subassemblies such as the fuel rod, structural assembly or spacers, and finally to the fuel assembly. The audit of changes in a single component should also include other components whose performance may be affected.
The first item in Table 1, “Items with small margins to design or licensing limits” applies to the audit of mechanical property limit margins, be they yield strength, creep strain, fracture toughness or other design limits. As an example, some of the most critical mechanical limits apply to the structural members that support the fuel assembly. In a PWR they are the guide tubes attached to the upper and lower nozzles. In a BWR, the structural components vary by vendor; water rods (GNF), the inner square water channel (Areva) or the water cross (Westinghouse) attach to the upper and lower nozzles and channels. Irradiation, corrosion and hydrogen pick-up have a significant effect on the properties and performance of these zirconium alloy components during service, and these factors must be taken into consideration in the audit.
After the familiarization with the drawings and design reports, auditors should select the components with the smallest margins. Their audit should include:
- Mechanical performance requirements, design assumptions, and design, licensing and failure limits
- Source of stresses, and in-reactor and ex-reactor qualification of this data
- Zirconium alloy database for ex- and in-reactor properties, considering all the effects of irradiation and corrosion, including ligament size reduction, and the source of the database
- Evaluation of resistance to bowing, potential stresses from differential extension of components, and stresses induced by hydraulic flow
- Mechanical design methodology, stress distribution analyses, and finite element analyses of high-stress concentration areas. In addition, attention should be paid to whether irradiation effects and corrosion and dimensional changes are included in the analyses.
- An excellent source of the latest data on zirconium alloys, which can serve as a reference library for the auditor, is the Annual Review of Zirconium Alloy Technology (ZIRAT)  and associated special topical reports covering significant data on zirconium alloy properties and performance.
The audit and review of materials specifications and proposed fabrication processes are best done as part of the mechanical design audit prior to the initiation of actual fuel fabrication. The opportunity to make improvements is considerably better at this early stage than later when fabrication is in process and on the way to completion.
The assurance of adequate heat removal from the fuel cladding surface will provide reliable thermal-hydraulic performance of the fuel assembly. The design parameters with the most significant influence on this respect are (1) the power distribution among the fuel rods determined in combination with the nuclear design, (2) the hydraulic flow characteristics of the coolant within and among the assemblies, and (3) the effect of the grid design on the coolant flow. The plant operating conditions, such as the core-wide and local power histories and exposures, and the environmental exposure effects, such as the coolant flow rates, temperature, and water chemistry, are also parameters that need to be considered.
Familiarity with the vendor’s analytical methodology is necessary for thermal-hydraulic, design, mechanical and nuclear audits. The methodology will vary significantly between vendors.
Consider for example an audit of the effect of a new spacer design on the margin to thermal limits. The audit would start with a review of the thermal-hydraulic performance requirements and the design and licensing limits, and then move on to the design assumptions, and the applicable ex-reactor testing data. This investigation is then followed by tasks including:
- The effect of mechanical design changes on assembly and core pressure drop, determined either analytically or by a flow test, and the potential effects of changes in pressure drop in assembly lift-off
- Validation of departure from nucleate boiling (DNB) or critical power ratio (CPR) either by calculation or critical heat flux (CHF) testing
- The potential effect of cross-flow or turbulence on neighbouring PWR assemblies, effects on DNB, and compatibility with existing assemblies in the core
- Effect of changes in coolant flow on bypass flow, component cooling and control rod insertion
- Coordination of the flow test with the mechanical design review for evaluation of grid-to-rod fretting potential
- Effect of modified turbulence on crud deposition coordinated with the mechanical design review
- Changes in the spacer material neutron absorption cross sections (nickel vs zirconium alloys) coordinated with the nuclear design review
The nuclear design is highly dependent on the plans for the fuel cycle. Those fuel cycle plans include cycle energy, length and exposure, fuel management scheme and core loading pattern, peaking factors, capacity factor and plant technical specifications.
The design of a reload must assure that the plant can produce the required energy in the next cycle, in combination with the existing core, while meeting the design and licensing criteria. Whether a reload complies with most of the design criteria will be determined by the fuel assembly and core power distributions and their reactivity characteristics. The selection of burnable absorber and enrichment levels and distribution, and the number of assemblies for the next reload will determine reload compliance and desirable operating conditions for the fuel. The process may be an iterative one until an acceptable design is established. The mechanical and thermal-hydraulic design criteria must also be compatible with criteria for nuclear design.
The nuclear design of the assembly and the core can be affected by mechanical design changes, such as for example an increase in the pellet density and pellet diameter. This change increases the amount of uranium per assembly and decreases fuel cycle costs. It affects nuclear design parameters such as the enrichment selection, loading pattern, power distributions, reactivity parameters and input to the mechanical and thermal-hydraulic design. An audit of this design change should include:
- Experience with a similar design change at other plants
- Nuclear cross-section generation
- Enrichment and burnable absorber selection and loading pattern development
- Cycle length predictions
- Reactivity coefficient and power distributions
- Input to the mechanical and thermal-hydraulic design
- Start-up physics and operating data generation
The three major design audit categories—namely, the mechanical/materials, the thermal-hydraulic and the nuclear—interact with each other most of the time. That a seemingly small design change can affect the other categories is an important consideration to take into account during an audit.
The principles of the design audits apply to fabrication audits as well. Some of the key items of the fabrication audits are summarized here; however, a detailed guidebook on what, why, and how, to audit, prioritized by degree of importance, has been published .
The auditors’ time is divided between a review of documentation and observation of the fabrication processes in the plant. An initial review of the fabrication process, process controls, QC inspection plan and procedures is essential to determine whether they are adequate. This process is similar to the reviews of the vendors’ design models, design tool verification, data input and design limits. As we have discussed above, these reviews are carried out prior to the design or production campaign so that any modifications may be made in time for the actual design or production.
Qualification programmes to establish adequate process and inspection parameters create a basis for the vendors’ fabrication and inspection processes. These qualification programmes extend to the qualification of the vendors’ personnel as well as of the vendors’ subcontractors. The programme provides the baseline against which each fabrication process in the plant is audited. Changes from the process parameters established by the qualification programme may be sufficiently significant to require a re-qualification of the entire process, including the inspection plan. A check should then be made of the operator instructions during a plant audit for compatibility with the qualified process.
Other important documentation includes the vendor’s detailed QA plan, the plan that audit findings will be compared to. QC results and their documentation should also be audited for comparison to specifications.
Time in the fabrication plant spent observing the fabrication and inspection processes should be maximized. While audit of the fabrication of all the components is important, the zirconium alloy and fuel are most critical and deserve the most time. Audits of zirconium alloy components should start with the ingot melting, which determines the alloy and impurity homogeneity for its lifetime. Heat-treating processes during the forming operations of the zirconium alloy components have been found to have a significant effect on corrosion resistance and require particular attention. The subsequent fabrication processes should be followed until the assembly is packed for shipping into a container that has been qualified to ship that assembly design, and which will assure its arrival at the power plant still meeting specifications.
Audits of the fuel should start with the QC of the enriched UF6 and continue through loading into the cladding, with a particular focus beginning at the UO2 powder blending stage—the step that determines the enrichment and powder homogeneity of the fuel—followed by pellet pressing and subsequent handling through pellet loading to maintain pellet integrity and the correct placement of enrichments and burnable absorbers.
Audits of the vendors’ subcontractors must assure that they adhere to the required QC/QA procedures. Identification of all subcontractors involved is needed at the start of the production campaign. This is especially true of large fuel vendors today, who are the product of international mergers and acquisitions. Their corporate structure results in the production of components in different countries with different languages but, hopefully, the same QA standards.
Alfred Strasser, Aquarius Services Corp, 17 Pokahoe Drive, Sleepy Hollow, New York, 10591 USA.
|Table 1: Generic selection criteria of specific items to audit|
- Items with small margins to design or licensing limits
 A. Strasser, K. Epperson, J. Holm, et al, Fuel Design Review Handbook, ANT International, May, 2010, www.tinyurl.com/37kl2c9