2017 Water Reactor Fuel Performance Meeting September 10 (Sun) ~ 14 (Thu), 2017 Ramada Plaza Jeju Jeju Island, Korea

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1 ASSESSMENT OF THE INTEGRITY OF THE FUEL ROD WITH SPALLED OXIDE UNDER HYPOTHETICAL TRANSPORTATION ACCIDENTS Alfonso Ascarza 1, Alberto Cerracín 1, Jorge Muñoz 1, Leo Carrilho 2, Guirong Pan 2. 1 Technology and Equipment Development, ENUSA INDUSTRIAS AVANZADAS S.A. S.M.E., Santiago Rusinol 12, Madrid, Spain, 28040, afa@enusa.es 2 Fuel Engineering And Safety Analysis, Westinghouse Electric Company Llc, 5801 Bluff Road, Hopkins, SC, United States 29061, carrilla@westinghouse.com ABSTRACT: The classification of the spent fuel for dry cask storage and/or transportation requires an evaluation of compliance of the fuel assembly with a number of safety functions. One of the mechanisms that could prevent the cladding from performing the fuel or system safety functions is the local embrittlement of the fuel rod cladding due to hydride lens, or hydride blister that may be found under an oxide spalling. Indeed, during in-reactor operation, if the waterside corrosion oxide spalls-off from the fuel rod surface, it could create a cold spot that may result in a local zirconium-hydride accumulation. This local hydride accumulation named hydride blister has been identified as a degradation mechanism during normal in-reactor operation and accident conditions, and it can be considered as a fuel defect that requires an evaluation of compliance with the safety functions during drying, storage and transportation to classify the fuel as undamaged. Fuel rods showing oxide spallation have been observed in Zircaloy-4 fuel irradiated in Pressurized Water Reactor (PWR) plants. Nevertheless, the compliance of its safety functions under transportation and storage scenarios has not ever been addressed. ENUSA and Westinghouse developed a methodology that allows the evaluation of the impact of the oxide spallation on the mechanical integrity of the fuel during the storage and transportation. This paper presents the methodology developed to assess the mechanical integrity of PWR fuel rods with spalled waterside cladding oxide layer under transportation scenarios in order to determine the conditions to classify the fuel rods with oxide spalling as undamaged. The evaluation is conducted with the ultimate goal assuring that the cladding integrity is maintained. Firstly, the fuel assembly finite element models for the nine (9) meters vertical and horizontal impacts of the cask system are described and, then, the acceptance criteria based on the Failure Assessment Diagram (FAD) is presented to demonstrate that the crack propagation does not occur below the allowable limits. KEYWORDS: Modeling, Accident, Transportation. I. INTRODUCTION The spent fuel dry storage and transportation requires demonstrating compliance with a number of safety conditions by the fuel-cask system: subcriticality, heat removal, shielding, confinement, and, in some countries such as Spain, fuel 1

2 retrievability. Therefore, by assuring the integrity of the spent fuel during storage and transportation, all these five safety functions are ensured. The fuel cask designer takes care of the verification of the safety functions during drying, storage, and transportation in both normal and accident conditions, assuming "healthy" fuel during safety evaluations. Nevertheless, fuel showing features other than those specifically associated with normal operation requires specific evaluation to determine whether or not the conditions of the fuel allows its classification as "undamaged" during dry storage and transportation. One mechanism that could prevent a fuel rod from performing these safety functions is the local embrittlement of the cladding due to a hydride blister that may form under certain irradiation conditions; underneath a sufficiently thick waterside oxide spalled area. The spallation of the oxide are potential cold spots that may attract the hydrogen picked-up by the cladding due to the corrosion process and that dissolved in the fuel rod tube. This high hydrogen concentration located at cold spots could precipitate during cooling once it reaches a concentration higher that the solubility limit, forming hydride lenses. Both the reduction of the thickness of the zirconium base cladding material and the local embrittlement caused by the hydride blister may degrade the cladding ability to maintain its integrity under mechanical and thermal loads that occur during storage and transportation. Although the impact of a hydride blister on fuel performance has been studied under normal and accident in-reactor conditions (Ref. 1-2), an analysis to demonstrate the cladding integrity of spalled PWR fuel under the transportation loads has never been reported. Zircaloy-4 clad fuel with spalled oxide layer has been only assessed under dry storage scenario, including mainly the creep mechanism (Ref. 3). According to the US NRC Code Federal Regulations (10 CFR 71.73), one of the most limiting loading condition for the fuel rod integrity under transportation accident is the nine (9) meters free drop impact of the cask system onto a flat, horizontal and unyielding surface, striking it in the worst case position (Ref. 4). Hence, in 2013, under the frame of a Joint Development Program, ENUSA and Westinghouse developed a methodology to assess the mechanical integrity of PWR fuel rods with spalled waterside oxide layer under dry storage and transportation loads. The purpose of this joint project is to generate a methodology to determine whether or not fuel rods with spalled oxide layer can be classified as undamaged, according to certain fuel rod characteristics and defect extension limits. In this paper, the developed methodology is presented and applied to a typical Westinghouse PWR 17x17 fuel assembly design in order to assess the spalled fuel rod mechanical integrity after vertical and horizontal nine (9) meter drop accident of the cask. II. METHODOLOGY The evaluation of the cladding integrity is based on safety requirements, in particular, on fuel assembly retrievability after a postulated drop accident during transportation. The methodology developed for this purpose is represented in Fig. 1 and described below: 2

3 Figure 1 Methodology II.A Defect Characterization In the proposed methodology, hydride blisters that may appear underneath spalled areas are considered as cracks oriented in the most limiting direction associated with limiting forces in each of the analyzed scenarios. That is, the crack is assumed to be normal to the principal stresses. Two crack orientations have been considered, according to the loading conditions at the critical load section: Transverse external crack is the most limiting defect when axial loads on the fuel rod are critical (bending stresses after vertical and horizontal drops) (Fig. 2-A). Longitudinal external crack is the most limiting defect when hoop stresses in the fuel rod clad are critical (pinch loads caused by grids over the fuel rod after horizontal impacts) (Fig. 2-B). 3

Figure 2-A Transverse External Crack Figure 2-B Longitudinal External Crack Figure 3 Defect Characterization The expected size and characteristics (shape) of the blister (Fig.

Although being able to obtain a very realistic prediction of the expected hydride blister in the analyzed fuel, a large range of defect characteristcs comprising all the three spacial dimensions has

It should be emphasized that the present study is also based on extensive hot-cell test program specifically carried out for hydride blisters produced during irradiation, which supports the initial

The flat-end morphology of the blister, far away from crack tip shape, provides a high degree of conservatism in the methodology described in this paper. II.B.

4 Figure 2-A Transverse External Crack Figure 2-B Longitudinal External Crack Figure 3 Defect Characterization The expected size and characteristics (shape) of the blister (Fig. 3) can be inferred from, e.g. fuel clad geometry, spalling characteristics, spalling location, operating temperatures, etc. (Ref. 5-6). Although being able to obtain a very realistic prediction of the expected hydride blister in the analyzed fuel, a large range of defect characteristcs comprising all the three spacial dimensions has been considered to define a safety allowance, suficient to cover several potential fuel rod defects. It should be emphasized that the present study is also based on extensive hot-cell test program specifically carried out for hydride blisters produced during irradiation, which supports the initial assumptions on size and shape of hydride lenses. The flat-end morphology of the blister, far away from crack tip shape, provides a high degree of conservatism in the methodology described in this paper. II.B. Finite Element Models Two finite element analysis (FEA) models have been developed to characterize the loading conditions and stresses in the fuel rod clad with hydride blister defects, during a transportation accident: a vertical model to analyze the response to vertical drop loads (Fig. 4-5) and a horizontal model to analyze the response to pinch loads associated with horizontal drop accident (Fig. 6-7). The main characteristics of both models are provided in the below sections. II.B.1. Vertical Model In order to assess the vertical drop accident, a 2-D finite element model has been developed in ANSYS to determine the fuel rod loads during vertical drop transportation accident (Ref.7). The model calculates the deflections (buckling) of a fuel rod under inertial axial loads. 4

INITIAL STATE 1st LOADSTEP 2nd and SUBSEQUENT Straight central rod Closing grid gap Inertial load Figure 4 Rod-to-Grid Contact Model Figure 5 FEA Model Load Steps The model consists of beam elements

The fuel rod to grid contacts are simulated with pre-compressed gap elements, two for the grid dimples and one for the springs at every grid location (Fig. 4).

5 INITIAL STATE 1st LOADSTEP 2nd and SUBSEQUENT Straight central rod Closing grid gap Inertial load Figure 4 Rod-to-Grid Contact Model Figure 5 FEA Model Load Steps The model consists of beam elements with tubular cross-section simulating the whole length of the fuel rod. The fuel rod to grid contacts are simulated with pre-compressed gap elements, two for the grid dimples and one for the springs at every grid location (Fig. 4). These gap elements connect the rod to fixed nodes, which represent the assembly skeleton. Additional gap elements along the fuel rod length simulate the fuel rod contact with the inner wall of the cask basket or with other rods, taking into account the available clearance for lateral deflections. The central rod is assumed to be representative, therefore a symmetric value for the lateral gap is considered. The nodes that represent the inner wall of the cask are fixed so that the stiffness of the basket is assumed to be rigid or much higher than the fuel rod stiffness. It is assumed that all deformations occur in a single plane. Therefore, all out of plane displacements and rotations are constrained for the beam elements. The lower end node of the rod is pinned. The FEA model uses a bilinear elastic plastic formulation for the material model. The yield and the ultimate stress values are based on irradiated Zircaloy-4 material properties at the assumed cladding temperature. The fuel weight is taken into account by adding a density value to the material model of the beams. This way, the weight of the fuel is distributed along the rod length. In the first load step, the fixed nodes that represent the fuel assembly skeleton are laterally displaced to the right in order to introduce some bowing (initial structural perturbation) in the fuel rod. The lateral displacements of these nodes follow a sinusoidal shape where the maximum value of the displacement occurs in the central grid and is equal to the gap between the grid and the inner basket wall. Subsequent analyses calculate the fuel rod responses to the impact loads. In order to accomplish this, the analyst applies incremental ramped accelerations in the axial direction until the maximum value of 60G is reached (Fig. 5). Finally, the model provides the bending moment and the axial reaction force results due to the fuel rod vertical impact loads. 5

This model is able to simulate horizontal impacts, in this case nine (9) meters free drop accident, and to calculate pinch loads, axial loads, and moments on fuel rods, as a result of the postulated

6 Figure 6 Fuel Assembly Model Figure 7 Fuel Assembly and Cask Model II.B.2. Horizontal Model The horizontal model consists of a bi-dimensional (2-D) representation of the fuel assembly inside a container package. This model is able to simulate horizontal impacts, in this case nine (9) meters free drop accident, and to calculate pinch loads, axial loads, and moments on fuel rods, as a result of the postulated accident. The 2-D fuel assembly model is based on nominal dimensions located inside the cask and separated from the upper and the lower sections of the cask by beam elements, whose lengths are equal to the upper and the lower fuel span lengths. The fuel assembly is free to move in the horizontal direction and includes gap elements between nozzles, grids, the fuel rod and the basket walls (cask). All nodes in the cask model are coupled for rotation to simulate the global rigidity of the system. The total mass of the cask system plus the mass of the fuel assemblies is uniformly distributed along the beam elements representing the cask. The 2-D fuel assembly model and the fuel assembly model inside the cask are shown in Fig. 6 and 7. The finite elements are represented as described below: The guide thimbles are represented by a two-column beam element model connected to the nozzles Fuel rod, grids, and nozzles are modeled using beam elements The fuel rod support system is modeled, as follows: The rod-to-grid normal contact is modeled with preloaded gap elements 6

The rod-to-grid rotational effect due to springs and dimples is modeled with rotational spring elements The rod-to-grid spring/dimple contact friction is modeled with axial slider elements Hold-down

The model also calculates the fuel rod pinch load response at each grid elevation.

7 The rod-to-grid rotational effect due to springs and dimples is modeled with rotational spring elements The rod-to-grid spring/dimple contact friction is modeled with axial slider elements Hold-down springs are modeled with two pre-compressed linear spring elements Figure 8 Typical FAD The FEA model calculates bending moments and axial reaction forces associated with horizontal impact loads. The model also calculates the fuel rod pinch load response at each grid elevation. Pinch load results are used in downstream analyses of circumferential stresses in the fuel rod clad at maximum fuel assembly lateral deflection. II.C. Fracture Analysis The integrity of the fuel rod is assessed using a powerful elastic-plastic fracture mechanics analysis tool: the Failure Assessment Diagram (FAD). The FAD approach correlates brittle fracture and plastic collapse criteria to evaluate failures in mechanical structures under different loading conditions and defect size. FAD helps to determine if a certain blister, assumed as a crack, is acceptable under certain loading conditions. The FAD approach was utterly presented in Ref. 8. Basically, the FAD curve K r =f(l r ) such as in Fig. 8, which is a function of yield strength of the material and crack geometry, is obtained and used as an acceptance criterion to evaluate mechanical components survivability (Ref. 9). The FAD approach correlates brittle fracture and plastic collapse (yield strength), which are specified by the normalized ratios K r and L r, as follows: Kr = The ratio of the applied linear elastic stress intensity factor, KI, to the material fracture toughness, KIC (Ref. 10). 7

8 Crack depth (%) 2017 Water Reactor Fuel Performance Meeting Lr = The ratio of the total applied load, giving rise to the primary stresses, to the material yield strength of the cracked structure (Ref. 10). 70% 60% 50% 40% 30% 20% 10% 0% Unsafe Safe Crack length (mm) Figure 9 Worst Case Crack Depth vs. Length Dimensions for Vertical Drop Analysis The FAD depends on the tensile properties of the material, fuel burnup, temperature and hydrogen content effects included in the equation Kr = f(lr ). It also includes a cut-off (cap) at Lr = L (r,max) = 1/2(1 + σu σys), which defines the plastic collapse limit of the structure, where σu is the ultimate strength and σys the yield stress. Once an assessment loading point (point A) is established, the structure can be evaluated as safe, if located under the FAD curve, or failed, if located above the FAD curve. A safety factor (SF) is defined as OB/OA ratio, where OB is the distance from the origin straight to FAD curve through point A, and OA is the distance from the origin to the loading point. Safety factors equal to or greater than one indicate safe conditions while factors lower than one indicate failure. III. RESULTS The methodology developed and explained in Section II was applied to a typical PWR 17x17 fuel assembly with spalled fuel rods under postulated transportation accidents. The major assumptions for the analysis are: The maximum impact acceleration is 60G. This is based on standard cask designs with impact absorbers. The simulation assumes end-of-life conditions for the fuel assembly. The rod model includes a waterside oxide layer of 100 μm, which is not load-bearing. The grid springs that support to the fuel rods are relaxed due to thermal-irradiation. The temperature is uniform, 300ºC (572ºF), which is a mean value at transportation conditions. The Zircaloy-4 material properties includes burn-up of 50 GWd/tU and hydrogen content in the base metal (outside the blister area) of ppm. The models include the total mass of the fuel pellet stack but conservatively use only the stiffness of the clad The model assumes an irradiated Zircaloy-4 fracture toughness of KIC = 20 MPa m at temperatures greater than 280 C. According to EPRI guide, for temperatures greater than 280 C, the value of fracture toughness is KIC = 30 MPa m, therefore, this assumption adds some conservatism to the analysis (Ref. 11). 8

9 Crack depth (%) Crack depth (%) 2017 Water Reactor Fuel Performance Meeting III.A. Vertical Drop A span-by-span analysis based on the fracture mechanics and plastic limit criterion has been performed. For each span, the most restrictive cross-section regarding transverse cracks is the one with maximum bending moment. Therefore, the FAD analysis is performed for the combined effect of the maximum bending moment and the axial reaction force Unsafe Safe Crack length (mm) Figure 10 Worst Case Crack Depth vs. Length Dimensions for Horizontal Drop Analysis Rod Bending 70% 60% Unsafe 50% 40% 30% Safe 20% 10% 0% Crack length (mm) Figure 11 Worst Case Crack Depth vs. Length Dimensions for Horizontal Drop Analysis Pinch Loads The FAD analysis is performed accounting for the depth and the length dimensions of the crack. In each case, a safety factor to failure is obtained from FAD diagram (Fig. 8). A curve correlating worst case depth and length for a crack can be obtained for each fuel rod span analyzed (Fig. 9). 9

10 III.B. Horizontal Drop Similarly to the vertical drop, under axial and bending loads, the worst case dimensions for the fuel rod span under maximum bending is presented in Fig. 10. Likewise, Fig. 11 represents the worst case dimensions for the fuel rod span under maximum pinch loads. Once the safety diagram is defined, the expected dimension of the blister (Section II) is used to check whether or not the defect falls in the safe or in the unsafe regions of the curve. IV. CONCLUSIONS ENUSA and Westinghouse have developed a methodology to assess the mechanical integrity of PWR fuel rods with spalled waterside oxide layer under dry storage and transportation load conditions. According to the methodology, the hydride blister that might appear underneath spalled areas of the fuel rod is supposed to behave as planar crack. This assumption leads to conservative assessment of cladding failure due to hydride blisters. Mechanical tests performed on irradiated spalled rods containing hydride blisters support this conclusion. FE models have been developed to determine the loads corresponding to vertical and horizontal drop impact loads. The integrity of the spalled fuel is assessed using a sophisticated elastic-plastic fracture mechanics analysis method. The FAD analysis is a very versatile tool that allows the evaluation for different combinations of crack length and depth dimensions. As a result, assessment curves are obtained for the evaluation of the cladding integrity. These curves indicate whether or not certain blister dimension, assumed as a planar crack, will cause the fuel rod to fail under postulated transportation accidents. These curves allow, together with spent fuel examination, the classification of spalled spent fuel as undamaged. REFERENCES 1. S. YAMANAKA and M. UNO, Analysis of the fracture behavior of a hydrided cladding tube at elevated temperatures by fracture mechanics, Journal of Alloys and Compounds, (2002). 2. M. KURODA and S. YAMANAKA, Assessment of the Combined Effects of Irradiation and Hydrogenation on the Fracture Behavior of Zircaloy Fuel Claddings by Fracture Mechanics, Journal of Nuclear Science and Technology, Vol.39-3, pp , (2002). 3. EPRI Creep as the Limiting Mechanism for Spent Fuel Dry Storage, Report Nº ,(2000). 4. Title 10, Code Federal Regulations, Part 71, Subpart 73, "Packaging and Transportation of Radioactive Material". United States Government Printing Office, Washington DC,(2004). 5. EPRI report TR , Hot Cell Examination of Extended Burnup Fuel from Calvert Cliffs-1, Volume Garde, Effects of Hydride Precipitate Localization and Neutron Fluence on the Ductility of Irradiated Zircaloy-4, STP J. MUÑOZ, A. CERRACÍN, CRISTINA MUÑOZ-REJA, MANUEL QUECEDO, Structural integrity of irradiated fuel rod cladding under axial loads from hypothetical transportation accident ", WRFPM 2014 (Top Fuel) Sendai, (2014). 8. J. MUÑOZ, A. CERRACÍN, L. A. CARRILHO, An assessment of irradiated fuel rod failure after vertical drop accident during fuel assembly transportation", WRFPM 2015 (Top Fuel) Zurich, (2015). 9. T. L. Anderson, "Fracture Mechanics: Fundamentals and Applications," 3rd edition, Taylor & Francis Group, (2005). 10. FITNET Fitness-for-Service (FFS) Procedure and FITNET Fitness-for-Service (FFS) Annex, Rev. MK8 11. EPRI , "Fracture Toughness Data for Zirconium Alloys, Application to Spent Fuel Cladding in Dry Storage," (2001). 10

MICROSTRUCTURE CHARACTERIZATION OF ZIRLO STRUCTURAL COMPONENTS IRRADIATED TO HIGH BURNUP

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