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1. Introduction
Various types of dry storage casks for spent nuclear fuel are used domestically and internationally but are primarily designed for low burnup fuel (with a burnup ratio below 45,000 MWD/MTU). However, the generation rate of high burnup fuels is increasing. Accordingly, developing a safety-significant high burnup dry storage cask capable of storing the increasing high burnup and damaged fuels for nuclear facilities in Korea is required.
This study conducted a preliminary evaluation of concrete and metal storage casks that store high burnup and damaged fuels. A structural evaluation aims to verify the performance of structural components that affect the safety of concrete and metal storage casks and, furthermore, ensure structural integrity and confinement of the storage casks under normal and off-normal conditions, accident conditions and natural phenomena. The evaluation method for this study is tip-over, which is considered the most conservative tip-over condition for storage casks set by NUREG-2215 and ANSI/57.9 [1, 2].
2. Dry Storage Cask for High Burnup and Damaged Fuel
The specifications of a dry storage canister are determined based on the results from analyzing high burnup fuel and damaged fuels generated in domestic nuclear power plants. In foreign countries, storing damaged and intact fuels together and occasionally damaged fuel only in dry storage cask is a standard practice. Also, high burnup fuel is often stored in dry storage casks, initially designed for low burnup fuel. Defective fuel is mainly placed outside the basket, and it has been analyzed that it is rarely placed in the center.
Based on the findings, the canister dedicated to storing high burnup and damaged spent nuclear fuels will be designed to have a storage capacity of 21 assemblies, considering its handling weight, radiation, and heat production, and one damaged fuel can in the middle. Considering the proportion of defective fuel in the amount of spent nuclear fuel generated, it is reasonable to load one bundle of defective fuel in the center of the dry storage cask.
3. Specifications of the Storage Cask
3.1 Canister
A canister is responsible for storing spent nuclear fuel and containing fissile materials to ensure nuclear criticality safety; it consists of a baseplate defining the containment boundary, a cylindrical canister outer shell, a lid and internal components distributing applied loads of spent nuclear fuel. Damaged fuel is stored and loaded in a Damaged Fuel Canister (DFC), measuring 236 mm in length and width and 4,625 mm in height, and weighing 755 kg (when including WH17×17 fuel). The body of the canister measures 4,880 mm in length, when the length of a DFC is taken into consideration and 25 mm in thickness; the inner diameter measures 1,636 mm in diameter and 4,580 mm in length and is also where the spent nuclear fuel and internal components are located. Internal components consist of 21 assemblies of spent nuclear fuel including damaged fuel, a basket assembly, a disc and rod. The total weight of a canister, including the internal components, is 33.84 tons.
3.2 Metal Storage Cask
A cask and a canister maintain confinement in a metal storage cask. The cylindrical cask is made of forged carbon steel (SA-350 Grade LF3), which is seal-welded with a cask lid made of stainless steel (SA-182 GRADE F6NM) using 40 hexagon head bolts. O-ring metal seals are used to ensure leak-tightness. The cask lid has exhaust and drainage openings and a closure lid with metal O-rings.
Table 1
Allowable stress intensities of the containment (ASME B&PV code, Sec.Ⅲ, Div. Ⅰ, Subsec. NB)
The body of the cask measures 5,340 mm in length and 220 mm in thickness. The inner shell of a cylindrical storage cask measures 1,750 mm and 5,000 mm in length. The canister is located within the cask body, and the metal storage cask weighs 74.87 tons. The cross-sectional view of a metal storage cask is shown in Fig. 1(a).
3.3 Concrete Storage Cask
A concrete cask body, made of carbon steel, is a hollow cylindrical shell structure, which will be filled with reinforced concrete for shielding between the internal and external shells. The body of the concrete cask is loaded with a canister holding 21 spent nuclear fuel bundles, and has 4 air flow paths at top and bottom of the cask body to eliminate the decay heat from the spent nuclear fuel. The concrete cask body has a length of 5,915 mm, an external diameter of 3,160 mm, an internal diameter of 1,910 mm, and a weight of 100.72 tons. The cross-sectional view of a concrete storage cask is shown in Fig. 1(b).
Table 2
Allowable stress intensities of the non-confinement boundary (ASME B&PV code, Sec.Ⅲ, Div. Ⅰ, Subsec. NG and NF)
| Stress intensity | Allowable stress | |||
|---|---|---|---|---|
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| Normal condition | Off-normal condition | Accident condition | ||
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| Primary membrane stress (Pm) | 1.0 Sm | 1.1 Sm | Elasticity | Min (2.4 Sm, 0.7 Su) |
| Plasticity | Max (0.7 Su, Sy + (1/3) (Su−Sy)) | |||
| Primary membrane stress + primary bending stress (Pm+Pb) | 1.5 Sm | 1.65 Sm | Elasticity | Min (3.6 Sm, 1.0 Su) |
| Plasticity | 0.9 Su | |||
| Membrane stress +Primary bending stress + Secondary stress (Pm+Pb+Q) | 3.0 Sm | 3.3 Sm | N/A | |
4. Allowable Stress
The allowable stress of canister internal components and the like that constitute the containment boundary (a baseplate, a cylindrical shell, and a canister lid) must be below the material characteristics of design stress computed from ASME B&PV code, Sec. Ⅲ, Div. 1, Subsec. NG and ASME B&PV code, Sec. Ⅲ, Div. 1, Subsec. NB, respectively [3, 4].
The allowable stress for cask body confinement components to ensure leak-tightness must be below the design stress computed from ASME B&PV code, Sec. Ⅲ, Div. 1, Subsec. NB, and for non-confinement components must be below that of ASME B&PV code, Sec. Ⅲ, Div. 1, Subsec. NF [5].
Furthermore, the reference temperature, shown in Table 3, for the stress evaluations of each component is determined based on the results of the thermal evaluation.
Table 3
Reference temperature for each component
| Component | Cask body | Canister | Basket and disc | Rod |
|---|---|---|---|---|
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| Concrete storage cask | 100℃ | 160℃ | 370℃ | 250℃ |
| Metal storage cask | 110℃ | 160℃ | 370℃ | 250℃ |
5. Structural Analysis Model
The structural analysis model of concrete and metal storage casks accounts for a cask body, a canister outer shell, a basket, a disc, and a rod. The internal fuel assembly is replaced with a dummy of an equivalent mass. The structural analysis model of a high burnup storage cask is shown in Fig. 2. The structural analysis model under the tip-over accident condition, shown in Fig. 3, took the concrete and metal storage casks colliding into a concrete pad into consideration since they are placed on a concrete pad in a dry storage facility. Structural analysis and stress evaluation postulated two crashing direction conditions for the metal storage cask: into the outer surface (Case 1) and the trunnion part (Case 2).
The structural analysis is computed using a general-purpose finite element computational analysis program called ABAQUS 2017 [6]. Solid elements used in the analysis are quadrangle 8-node elements (C3D8R, 8-node linear brick, reduced integration with hourglass control). Elastic analysis was applied to a canister, a metal storage cask lid, and a body that constitutes the confinement boundary; Elasticplastic analysis was applied to a concrete storage cask and its internal components. All analyses executed a dynamic analysis and applied the general contact condition offered by ABAQUS. The coefficient of friction between the steel materials was 0.3 applied. Considering the symmetry, the cross section was subjected to axisymmetric boundary conditions, considering its symmetry, while vertical constraints were applied to the underside of the concrete pad. The sealwelded canister lid and the canister shell share a node, and full constraints between elements were applied. Component elements such as the port cover that do not affect the structural performance aren’t modeled in. The mechanical characteristics of the main materials that are subject to the structural evaluation are referred to ASME B&PV Code, Sec.II, Part A and Part D [7, 8].
6. Loading Conditions
Tip-over accidents of concrete and metal storage casks may happen due to environmental factors such as typhoons and earthquakes, and therefore, the structural integrity shall be assessed for impacts caused by tip-over.
Since tip-over of a storage cask will occur when the center of gravity moves past the pivot point as shown in Fig. 4, the initial tip-over start angle has been given by calculating the center of gravity. Only considering the dead load of concrete and metal storage casks, the initial tip-over start angle is 63.0° and 65.6°. However, to shorten the analysis process of studying the tip-over angle of the storage casks caused by the rigid body motion, the initial angular velocity of tip-over is provided in Table 4.
Table 4
Initial angular velocity of concrete and metal storage casks in tip-over accident
| Data | Concrete storage cask | Metal concrete cask |
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| ∙ Total Weight (M) (ton) | 134.56 | 108.68 |
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| ∙ Center of the x-axis (Cz) (mm) | 1,560.0 | 1,095.0 |
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| ∙ Center of the y-axis (Cy) (mm) | 3,063.92 | 2,701.49 |
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| ∙ Distance to the Pivot Point (r) (mm) | 3,438.2 | 2,914.97 |
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| ∙ Inertia moment about the center of mass | 5.08 107 | 3.20 108 |
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| ∙ Rotational Inertia (ton-mm2) | 1.6415 109 | 1.2435 109 |
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| ∙ Tip-over position angle from the center of gravity (θ) | 63.02 | 65.60 |
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| ∙ Angular velocity of cask tip-over (rad/sec) | 1.6783 | 1.7279 |
7. Structural Analysis Results
Figs. 5~7 and Tables 5~6 illustrate the results of the stress distribution and evaluation for the concrete and metal storage casks under tip-over accident.
Table 5
Stress evaluation results for concrete storage cask
| Component | Material | Stress | Allowable (MPa) | Analysis (MPa) | Safety factor | |
|---|---|---|---|---|---|---|
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| Cask | Lid | SA-36 | Pm | 344.4 | 284.7 | 1.21 |
| Pm+Pb | 442.8 | 351.6 | 1.26 | |||
| Body | SA-36 | Pm | 344.4 | 284.3 | 1.21 | |
| Pm+Pb | 442.8 | 304.3 | 1.46 | |||
| Baseplate | SA-36 | Pm | 344.4 | 242.5 | 1.42 | |
| Pm+Pb | 442.8 | 290.5 | 1.52 | |||
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| Canister | Lid | SA-240 | Pm | 273.1 | 166.6 | 1.64 |
| Type 316 L | Pm+Pb | 409.7 | 214.3 | 1.91 | ||
| Shell | SA-240 | Pm | 273.1 | 230.0 | 1.19 | |
| Type 316 L | Pm+Pb | 409.7 | 284.1 | 1.44 | ||
| Baseplate | SA-240 | Pm | 273.1 | 5.1 | 53.55 | |
| Type 316 L | Pm+Pb | 409.7 | 74.1 | 5.53 | ||
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| Rod | SA-479 | Pm | 428.3 | 217.1 | 1.97 | |
| Type 304 | Pm+Pb | 550.6 | 227.1 | 1.48 | ||
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| Disc | SA-240 | Pm | 428.3 | 363.1 | 1.18 | |
| Type 304 | Pm+Pb | 550.6 | 372.2 | 1.48 | ||
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| Basket | SA-240 | Pm | 428.3 | 124.4 | 3.44 | |
| Type 304 | Pm+Pb | 550.6 | 289.0 | 1.91 | ||
Table 6
Stress evaluation results of metal storage cask
| Component | Material | Stress | Allowable (MPa) | Case 1 | Case 2 | |||
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| Analysis (MPa) | Safety factor | Analysis (MPa) | Safety factor | |||||
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| Cask | Lid | SA-182 | Pm | 555.1 | 101.6 | 5.46 | 145.6 | 3.81 |
| F6NM | Pm+Pb | 793.0 | 272.1 | 2.91 | 274.6 | 2.89 | ||
| Body | SA-350 | Pm | 338.1 | 76.2 | 4.44 | 99.7 | 3.39 | |
| LF3 | Pm+Pb | 483.0 | 399.3 | 1.21 | 381.8 | 1.27 | ||
| Baseplate | SA-350 | Pm | 338.1 | 40.0 | 8.45 | 61.4 | 5.51 | |
| LF3 | Pm+Pb | 483.0 | 60.7 | 7.96 | 122.9 | 3.93 | ||
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| Canister | Lid | SA-240 | Pm | 273.1 | 247.5 | 1.10 | 48.4 | 5.64 |
| Type 316 L | Pm+Pb | 409.7 | 267.9 | 1.53 | 57.5 | 7.13 | ||
| Shell | SA-240 | Pm | 273.1 | 237.6 | 1.15 | 199.9 | 1.37 | |
| Type 316 L | Pm+Pb | 409.7 | 264.8 | 1.55 | 201.2 | 2.04 | ||
| Baseplate | SA-240 | Pm | 273.1 | 69.9 | 3.91 | 25.1 | 10.88 | |
| Type 316 L | Pm+Pb | 409.7 | 96.2 | 4.26 | 32.0 | 12.80 | ||
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| Rod | SA-479 | Pm | 428.3 | 197.9 | 2.16 | 55.4 | 7.73 | |
| Type 304 | Pm+Pb | 550.6 | 207.7 | 2.65 | 254.7 | 2.16 | ||
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| Disc | SA-240 | Pm | 428.3 | 297.9 | 1.44 | 112.8 | 3.80 | |
| Type 304 | Pm+Pb | 550.6 | 301.6 | 1.83 | 355.9 | 1.55 | ||
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| Basket | SA-240 | Pm | 428.3 | 125.6 | 3.41 | 170.3 | 2.51 | |
| Type 304 | Pm+Pb | 550.6 | 281.7 | 1.95 | 340.8 | 1.62 | ||
The stress evaluation involved linearization at the location where the stress was maximized in the analysis results, yielding values of Pm and Pm+Pb.
The analysis of the tip over conditions showed that the largest stresses occurred in the lid farthest from the rotation point. In the case of the canister, the impact is cushioned by the deformation of the channel located inside the concrete storage cask, but due to the large mass of the canister lid, the stresses are somewhat larger in the upper part of the canister.
The maximum stress in the concrete storage casks reads 351.6 MPa on the cask body, 284.1 MPa on the canister, 372.2 MPa on the disc that supports the fuel and basket assemblies.
The maximum stress in the metal storage cask in Case 1 reads 399.3 MPa on the cask body, 267.9 MPa on the canister, and 301.6 MPa on the disc that supports the fuel and basket assemblies. The maximum stress in the metal storage cask in Case 2 reads 381.1 MPa on the cask body and 355.9 MPa on the disc.
For the Case 2, where the localized collision of the trunnion is caused by the difference in the side collision conditions of the metal storage cask, the stresses in the cask body are rather large, while the internal components experience relatively small stresses.
8. Conclusion
This paper described a structural integrity evaluation of the design basis and design load for dry storage cask for high burnup fuels. The evaluation is conducted via the most conservative tip-over condition and compared with the allowable stress intensity set by the ASME B&PV Code.
The results of the stress evaluation show that both concrete and metal storage casks have secured structural integrity as they performed higher than 1.0 in all aspects regarding primary membrane stress (Pm) and primary membrane stress plus primary bending stress (Pm+Pb).
In the Case 2 condition, where the metal storage cask tip-over toward the trunnion part, stress tends to concentrate on the cask body and relatively milder stress on the internal component elements.
Concrete and metal storage casks that store a bundle of high burnup and damaged fuels in the middle have secured structural integrity in tip-over conditions, and thus, the evaluation results of this study can serve as baseline data for designing storage casks.
