1. Introduction

Spent fuel generated in PWR is stored in the pool filled with water. Dropping accidents of a fuel element on top of the rack may occur during fuel handling in SFP. In the NUREG-0800 APPENDIX D 3.8.4, it requires that a review of the functional capability of the racks and the fuel pool, including the fuel pool liner, be performed for hypothetical accidents that could occur during the handling of spent nuclear fuel [1]. In the NUREG-0800, functional capability refers to the ability of structures, systems, or components (SSCs) to perform their intended safety functions under various conditions, including normal operations, postulated accidents, and structural integrity concerns the physical condition of the SSCs being sufficient to prevent structural failure that could lead to safety hazards. Structural integrity ensures that SSCs remain intact and serviceable under all anticipated loadings, including environmental impacts and operational stresses. Among the two evaluation methods, the structural integrity of the racks and the fuel pool, including the fuel pool liner was evaluated in the technical reports (Pool liner qualification report for spent fuel storage racks and Seismic, Structural and Fatigue Evaluation of the Spent Fuel Racks) according to ASME B&PVC, Section III, Division 1, Subsection NF [2-4].

This study evaluated the critical effects and additional damage caused by rack failure and the functional capability of the racks for postulated fuel drop accidents using the tresca stress basis.

2. Dropping Accident of Spent Fuel

2.1 Evaluation Conditions

The accident of spent fuel dropping on the storage racks is assumed to occur in the following three scenarios.

The consideration of an inclined drop scenario, where the impacting spent fuel assembly falls in a sloped manner, is conceivable. However, in such instances, the spent fuel assembly undergoes a redistribution of its center of mass during the initial collision with the upper part of the storage rack, leading to subsequent collisions with multiple cells and secondary impacts. Consequently, the kinetic energy of the spent fuel assembly is dissipated and absorbed across the entirety of the storage rack cells. Therefore, due to the significantly lower amount of energy absorbed by a single cell compared to vertical drop accidents, this scenario has been omitted from the evaluation. The assessment focused on a storage rack measuring 15×10 in its horizontal dimension, chosen for its largest size, with postulated fuel-dropping accidents impact regions depicted as illustrated in Fig. 1. The CE16×16 PLUS with the largest weight among the stored spent fuels was selected. The weight of spent fuel is 640 kg.

Fig. 1

Impact regions of Postulated fuel-dropping accidents.

2.2 Shallow Drop

The shallow drop accident assumes that fuel assembly and handling tools fall vertically together, colliding with the upper surface of the outer cells of the storage rack module. Since the storage rack has a square grid structure, parts, where displacement occurs, are restricted to the impact part. Additionally, the depth of damage to the cell walls after the drop accident should be less than that of the upper part of the effective fuel area. Therefore, the permissible range of plastic deformation on the cell walls of the storage rack should be less than 187.5 mm from the top of the cell, where the neutron absorber is installed.

2.3 Deep Drop-1

The deep drop-2 scenario involves fuel assembly passing through an empty cell of the storage rack and striking the center of the base plate. Consequently, it could cause damage to the concrete plate is penetrated or deformed significantly. Therefore, the functional capability of the base plate is essential in the event of a drop accident. Additionally, evaluation should be conducted to assess the potential damage to the concrete plate due to secondary collisions between the base plate and the concrete plate when maximum displacement occurs.

2.4 Deep Drop-2

In the deep drop-2 scenario, fuel assembly pass through an empty cell of the storage rack and strike the support structure directly. As the base plate is supported by the support structure, the impact load is transmitted directly to the concrete plate of SFP, potentially causing damage. Therefore, functional capability of the base plate and support structure is crucial in the event of a drop accident, and assessment of potential damage to the concrete plate of SFP is necessary.

3. Criteria for Mechanical Accidents

4. Analysis of Accidents

4.1 Assumptions

4.2 Analysis Method

Drop impact analysis used the ABAQUS/Explicit computational program for the Region Ⅱ 15×10 module [7].

4.2.1 Analysis model

The base plate, pedestal and bearing pad were modeled using 8-node solid elements, while the rack cells were modeled using 4-node shell elements. The spent fuel assembly was modeled as a rigid body, conservatively assuming no energy absorption. Detailed geometric shapes were used to model the impacting objects, including the rack cell, base plate, pedestal and bearing pad. Contact conditions were applied to areas where contact with the spent fuel assembly was expected. Tie constraints shared nodes between the rack cell and the base plate. For the shallow drop scenario, the analysis model was partially refined for the cell involved in the collision, with a total of 177,527 nodes and 168,089 elements. The analysis model for the deep drop-2 scenario included the concrete plate supporting the storage rack to evaluate stresses. It was based on the deep drop-1 model, with a total of 179,193 nodes and 165,356 elements. Material properties of concrete were considered. The geometric configurations of the finite element models for shallow drop, deep drop-1, and deep drop-2 structural analyses are depicted in Fig. 2.

Fig. 2

Finite element analysis models.

4.2.2 Boundary and initial conditions

For the shallow drop and deep drop-1 analysis models, the boundary conditions constrained the vertical and horizontal degrees of freedom of the nodes on the underside of the support structure. For the deep drop-2 analysis model, the boundary conditions constrained the vertical and horizontal degrees of freedom of the nodes on the underside of the concrete slab supporting the storage rack. The initial conditions applied a drop velocity to the spent fuel assembly, considering water resistance, with velocities of 3,142, 6,978, and 6,585 mm·sec⁻¹ for the shallow drop, deep drop- 1, and deep drop-2 scenarios, respectively.

5. Analysis Results

5.1 Shallow Drop

The deformation shape and stress distribution at the collision area are illustrated in Fig. 3. The maximum deformation depth, as shown in Fig. 4, is 44.0 mm in the rack cell. The maximum Tresca stress at the collision area is 557.5 MPa in the rack cell colliding with the spent fuel assembly, which does not exceed the yield strength (676.2 MPa). Plastic deformation is observed in the upper part of the storage rack cell, extending to a depth of 261 mm from the top. However, it does not reach the effective fuel area starting from a depth of 625 mm.

Fig. 3

Deformation shape and stress distribution for shallow drop.

Fig. 4

Deformation depth of rack cell for shallow drop.

Fig. 5

Deformation shape and stress distribution for deep drop-1.

5.2 Deep Drop-1

The maximum vertical displacement of the base plate directly impacted by the spent fuel assembly is 19.3 mm, as shown in Fig. 6. This deformation does not exceed the threshold (150 mm) to cause damage to the concrete plate. The maximum Tresca stress at the collision area is 301.6 MPa in the rack cell and 213.6 MPa in the base plate. Although yielding occurs in the rack cell and base plate, the stress does not surpass the ultimate strength (676.2 MPa), indicating no fracture.

Fig. 6

Displacement of base plate for deep drop-1.

5.3 Deep Drop-2

The deformation shape for the deep drop-2 collision analysis is depicted in Fig. 7. The maximum Tresca stress at the collision area is 903.9 MPa at the male pedestal, indicating localized plastic deformation around the bolt bottom and bearing pad top. However, it remains below the ultimate strength (1,100.0 MPa), suggesting no failure. Additionally, the maximum stresses of 201.4 MPa and 237.5 MPa are observed in the base plate and bearing pad, respectively, below the ultimate strength of 676.2 MPa.

Fig. 7

Deformation shape and stress distribution for deep drop-2.

The axial maximum stress on the concrete plate supporting the storage rack reaches 76.9 MPa, exceeding the design compressive strength of 28 MPa. This is expected to cause localized surface cracking within the thickness of the concrete plate, with no significant impact on the overall functional capability of the base plate.

A summary of the collision analysis results for the virtual drop accidents is presented in Tables 2 and 3.

6. Conclusion

The evaluation of potential mechanical accidents on the storage rack yields the following results:

In conclusion, the storage rack is deemed to maintain functional capability against all anticipated mechanical accidents.