Effects of Boundary Conditions on PWR SNF Rod Stiffness and Stresses Under Pinching Loads

September 13, 2019
Publication: 25th Conference on Structural Mechanics in Reactor Technology
Author(s): Ricardo Medina Elmar Eidelpes Luis F. Ibarra

The effects of rod boundary conditions (BCs) on the stiffness and cladding stresses of pressurized water reactor (PWR) spent nuclear fuel (SNF) rods under pinching loading are investigated. Pinching may occur at the assembly spacer grids during an SNF cask transportation accident. The pinching loading capacity of PWR SNF cladding is affected by material degradation effects, such as hydride precipitation and radial hydride reorientation. Results of ring compression tests (RCTs) on empty, hydride-containing PWR cladding indicate that hydride-related embrittlement reduces the diametric cladding ductility. Pinching loading in RCTs deforms the cladding samples radially, causing high circumferential stresses that can trigger cladding cracking along hydride-induced fault lines. The BCs in such RCTs, however, do not fully reproduce the complex stress state in cladding under real pinching loading conditions. Ring compression tests on short tube samples may underestimate the rod pinching stiffness because in a long cladding tube, a load applied at the spacer grid springs or dimples triggers a membrane effect and consequently, a cladding stress in the longitudinal direction of the tube. This membrane effect increases the pinching stiffness.
Further, the supporting effect of the fuel pellet dominates the pinching stiffness of SNF rods once pelletcladding contact is established. To evaluate effects related to fuel-rod BCs on the rod pinching stiffness and cladding stress state, finite element (FE) models were developed using different geometrical simplifications, and their response to pinching loading was compared with load-displacement curves recorded in RCTs and with theoretically computed stiffness values. A three-dimensional FE model was created using solid elements for the pellet, and shell elements for the cladding. This Shell Model (SHM) reproduces well load-displacement curves of cladding before pellet-cladding contact is established, but it cannot capture cladding deformations related to pellet-cladding bearing. Different cladding lengths were modeled to investigate the effect of the tube length on the deformation response. Further, a computationally inexpensive plane strain model (PSM) was developed, capable of capturing cladding deformations related to pellet-cladding bearing, and the pinching stiffness of uniformly loaded, long cladding tubes. However, the PSM cannot reproduce cladding membrane effects related to a locally applied pinching load. Finally, another three-dimensional model was developed, using solid elements for both the cladding and the pellet.
This Solid Model (SOM) is the most computationally expensive model but can reproduce effects related cladding tube length, membrane stresses, and pellet-cladding bearing. The results of the investigations of this study indicate a complex stress state in the cladding of SNF rods under localized pinching loading, which is dominated by circumferentially oriented tensile principal stresses, similar to the stress state of a short RCT sample. However, the results also confirm that RCTs on short, empty cladding samples do not necessarily represent the mechanical behavior of SNF rods under pinching loading and that the membrane effect increases the cladding pinching stiffness significantly. This could lead to higher tensile cladding stresses before the pellet is engaged in the load transfer. After pellet-cladding contact, the pellet support further increases the fuel rod pinching stiffness by orders of magnitude.

Markets: Nuclear