Evaluation of missing pellet surface geometry on cladding stress distribution and magnitude

Abstract

INTRODUCTIONPellet cladding interaction (PCI) in light water reactor (LWR) fuel is a coupled thermal-chemicalmechanical process that can lead to cladding breach and release of radioactive fission products into the coolant under certain conditions of operating history, power change, and fuel rod design characteristics [1][2][3][4]. Reactor operating restrictions, which limit power maneuvering, have been established to mitigate PCI, but they restrain operational flexibility and lead to loss of power generation. The Consortium for the Advanced Simulation of Light water reactors (CASL) has selected PCI as a key challenge problem and is developing an advanced, 3-dimensional fuel rod simulation capability (referred to as Bison-CASL) to evaluate fuel performance in general and provide PCI failure assessments in particular. With an advanced fuel rod modeling capability that considers the underlying mechanisms leading to cladding failure, fuel designers and engineers can investigate improved fuel concepts for PCI-resistance and better quantify margins to PCI for operating existing fuel rod designs.. PCI failures generally occur following an increase in the local power over a short period of time, and in fuel that has been previously exposed to irradiation. Classical PCI is driven by the localized strains in the vicinity of a pellet crack, as well as the presence of a chemical species, such as iodine, that drive stress corrosion-induced cracking of the cladding [5]. Fuel pellet cracks that form in brittle ceramic pellets by large temperature gradients, are believed important in the PCI failure mechanism [5]. During a local power increase, pellet expansion produces a high contact force between the fuel pellet and cladding material, when a reduced or eliminated residual pellet-clad gap is present because of previous irradiation. Furthermore, during the rapid thermal expansion of the pellet, the fuel cracks can further open, which transfers tangential shear forces onto the cladding. These tangential shear forces are a function of the equilibrium pellet-clad gap or residual contact pressure at the start of the power increase, the power level at gap closure, the interfacial friction, and the maximum local power.Non-classical PCI failure is associated with the presence of a missing pellet surface (MPS) defect [5]. These MPS defects form through mishandling or the manufacturing process, where the pellet is chipped leaving a flaw on the outer surface. The presence of an MPS defect during a localized power ramp can cause severe bending moments in the clad in the vicinity of the MPS when the fuel undergoes rapid thermal expansion due to this increase in local power. Furthermore, the localized region near the MPS also experiences a different temperature distribution compared to when the MPS is not present. The result is a localized hot spot in the fuel and cold spot in the clad.Both classical and non-classical PCI are significantly influenced by the geometry of fuel pellet flaws (i.e pellet cracks and MPS). The purpose of the current...

BISON: A Flexible Code for Advanced Simulation of the Performance of Multiple Nuclear Fuel Forms

“…BISON has been applied to the simulation of a variety of LWR fuel problems encompassing fuel rod irradiation experiments under normal operating conditions and power ramps, 7 LOCA experiments including both separate-effects cladding ballooning tests and integral fuel rod tests, 23,31,35,36 RIA experiments, 32,34,67 and idealized problems involving specific multidimensional aspects such as MPS fabrication defects. 15,68,69 As mentioned previously, BISON has been coupled to the MPACT (neutronics) and CTF (thermal hydraulics) codes through VERA, and in one application 70,71 was used to perform quarter-core simulations of all fuel rods to identify specific rods most susceptible to PCMI failure. While most BISON applications for integral LWR fuel rod analysis used 2-D geometrical representations, the multidimensional modeling capability of BISON implies the potential to investigate inherently 3-D aspects.…”

3-Dimensional Multiphysics Modeling of Behaviors of Pressurized Water Reactor Fuel Rods With Missing Pellet Surface

“…In the simulation of SCC behavior, the model used in this paper does not consider the influence of gas corrosion behavior in the gas cavity on the crack propagation of the cladding, so the simulation results are still conservative. In addition, this paper does not consider the influence of different shapes of MPS defects on fuel performance, if interested, check in Nathan Capps' research (Capps et al, 2016). When the edge of the defect is sharper, the threat of the defect to the integrity of the cladding may be greater.…”

“…In 2016, Nathan Capps studied the influence of MPS defect size on the hoop stress distribution of the cladding using the BISON-CASL program, and calculated the stress reduction factor of 7-16% for a finite length MPS by BISON-CASL (Capps et al, 2016). B.W.…”

Full Core LOCA Safety Analysis for a PWR Containing High Burnup Fuel

“…However, these results can serve as a reasonable assumption until high burnup core designs and operating conditions are available for different types of PWRs, as well as for 18-month high burnup fuel cycles. Formation of radial and circumferential cracks in UO 2 has been observed and understood under reactor startup, power changes, and shut down conditions [50][51][52][53][54][55]. Furthermore, UO 2 cracking has been evaluated using various analysis techniques such as finite element and extended finite element to assess the impact of UO 2 cracking on the overall fuel performance [51][52][53][54][55][56][57][58].…”