Dynamic Deformation Simulation of Spent Nuclear Fuel Assembly and CIRFT Deformation Sensor Stability Investigation

Abstract

In a horizontal lay-out of a spent nuclear fuel (SNF) assembly under normal condition of transport (NCT), the fuel assembly’s skeleton formed by guide tubes and spacer grids becomes the primary load bearing members for carrying and transferring the vibration loads within a SNF fuel assembly, including interaction forces between the SNF assembly and the canister basket walls. Therefore, the integrity of guide tubes and spacer grids will critically affect the vibration intensity of fuel assembly during transport and consequently affect multipurpose purpose canister (MPC) design concept development to ensure safe SNF transport. Random vibration experienced/registered at SNF transport cask, excited from the railcar bed, provides the external loading driver to vibrate the SNF assembly. In addition to this external vibration driver, another form of cask internal transient shocks driver existed from the dynamic contacts interactions among the fuel assembly components within the cask, including skeleton, fuel rods, and canister basket walls. This SNF assembly components dynamic interactions phenomenon has potential to significantly increase the high-rate impact loading intensity within fuel assembly components during NCT. This research is designed to investigate the SNF assembly deformation dynamics during normal vibration mode as well as cask internal transient shock mode of NCT. Finite element method was utilized to develop vibration simulation modeling for the targeted SNF sub-assembly. Dynamics analyses were performed both in frequency domain to study frequency characteristic of the fuel assembly system, and in time domain to simulate the transient dynamic response of the fuel assembly. FEA simulation results are used to determine the stress and strain states of sub-model components, such as fuel rods, guide tube, spacer grids, nozzles and basket wall under vibration loading during normal vibration condition and transient shocks of NCT. The resultant stress/strain would be used in further fuel assembly component failure analyses. The contact interactions between system components during transient dynamic simulation have also been investigated. To further evaluate the intensity of contact interaction induced by the local contacts impact loading at spacer grid, the detailed models of spring and dimples of spacer grids were established. The impacts between fuel rod and spring/ dimples of spacer grids were simulated with a 20g transient shock load. The associated contact interaction intensities in terms of reaction forces were estimated from FEA results. The corresponding bending moment that generates the same resultant stress on the clad experienced with a 20 g acceleration transient shock was also determined for the purposed of defining the same intensity pure bending loading to be used in a cyclic integrated reversible-bending fatigue tester (CIRFT) vibration testing. In order to estimate the damage potential of the transient shock to the SNF vibration lifetime, the drop tests were performed on the CIRFT specimens. The combined drop-impact and CIRFT test results show reduced fatigue strength from SNF CIRFT specimens. FEA was used to investigate the contact reaction at CIRFT test samples during impact loading induced by drops and the result was compared with that from a 20 g acceleration transient shock load. This report also provides CIRFT system dynamic simulation to predict testing system dynamic performance and verify the system efficiency under different vibration frequencies. The CIRFT deformation sensor dynamic stability was studied in details. The adjustment factors for correcting CIRFT deformation probe geometry stability were developed accordingly for both static and dynamic CIRFT testing, based on SNF CIRFT test results.