Evaluation of CANDU Spent Nuclear Fuel Bundle Structural Integrity During Normal Transport Conditions
Authors: Morandin Greg , Araujo Eric , Ribbans David J. 
Published: August 2008

The International Atomic Energy Agency requires that the transport of spent nuclear fuel in containers be able to handle certain loads in the axial, lateral and vertical direction under normal off-site handling scenarios. During transport, CANDU nuclear fuel bundles may experience axial impact loads due to possible sliding within a transport tube resulting in impact with the container wall. This paper presents a series of postulated fuel bundle impact scenarios in order to determine the enveloping dynamic g load that a bundle can experience before possible plastic deformation to the bundle fuel sheath. The IAEA load factors for envelope design are used as a reference to ramp the impact velocities and are not equivalent to the dynamic loads used in the analysis. Based on the transportation induced g loads outlined in the IAEA regulations for safe transport of spent fuel under normal handling conditions (IAEA 1985), these g loads are used to calculate a terminal velocity for the bundle whose motion impacts a rigid plate. One type of CANDU nuclear fuel bundle consists of 28 Zircaloy-4 fuel pencils loaded with Uranium Dioxide fuel pellets. The ends of the pencils are fitted with end caps and each end cap is spot welded to a Zircaloy-4 end plate at either end. The finite element model of the fuel bundle consists of 4-noded shell elements representing the fuel sheaths and end plates and 8-noded continuum elements representing the Uranium Dioxide pellets. For the purpose of the analysis, the fuel bundle is housed inside a transport tube, which limits the bundle lateral and vertical motion during impact rebound. The impact target is conservatively modelled as an infinitely rigid plate. Contact surfaces are modelled between the fuel bundle and transport tube, between the fuel bundle and impact plate and between each individual fuel pencil. Two bundle scenarios are considered. The first is a single fuel bundle impacting the plate and the second is two fuel bundles in series in a single transport tube impacting the plate. The second scenario considers the interaction between the two bundles during initial impact and rebound. The analysis covers these scenarios under various magnitudes of applied dynamic loading including 2g, 5g, and 8g. The objective is to determine at what applied load the fuel bundle will experience plastic damage to the fuel pencil sheath. This will effectively provide a bounding g load for CANDU spent fuel transport. The results of the analysis show that for a single bundle in a transport tube, a dynamic load of 8g results in plastic deformation of and the target are modeled using 4-noded shell elements. The pencil end caps are attached to the endplates using an area of common nodes (Fig. 3). Although the actual endcap to endplate connection is through a round spot-welded cross section, for modeling ease the interface is several fuel pencil sheaths. For the two-bundle case, a dynamic load of 8g does not result in any plastic deformation in the fuel pencil sheaths. Thus, a limiting dynamic load between 5g and 8g is determined for the fuel handling scenarios. This paper presents the methodology and models used in the analysis as well as the results of the simulations.