AXLOAD: A Computer Code to Model Constrained Fuel String Axial Expansion

Abstract

The computer code AX-LOAD was developed to analyze constrained axial expansion of a fuel string in a CANDU reactor under large break loss of coolant accident (LOCA) conditions. For certain large break LOCAs, the fuel experiences rapid heat up and the differential axial thermal expansion can exceed the available axial gap in the channel. Constrained expansion of the fuel string will then result and further expansion is mainly accommodated by deformation of the fuel string. The axial loads generated during constrained expansion and the consequential axial deformations are assessed by the AXLOAD code. The compliant components modelled include the fuel elements, latch bundle, pressure tube, and bundle junctions. The heat up of the bundle junctions during constrained expansion, as a result of contact between the end fuel pellet and element endcap, is considered. Effects of initial fuel conditions and external forces such as flow, pressure and friction are also modelled.