A Simplified Method of Restrained Distortion Assessment of Reactor Core Components

Owing to the appreciable variations in intensity of fissile power and fast neutron damage flux which occur in prismatic components of reactor cores under steady state operating conditions, it is necessary to assess the stresses and deformations arising. These deformations may become unacceptably large in graphite structures in which the temperature and damage flux variation is unsymmetrical with respect to the geometry of the component. Even with the simplifying assumption of an axi-symmetrical core power loading the problem is complicated by the wide variations obtaining in both axial and radial directions within any single fuel carrier or reflector structure. In order to reduce this complex time dependent problem in three dimensions to one that is within the scope of a relatively small computer, a simplified approach was developed starting from the concept of bi-metallic strip theory. The main assumption made in this simplification is that the variations in temperature and irradiation dose (on which hinges the mechanical and thermal property changes) occur only in the direction of a single vertical plane drawn through the centre of the components. By dividing the body into a number of parallel layers normal to this plane and also into a number of separate vertical sections, it is possible to ascribe average temperature and damage flux intensities to each of the laminated lengths thus formed. A computer code CURVO has been written using a 'spring and plate' analogy to evaluate both the free and the restrained distortion behaviour of such components in which variation of temperature and dose dependent structural parameters can be taken into account throughout the residence time of the component in the reactor. An example of the application of this analysis to a vertical reflector column bounding the core of the Dragon Reactor Experiment is presented in some detail together with comparisons of the predicted and observed distortions. A derivation of the method has been used in the preparation of a computer subprogramme PISC for use in conjunction with the finite element code STAG to predict post-irradiation distortion of cut sections of fuel element structures. Brief mention is also made of the application of the method of fuel element bowing problems. (author)