Core restraint system design experience has shown the irradiation creep and swelling design equations to be valuable design tools. The principal difficulty encountered in using these equations occurs when the two effects must be superposed, particularly when creep is used to offset the effects of swelling. The difficulty arises due to the wide range of results obtainable within the swelling-creep confidence limit envelope. An improved understanding of swelling-creep superposition behavior would be of considerable benefit to the core restraint system designer. The slit-tube test offers a near term means of obtaining some swelling-creep superposition data.