The design, analysis, and operation of an experiment vehicle to test Fast Ceramic Reactor fuel under prototypical conditions in a thermal test reactor are discussed. The experiments are designed as capsules, with concentric annuli providing a closed loop sodium flow path and an electromagnetic pump to force circulation of the sodium coolant through the multiple fuel pin bundle. The capsules may be irradiated in either the pool or the core of the General Electric Test Reactor, and flux filters may be used to obtain the appropriate neutron spectrum. Pool experiments may be positioned with a movable facility which follows the reactor flux profile and allows adjustment of the experiment power. Additional flexibility in capsule performance is gained by using a binary gas control system which controls the capsule temperature by varying the composition of a gas mixture flowing through an annulus in the capsule. Given the above requirements of adjustable coolant flow and coolant temperature, and either variable power or neutron spectrum, plus practical considerations regarding configuration and fabrication, a design for an experiment can be realized. However, the task of reliably and accurately predicting capsule performance is formidable. Analytical techniques using advanced numerical and computer methods were developed which account for the significant factors influencing capsule performance. The program's capabilities include: conduction, convection, and radiation heat transfer for steady and transient cases, arbitrary three-dimensional lumped-parameter geometry, variable material properties, variable heat generation, computation and use of hot dimensions, and computation of thermal properties of a binary gas mixture. Results obtained from the first in-pile experiment confirm the concept, the manufacturing techniques, and the analytical model.