This paper investigates the feasibility of testing fast-reactor fuel elements in the core of a thermal reactor. A fast-reactor environment is approximated by placing an assembly of fast-reactor fuel elements surrounded by a neutron filter in the core of a large thermal reactor. The scope of tests which can be performed in currently available thermal reactors is explored, and the effects of such parameters as the number of elements in a test assembly, the enrichment of each element, and the characteristics of neutron filters are investigated. The scoping calculations are based on a heterogeneous arrangement of fuel elements of a type currently being considered for use in large fast reactors. Hypothetical accidents resulting from the disintegration of the filter and/or loss of the sodium coolant are analyzed for the purpose of determining the period of time from the initiation of the accident to the loss of structural integrity of the fuel elements. Useful information pertaining to the operating characteristics and safety aspects of various fast-reactor fuel elements can be obtained from this method of testing