A concept of a facility for carrying out a series of in-pile tests of varying sizes for ensuring LMFBR safety is needed. The chief problems to be overcome are those arising from nuclear feedback from the test zone and the need to contain the test in a loop. The suggested solution involves using a zoned core coupled to the test region by a nickel reflector. This suggestion is based on the observation that, as the test size is changed, the reactivity changes are easily accommodated by control rod movement if the loop wall thickness does not change much. To increase the test size safely without increasing test loop wall thickness requires the successful extrapolation of knowledge of energy yield and absorption gained on earlier, smaller tests. Such a procedure is termed a bootstrap procedure.