[en] A two-loop test apparatus to be used in the BWR Blowdown/Emergency Core Cooling Program is described. The apparatus will be used for the investigation of reactor transient heat transfer featuring the operation of the core spray systems following the hypothetical recirculation line break (design basis accident). For physical scaling purposes, a typical boiling water reactor is modeled. Program background is outlined and program objectives are defined. Design basis is presented together with a detailed description of the test apparatus' operating performance. Measurement systems and data handling equipment are described in detail. Finally, performance predictions for the test apparatus are compared with boiling water reactor expected performance and with the design basis accident analysis performance