[en] There has been an extensive reorientation of the light-water reactor (LWR) safety research program since the TMI-2 accident and the emphasis is now on small break loss-of-coolant accidents (SBLOCAs) and transients. The Japan Atomic Energy Research Institute (JAERI) has institued the Rig of Safety Assessment Number 4 (ROSA-IV) program for the purpose of studying SBLOCAs and transients. JAERI is constructing the Two-Phase Test Facility (TPTF) for the separate effect tests and going to construct the Large Scale Test Facility (LSTF) for the system effect tests. The design philosophy and the primary specifications of the LSTF system are described in this report. (author)