The RELAP-3D (version bt03) computer program was used to assess the LOFT-Wyle blowdown test (WSB03R). The primary goal of this new assessment is to represent faithfully the experimental facility and instrumentation using the latest three-dimensional fluid flow modeling capability available in RELAP5-3D. In addition, since RELAP5-3D represents a relatively new and significant upgrade to the capabilities of the RELAP5 series of computer programs, this study serves to add to its growing assessment base. The LOFT-Wyle Transient Fluid Calibration test facility consisted of an approximately 5.4m3 pressure vessel with a flow skirt which created an annulus that acted as a downcomer. An instrumented blowdown loop with an orfice was connected to the downcomer. This facility, built to calibrate the orfices used in several of the LOFT experiments, simulated the LOFT reactor vessel and broken loop cold leg. For the present assessment an existing RELAP5 model developed at INEEL was corrected and upgraded. The model corrections included: (1) employing the proper measured downcomer thickness, (2) positioning the experimental instrumentation in its correct location, and (3) setting the fluid conditions to their measured initial values. Model upgrades included: (1) use of a more finely-detailed fluid component nodalization, (2) explicit modeling of the experimental …