Culminating in the participation of the Universitat Politècnica de Catalunya in the Organization for Economic Cooperation and Development-Committee on the Safety of Nuclear Installations/Nuclear Science Committee pressurized water reactor (PWR) main-steam-line-break (MSLB) benchmark, we present the analysis with RELAP/PARCS of a double-ended MSLB assumed to occur in the Ascó nuclear power plant (NPP). This Spanish NPP, a two-unit 1000-MW(electric) PWR plant of Westinghouse design, has been in normal operation since 1983. The utility uses the RELAP model developed by its analysts to study transients that occurred (or postulated), following its own procedures, giving response to operation-related issues, as well as serving licensing and training purposes. The model is well validated. The present study tests the RELAP/PARCS model of the Asco NPP and, in particular, tests the coupling between the neutronics and the thermal hydraulics; its focus is not licensing or validation.