The MYRRHA (Multi-purpose hYbrid Research Reactor for High-tech Applications) project aims at the construction of a pool-type subcritical accelerator-driven system that could also operate as a critical reactor. The primary system, enclosed in the primary vessel, is filled with lead-bismuth eutectic (LBE). The secondary cooling fluid is two-phase water operating at relatively low pressure (16 bars). Four aerocondensers act as heat sinks.

The code version used for the development of the MYRRHA models is RELAP5 MOD 3.3; this version has been properly modified to allow the use of LBE as a fluid.

Since the RELAP5-3D code is already equipped with LBE as working fluid, RELAP5-3D has recently been acquired by SCK•CEN in anticipation of the licensing process.

The first important action taken consisted of comparing the two codes by running the existing MYRRHA model input deck, developed for RELAP5 MOD 3.3, on RELAP5-3D.

From the steady-state comparative analysis, it appears clear how the two code versions are using different physical models since the steady-state predictions show several differences. Several code issues have been found, mainly about LBE physical properties, initial noncondensable gas computation at LBE free surface level, and LBE heat transfer coefficient correlations.

For what concerns the transient analysis, the protected loss-of-flow (PLOF) accident has been taken as reference. Also, in PLOF conditions the mass flow rates and temperature distributions are affected by physical properties and heat transfer model differences.