Experimental investigation of thermal limits in parallel plate configuration for the future material testing reactor (JHR)

Full text of publication follows: The design of the future material testing reactor, named Jules Horowitz Reactor and dedicated to technological irradiations, will allow very high performances. The JHR will be cooled and moderated by light water. The preliminary core of JHR consists of 46 assemblies, arranged in a triangular lattice inside a rectangular aluminium matrix. It is boarded on two sides by a beryllium reflector. The other two sides are left free in order to introduce mobile irradiation devices. The JHR assembly would be composed of 3 x 6 cylindrical fuel plates maintained by 3 stiffeners. The external diameter of the assembly is close to 8 cm with a 600 mm heated length, coolant channels having a 1.8 mm gap width. The JHR core must be designed to accommodate high power densities using a high coolant mass flux and sub-cooling level at moderate pressure. The JHR core configuration with multi-channels is subject to a potential excursive instability, called flow redistribution, and is distinguished from a true critical heat flux which would occur at a fixed channel flow rate. At thermal-hydraulic conditions applicable to the JHR, the availability of experimental data for both flow redistribution and CHF is very limited. Consequently, a thermal-hydraulic test facility (SULTAN-RJH) was designed and built in CEA-Grenoble to simulate a full-length coolant sub-channel representative of the JHR core, allowing determination of both thermal limits under relevant thermal hydraulics conditions. The SULTAN-RJH test section simulates a single sub-channel in the JHR core with a cross section corresponding to a mean span (∼50 mm) that has a full reactor length (600 mm), the same flow channel gap (1.5 mm) and Inconel plates of 1 mm thickness. The tests with light water flowing vertically upward will investigate a heat flux range of 0-7 MW/m2, velocity range of 0.6-18 m/s, exit pressure range of 0.2-1.0 MPa and inlet temperature range of 25-180 deg. C. The test section is instrumented on the back of the channel plates (i.e. the surface of the insulator) with 40 thermocouples at the center span of one plate. The spacing is staggered to provide improved definition in the region close to the channel exit: every two centimeters at the bottom and every one centimeter at the top. Pressure and temperatures are measured at the test section inlet and outlet as the mass flow rate. Differential pressure measurements are located axially along the channel. The total differential pressure is also measured. A particular attention is devoted to calculate the plate temperature on the fluid side from the temperature measurements, taking into account the Inconel thermal conductivity. The main objective of the SULTAN-RJH facility is to determine the thermal limits (flow excursion and the critical heat flux) in JHR thermal-hydraulics conditions and to validate thermal-hydraulics codes (FLICA4 and CATHARE2) by developing new correlations. For that purpose, tests are conducted by decreasing the mass flow rate step by step under a constant heat flux and at a constant exit pressure. The potential for flow redistribution is determined by detecting the test section pressure drop minimum. However, the facility is also designed to examine other thermal-hydraulics phenomena, including onset of incipient boiling, single-phase heat transfer coefficients and friction factors, as well as two-phase heat transfer and pressure drop characteristics. Tests are performed in nominal conditions for normal conditions and safety margin analysis, and also in conditions encountered during transient scenarios (loss of coolant accident, loss of offsite power..). This paper will report and discuss the first experiments focused on the pressure drop characteristics, the heat transfer correlation and the flow redistribution phenomena. (author)