Heat Transfer to a Dispersed Two-Phase Flow and Detailed Quench Front Velocity Research

Summary

During the blow-down phase of a loss-off coolant accident (LOCA) in a large pressurized water reactor (LWPR) toe core will heat up dramatically. Water will be injected in the system, and by bottom flooding the core will be cooled.

The objectives of this experimental programme are, to obtain a data base for “Boildown and Reflood” computer code development and to obtain information concerning the influence of non uniform temperature and/or power profile on the quench front velocity and prequench heat transfer, including unheated wall and grid effects.

This programme can be divided into two parts: firstly the tube, annulus and a 4-rod bundle experimental programme and secondlythe 36-rod bundle reflood test programme.

The experiments performed in the first part of the programme show an early wetting of the unheated shroud, which leads to a quench front velocity increasement and an increase in liquid transport downstream the quenchfront. Furthermore an increased efficiency of inverted annular flow regime in the cooling of the heater pin is found. For the inverted annular flow regime the extended Bromley correlation gives rather good agreement with the experimental data. In the second part of the programme, boildown and reflood experiments have been performed, at 0.2 MPa pressure in the ECN 36-rod bundle test facility with axial uniform heated rods (3m length) The experiments show the following: The considerable wall-temperature differences in the radial direction either present initially or obtained during the quench process deform the quench front. Under these process conditions high increase of prequench heat transfer is observed on the hotter surfaces, suggesting an inverted annular or transient flow regime to be induced. This intensive heat transfer considerably levels the wall temperatures just before quenching and helps to smooth out the quench-front velocity differences. Heat transfer improvement induced by the presence of spacer grids is observed.