The proposed Indian AHWR is a vertical pressure tube type, heavy water moderated, boiling light water cooled reactor employing two-phase natural circulation as the normal mode of coolant circulation. It also relies on natural circulation for heat removal during shutdown and accident scenarios such as LOCA. Both the passive shutdown heat removal system and the passive containment cooling system employ condensers, which reject heat to large pools of water. To test the adequacy of the natural circulation systems employed and to validate the performance of the various computer codes developed several experimental facilities have been proposed. Some of them are already in operation and others are in various stages of implementation. A brief description of the various test facilities is given in the following to subsections.

The integral test loop is being set up to simulate the primary system thermal hydraulic behavior of AHWR during normal operating and accident conditions. The facility has been designed based on a 3-level approach. Primary system scaling is based on the power-to — volume scaling philosophy (level-1). Important local phenomena, which can potentially influence the integral system behavior (e. g. CHF, flow pattern transition, etc.), have been simulated (level-2). Boundary flows (accumulator injection, break flow, etc.) of energy and mass, which can significantly modify the integral system behavior have also been simulated (level-3). It is a full elevation and full pressure facility simulating the AHWR with a volume scale ratio of 400. The facility incorporates all the essential components of AHWR with velocity simulated to 1:1 scale. It can be used to generate experimental data on the steady state and stability behavior of AHWR. It also incorporates safety systems such as the shutdown heat removal system and emergency core-cooling system, which includes an advanced accumulator and the gravity driven cooling system (GDCS).

The high-pressure natural circulation loop is a simple rectangular loop, having similar geometry as one of the parallel channels of AHWR. It is a full pressure test facility set up to investigate the steady state and stability behaviour of natural circulation. The facility is already in operation. Experiments conducted in this facility include steady state two-phase natural circulation and stability of two-phase natural circulation. The parametric influences investigated include the effect of pressure, power and inlet sub-cooling. The facility has provisions to investigate the geometry effects such as loop height, pipe diameter, inlet and exit orificing. The facility is designed to operate up to 115 kg/cm2 pressure, at 315oC.

An experimental facility to study the flow pattern transition instability under 2-phase natural circulation conditions has been designed. The design pressure corresponds to 125 bar. It is proposed to carry out experiments on flow pattern transition instability with four different diameter tubes. This will also facilitate investigation of the effect of scaling on two-phase natural circulation. The facility has been set up and commissioned with instruments. To investigate the flow pattern transition, visualization of the flow patterns is possible using neutron radiography. This technique will also facilitate the measurement of the cross-sectional average void fraction and its profile, which will also be compared with similar measurements, obtained by conductance probes.