An experimental assessment of cooling of a 54-rod bundle by in-bundle injection

The performance of an in-bundle coolant injection system for the quenching of dry heated rods has been experimentally investigated. The rod bundle contains 54 fuel rods of 11.2 mm diameter, 3700 mm long, arranged in three concentric rings with a central coolant supply tube. The coolant tube supplies the coolant in the form of jets through a series of circumferential holes at different axial levels inside the rod bundle. Visualization during cold state injection tests ensures that the liquid spray can reach different levels of all the rods above a certain flow rate of water through the coolant tube. Extensive cooling experiments were done to assess the suitability of the proposed scheme of in-bundle coolant injection. Time–temperature curves have been derived from rods at different locations, from different heights of the rods, over a range of coolant flow rate as well as for different rod temperatures. The effect of the presence of the spacers on local cooling has also been investigated. The cooling curves follow a general trend of a rapid temperature drop up to almost 100 °C of the rod surface temperature irrespective of the operating parameters and the location of the rod. Thereafter, the temperature falls slowly reaching the coolant temperature almost asymptotically. Moreover, the second phase of cooling is often marked by temperature fluctuations of random nature. It was also observed that a large volume of steam generates during cooling and comes out through the top of the test section expelling a significant amount of the coolant. This counter current flow of steam–water mixture has an adverse effect on cooling. Further, comparison of the cooling curves obtained from the present experiment with those obtained from quenching of single rod by top flooding brings out the similarities and differences of the two processes.

► Rewetting of a 54-rod bundle assembled with a central coolant tube is investigated.
► The coolant tube injects the coolant radially outwards at different axial levels.
► Above a minimum flow rate, coolant quenches all the rods throughout their length.
► Rapid cooling of rods occurs up to around 100 °C of the rod surface temperature.
► Counter current flow of steam–water gets generated which affects cooling adversely.