Radial jet induced rewetting study for heated rod

Abstract

Establishment of rewetting of hot nuclear fuel pins under a pipe break event is an important design aspect of Nuclear Power Plant (NPP). The successful rewetting of heated pins through Emergency Core Cooling System (ECCS) injection stands to be a major requirement as this action mitigates the accident progression. In Advance Heavy Water Reactor (AHWR), ECCS is designed to inject water from a central water rod of the fuel bundle in form of jets to rewet hot surface of fuel pin. This kind of design to reflood the fuel bundle is different than bottom and top spray reflooding practiced in PWR and BWR type of nuclear reactors. The success of the proposed ECCS injection type has been assessed in a separate effect test where the study has been carried out with a simulated single fuel pin. Influence of issuing radial jet on circumferential and axial conduction and associated rewetting pattern has been studied. As ECCS injection may take place at different decay power levels and at different fuel initial temperatures hence experiments are conducted at different power levels (212W, 600W, 780W) and at different initial clad temperatures (400°C, 600°C, 660°C). The injection flow rate has been kept constant at a design flow rate of 1.8lpm for all the experiments. In all the experiments it is observed that a steep circumferential temperature gradient is established at the beginning of injection and the gradient comes down with the time advancement. A maximum gradient of 500°C is observed for a short period of time (∼4s) for the initial temperature of 660°C. The influence of variation of jet velocity arising from height of it’s injection is reflected on circumferential rewetting velocities which are found to vary from 2mm/s to 10mm/s. A correlation developed from experimental data has been proposed to estimate the average fuel temperature at any time instant during rewetting process. The study concludes that the design of multi point injection within the bundle will successfully be able to rewet the simulated fuel pin over the maximum period of time of ∼28.6s.