Development of honeycomb type orifices for flow zoning in PFBR

The prototype fast breeder reactor (PFBR) is in its advanced phase of construction at Kalpakkam, India. It is a sodium cooled, pool type reactor with two loop concept where each loop have one primary sodium pump (PSP), one secondary sodium pump (SSP) and two intermediate heat exchangers (IHX). PFBR core subassemblies (SA) are supported vertically inside the sleeves provided in the grid plate (GP). The GP acts as a coolant header through which flow is distributed among the SA to remove fission heat. Since the power profile of the reactor core is not uniform, it is necessary to distribute the coolant flow (called flow zoning) to each subassembly according to their power levels to get maximum mean outlet temperature of sodium at core outlet. To achieve this, PFBR core is divided into 15 zones such as fuel, blanket, reflector, storage, etc. according to their respective power levels. The flow zoning in the different SAs of the reactor core is achieved by installing permanent pressure dropping devices in the foot of the subassembly. Orifices having honey-comb type geometry were developed to meet the flow zoning requirements of fuel zone. These orifices being of very complex geometry requires precision methods of manufacturing to achieve the desired shape under specified tolerances. Investment casting method was optimized to manufacture this orifice plate successfully. Hydraulics of these orifices is important in achieving the required pressure drop without cavitation. The pressure drop across these orifice geometries depends mainly on geometrical factors and Reynolds number (Re) of the flow. Experiments were carried out on full scale model using water as simulant. Re and Eu (Euler number) similitude was followed for the experiments. Cavitation test were carried out to find the incipient cavitation index and were compared to operating cavitation index to find the suitability of these devices in the PFBR. This paper presents the details of the need for these devices, manufacturing methods, similarity criteria followed for the experiment, experimental methodology, instrumentation involved and the results obtained from the experiments with their transposability to the reactor conditions.