Development of a novel nitriding plant for the pressure vessel of the PBMR core unloading device

Abstract

The Pebble Bed Modular Reactor (PBMR) is one of the most technologically advanced developments in South Africa. In order to build a commercially viable demonstration power plant, all the specifically and uniquely designed equipment must first be qualified. All the prototype equipment is tested at the Helium Test Facility (HTF) at Pelindaba. One of the largest components that are tested is the Core Unloading Device (CUD). The main function of the CUD is to unload fuel from the bottom of the reactor core to enable circulation of the fuel core. The CUD housing vessel forms part of the reactor pressure boundary. Pebble-directing valves and other moving machinery are installed inside its machined inner surface. It is essential that the interior surfaces of the CUD are case hardened to provide a corrosion- and wear-resistant layer. Cold welding between the moving metal parts and the machined surface must also be prevented. Nitriding is a case hardening process that adds a hardened wear- and corrosion-resistant layer that will also prevent cold welding of the moving parts in the helium atmosphere. Only a few nitriding furnaces exist that can house a forging as large as the CUD of the PBMR. Commercial nitriding furnaces in South Africa are all too small and have limited flexibility in terms of the nitriding process. The nitriding of a vessel as large as the CUD has not yet been carried out commercially. The aim of this work was to design and develop a custom-made nitriding plant to perform the nitriding of the first PBMR/HTF CUD. Proper process control is essential to ensure that the required nitrided case has been obtained. A new concept for a gas nitriding plant was developed using the nitrided vessel interior as the nitriding process chamber. Before the commencement of detail design, a laboratory test was performed on a scale model vessel to confirm concept feasibility. The design of the plant included the mechanical design of various components essential to the nitriding process. A special stirring fan with an extended length shaft was designed, taking whirling speed into account. Considerable research was performed on the high temperature use of the various components to ensure the safe operation of the plant at temperatures of up to 600°C. Nitriding requires the use of hazardous gases such as ammonia, oxygen and nitrogen. Hydrogen is produced as a by-product and therefore safety was the most important design parameter. Thermohydraulic analyses, i.e. heat transfer and pressure drop calculations in pipes, were also performed to ensure the successful process design of the nitriding plant. The nitriding plant was subsequently constructed and operated to verify the correct design. A large amount of experimental and operating data was captured during the actual operation of the plant. This data was analysed and the thermohydraulic analyses were verified. Nitrided specimens were subjected to hardness and layer thickness tests. The measured temperature of the protruding fan shaft was within the limits predicted by Finite Element Analysis (FEA) models. Graphs of gas flow rates and other operation data confirmed the inverse proportionality between ammonia supply flow rate and measured dissociation rate. The design and operation of the nitriding plant were successful as a nitride layer thickness of 400 μm and hardness of 1 200 Vickers hardness (VHN) was achieved. This research proves that a large pressure vessel can successfully be nitrided using the vessel interior as a process chamber.