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an experimental Pebble Bed Modular Reactor (PBMR1) core unloading device (CUD) was required. The CUD housing is a pressure vessel measuring 3 × 2.2 × 1.5 m and weighs 14 t. It forms the housing and structure for a large machine assembly with a number of moving metal parts and valves which make contact with the housing interior surface. The most important reason for nitriding the CUD housing was to prevent cold welding between the moving metal parts and its lining surface, especially inside the PBMR helium gas pressure boundary. Friction in a helium environment causes cold welding between components made of similar steels, since there is no oxygen to replenish the oxide layer on the surface of the steel when wear takes place; a similar phenomenon is encountered in space and high-vacuum applications (Johnson and Keller, 1996; Moeller and Noland, 1968; Lúcia et al., 2006). A nitride layer also provides improved wear, corrosion, and fatigue resistance (Pye, 2003; Nisbett, 2005; Ashrafizadeh, 2003; Kenan, Mehmet, and Mehmet, 2000; Nathalie and Yves, 2006). However, there are no nitriding facilities in South Africa that are able to nitride such a large pressure vessel. Furthermore, no records or evidence could be found that such a large workpiece had ever been nitrided before. Most commercial off-the-shelf nitriding systems (e.g. Nitrex) cannot contain a component as large as the CUD housing. The cost of a custom-built commercial system was prohibitive, and therefore an alternative methodology had to be adopted. The objectives of this work were the purposeful design, building, commissioning, and qualification of a suitable nitriding plant to nitride the housing of the first PBMR CUD prototype. The CUD housing was designed as an ASME VIII (Division 1) pressure vessel (American Society of Mechanical Engineers, 2003a) and was constructed from ASME pressure vessel code steel. The CUD housing forging material is a Cr-Mo steel alloy that meets the ASME specification of SA 336 Grade F22, with chemical composition shown in New gas nitriding technique using the interior of the nitrided pressure vessel as the process chamber by R.W. Nell*, M. Kleingeld, and L. Liebenberg