An investigation of the wear resistance of bearing materials for nuclear applications

Abstract

Safety is the most important factor in the design of nuclear reactors. The impact of the high temperature, high pressure, corrosive and radioactive environment of a nuclear reactor core is a significant consideration in the selection of materials for application in it. Such materials must have exceptional mechanical properties, critical to safe and reliable operation of the nuclear reactor. Cobalt-based alloys offer excellent resistance to wear and chemical corrosion in such environments. Due to their exceptional mechanical properties, they are often applied as bearing surfaces in nuclear applications. However, when cobalt is exposed to the radioactive products of the reactions taking place in the nuclear reactor core, an isotope of cobalt, cobalt-60, is produced. Cobalt-60 is a leading contributor to occupational based radiation exposure of maintenance personnel. The impact of this potentially hazardous behaviour would be reduced by minimising the presence of cobalt in the reactor core. To do this, low cobalt or cobalt-free alloys must be assessed for their suitability to replace the existing cobalt-based alloys currently applied in nuclear power plants. Any such replacement alloys must exhibit equal to or improved mechanical and corrosion resistant properties by comparison. The main objective of this project was to identify such alloys that have the potential to replace current in-service alloys, specifically Stellite 20 and Haynes 25, two cobalt-based alloys conventionally applied in rolling element bearings (REBs) that operate in nuclear reactor cores. The first step taken to achieve this objective was to study and explore the wear resistance and tribological performance of Stellite 20 and Haynes 25, used as rolling and raceway components in the REBs. The sliding wear behaviour of the alloys was assessed using a bespoke ball-on-disc tribometer, which was enclosed in an autoclave, designed to simulate the environment of a nuclear reactor core. Results from these experiments set a performance benchmark from which the wear behaviour of any alternative alloy could be compared. The wear behaviour of a cobalt-free, iron based, developmental alloy, RR2450 was assessed using the same tribometer and compared to Stellite 20. The results indicated good wear resistance of RR2450, relative to Stellite 20. However, concerns were highlighted as to the machinability of the alloy. Experiments conducted in the bespoke tribometer were time intensive and in order to increase time efficiency of the testing programme, a preliminary screening experiment was developed, using a commercially available tribometer that wasn’t enclosed in an autoclave. The sliding wear resistance of two potential Haynes 25 replacement alloys, Cronidur 30 and Haynes 230, were also conducted in less extreme conditions than those for experiments conducted in the autoclave. Following these experiments, it was recommended that the wear behaviour of Cronidur 30 should be further assessed in the simulated reactor ball-on-disc set-up. The form of Haynes 230 assessed demonstrated very poor wear resistance and no further assessments were deemed appropriate. Experimental results were then used to develop two sets of wear prediction models. The first set made use of a semi-analytical methodology and the second applied finite element (FE) methods. Further development of the FE model was also conducted, where an interaction in which a component surface, influenced from both mechanical wear and surface oxidation, was simulated. The growth of oxide layers has been linked, in literature, with reducing the impact of mechanical wear on the surfaces of components made from cobalt-based alloys in the nuclear reactor environment. The objective of this development was to explore and gain an understanding of how the FE wear models could be used to reproduce the working environment and the response of bearings in harsh environments. These environments are complex and tribo-chemical characterisation and tests in them are difficult to conduct, therefore design strategies could be improved by adopting FE simulation techniques.