Hydrogen in Metals

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Hydrogen is ubiquitous in metals, it enters during production and from corrosion during service. Hydrogen causes embrittlement, hardening and internal damage. Hydrogen can reduce fracture toughness and, in some metals, hydrogen can precipitate forming brittle hydrides that crack under load. The process is called Delayed Hydride Cracking (DHC) and it is a particular problem for zirconium alloys that are used in chemical and nuclear plants as pressure boundaries. The consequence of cracking under pressure can be violent release of high temperature steam. Failure of zirconium alloys used to contain nuclear fuel could mean release of radioactivity during reactor operation and later when the spent fuel is in long-term storage. Currently, safe operation with zirconium alloys is ensured by placing very conservative limits on operating conditions.*** In spite of decades of research there is still much to learn about hydrogen in zirconium. Recently, the applicant's research group has reported hydrides in zirconium dissolving when cooled, and precipitating when heated, which is truly bizarre. We expect the opposite to happen: things dissolve when heated and precipitate when cooled. These observations are unprecedented, and so is the explanation being developed. This bizarre behavior only happens over a small temperature range, but it happens at the temperatures where reactors and chemical plants operate. The implications for how properties of zirconium change with hydrogen ingress are beginning to show: hydrogen diffusion changes, and delayed hydride cracking occurs at temperatures where you do not expect hydrides to form.*** Models of DHC have been developing since the mid-70s. In the last 10 years these models have been critically questioned. The applicant's research group have introduced a new model for DHC that has successfully been used to predict a broad spectrum of experimental results; this is unprecedented and exciting, and incorporates a better understanding of the underlying physics. A reliable robust physical model of DHC will lead to better predictions of failures of zirconium components and, thus, improved safety for chemical plants, nuclear reactors and spent nuclear fuel in long-term storage.***A new understanding is emerging of how hydrogen and hydrides behave in metals, and in zirconium alloys in particular. The goal of this proposal is to have fun with a group of talented students systematically exploring the implications of this new understanding, and taking it to a higher level.******