Many current and next generation energy systems are reliant on the production, transportation, storage and use of gaseous hydrogen, often at high pressure. The safety, durability, performance, and economic operation of such systems are challenged due to the reality that hydrogen promotes a variety of degradation modes in otherwise high performance materials. Such degradation is often manifested as cracking which compromises the structural integrity of metals and polymers; a behaviour complicated by time and operating cycle (e.g., stress, hydrogen pressure, and temperature) dependencies of degradation. As an example, concurrent stressing and hydrogen exposure at typical pressure vessel or pipeline environmental conditions can promote cracking in modern metallic systems at one-tenth the fracture toughness. Such hydrogen-induced degradation phenomena are generally categorised as hydrogen embrittlement. The breadth and importance of hydrogen damage phenomena have not gone unnoticed in the scientific community with an immense amount of work conducted over the past 100 years. The problem is broadly interdisciplinary and such work has involved metallurgy, chemistry, solid mechanics and fracture mechanics, surface science, molecular and atomic hydrogen physics, non-destructive inspection, materials characterisation, and mechanical-properties testing. This important work notwithstanding, major challenges face those tasked with managing complex engineering structures exposed to demanding environment and mechanical loading conditions. The challenge here is to transform debate on mechanisms of hydrogen damage into a focus on quantitative, predictive models of material cracking properties. Overarching these challenges is the inescapable fact that hydrogen damage problems are immensely complex, requiring understanding of time-cycle dependent processes operating at the atomic scale to impact behaviour manifest at the macroscopic scale.
Light water nuclear reactors, in particular, use gaseous hydrogen additions to their primary coolant to scavenge oxygen and control corrosion in plant components. However, these hydrogen additions can degrade the fracture toughness of many structural alloys. Most notably in PWR and BWR systems, the majority of precipitation-hardened nickel based alloys show a large loss in fracture toughness in hydrogen deaerated water at temperatures less than 150°C. This phenomenon is known as low temperature crack propagation (LTCP). In this study we will attempt to characterise the hydrogen solubility, transport and trapping which govern embrittlement associated with LTCP of a range of Nickel-based alloys commonly used in current reactors. Alloys of interest could include Alloy 600, Alloy 690, and weld filler metals 82, 182, 52 and 152. The proposed work will build on earlier work by Mills and Brown which showed that EN82, EN52 and Alloy 690 were susceptible to LTCP while Alloy 600 was resistant. This PhD programme will be co-supervised at the University of Manchester by Prof Grace Burke who was involved in the earlier, seminal work programme led by Mills and Brown in the US.
Initial experiments will include hydrogen permeation studies and thermal desorption analysis of the above-mentioned Ni alloys as a function of heat treatment (electrochemically charged). Subsequent experiments to evaluate hydrogen trapping would look to charge the alloys in PWR environments in two temperature regimes: low temperature (60-150°C) and high temperature (280-320°C). It should be noted that the effect of temperature on hydrogen uptake leads to a maximum above 150°C where LTCP is a concern, It is likely that exposure to a temperature of maximum hydrogen uptake followed by cooling into the regime where LTCP is operative may be the worst case scenario for fracture toughness in HE susceptible alloys /heat treatments.
Further experimental work could include fracture mechanics / fracture toughness testing as well as analytical electron microscopy. Further work could also look at the effect of dissolved hydrogen amounts within the high purity water coolant on the uptake, solubility, trapping and fracture toughness properties. Additional studies, time permitted could look into the effect of grain boundary segregation and microstructure on HE susceptibility.
This research project is funded in part by EDF-Energy who will also provide co-supervision. The candidate will make use of the state-of-the-art testing facilities located within the Henry Royce Institute for Advanced Materials Research and Innovation at the University of Manchester. Taught elements for the project will be made available to candidates with non-materials science backgrounds.
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