Research for Trident

Materials World magazine
3 Oct 2015

Dr Ewan Galloway and Paul Sagoo explain the crucial role of research and technology in maintaining warheads for Trident, the UK’s nuclear deterrent.

Dr Ewan Galloway studied chemical physics at the University of Glasgow and went on to continue his studies into reaction surface chemistry during his PhD at the University of Cambridge. His role at AWE involves studying the ageing and compatibility of inorganic materials using fundamental surface science applications.

Paul Sagoo works in Corporate Communications at AWE. He specialises in technical conferences and events and provides editorial expertise in news and feature articles about AWE.

Materials science and metallurgy are key areas of research at the Atomic Weapons Establishment (AWE) and are fundamental to understanding the performance, reliability and characteristics of a nuclear warhead. This has become increasingly important since the UK’s ratification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) in April 1998, which prevents data from being collected from nuclear tests for research. This means that we have had to develop new methods to ensure that Trident is an effective deterrent, and increase our knowledge of the science and engineering involved in maintaining it.

The method

As part of understanding how materials behave in a nuclear warhead over time in the post-CTBT era, AWE has commissioned the inorganic surface science capability (ISSC). The ISSC is an instrument designed to investigate the chemical and corrosive characteristics of materials. We need this capability to assess whether multiple materials are compatible in proximity over a period of decades. A high level of fidelity is needed to study the slow corrosion mechanisms involved. 

The ISSC is an ultra-high-vacuum (lower than one millionth of a millionth of atmospheric pressure) surface-science instrument that provides data on the surface properties of materials – typically studying the top five nanometres of material surface in fine detail, but also providing the broadest and most detailed means for AWE to determine the potential for material corrosion. It enables our scientists to use a model-based approach in order to predict the lifetime of the warhead safely and reliably.

The practice

This approach enables AWE scientists and engineers to predict the behaviour of the warhead in a range of environments and theatres. Post CTBT, this approach is invaluable in underwriting the reliability and performance of Trident.

Our scientists are able to quantify the mechanisms of ageing and surface corrosion of materials such as inorganic hydrides, metals and composites through an understanding of the chemical degradation processes like never before. These materials can be oxidised by water or gases such as oxygen, carbon dioxide, or even hydrogen. Predicting the material corrosion and its effect on existing systems is essential to the maintenance of warheads for the UK’s deterrent.

One of a number of techniques that will be available to scientists is the capability to yield chemical information of a material surface from approximately 0.1–5nm thicknesses. The slightest reaction involving a material surface can therefore be detected. This technique enables us to obtain material composition and chemical kinetic and thermodynamic properties, to describe the life cycle of the deterrent with increased confidence.

For the first time, AWE scientists will be able to conduct experiments using a supersonic, highly-focused beam of gas directed at a material’s surface. The system and operators can then investigate the interaction of the gas beam as it impacts the material’s surface using a rotatable mass spectrometer, which can determine the likelihood of a sticking or corrosive event (between the gas beam and the surface).

This molecular beam is generated by the expansion of a high-pressure gas into a vacuum as it passes through a small nozzle (~40 micrometres diameter). The expansion of the gas as it passes through the nozzle causes compression waves and a cooling effect of the gas, in the axes perpendicular to its direction of travel to as low as a few degrees Kelvin. The axis of travel, however, still retains a speed (typically 1.7 km/s for helium) associated with the original temperature of the gas. The gas becomes a beam that can be directed onto a sample, which enables the study of the interaction between a gas and a sample to enable corrosion assessment. Molecular beams are common in academia and are used in techniques such as molecular beam epitaxy. However, this is one of the few examples of where molecular beam scattering has found an application in industry.

The future

The creation and modification of surfaces using conventional vacuum techniques, such as e-beam physical vapour deposition, sputtering and annealing, or studying new materials made through conventional or additive manufacturing are further areas of scientific research that the ISSC brings to AWE. These materials can then be assessed for chemical, structural or corrosive properties that can reveal their disposition towards reaction or inertness in a given environment, or reveal other surprising properties in the nanometre scale.

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The ISSC is a surface-science instrument designed to incorporate a large number of in situ sample preparation and analytical techniques – it enables a full assessment on whether certain materials will corrode in a given chemical and temperature surrounding. The ISSC is modular in design with different chambers performing different operations. It typically operates in ultra-high vacuum (<1x10-9 mbar) and is pumped by 21 different vacuum pumps. The sample loading and preparation chambers of the ISSC are capable of common sample preparation techniques such as physical vapour deposition, argon sputtering, cooling and annealing cycles, and gas dosing and cracking – a process where diatomic gases are split into radicals to increase their reactivity with samples or substrates. The central part of the ISSC is a bespoke molecular beam scattering chamber, and is used to analyse the reaction and dispersion of a beam of gas as it impinges on a substrate to assess for corrosive behaviour. The analytical chamber comprises techniques based on the photoelectric effect such as X-ray and UV photoelectron spectroscopies, and also Auger spectroscopy and scanning electron microscopy