Tag Archives: nuclear energy

Graphite for Very High Temperature Reactors (VHTR)

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One of the implications of the second law of thermodynamics is that the thermal efficiency of power stations increases with their operating temperature.  Thus, there is a drive to increase the operating temperature in the next generation of nuclear power stations, known as Generation IV reactors.  In one type of Generation IV reactors, known as the Very High Temperature Reactor (VHTR), graphite is designed to be both the moderator for neutrons and a structural element of the reactor.  Although the probability of damage in an accident is extremely low, it is important to consider the consequences of damage causing the core of the reactor to be exposed to air.  In these circumstances, with the core temperature at about 1600°C, the graphite would be exposed to severe oxidation by the air that could change its material properties and ability to function as a moderator and structural element.  Therefore, in recent research, my research group has been working with colleagues at the UK National Nuclear Laboratory (NNL) and at the National Tsing Hua University (NTHU) in Taiwan to conduct experiments on nuclear graphite over a range of temperatures.  Our recently published article shows that all grades of nuclear graphite show increased rates of oxidation for temperatures above 1200°C.  We found that large filler particles using a pitch-based graphite rather than a petroleum-based graphite gave higher oxidation resistance at these elevated temperatures.  This data is likely to be important in the design and operations of the next generation of nuclear power stations.

The work described above was supported by the NTHU-University of Liverpool Dual PhD Programme [see ‘Citizens of the world‘ on November 27th, 2019] and NNL.  This is the fifth, and for the moment last, in a series of posts on recent work published by my research group.  The others are: ‘Salt increases nanoparticle diffusion‘ on April 22nd, 2020; ‘Spatio-temporal damage maps for composite materials‘ on May 6th, 2020; ‘Thinking out of the box leads to digital image correlation through space‘ on June 24th, 2020; and, ‘Potential dynamic buckling in hypersonic vehicle skin‘ on July 1st, 2020.

The image is figure 5: SEM micrographs of the surface of petroleum-based IG-110 graphite samples oxidized at various temperatures from Lo IH, Tzelepi A, Patterson EA, Yeh TK. A study of the relationship between microstructure and oxidation effects in nuclear graphite at very high temperatures.  J. Nuclear Materials. 501:361-70, 2018.

Source:

Lo I-H, Yeh T-K, Patterson EA & Tzelepi A, Comparison of oxidation behaviour of nuclear graphite grades at very high temperatures, J. Nuclear Materials, 532:152054, 2020.

Thinking out of the box leads to digital image correlation through space

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This is the third in a short series of posts on recent engineering research published by my research group.  Actually, two have already been published: ‘Salt increases nanoparticle diffusion‘ on April 22nd, 2020; and ‘Spatio-temporal damage maps for composite materials‘ on May 6th, 2020 and then I got distracted.  This third one arose from the same project as the time-damage maps which was sponsored by the United States Air Force.  The time-damage maps allow us to explore the evolution of failure in complex materials; however, we already know that damage tends to initiate from imperfections or flaws in the microstructure in the material.  New continuous fibre reinforced composite (CFRC) materials based on ceramics are very sensitive to defects or anomalies in their microstructure, such as misalignment of fibres.  However, they are capable of withstanding temperatures in excess of 1500 degrees Centigrade, which offers the opportunity to use them in jet engines or nuclear power plants to help generate energy more efficiently.  Therefore, it is worthwhile investigating effective methods of inspecting their microstructure which we can do either destructively by repetitively polishing away the surface of a sample and viewing it in a microscope, or non-destructively using x-ray tomography.  In both cases, the result is hundreds of ‘images’ containing millions of data values from which it is challenging to extract useful information.  In our work, we have used a little lateral thinking, to show how digital image correlation, usually used to track deformation of structures using multiple images collected over time [see ‘256 shades of grey‘ on January 22nd, 2014] , can be used to track fibres through the multiple images of the layers of the microstructure.  The result is the sort of ‘stick’ diagram in the image showing the orientation of fibres through the sample.  We have demonstrated that our new algorithm was more reliable and 30 times faster than its nearest rival.

The image shows, at the top, a typical stack of images from the microscope of a ceramic matrix composite; and, at the bottom, a plot of 3d profiles of the fibres tracked using the DIC-based method with the fibres orientated nominally at ±45° from the sectioning (x-y) plane shown in red and green colours.

Source:

Amjad K, Christian WJR, Dvurecenska K, Chapman MG, Uchic MD, Przybyla CP & Patterson EA, Computationally efficient method of tracking fibres in composite materials using digital image correlation, Composites Part A, 129:105683, 2020.

 

Reinforcement ensures long-term structural integrity

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Last month when I was in Taiwan [see ‘Ancient Standards‘ on January 29th, 2020] , I visited Kuosheng Nuclear Power Plant which has a pair of boiling water reactors that each generate 986 MWe, or between them about 7% of Taiwan’s electricity.  The power station is approaching the end of its licensed life in around 2023 after being constructed in 1978 and delivering electricity commercially for about 40 years, since the early 1980’s.  There is an excellent exhibition centre at the power station that includes the life-size mock-up of the reinforcement rods in the concrete of the reactors shown in the photograph.  I am used to seeing reinforcing bar, or rebar as it is known, between 6 to 12mm in diameter on building site, but I had never seen any of this diameter (about 40 to 50mm diameter) or in such a dense grid.  On the other hand, we are not building any nuclear power stations in the UK at the moment so there aren’t many opportunities to see closeup the scale of structure required.

Fake facts & untrustworthy predictions

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I need to confess to writing a misleading post some months ago entitled ‘In Einstein’s footprints?‘ on February 27th 2019, in which I promoted our 4th workshop on the ‘Validation of Computational Mechanics Models‘ that we held last month at Guild Hall of Carpenters [Zunfthaus zur Zimmerleuten] in Zurich.  I implied that speakers at the workshop would be stepping in Einstein’s footprints when they presented their research at the workshop, because Einstein presented a paper at the same venue in 1910.  However, as our host in Zurich revealed in his introductory remarks , the Guild Hall was gutted by fire in 2007 and so we were meeting in a fake, or replica, which was so good that most of us had not realised.  This was quite appropriate because a theme of the workshop was enhancing the credibility of computer models that are used to replicate the real-world.  We discussed the issues surrounding the trustworthiness of models in a wide range of fields including aerospace engineering, biomechanics, nuclear power and toxicology.  Many of the presentations are available on the website of the EU project MOTIVATE which organised and sponsored the workshop as part of its dissemination programme.  While we did not solve any problems, we did broaden people’s understanding of the issues associated with trustworthiness of predictions and identified the need to develop common approaches to support regulatory decisions across a range of industrial sectors – that’s probably the theme for our 5th workshop!

The MOTIVATE project has received funding from the Clean Sky 2 Joint Undertaking under the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 754660 and the Swiss State Secretariat for Education, Research and Innovation under contract number 17.00064.

The opinions expressed in this blog post reflect only the author’s view and the Clean Sky 2 Joint Undertaking is not responsible for any use that may be made of the information it contains.

Image: https://www.tagesanzeiger.ch/Zunfthaus-Zur-Zimmerleuten-Wiederaufbauprojekt-steht/story/30815219