Tag Archives: graphite

Graphite for Very High Temperature Reactors (VHTR)

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.

Christmas diamonds

If you enjoyed a holiday dinner lit by candles then you might be interested to know that the majority of the light from the candle does not come from the combustion of the candle wax in the flame, but from the unburnt soot glowing in the intense heat of the flame.  The combustion process generates the heat and the blue colour in the centre of the flame. However, due to the lack of sufficient oxygen, the combustion of the candle wax is incomplete  and this produces particles of unburnt carbon.  The unburnt carbon forms soot or graphite, but also more exotic structures of carbon atoms, such as nano-diamonds.  The average candle has been estimated to produce about 1.5 million nano-diamonds per seconds, or maybe 10 billion nano-diamonds per Christmas dinner! Unfortunately, they are too small to see otherwise they would add a lot of sparkles to festive occasions.

The picture is an infrared image of a 1cm diameter candle.  About 2cm of the candle height extends from the bottom of the picture and the visible flame is about 2cm high.

Source:

Helen Czerski, Storm in a Teacup: The Physics of Everyday Life, London: Penguin Random House, 2016.