Fred Starr recollects: real science fiction
Fred Starr recollects…
In the days before a flight delay meant spending half of one’s holiday money in a glorified shopping mall, we got stuck for three hours in Lisbon Airport, Portugal. All I could find to read was a copy of Analogue, a science fiction magazine that specialised in stories where technological change was impacting on society, but I didn’t buy the magazine for those. The attraction was an in-depth piece on a non-fictional American programme to build a nuclear powered space rocket. Unlike today, where hundreds of millions are spent on paper studies, the Nuclear Engine for Rocket Vehicle Applications (NERVA – a tongue-in-cheek acronym, if I ever saw one) was built and tested. The rocket’s reactor core consisted of white hot plates of enriched uranium dioxide, the propellant being liquid hydrogen. Exhaust temperature was an incredible 2,100°C.
Although the challenge to materials science sounds a killer, it was, according to the author, poor heat transfer making the engine too heavy that ended the programme. In his view, the channels taking the hydrogen through the reactor were too wide. He argued that if the passages had been narrower, the flow would have been laminar, something normally viewed with horror by heat transfer experts. They favour turbulent flow. However, narrow channels meant that hydrogen was in very close contact with the plates of UO2, so the heat didn’t have far to go. It made sense to me. At the time, the article was no more than an intriguing read.
As I mentioned in my last recollection (Materials World, August 2016, page 25), around 10 years later, when deciding on the equipment to build a closed cycle gas turbine, I was traumatised about the likely size and cost of the recuperator. It had to operate at 20 bar pressure, with metal temperatures of up to 720°C. Furthermore, it had to be extremely effective at transferring heat. Assuming that the recuperator was to be of the tube-and-shell type, it would be two metres in diameter and eight metres long, containing about 200 tubes. The whole lot would go inside an internally insulated pressure vessel, adding another metre to both diameter and length. There would be a multitude of detailed design issues, mostly because of thermal expansion and fatigue.
At this juncture, we were told that another part of British Gas was recommending the use of the Printed Circuit Heat Exchanger (PCHE) for pressure reduction stations on natural gas pipelines. It was very compact and could withstand tremendous pressures. Would it be of any interest? Out of desperation, I asked the sales rep to call.
After five minutes, I was selling him the idea. The PCHE had just the type of narrow passages advocated by the man who had so criticised the NERVA nuclear rocket programme, promising an extremely compact job. There was no pressure vessel. The PCHE is made by etching tiny heat transfer passages into stainless steel plates. These are then diffusion bonded together, giving an immensely strong monolithic assembly. Alternate rows take the inlet and outlet streams.
When our PCHE arrived, it surpassed all expectations. The PCHE assembly was dwarfed by the inlet and outlet manifolds. It performed well in the limited trials we could run, before downsizing within British Gas brought our programme to a very abrupt stop.
Germany and Britain once envisaged, as the next step in nuclear technology, a high temperature reactor incorporating a closed cycle gas turbine. I drew heavily on what these workers had been doing, visiting KFA Juelich and the UK National Gas Turbine Establishment. The PCHE was not around at that time, making the recuperator an intractable design issue. It is no longer. So, can I plead with the nuclear establishment to be a bit more innovative and get over their fixation with the pressurised water reactor?
Let’s face it – after 50 years of playing around with water-cooled nuclear reactors, we have ended up with the economic and political mess that is Hinkley Point C. Scheduled commissioning date and electricity price? Now that’s pure science fiction.