Early-stage research into the possible revival of the Pebble Bed Modular Reactor (PBMR) has yielded a revamped reactor concept that incorporates an entirely new operational philosophy, as well as the use of construction materials and manufacturing techniques that were either not considered or unavailable during the development of the PBMR.
The PBMR programme was officially closed in 2010, owing to funding constraints and an inability to attract an investment partner, but the intellectual property was retained the by PBMR Company, a wholly owned subsidiary of South Africa’s State-owned utility, Eskom.
The new “conceptual outline” is described as being more efficient and cheaper to build than the PBMR and the changes are so material that the project is now being referred to within Eskom as the Advanced High Temperature Reactor (AHTR), rather than the PBMR.
Chief nuclear officer David Nicholls tells Engineering News that the work, which has been under way for a year, is being conducted within Eskom’s existing research and development budget and is being carried out by a handful of internal engineers and scientists, together with university researchers.
Nicholls is not willing to disclose the budget, but reports that Eskom has teamed up with researchers at the North-West University, the Nelson Mandela Metropolitan University and the University of Cape Town to advance the research effort. In 2017, the group of involved universities will be expanded to include researchers from the universities of Stellenbosch, Pretoria, Witwatersrand and Johannesburg.
“We started with a clean sheet of paper, with the aim of using the PBMR as a basis for a radically more advanced reactor,” Nicholls explains. “We’ve looked at the temperatures, we’ve looked at the layout and, over the last year, we have developed a conceptual outline, which is substantially simpler, more efficient and cheaper to build than the PBMR.”
He stresses, though, that the AHTR concept could not be termed a design, describing the current visual representations of the reactor a little more than a “cartoon”.
Unlike the original PBMR, the brief is not the immediate development of a commercial reactor. Instead, the researchers have been asked to incorporate core aspects of the PBMR’s passively safe reactor design (fuelled by tennis-ball-sized graphite pebbles containing low-enriched uranium-oxide particles) to develop a high-temperature modular plant, which could be commercialised during the 2030s.
The more immediate ambition, though, is to progress the AHTR design to a point whereby a 50 MW “proof-of-concept plant” can be built, possibly during the mid-2020s.
The conceptual outline for the AHTR is significantly different from the original PBMR. The layout has been revised; the top-to-bottom helium gas flow reversed; higher operating temperatures proposed, along with a combined-cycle power-generation solution; and alternative construction materials (including a concrete rather than a steel pressure vessel) incorporated.
“We could have simply restarted the PBMR as it was. But that’s going back 20 years in design and probably 35 years in technology. What we are saying instead is, ‘What can we do better than that?’” Nicholls explains.
A major deviation from the PBMR is the AHTR’s far higher operating temperatures. Under the current proposal, the helium gas enters the reactor at 570 ºC and exits to a gas turbine, which will deliver about 40% of the electrical output, at 1 200 ºC. The temperature of the exhaust gas is 600 ºC, which, while still high, can be deployed in a steam cycle, using a molten-salt circuit, to deliver the 60% electrical balance.
“Adding the steam cycle gives the machine an impressive thermal efficiency of 60%,” Nicholls enthuses.
The researchers have also been asked to draw inspiration from outside the nuclear industry and seek to incorporate changes in manufacturing techniques, such as three-dimensional printing, as well as recent advances in material science.
“Where is this going to end up? I don’t know. But our current view is that the first machine will be a proof-of-concept plant, with the ultimate goal of bringing 150 MW high-temperature commercial reactors on line in the 2030s.”