As nuclear energy attracts increasing attention worldwide, because of its extremely low life-cycle greenhouse-gas emissions profile, three new or improved atomic fission-based tech-nologies are available or under development to meet the planet’s energy needs.

These are advanced light (or heavy) water reactors (LWRs or HWRs), high temperature gas-cooled reactors (HTGRs), and fast breeder reactors (FBRs). (South Africa’s pebble-bed modular reactor – PBMR – falls into the category of HTGRs.)




In most parts of the world, the LWR or HWR has been the dominant design for nuclear power reactors for many years now, and the advanced LWRs are the latest generation of this technology. “This new generation of LWRs came into being during the 1990s,” explains Stanford University Centre for International Security and Cooperation consulting professor Dr Chaim Braun.

“They were developed mostly to retain the best practices of the older generations of LWRs, that had operated up until that time, and improve the performance of the reactor, in terms of its overall safety, ease of operation, and reliability, as well as incorporating the lessons learned in the Three Mile Island accident, in the US, and the Chernobyl accident, in the Ukraine, ensuring that such accidents do not happen again. “What went wrong in the past would be corrected by better designed reactors and in part by better training of the plant operators.”




HTGRs, unlike LWRs, use most of the thermal energy generated in their reactor cores, and this is their great promise. “In a LWR,” elucidates Braun, “only a third of the thermal energy generated is used effectively, to make electricity. “The other two-thirds of the heat generated in the reactor core is essentially released into the atmosphere or into bodies of water.” HTGRs are thus much more efficient than LWRs.

This makes them superior in terms of the sustainability of uranium resources. Also, because HTGRs produce a lot of high-tem- perature heat, they can be used to provide process heat for industrial applications, such as in chemical plants, the extraction of oil from tar sands, the production of hydrogen, and even in steelmaking. As this process heat cools, it could also be used in applications requiring lower temperatures, such as refining and water desalination.

Thus, HTGRs could form the core of major industrial complexes. South Africa’s PBMR – one of several designs of the HTGR – would be especially suitable for process heat applications, owing to its modular nature.

FBRs are remarkable in that they produce – or breed – more fuel than they consume.

Neutrons play a key role in the process of atomic fission in the cores of all reactor designs. Normally, every reactor contains a moderator – such as water or carbon – to slow down the neutrons, but FBRs do not have moderators, so they employ fast-moving neutrons to bring about fission – hence, the term ‘fast breeder reactors’. FBRs are cooled by liquid sodium (LWRs are cooled by water, and HTGRs by helium gas) and use a mixture of plutonium and uranium oxide as fuel.

The core of an FBR is surrounded by a blanket of uranium oxide and, during operation, some of this is converted into plutonium, which can be extracted and used as fuel. An FBR produces more plutonium than it consumes. Futher, an FBR can extract about 60 times the energy from uranium than LWRs can. “Eventually, an FBR will produce not just enough fuel to keep its own cycle running and operating for its lifetime, but will produce enough fuel to start another FBR – and so on and so on,” points out Braun.

However, an FBR is only really practical if the country, or operator, already has operational LWRs, which are needed to produce the plutonium required to fuel the FBR, and if the country of operation has developed commercial technologies for used fuel reprocessing and fresh fuel fabrication. (Plutonium is a by-product of the nuclear reactions that take place in the core of a LWR; it is part of the used fuel and is extracted by reprocessing that spent fuel.) “But, eventually, FBRs could, if they got enough fuel of their own, keep their own cycle going,” he says. “However, we are talking about tens of years, perhaps, a hundred years, to set up a wholly independent self-sustaining system of FBRs.”

To watch a video in which Dr Chaim Braun discusses the different nuclear technologies, click here.