High-temperature gas-cooled reactors inc pebble-bed modular reactors

The checkered history of high-temperature gas-cooled reactors

Academic M.V. Ramana has written a useful summary of the troubled history of high-temperature gas-cooled reactors (HTGR) including the pebble-bed reactor sub-type. In the past, both Germany and the United States spent large amounts of money to design and construct HTGRs, four of which fed electricity into the grid. Other countries have also invested in HTGR technology. Ramana’s analysis is of more than historical interest as several countries are either considering the construction of new HTGRs or pursuing research into the field.

Ramana writes:

“Proponents of HTGRs often claim that their designs have a long pedigree. … But if one examines that very same experience more closely – looking in particular at the HTGRs that were constructed in Western Europe and the United States to feed power into the electric grid – then one comes to other conclusions. This history suggests that while HTGRs may look attractive on paper, their performance leaves much to be desired. The technology may be something that looks better on paper than in the real world …

“Although Germany abandoned this technology, it did migrate to other countries, including China and South Africa. Of these, the latter case is instructive: South Africa pursued the construction of a pebble-bed reactor for a decade, and spent over a billion dollars, only to abandon it in 2009 because it just did not make sense economically. Although sold by its proponents as innovative and economically competitive until its cancellation, the South African pebble-bed reactor project is now being cited as a case study in failure. How good the Chinese experience with the HTGR will be remains to be seen. …

“From these experiences in operating HTGRs, we can take away several lessons – the most important being that HTGRs are prone to a wide variety of small failures, including graphite dust accumulation, ingress of water or oil, and fuel failures. Some of these could be the trigger for larger failures or accidents, with more severe consequences. … Other problems could make the consequences of a severe accident worse: For example, pebble compaction and breakage could lead to accelerated diffusion of fission products such as radioactive cesium and strontium outside the pebbles, and a potentially larger radioactive release in the event of a severe accident. …

“Discussions of the commercial viability of HTGRs almost invariably focus on the expected higher capital costs per unit of generation capacity (dollars per kilowatts) in comparison with light water reactors, and potential ways for lowering those. In other words, the main challenge they foresee is that of building these reactors cheaply enough. But what they implicitly or explicitly assume is that HTGRs would operate as well as current light water reactors – which is simply not the case, if history is any guide. …

“Although there has been much positive promotional hype associated with high-temperature reactors, the decades of experience that researchers have acquired in operating HTGRs has seldom been considered. Press releases from the many companies developing or selling HTGRs or project plans in countries seeking to purchase or construct HTGRs neither tell you that not a single HTGR-termed “commercial” has proven financially viable nor do they mention that all the HTGRs were shut down well before the operating periods envisioned for them. This is typical of the nuclear industry, which practices selective remembrance, choosing to forget or underplay earlier failures.”

M. V. Ramana, April 2016, ‘The checkered operational history of high-temperature gas-cooled reactors’, Bulletin of the Atomic Scientists, http://dx.doi.org/10.1080/00963402.2016.1170395

Accident Scenarios Involving Pebble Bed High Temperature Reactors

Matthias Englert, Friederike Frieß and M. V. Ramana, Feb 2017, ‘Accident Scenarios Involving Pebble Bed High Temperature Reactors’, Science & Global Security, Vol.25 Iss.1, pp.42-55, http://dx.doi.org/10.1080/08929882.2017.1275320

Proponents of high temperature gas cooled reactors argue that the reactor type is inherently safe and that severe accidents with core damage and radioactive releases cannot occur. The argument is primarily based on the safety features of the special form of the fuel. This paper examines some of the assumptions underlying the safety case for high temperature gas cooled reactors and highlights ways in which there could be fuel failure even during normal operations of the reactor; these failures serve to create a radioactive inventory that could be released under accident conditions. It then describes the severe accident scenarios that are the greatest challenge to high temperature gas cooled reactor safety: ingress of air or water into the core. Then, the paper offers an overview of what could be learned from the experiences with high temperature gas cooled reactors that have been built; their operating history indicates differences between actual operations and theoretical behavior. Finally, the paper describes some of the multiple priorities that often drive reactor design, and how safety is compromised in the process of optimizing other priorities.


2013 summary by Friends of the Earth Australia

Pebble Bed Modular Reactors (PBMR) are helium-cooled and graphite-moderated and intended to be built in small modules (Thomas, 1999; Harding, 2004; Hirsch et al., 2005). Pressurised helium heated in the reactor core drives turbines that attach to an electrical generator.

While the PBMR is in some respects innovative, it also shares features with high temperature gas cooled reactors (HGTR). The HTGR line has been pursued until the late 80s in several countries; however, only prototype plants were ever operated (in the USA, UK and Germany), all of which were decommissioned after about 12 years of operation at most.

A South African nuclear utility has been at the forefront of developing pebble bed reactors but the project was postponed indefinitely as a result of economic factors as well as technical factors, some with safety consequences. Unless the South African project is revived, that leaves only China developing pebble bed concepts (with one small prototype operating and one 200 MW ‘demonstration reactor’ planned or in the early stages of construction).

These articles discuss the demise of PBMR technology in South Africa:

PBMR proponents claim major safety advantages resulting from the heat-resistant quality and integrity of the small fuel pebbles, many thousands of which are continuously fed from a silo. Each spherical fuel element has a graphite core embedded with thousands of small fuel particles of enriched uranium (up to 10% uranium-235), encapsulated in layers of carbon.

The safety advantages of PBMR technology include a greater ability to retain fissile products in the event of a loss-of-coolant accident. While this configuration is potentially advantageous compared to conventional reactors, it does not altogether avoid the risk of serious accidents; in other words, claims that the system is ‘walk-away safe’ are overblown. The safety advantages can be undermined by familiar commercial pressures; for example there are plans to develop PBMR reactors with no containment building.

In relation to weapons proliferation (Harding, 2004):

  • The nature of the fuel pebbles may make it somewhat more difficult to separate plutonium from irradiated fuel, but plutonium separation is certainly not impossible.
  • Uranium (or depleted uranium) targets could be inserted to produce weapon-grade plutonium for weapons, or thorium targets could be inserted to produce uranium-233.
  • The enriched uranium fuel could be further enriched for weapons – particularly since the proposed enrichment level of 9.6% uranium-235 is about twice the level of conventional reactor fuel.
  • The reliance on enriched uranium will encourage the use and perhaps proliferation of enrichment plants, which can be used to produce highly-enriched uranium for weapons.


Harding, Jim, 2004, “Pebble Bed Modular Reactors—Status and Prospects”, www.rmi.org/sitepages/pid171php#E05-10

Hirsch, Helmut, Oda Becker, Mycle Schneider and Antony Froggatt, April 2005, “Nuclear Reactor Hazards: Ongoing Dangers of Operating Nuclear Technology in the 21st Century”, Report prepared for Greenpeace International, www.greenpeace.org/international/press/reports/nuclearreactorhazards

Thomas, Steve, 1999, “Arguments on the Construction of Pebble Bed Modular Reactors in South Africa”, www.sussex.ac.uk/Units/spru/environment/research/pbmr.html