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High-temperature, gas-cooled zombie SMRs

Nuclear Monitor Issue: 

High-temperature gas-cooled reactors (HTGR) and their pebble-bed modular reactor (PBMR) sub-type have a long and troubled history. But the zombie HTGR concept refuses to die: each failure is followed by another attempt and another failure.

Here is Nuclear Monitor's 2010 report on the failure of South Africa's PBMR project:1

The Pebble Bed Modular Reactor. Remember? It was globally heralded as the perfect nuclear reactor: small, safe and cheap. Dozens would be built in South Africa alone and in 1999 the company expected to sell 30 reactors annually from 2004 on.

Now, the South African government announced it is expected to close operations at PBMR (Pty) Ltd. finally 'within a few weeks' (that is August). The company once planned to build up to 24 165-MW high-temperature gas-cooled reactor modules for state-owned utility Eskom and export the modular HTR worldwide, but hasn't built even the demonstration model.

The government has invested an estimated South Africa Rand 9 billion (US$1.23 billion at current rates) in PBMR Ltd. over the 11 years since it was founded as an Eskom subsidiary. PBMR Ltd. is formally owned by Eskom, the Industrial Development Corp. and Westinghouse, but they have put no equity in the company for several years.

In a July statement, the Department of Public Enterprises, which has responsibility for the PBMR company, said PBMR "has not been able to acquire additional investment in the project since government's last funding allocation in 2007, nor has it been able to acquire an anchor customer despite revising its business model in 2008/09."

The company is operating on funds that were left over from the 2007 allocation and has downsized from about 800 staff to about 25. Although the PBMR website doesn't show anything about the current situation, it says there are "no career opportunities at the moment."

The company was set up in 1999 as Pebble Bed Modular Reactor (Pty) Ltd. to develop and deploy German technology it had acquired for small HTRs with coated pebble-shaped fuel elements. Besides British Nuclear Fuels plc (BNFL), Exelon, the largest nuclear fleet operator in the US, also made an early equity investment, and the company was broadly touted as the herald of a new nuclear age for the developing world based on small reactors that could be set up quickly under various site conditions. BNFL's stake was transferred to Westinghouse when the latter was sold to Toshiba.

But the PBMR partners never agreed on a new equity structure and the company remained the property of the South African government. The Department of Public Enterprises believes the R9-billion spent on the PBMR project has not been lost, as the skills developed "will contribute significantly in any future nuclear programs and save the country huge amounts of money in the process".

One of the critics, Stephen Thomas, professor of energy policy at the University of Greenwich in the UK, told the Cape Times that it was clear at least six years ago that the PBMR project was "going badly wrong. Yet the government continued to pour public money into it, indeed about 80 percent of all the money spent on the pebble bed was spent in the past six years."

Tristen Taylor, of Earthlife Africa, said "We hope that this will also mark the end of the South African government's love affair with nuclear energy and that taxpayer funds can now be spent on clean, proven and reliable forms of renewable energy".

The demise of PBMRs ... and China's attempted revival

Steve Thomas, Professor of Energy Policy at the University of Greenwich, wrote about the demise of PBMRs in the Bulletin of the Atomic Scientists in 2009.2 Thomas covers the failure of PBMR projects in Germany and South Africa. He notes that the cost of the proposed PBMR demonstration plant in South Africa was initially US$223 million but the estimate had escalated eight-fold to at least US$1.8 billion by the time the project was abandoned.

Thomas concluded:2

"All the major countries involved in designing reactors, including the United States, Germany, France, Japan, and Britain, have put major time and effort into developing high-temperature, gas-cooled reactors such as the PBMR. Despite more than 50 years of trying, however, no commercial-scale design has yet been produced. Yet China and South Africa have found the allure of pebble bed technology irresistible, as if it were an "unpolished gem" waiting to be developed, regardless of the consistent engineering problems it has had since the beginning.

"South Africa took a particularly aggressive approach, believing that it could develop a commercial-size PBMR design without even operating a prototype. If the PBMR is proved to be fundamentally flawed, as indicated in the Jülich report3, South Africa's $980 million investment in the project will be seen in hindsight as wasteful, one that the country, plagued with many more pressing and basic problems, could ill afford."

The Jülich report mentioned by Thomas is the Jülich Center's 2008 review of its previous PBMR work.3 It was Jülich's design ‒ specifically the prototype PBMR ‒ which South Africa had taken as the basis for its PBMR. It seems that one after another nuclear nation is destined to find out for themselves that HTGR/PBMR designs are technically challenging and are best avoided.

China is building one demonstration HTGR/PBMR: twin reactors driving a single 210 MWe turbine.4 Further HTGR feasibility studies are underway in China5, but plans for 18 additional HTGR/PBMRs (with total capacity of 3,800 MW) at the same site as the demonstration plant have been "dropped" according to the World Nuclear Association.4 In 2016, completion of the demonstration reactor was anticipated the following year, and China's HTGRs would be on the world market within five years.6,7 But the demonstration reactor has not been completed as of February 2019, construction of larger HTGRs in China has not yet begun, and the world will simply have to wait for Chinese HTGRs … or find alternative power sources. Construction cost estimates of the demonstration HTGR have approximately doubled.8

The checkered history of HTGRs

University of British Columbia academic M.V. Ramana has written a summary of the troubled history of HTGR / PBMR projects.9 An excerpt from Ramana's article is reproduced here:

"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."


1. Nuclear Monitor #714, 20 Aug 2010, 'The end is near for the PBMR',

2. Steve Thomas, 22 June 2009, 'The demise of the pebble bed modular reactor',

3. R. Moormann, "A Safety Re-evaluation of the AVR Pebble Bed Reactor Operation and Its Consequences for Future HTR Concepts," Forschungszentrum Jülich, 2008,

4. World Nuclear Association, 21 March 2016, 'First vessel installed in China's HTR-PM unit',

5. World Nuclear Association, 19 Sept 2017, 'China plans further high temperature reactor innovation',

6. Dan Yurman, 13 Feb 2016, 'Nuclear News Roundup for 2/14/16',

7. Richard Martin, 11 Feb 2016, 'Two high-temperature, gas-cooled reactors under construction in Shandong will make up the first commercial-scale plant of its type in the world',

8. 1 Dec 2016, 'China's plans to begin converting coal plants to walk away safe pebble bed nuclear starting in the 2020s',

9. M. V. Ramana, April 2016, 'The checkered operational history of high-temperature gas-cooled reactors', Bulletin of the Atomic Scientists,