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July 22, 2009

Pebble Bed Reactors Safety and Feasibility

There have been some articles that are critical of the design and safety of pebble bed reactors and claim the demise of pebble bed reactors.

Here is the rebuttal of the criticism of pebble bed reactors. Dr Albert Koster, PBMR (Pty) senior consultant, nuclear safety, replies directly to criticisms of the PBMR reactor.

Moormann’s views are old news and not supported by new, advanced work or the preponderance of contemporary evidence, analysis and expert opinion established during the AVR and THTR operating periods and more recently. Further, modern tools and techniques are highly reliable tools only conceived of just a short time ago that make the ability to accurately predict system and component performance in ways unimagined when earlier designs were developed.


On 5 February 2009 PBMR issued a news bulletin stating that the company will be focusing on the design of a plant to service both the electricity and process heat markets. In the March issue of this magazine [pp22-3] Prof. Stephen Thomas insinuates that the change was motivated not by commercial considerations but because PBMR was aware of problems with pebble bed performance at high temperatures (see article), as alleged by Dr. Rainer Moormann in a second article [pp16-20]. In order to set the record straight it is necessary to chart a short history of the development of HTRs and PBMR in particular.

This brings us to the allegations of hiding facts and supposed safety problems in pebble bed reactors as so stridently described in the March issueI. To this end, only the safety concerns raised by Moormann need to be addressed as Thomas based his argument on the premise that Moormann is correct: that PBMR knew about safety problems all the time and opted to keep quiet about it.

In his articles, Moormann presents a number of different arguments but by some analysis, these can be boiled down to two major issues. The first is that both the AVR and THTR were shut down because of safety concerns. The second is that the AVR was highly contaminated and this was due to high fuel temperatures that caused excessive release of caesium and strontium from the fuel. He then advances reasons why the fuel temperatures might have been high and draws conclusions about the safety of future pebble bed reactors based on his speculations. The major contentions are addressed below; others have been covered at the HTR 2008 conference or in prior published articles.




Technical arguments aside, it appears that China should be proceeding with their
commercial pebble bed in Sept of this year. Presumably the people on this project, which involves a few thousand people and few hundred million dollars are doing their homework. The site of the Shidaowan project will install 18 additional modules, which will total 3,800 MWe.

Upon startup of the Shidaowan plant, China will become the first country to commercially venture into HTR nuclear technology. The plant will be owned and operated by Huaneng Group, one of China’s largest independent utilities; China Nuclear Engineering and Construction Corp., China’s construction company for the nuclear island; and Tsinghua University.

The Shidaowan project received environmental clearance in March 2008 for construction start in 2009 and commissioning by 2013. The 200 MWe (two reactor modules, each of 250 MWt) plant will drive a single steam turbine at about 40 percent thermal efficiency. The reactor module, which was originally planned for 458 MWt, was reduced to 250 MWt in order to retain the same core configuration as the prototype HTR-10.

The HTR-10 is powered by graphite balls about the size of standard billiard balls packed with tiny flecks of uranium, rather than with the conventional white-hot fuel rods used in existing nuclear reactors. Instead of water, the core is bathed in inert helium, which can reach much higher temperatures. The HTR-10 reached full power in 2003 and has an outlet temperature of 700 C to 950 C.

“First and foremost, this generator will be the safest nuclear power plant ever designed and built,” said Wu. The major safety issue regarding nuclear reactors lies in how to cool them efficiently, as they continue to produce residual heat even after shutdown. Gas-cooled reactors discharge surplus heat and don’t need additional safety systems like water-cooled reactors do. The HTR-10 was subject to a test of its intrinsic safety in September 2004 when, as an experiment, it was shut down with no cooling. Fuel temperature reached less than 1600 C and there was no failure.

“Using the existing operating HTR-10 reactor at the Institute of Nuclear and New Energy Technology of Tsinghua University in Beijing, we have already done what would be unthinkable in a conventional reactor—we switched off the helium coolant and successfully let the reactor cool down by itself,” said Wu.

Second, the modular design enables the plant to be assembled much quicker and cost-effectively than traditional nuclear generators. Its streamlined construction timetable is also a first for the nuclear power industry, where designing and building generators usually take decades, rather than years.

The modules are manufactured from standardized components that can be mass-produced, shipped by road or rail and assembled relatively quickly


The pebble bed reactor program is number six in terms of priority for national projects. There have been some delays but it appears on track now.

South Africa continues to press forward. Their new version of the project is to try to get an 80MW version going in 2018.

There are 25 nuclear plants (mostly Westinghouse AP1000 reactors and CPR-1000. CPR-1000 are based on transferred Areva nuclear reactor) forecast to be built in the next five years in China, compared to only two new plants scheduled to be built in the next 10 years in the U.S


Moormann proposes areas where he feels more research is needed, some of which are addressed below.

Full evaluation of the operational experience and problems of AVR and THTR300.

PBMR (Pty) Ltd. has been in the process of evaluating the AVR for the last two years. The starting point was to collect all the design drawings and descriptions to (for the first time) enable the AVR to be modelled in detail. The latest results were presented at HTR-2008 [5]. Additional results explaining the fuel temperatures will appear soon in print.

Components from the AVR continue to be examined, at the request of PBMR (Pty) Ltd., for dust characterization, concentration and dust adherence to better understand the mobility of agglomerated dust, or the lack of it.

Experiments on iodine release from fuel elements in core heat-up accidents.

This is part of the planned PBMR fuel qualification tests where fuel will be placed in test reactors and subjected to expected operating temperature conditions. Afterwards the irradiated fuel will be subjected to post-irradiation heat-up testing to simulate design-based accident events where measurements of all significant nuclides will be made.

Full understanding and reliable modelling of core temperature behaviour, and of pebble bed mechanics, including pebble rupture.

The publications presented at HTR-2008 and the final model results show that this is already achieved. The terminology ‘pebble rupture’ is misleading; pebbles do not rupture. A very small percentage may fail due to mechanical handling and movement and the faulty pebbles are automatically removed from the core when they exit at the bottom of the core.

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