Nuclear Power Plant High-Temperature Gas-Cooled Reactor with Spherical Fuel Elements – A German Development Implemented Abroad

[Ed — This is a guest post authored by Dr. -Ing. Heinrich Bonnenberg, member of the German Council on Foreign Relations (DGAP), Berlin. In one segment of his paper Dr. Bonnenberg documents the sad history of the termination of successful German HTR programs, which ranks with the similarly misguided termination of the American IFR in 1994.]

The following is the paper I wrote in my capacity as a member of the DGAP Deutsche Gesellschaft für Auswärtige Politik e.V. (German Council on Foreign Relations), on the occasion of the Shell Energy Dialogue, Strategische Herausforderungen für die europäische Energiepolitik (Strategic Challenges for European Energy Policy), Berlin, February 1, 2007 organized by the Deutsche Gesellschaft für Auswärtige Politik e.V. (DGAP) and Shell in Germany. May 31, 2007: I have included all the comments I have received to date.


In order to develop the energy economy of the future, five equally important key questions must be answered, all of which require the development of certain technologies:

1. Will we succeed in separating CO2 from gaseous emissions at an acceptable cost and storing it safely and permanently?

2. Will we succeed in making renewable energy processes economical, i.e. practicable without the support of subsidies?

3. Will we successfully solve the problem of storing electrical energy?

4. Will we succeed in building continuously and economically operating nuclear power plants that use nuclear energy produced by nuclear fusion?

5. Will we succeed in devising catastrophe-proof nuclear power plants that use nuclear energy produced by nuclear fission?

The first four questions are still open.

The fifth question, by contrast, has already been answered with a resounding YES. However, many of my contemporaries, especially those in Germany, are not yet prepared to accept this fact.

The answer to the fifth question is the high-temperature gas-cooled reactor with spherical fuel elements (pebbles), called the HTR pebble bed reactor for short.

This type of reactor is also referred to internationally as the PBMR (Pebble Bed Modular Reactor).

The high-temperature pebble bed reactor is a German development that was created starting in the 1950s, with the integration of know-how from the USA and the UK and some additional research done in Italy, Sweden and Switzerland.

The industrial development of this future-oriented technology towards market launch was discontinued in Germany at the end of the 1980s.

It was very successfully continued, and is still going on today, in China, South Africa, the USA, Japan, Russia, South Korea and our neighboring countries the Netherlands and France.

At the university level, work on the high-temperature reactor is being conducted at

• Massachusetts Institute of Technology (MIT), Cambridge/Boston, USA,

• Tsinghua University, Beijing, China, and

• Rheinisch-Westfälische Technische Hochschule (RWTH), Aachen, Germany.

The high-temperature reactor is recognized as the most promising representative in the international project GENERATION IV, which was commissioned by the U.S. Department of Energy (DOE), Washington, and in which all of the countries that use nuclear energy are participating,

except Germany.

One of the effects of Germany’s noninvolvement in the GENERATION IV project is that the information about modern safety technologies for nuclear power plants that is generated by this project reaches Germany only indirectly and with delays.

The most important components of a nuclear power plant are the fuel elements.

The fuel elements contain

• the fissile material for generating the desired energy, and

• the fission and decay products (the radioactive waste), which are the sources of dangerous radioactivity and considerable amounts of (delayed) decay heat.

The more robust the fuel element, the safer the nuclear power plant!

In a high-temperature reactor, the fuel is located in billions of tiny particles, each of which is approximately as large as the head of a pin and has a power output of approximately 0.2 watts per particle. These fuel particles are coated with several layers of a ceramic material that is pressure-resistant, leakproof even at extremely high temperatures, and non-combustible (silicon carbide). Thus in the HTR the source of danger is fragmented into tiny amounts, each of which harbors only marginal danger, in robustly coated particles.

The basic concept used in the high-temperature reactor to eliminate risks is brilliant:

mini-sources of danger in mini-containments.

The fuel rods normally used in other types of nuclear power plant contain billions of times more material per fuel rod than is contained in each of the particles used in the HTR. In addition, the fuel rods use a metallic cladding. They are therefore extremely sensitive, especially with regard to high temperatures, very much in contrast to the HTR particles.

The particles in the HTR are embedded in pressure-resistant, robust graphite pebbles as large as tennis balls with a power output of approximately 3 kilowatts per pebble, with each pebble containing approximately 15,000 particles. The fuel rods in other types of nuclear power plants are combined into metallic fuel elements, which are far less robust.

There are several hundred thousand pebbles in the nuclear reactor. The number of pebbles depends on the output of the power plant. The pebbles form a pebble bed that is loaded in from above and withdrawn from below. The pebble bed reactor is thus operated by means of continuous charging with the fissile material. As a result, the reactor always contains only the precise amount of fissile material that is required for the current operation of the nuclear power plant. In other words, there is no “threatening” reserve supply of fresh fissile material, as there is in conventional nuclear power plants that are charged in batches in order to compensate for the burn-out of the fissile material during the lifetime of the fuel elements.

In addition, the continuous operation of the pebble bed reactor makes it possible to achieve a very high utilization of the fissile material.

The heat generated by the high-temperature reactor is drawn off using helium, an inert gas that is reaction-resistant.

Parallel to the German development of the pebble bed reactor, a high-temperature reactor was developed in the USA in which the fuel particles are embedded in blocks of graphite.

The high-temperature reactor generates electricity at high efficiency, using modern steam turbine processes; the use of gas turbines is also possible.

In addition, the high-temperature reactor can provide heat at high temperatures for technical processes. The main processes in question are

• the production of fuels and natural gas through the gasification of lignite and hard coal, and

• the production of hydrogen through the thermal fission of water,

both for the propulsion of motor vehicles and for heating.

The particular potential of the high-temperature reactor for processing coal, as well as its outstanding safety, were the main reasons why the German federal state of North Rhine-Westphalia was so intensely involved until the end of the 1980s in developing the high-temperature pebble bed reactor.

There is further potential in the use of the heat (from smaller high-temperature reactors) for extracting oil through steam flooding and from oil sand and oil shale.

The development of the German pebble bed reactor was mainly financed by

• the European Atomic Energy Community (EURATOM),

• the Federal Republic of Germany, and

• the federal state of North Rhine-Westphalia.

Note that the funding came from taxes and was supported by all the governments, independent of their respective political orientation.

This support was especially strong after the shock of rapidly rising oil prices in 1974, when the German chancellor Helmut Schmidt advocated a policy of building new nuclear power plants in order to alleviate Germany’s “energy vulnerability”, to the point of threatening to resign at the party conference of his SPD (Social Democratic Party of Germany) in Berlin in December 1979 if his party refused to adopt this policy.

The development of the high-temperature pebble bed reactor was spearheaded by the Nuclear Research Institute Jülich in North Rhine-Westphalia, which was founded in 1956 by the state government of North Rhine-Westphalia under Minister-President Fritz Steinhoff, SPD. It should be mentioned that the further development of the use of nuclear energy (fusion and fission) was supported as a matter of prime importance by the State Secretary at that time, Professor Dr. Leo Brandt, SPD, a visionary of modern technology and sponsor of the HTR who was then the head of the technology-oriented State Office for Research under the Minister-President of the state of North Rhine-Westphalia. The politicians of all parties had justifiably identified nuclear power as an economical and environmentally friendly source of energy whose supply was secure.

The following prototype nuclear power plants of the high-temperature pebble bed reactor type were operated, both of them in the “energy state” North Rhine-Westphalia and both of them supported by the politicians in office at that time:

• AVR 15 MW near Jülich

• THTR 300 MW near Hamm

Unfortunately, both prototype nuclear power plants were closed down in the late 1980s.

A key reason for the decision to close down the plants was the resolution passed at the SPD party congress held in Nuremberg in August 1986, “Abandonment of nuclear energy within ten years”. This resolution – which was passed about four months after the catastrophe in Chernobyl – must be regarded as a prelude to the Kohl (CDU) – Rau (SPD) election campaign that began on January 25, 1987.

In North Rhine-Westphalia this resolution was implemented by the state government headed by Johannes Rau, SPD, by closing down, for political reasons, the future-oriented project of the HTR pebble bed reactor (and the equally future-oriented project of the fast breeder) after more than 30 years of very successful work. This was done with the approval of the national government, represented by the Federal Ministry of Research and Technology under Minister Dr. Heinz Riesenhuber, CDU. The two prototype projects were granted no more public funding by either the state government or the national government. The funding was canceled even before this type of nuclear power plant had reached market readiness.

There was no party for whom the politicians needed to show any consideration.

This measure did not provoke much resistance. There had never been a really sound demand in the economy of North Rhine-Westphalia for electricity produced by nuclear power plants. In this state, which is rich in lignite and hard coal, the contrary had been the case: the state’s mines produced the coal that was needed by Germany’s coal-fired power plants. There was only one commercial nuclear power plant in North Rhine-Westphalia: Würgassen, a first-generation boiling-water reactor located far away from the powerful North Rhine-Westphalian coal centers in the region bordering on two other federal states, Hesse and Lower Saxony.

In addition, the potential supplier industry of the HTR pebble bed reactor was not ready or able to engage in an objective discussion with the politicians. Its inadequate future orientation was obvious, and the light-water reactor lobby, which was afraid of competition from the HTR, was too powerful.

Because of this political opportunism, the anxiety of the public after the catastrophe of Chernobyl on April 26, 1986, was criminally exploited.

Every interested and responsible individual was aware that the damaged Soviet nuclear power plant of the RBMK type did not comply in any way – neither physically nor technically, in other words not in the least – with the safety standards that apply to the other nuclear power plants operating all over the world, and especially not with the normal safety standards that apply in Germany.

Everyone familiar with the sector knew that the Russian nuclear power plant of the type RBMK is demonstrably unsafe.

By contrast, the HTR pebble bed reactor is indisputably the safest nuclear power plant in the world.

The reason for this fact is that the pebble bed reactor was developed in response to the specific commission to design a nuclear power plant with the high degree of safety that is required to generate electricity through nuclear fission in densely populated regions, even in cities, and also to generate combined heat and power for heating households and supplying process steam in industrial plants, e.g. in the chemicals industry.

No other nuclear power plants have been specifically commissioned to measure up to this safety standard. They are derived from nuclear reactors commissioned for military use, either reactors for submarines (objective: high compactness) or reactors for the production of weapons-grade plutonium (objective: high yield of plutonium). Through the addition of actively operating safety equipment, these types of nuclear power plant were adapted to generate electricity in the civilian sector.

The high-temperature pebble bed reactor is called “inherently safe”.

In other words, it is “passively” safe (as a result of the laws of nature) rather than being made “actively” safe (through technical equipment). Technical equipment always harbors the possibility of failure, small though it might be.

The outstanding safety of the HTR pebble bed reactor is due primarily to

• its robust fuel particles, which retain the dangerous radioactive products even during very high overheating (e.g., after loss of the coolant), and whose coatings do not melt,

• its basic physical design, which does not permit an uncontrolled intensification of the nuclear fission process, and

• its low power density (ratio of power output to structural volume), which makes uncontrolled overheating – and this includes the decay heat – impossible.

These advantages were demonstrated by conducting “planned” accidents in a ratio of 1:1 in the AVR high-temperature reactor near Jülich. The catastrophe-proof safety behavior of the high-temperature reactor was thus demonstrated in actual operation, not only through theoretical investigations and studies.

In addition, it is impossible for the reactor to be penetrated by air that could lead to combustion of the fuel elements, thanks to the laws of nature, which have been taken into account in the technical construction of the reactor. The essential elements of this type of construction were largely implemented in the THTR mentioned above.

It is impossible to divert weapons-grade material from the fuel particles of the high-temperature reactor.

A further, very significant safety advantage is the fact that the spent pebble fuel elements can be taken out of the reactor and transferred to a final repository without intermediate treatment, because

• their fissile material is sufficiently burned out,

• they do not require any technically designed, active removal of decay heat,

and because

• the coatings prevent their fuel particles from releasing the very long-lived alpha-ray emitters, i.e. they keep these poisonous substances safely “imprisoned”.

In addition, the coatings

• do not deteriorate, even under high pressure, and

• they cannot be corroded by water.

In every kind of final storage, gamma radiation is generally insignificant in the long term. It decays relatively quickly.

Because the coatings of the fuel particles keep the alpha-ray emitters so well encapsulated, a pebble bed fuel element could be safely picked up in a person’s hand after 200 years.

In order to store the radioactive waste of the HTR pebble bed reactor in a final repository, it is not necessary

• to separate out the remaining fissile material and the radioactive waste from the fuel elements, and to separate these from each other (reprocessing),

and therefore it is also not necessary

• to subsequently condition the radioactive waste (e.g. through vitrification) for final storage,

by contrast to the requirements of conventional types of nuclear power plant.

The risks harbored by reprocessing and conditioning installations are thus eliminated by the HTR pebble bed reactor.

The pebbles can be transported to the final repository without being crushed.

If above-ground interim storage of the pebbles is required for a limited period of time for logistical reasons, only the normal protective measures are necessary.

Final repositories for the pebbles with their radioactive waste can be found in suitable geological structures and at depths that geophysically (i.e. through the laws of nature) prevent the radioactivity from ever returning into the biosphere.

The high-temperature pebble bed reactor system can therefore also be called catastrophe-proof with regard to the disposal of its radioactive waste products.

Finally, it must be pointed out that only very small total volumes of spent pebbles need to be transported to the final repository. Per 1,000 MW of output from high-temperature reactors, the volume of used pebbles would be at most approximately 30 m³ per year, but probably even less; in mathematical terms, this would amount to a cube measuring approximately 3 m x 3 m x 3 m.

A modern constant-load power plant fueled by coal with an output of 1,000 MW produces about 5 million tons of CO2 per year. This is equivalent to approximately 2.5 billion m3 per year; in mathematical terms, this would amount to a cube measuring approximately 1.4 km x 1.4 km x 1.4 km. CO2 can be liquefied at high pressure, whereby the volume is reduced to 0.27% of the initial volume. In the example just cited, the CO2 would be reduced to a volume of 6.75 million m3, amounting to a cube measuring approximately 190 m x 190 m x 190 m.

The above-ground storage of such great volumes of liquefied CO2 would not be possible, because it would require the use of gigantic pressurized containers, which are not feasible.

The question that suggests itself at this point is: Would the subterranean final storage of such huge volumes of liquefied CO2 under high pressure be equally safe for human beings as the final storage of the fuel elements of the high-temperature reactor?

And that’s not to mention the long-term damage that will very probably be caused by that proportion of the gaseous waste product CO2 which must be emitted into the atmosphere, as it has been so far, because, among other reasons, there are not enough caverns available that would be suitable for the final storage of liquefied CO2.

The argument often expressed in the political discussion, that there is too little nuclear fuel for a future energy supply generated by nuclear power plants, is simply false, even with regard to the conventional types of nuclear power plants used today.

In the case of the high-temperature reactor, there is also the additional advantage that it can itself generate some of the fissile material it needs, starting from thorium, which is additionally loaded in the reactor, and of which there is a surplus in nature. This potential of the high-temperature reactor was exploited in the THTR (Thorium High-Temperature Reactor) near Hamm in North Rhine-Westphalia, but the advantages could not be completely demonstrated because the THTR was closed down prematurely.

However, thanks to the AVR, the utilization of the “breeding” of the fissile material uranium 233 from thorium 232 could be demonstrated at this reactor to the full extent, thus proving its feasibility. That was yet another record set by the high-temperature pebble bed reactor AVR!

The utilization of the fuel in the high-temperature pebble bed reactor reaches a thermodynamic efficiency that is considerably higher than that of the conventional light-water reactor in use today. The low efficiency of the light-water reactor is mainly due to the weakness of its fuel rods.

The efficiency of the HTR pebble bed reactor matches that of modern coal and gas-fueled power plants. It would also be possible to design combined power plants that use gas and steam turbines (Combined Cycle Power Plants, CCPPs) with the HTR, thus achieving thermal efficiencies of up to 46%.

Additional advantages include the higher degree of utilization of the fissile material, which was already mentioned above, through

• the continuous operation of the HTR pebble bed reactor and through breeding, as well as

• the possibility of using the combined generation of power and heat (co-generation).

The HTR pebble bed reactor thus saves fissile material resources, by contrast to the conventional light-water reactor.

Wherever carbohydrates are combusted (coal, heating oil, gasoline, diesel fuel, natural gas, wood, peat, refuse, biomass), CO2 emissions are generated. By contrast, wherever nuclear energy is used directly or indirectly as an energy source, there are no CO2 emissions.

The high-temperature reactor can accomplish the desired reduction of the gaseous pollutant CO2 in all segments of the energy economy (electricity, fuel, heating and industrial heat supply).

Electricity generation by the HTR pebble bed reactor, as compared to generation by the light-water reactor, was variably calculated, sometimes as slightly more expensive and sometimes as equally expensive. However, such marginal differences are a negligible factor in the electricity prices the consumer ultimately pays. Since the price for uranium will rise, the difference will in fact shift to the advantage of the HTR because of its more efficient utilization of the fissile material, and also because of the ever-increasing safety requirements for the light-water reactor.

Electricity generated by the HTR pebble bed reactor will in the future also be more cost-effective than electricity generated by thermal power plants, should the latter have to be refurbished in view of the CO2 problem. And that applies even more to electricity generated using renewable energies in non-subsidized power plants.

It is fortunate that the HTR pebble bed reactor, as a small unit, is economical. It thus offers the advantages of modular construction. An HTR module of 200 MWthermal is feasible, and it offers all of the advantages noted above — especially that of safety, as the Nuclear Technology department of the Technical Inspection Authority (TÜV) Rhineland already concluded in a very detailed analysis in June 1982.

The technology of the high-temperature reactor with pebble fuel elements, which is so important for the future, was initially hailed, then promoted, but ultimately betrayed by German politicians.

The decision to scrap the sophisticated HTR pebble bed reactor program in Germany was a senseless abandonment of an environmentally friendly, economical technology with a secure fuel supply, and it has resulted in a dramatic loss of scientific stature for Germany. This abandonment is a scandal for which politics and industry bear equal amounts of responsibility.

Our gratitude goes to China, South Africa and the other countries that are continuing their commitment to the future-oriented technology of the high-temperature reactor!

A total of 210 nuclear power plants with 437 reactor blocks are currently operating all over the world. In nearly all cases, the decision to construct them was made before 1985, and most of them also became operational before 1985. After 1985, only about 30 decisions were made to build new nuclear power plants, with only 8 of these nuclear power plants to be built in the USA and Canada and only 6 in Europe (not including Russia, Ukraine and new EU member countries).

This slump in demand was caused by a temporary saturation of the need for new power plants in general. Since that time the situation has changed, on account of the necessary modernization of power plants in the industrialized countries and the growing demand for power plants in the emerging countries.

There is growing awareness that a future energy supply (electricity, fuel, heating and industrial heat supply) is unimaginable without nuclear power. In some places this realization is being reached sooner, in others later.

In view of this development, the following list summarizes some conclusions that the rest of the world regards as obvious but Germany continues to repress:

The German high-temperature pebble bed reactor

• is catastrophe-proof at every stage, including the disposal of its radioactive waste,

• operates without environmentally harmful emissions,

• uses the fissile material very efficiently,

• can be used in the entire energy market (electricity, fuel, heating and industrial heat supply),

• can be constructed in modular form,

• is economical, and

• offers no possibility of a diversion of weapons-grade material!

In Germany, it is imperative that we rethink our attitude toward the high-temperature pebble bed reactor, a versatile future-oriented system whose development, as we know, was unnecessarily canceled in the German state of North Rhine- Westphalia at the end of the 1980s.

However, there is still – just still – technical and scientific expertise concerning the high-temperature pebble bed reactor in Germany!

The situation today (figures in MWthermal)

The following high-temperature reactors are in operation:

• China: HTR 10 10 MW (with pebbles)

• Japan: HTTR 30 MW (with blocks)

The following high-temperature pebble bed reactors are being planned:

• South Africa: PBMR 400 MW (with gas turbine)

• China: HTR module 250 MW (with steam turbine)

The following programs for the future utilization of the high-temperature reactor are being processed:

• International cooperation: GENERATION IV

• China: 30 HTR 250 until 2020

• South Africa: 20-30 HTR 400 until 2050

• France (EU program): ANTARES project (with gas turbine)

• USA, Russia: MHTGR for the destruction of plutonium

• USA: HTR for generating hydrogen

• South Korea: HTR for generating hydrogen

• Japan: HTR for generating hydrogen

• Netherlands: HTR for powering ships


1. TÜV Rheinland, Fachbereich Kerntechnik: Studie zu den sicherheitstechnischen Auslegungsanforderungen an den Hochtemperaturreaktor-Modul der GHT zur Strom- und Prozessdampferzeugung, Cologne, June 1982

2. Kugeler K., R. Schulten: Hochtemperaturreaktortechnik, Springer Verlag, Berlin, Heidelberg, New York, 1989

3. AVR – Experimental High-Temperature Reactor, 21 years of successful operation for a future energy technology, Association of German Engineers (VDI), VDI-Verlag GmbH, Düsseldorf, 1990, ISBN 3-18-401015-5

4. Schulten R., H. Bonnenberg, Brennelement und Schutzziele, Jahrbuch 91, VDI-GET, VDI-Verlag GmbH, Düsseldorf, 1991, p. 175

5. Kugeler K. et al., Fortschritte in der Energietechnik, Prof. Rudolf Schulten zum 70. Geburtstag, Monographien des Forschungszentrums Jülich, Vol. 8, 1993

6. Kugeler K., H. Bonnenberg, Der Hochtemperatur-Reaktor, VDI-Bericht No. 1493, Düsseldorf, 1999, p. 147

7. Nickel H. et al., Long Time Experiments with the Development of HTR Fuel Elements in Germany, Nuclear Engineering and Design 217 (2002), pp. 141-151

8. Röhrlich, Dagmar, China baut Kugelhaufen-Kernreaktor, DIE WELT, February 19, 2005, p. 31

9. Pohl, P., The Importance of the AVR Pebble-Bed Reactor for the Future of Nuclear Power, CD-ROM Proceedings PHYSOR 2006, ANS Topical Meeting on Reactor Physics, Vancouver, Canada, September 10-14, 2006, B085

10. Kugeler K., Moderne Konzepte für eine sichere Kernreaktortechnik, presentation to the Deutsche Physikalische Gesellschaft, Magnus-Haus, Berlin, February 13, 2007

11. WIKIPEDIA – Internet: pebble bed reactor (PBR), pebble bed modular reactor (PBMR) and high-temperature reactor (HTR)