The pebble bed reactor developed in Germany is regarded by the anti-nuclear scene as a prime example of an irresponsible energy project that has failed forever. In China, the view is different. There it was replicated, tested, further developed and put into operation. The main reason - both in Germany and in China - was its unusual safety features. However, this type of reactor still has a flaw.
by Udo Pollmer July 11, 2024
When the wailing, and gnashing of teeth after the collapse of the hydrogen economy is over and reconstruction begins, solid energy technology will be in demand. One type of reactor with a future is the pebble bed reactor. In a way, it is the link between the old light water reactors and the new fast breeder reactors. However, our eco-nuclear experts had already confirmed its final demise: The decommissioned pebble bed reactor in North Rhine-Westphalia is "a symbol of failure for eternity"!1
As if in defiance, a similar reactor went online in China in Shidaowan on...
...the Yellow Sea at the end of 2023.2 If you read the writings of the environmental scene, then the actions of the Chinese are stupid and irresponsible. After all, from the world view of our green baizuos, as the Chinese affectionately call this type of contemporary, the pebble bed reactor is one of the worst of all.
It is certainly true that the first of their kind - first in Jülich and later in Hamm-Uentrop - were not under a lucky star. Technical defects, operating errors and disregard for safety regulations led to considerable releases of radioactivity.3 At one point, the operators probably thought they could conceal such an accident because the Chernobyl cloud was blowing over the country. Some "accidents" look like sabotage. In 1989, after the power plant in Hamm-Uentrop, the Jülich reactor was finally shut down. The events show that the fierce public protests against what was then an autocratic nuclear lobby were only too justified. But that was a long time ago.
The reactor in Jülich was the first power plant that was no longer operated with fuel rods, but with fuel balls the size of tennis balls. In addition to inactive uranium-238, each contained a few grams of uranium-235 and graphite. At that time, double-coated particles were still used (BISO). This resulted in a lot of abrasion. If graphite dust comes into contact with oxygen, a spark can ignite it.4 The operators were lucky, nothing happened.
Much better fuel pellets were available from 1980. Once again, the fuel was contained in tens of thousands of silicon coated particles as tiny as poppy seeds. But now the graphite spheres were also coated with silicon carbide. These spheres, known as TRISO, are still in use around the world today; they are ultra-stable and can withstand temperatures of 1600 degrees.5 They were not allowed to be used in Hamm-Uentrop for authorisation reasons.
More security
Replacing the fuel rods in older nuclear reactors is a costly and risky operation: the pressurised reactor has to cool down before it can be opened, which requires a shutdown lasting several weeks. To delay this, the reactors were loaded with an extra portion of fuel elements. This so-called excess reactivity is compensated for by additional moderators. In the pebble bed reactor, on the other hand, only as many pebbles are added as necessary. Spent fuel can be removed at the bottom during operation.
Cooling is another Achilles heel of light water reactors. In the event of a failure, the temperature in the reactor rises. The resulting water vapour can cause the pipes to burst. A core meltdown followed by a severe hydrogen explosion cannot be ruled out. Not so in the pebble bed reactor. The helium used here is not pressurised and can neither burn nor explode, nor does it become radioactive.
Even more important is a special physical feature: the "negative temperature feedback".6 As the temperature rises, the hit rate of the neutrons decreases due to the expansion and thus also the nuclear fission. As a result, the reactor cools down. Once it has cooled down a little, the hit rate increases again and the temperature rises again. This effect would be intensified by the "Doppler broadening" of the atoms.
But how do the technicians know that the physicists' paradoxical-sounding theories are correct? Quite simply: they tried it out, they simply switched off the cooling at full load. The reactor first got a little hotter, then it calmed down and cooled down as expected. No damage, no leakage of radioactivity. The tests were first carried out in Jülich in Germany and then in China.3
The World Nuclear Association reported: "In 2004, the reactor was subjected to an extreme safety test when the helium circuit was deliberately shut down without the reactor being shut down. The temperature rose steadily, but the physics of the fuel caused the reaction to gradually decrease and eventually shut down after three hours," meaning it had cooled back down to its lower normal temperature.7
China: Finally a "great leap forward"
The first pebble bed reactor had already gone into operation in Jülich in 1967. This was soon replicated and further developed by Beijing's Tsinghua University. A special feature of the Chinese version is that it not only burns uranium-235 as fuel, but also thorium-232: if thorium spheres are positioned around the active core of the reactor, they act as neutron scavengers and convert into fissile uranium-233. When they roll into the inner area in a controlled manner after passing through the breeding zone, they serve as new fuel.8,9
The industry's goal is, of course, a reactor that produces as much new fissile material as it consumes. The fact that Tsinghua University has spent around 50 years developing its experimental reactor is certainly also due to this desire. Even if the Chinese are reluctant to show their cards, it can be assumed that they have developed a "near-breeder" that can breed its own fuel for years.10,11
Even if pebble bed reactors are far superior to light water reactors in terms of safety, they still have a serious disadvantage: The considerable amounts of nuclear waste. The burnt-out spheres are currently not reprocessed due to their high stability. All fission products are trapped inside them and the reactor remains clean. Nevertheless, fission products in the spheres (pebbles) will sooner or later bring the nuclear reactions to a standstill.12 This limits their utilisation in a similar way to fuel rods.
As far as information on the Chinese pebble bed reactor HTR-PM is available, it still works with an "open fuel cycle". This requires interim storage of the old fuel pellets for a hundred years, after which final disposal is possible. Sooner or later, however, the Chinese will reprocess their spent fuel pellets as soon as the quantity is worthwhile. It is certainly not rocket science to break up these ceramic elements, recycle the graphite to reduce the sheer volume and process the radionuclides back into fuel pellets or for medical purposes.13
The first reactor block went into operation in Shidaowan in 2022, followed by Block 2, 18 further pebble bed reactors are now planned in China. At the beginning of 2016, the Chinese agreed with Saudi Arabia to build such a plant to distil seawater. Indonesia followed suit in August 2016 to generate energy in remote parts of the country. These reactors lend themselves to a modular design consisting of smaller units. This allows the plants to be industrially pre-produced and set up on site.
Such systems would be a great benefit for the quality of life and health of the population of less developed countries. In many regions of the world, people still cook with wood. The smoke that is produced is the cause of many health problems.14,15 Women in particular suffer from serious lung diseases such as COPD (a chronic inflammation with increasing narrowing of the airways and emphysema).16 Cooking with wood also causes many women to go blind.17
Likewise, refrigerators also improve health. Foodborne infections are a constant danger, especially in tropical countries. As animal foodstuffs cannot be transported and stored fresh, livestock is often driven long distances to the markets. Immediately after slaughter, the meat is put on the fire. The pieces of meat are usually small because the climate means that the meat does not have to be matured, which would make the tissue more tender. China's reactor modules could therefore prove to be a blessing for people in less developed countries.
Wikipedia explains the world differently: for its German readership, the pebble bed reactor is "one of the biggest mistakes in German projects of the past 55 years".18 But the ugly duckling is turning into a proud swan. No matter how erratic the events surrounding the prototypes in NRW may have been, prototypes are not at the end of a development, but at the beginning. Just as ingenious would be references to the safety problems of the first cars. Benz's motorised carriage from 1886 had neither seat belts nor airbags.
No risk, no fun
Rainer Moormann, probably the most critical expert on pebble bed reactors, nevertheless warns against rash euphoria despite all the successes: after all, no complex industrial plant is immune to errors and risks, even if the pebble bed reactor offers far greater safety than previous reactors. Time and again, the test facilities have shown unexpected reactions. Defective spheres that break and then contaminate the reactor core with gaseous and dust nuclides should also be considered.19
The fundamental concerns naturally apply to all types of energy generation. If the cement is mixed incorrectly during dam construction, the risk of a breakage increases. A severe earthquake will cause any dam to burst. What might the consequences be for the Three Gorges Dam on the Yangtze? Even seemingly harmless technologies, such as wind energy, have developed into an energy policy and ecological nightmare. Or geothermal energy, which is destroying a small German town called Staufen in the German Federal State of Baden-Württemberg in slow motion:
The people of Staufen actually only wanted to heat their town hall with the water bubbling up from the depths in order to save energy. Now the citizens are living on a powder barrel. To date, the buildings in the old town have risen by 70 cm and shifted by half a metre - in different directions. Pumps are being used to pump water from new, even deeper boreholes, water that previously caused a layer of calcium sulphate to swell. Without powerful pumps, the ground is likely to rise by another two metres or more - but not evenly.20
It is not a question of downplaying or dramatising risks, but of weighing them up and accepting that even at first sight attractive ecological processes on a very small scale can have devastating consequences for people and the environment. In the meantime, numerous other serious problems with geothermal energy have come to light.21 As a consequence, the safety technology for new boreholes has been drastically improved. So far, every successful technology has taken this route.
The Swiss: Critical, but please don't be undercritical
The Swiss take a very critical view of nuclear energy. Reason enough for the scientists there to develop an elegant reactor that gets by without the irritating words that make the eco-roosters' crests swell. The most important difference to all other reactors: it always works "subcritically".22 This means that it is not capable of starting or maintaining a chain reaction on its own. It is not kept going with neutrons from radionuclides, as is usually the case, but from outside by a particle accelerator, a high-energy cyclotron. Small versions of cyclotrons have long been used in many hospitals to treat cancer patients.
This reactor is also based on thorium, which is converted into fissile uranium-233. The cyclotron does this by accelerating protons to almost the speed of light. When these protons hit a target metal such as lead inside the reactor, it is broken down into many fragments (spallation). In the process, plenty of neutrons are shot out of the nuclei. These neutrons in turn set the process of transmutation in motion, turning lead into uranium. Together with bismuth, lead is also a coolant.23 This makes it possible to work under atmospheric pressure, which benefits safety.
As soon as the accelerator is switched off, the reaction stops.24 The reactor, like others, only generates some decay heat because the (short-lived) fission products continue to "radiate" for some time and also emit a small amount of neutrons.25 Just as a tiled stove continues to emit heat after the fire has gone out.
In addition to thorium, nuclear waste can also be used as an energy source. The fast neutron flux burns most long-lived waste, including plutonium, neptunium and americium.26 This reduces the mass of spent fuel rods. Of course, spallation in the reactor also produces transuranics and fission products. Most of them, such as technetium-99 or iodine-129 with "biblical" half-lives of millions of years in some cases, are broken down into smaller fragments in the same way, thereby gaining energy. The cyclotron uses these to produce the short-lived isotopes technetium-100 and iodine-130, from which stable ruthenium and stable xenon-130 are produced.27
Although the experiments carried out for this purpose have worked without exception, there is a downside here too: the Swiss people decided against it in a referendum. It prefers to sit on its nuclear waste and guard it until the end of all days. Now the inventors are looking abroad for ways to realise it.
References
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English Editor: Josef Hueber
