A Surprising Advance in Nuclear Power

While nuclear fusion technologies remain elusive, China just demonstrated a breakthrough in safe fission.

Over the past few months, while public attention was squarely focused on the many struggles of wind energy, there’s been a convergence of news which point to increasing momentum of a different kind of energy transition.

Scientists and engineers have been working for more than two decades on new technologies for nuclear fission power, summarily called “generation-IV” designs. A key objective: gen-IV reactors should operate with technologies which prevent core damages passively. In other words, chains reactions won’t escalate in case of control failures.

An international working group set an ambitious goal of 2030 for first results. But just a few weeks ago, engineers in China ran a successful demonstration. This would put China more than six years ahead of an optimistic schedule, and more than ten years ahead of international competition.

Hype or breakthrough? Let’s take a look at the broader picture. Hope this is interesting and helps you stay ahead in your discussions!

(HTR-PM in Shandong, China)

It’s well known that China is the country with the largest growth of energy production and consumption. Growth is largely driven by coal burning, and it is more than enough to off-set any western attempts to cut carbon emissions. However, China is also working on the development of low-carbon energy sources. Little noticed, the country has achieved leadership in a technology which may become central to a global energy transition: the energy density of a fission reactor, but without the risk of a core meltdown. It is a revolutionary design and a technological triumph, but also a story about the absurdities of public policy and environmentalism gone wrong.

Conventional reactors

Essentially all nuclear reactors in commercial operation are designed in the same operating principle: nuclear fuel like uranium, a very heavy element, decomposes into lighter elements like krypton and barium, in a process which releases neutrons. Those neutrons hit more uranium cores causing them to decompose as well – a chain reaction which releases a lot of energy. LOTS of energy: a kilogram of uranium-235 produces three million times more energy than a kilogram of coal.

The engineering challenge therefore is to control this amount of potential energy. The key to control lies in the process described above: limit the number of free neutrons, and you limit the speed of the chain reaction. In a reactor, this is typically done with control rods made of graphite (a form of carbon) which catch neutrons. Driving control rods in between fuel rods, the reactor slows down.

It sounds simple, if still dangerous, but in reality is a very complex problem. Failures in operating protocols have escalated into grave accidents.

First of all, processes in a reactor don’t run as cleanly as described above. The chain reactions produce all kinds of products, some of them for milliseconds only, and some of them may accumulate over time, interfering with other processes. The most famous example of that problem is the Chernobyl accident in Ukraine in April 1986. During a test shutdown of power block 4, the Chernobyl plant received a request to keep the block on hold to jump in for another power plant which went offline after a fire. As power block 4 idled below minimum operating load, a whole zoo of intermediate chemical elements built up in incomplete reactions. One of them was xenon, an element which absorbs neutrons and slows down the chain reaction just like graphite. Reactor block 4 was dying down, and the engineers had no clear idea what was going on. They removed one control rod after the other without much effect, until suddenly the reaction kicked in again, and with nothing to slow it down. The pressure in the reactor blew up the housing, and in a second explosion the roof of the building. The reactor core lay open to the environment, releasing radioactive isotopes.

The vulnerability of contemporary control processes was demonstrated again almost exactly twenty-five years later, in March 2011, when a large earthquake near Japan’s main island Honshū damaged the external power supply of the Fukushima nuclear power plant. The earthquake also triggered a tsunami which overflowed a protective dam and flooded backup diesel generators. Without electrical power, cooling processes failed and three of the plant’s six reactors overheated, leading to a core meltdown.

Enter the pebble-bed reactor

The Shidao Bay Nuclear Power Plant in Shandong Province has been in commercial operation since December 2023. It was developed under the leadership of Tsinghua University and follows radically different design principles. Most fundamentally, the design eliminates the complex cooling procedures, the breakdown of which caused the Fukushima accident; and secondly, the chain reaction is self-modulating which prevents an uncontrolled escalation like at Chernobyl.

It relies on two principles:

1. The reactor uses helium gas as a coolant instead of water. Helium is chemically inert (it doesn’t chemically change e.g. absorbing particles) under a wide range of environmental conditions, and it can be heated up to much higher temperatures than water. Because of the design of the reactor, helium circulates naturally.Therefore, even if control pumps are switched off, helium transports heat away.

2. Instead of fuel rods, the Shidao reactor uses fuel “pebbles”, which are spheres about the size of a tennis ball.

(Pebble fuel. Size comparison and construction. Source: x-energy)

The pebbles are constructed with three layers of carbon (remember, a reaction moderator used in conventional reactors) around a uranium core. When the pebbles heat up, their capacity to absorb neutrons increases, and the reaction slows down. The pebbles can withstand temperatures of up to 1,600°C (3,000°F), which is above the heat the reactor can develop even if gas circulation pumps go offline, because of design principle 1.

(HTM-PM design principles)

In July 2024, the operators published their results of several tests, in one of which they turned off the helium pumps. The reactor cooled down naturally, i.e. without any intervention of operators and without active cooling procedures. This was the first demonstration of an inherent-safety feature preventing meltdowns on a commercial scale reactor – a major breakthrough in fission reactor design.

The temperature range of its heated helium is similar to steam in coal-fired power plants, which raises the operational efficiency a lot. The Shidao reactor (“HTR-PM”/ high-temperature gas reactor, pebble bed module) has a capacity of 100MW electrical energy/ 250MW thermal energy, i.e. a 40% thermal efficiency.

HTR-PM was developed to be built in modules. In the next development step, engineers plan to make the modules compatible with steam generators at coal plants. The goal is to replace coal energy but use all the other existing infrastructure for power generation.

The bigger picture

The Shidao reactor is one of six candidate technologies¹ for gen-IV reactor designs.

(4 generations of nuclear power designs. Western countries mostly have generation-II reactors in operation. The reactors which just went on-stream at the Vogtle Electric Generation Plant, Georgia, are generation-III+)

In the U.S., a generation-IV reactor just started construction. In June 2024, TerraPower, an energy company founded and chaired by Bill Gates, began building a sodium-cooled reactor. It uses natrium (also known as sodium) as a coolant. One of the key design features is that molten salt can be used to store energy and release it on demand, so the combined reactor/ storage unit can act as a swing producer with more flexibility than conventional nuclear plants. However it will take up to 10 years to go into commercial operation. It is scheduled to replace a nearby natural gas plant (Naughton Power Plant) in 2036. From that perspective, China has a technological lead of at least 10 years.

Scientist and engineers have long known about the design principles, but developing real-world fuel pebbles which deliver the right balance of neutron emission and capture, in the preferred temperature bands, took decades. Unfortunately, it turns out that a lot of time was lost.

Germany dropped the ball

The idea of using helium as a coolant had been around since the early days of nuclear power. The U.S. built a first plant near Delta, Pennsylvania, in operation from 1969-1974, and a second unit at Fort St. Vrain in Colorado (construction began in 1968. Commercial operation 1979-1988). However they were shut down as they were not commercially viable.

Germany however was already ahead. A German researcher, Rudolf Schulten, developed the idea of using pebbles, the key missing link. A team of researchers built the AVR (German “Arbeitsgemeinschaft Versuchsreaktor” = collaborative test reactor) from 1961, a test site which was in operation from 1969-1988. Similarly to the U.S. reactors, it experienced a number of operational failures which made the test reactor expensive to operate.

The Chernobyl accident in 1986 led to a huge change in the perception of the broader public. Sentiment became strongly anti-nuclear up to current times. To some degree it is understandable, as Germany and Austria were the West European countries most exposed to radioactive fallout.

(Chernobyl fallout Cs-137, Source: Austrian Institute for Meteorology and Geophysics)

AVR was shut down in 1988. Dismantling the reactor and storing the remaining fuel became an expensive and difficult operation, adding to the negative public sentiment towards nuclear power in Germany.

China’s HTR-PM is based directly on Germany’s AVR. It is ironic that Germany, paralyzed by fear of core meltdowns, missed the chance to develop a meltdown-proof reactor. Given its know-how in engineering and chemistry, sectors in which Germany is a world leader, Germany would have been predestined to come up with an operable technology many years before China. This would also have addressed the country’s obsession with reducing carbon emissions.

Germany does have a remaining role in pebble-bed energy. SGL Carbon produces the fuel pebbles for China’s reactor.

Despite the recent advances in China and the U.S., it will most likely be decades until the generation-IV reactors will have a meaningful impact on total electricity generation. It’s going to be interesting to see how the gap will be closed until then – more fossil fuels (“peak oil” is moving many years into the future, once again), gen-III+ reactors (considering it takes about ten years from beginning construction to going online), or renewables?

Let me know what you think!

All the best,

John

1. In case you want to follow up, the six technologies are: the gas-cooled fast reactor (GFR), the lead-cooled fast reactor (LFR), the molten salt reactor (MSR), the sodium-cooled fast reactor (SFR), the supercritical-water-cooled reactor (SCWR) and the very high-temperature reactor (VHTR).