X-Energy's Immodest IPO
What China's 50-year pebble bed reactor program can teach us about the valuation of the West's most recent publicly traded reactor company.
X-Energy was listed on the Nasdaq under the ticker “XE” on April 24th, raising $1.02 billion with shares surging 30.9% on debut to give the company a valuation of $11.9 billion.
The pitch is Dow Chemical as anchor customer, Amazon as both lead equity investor and holder of power purchase commitments for more than 5 GWe, and Centrica as a UK utility partner, all anchored to an 80 MWe pebble-bed high-temperature gas-cooled reactor.
X-Energy is a 916-person development-stage company promising rapid construction of high reliability reactors fuelled by proprietary TRISO fuel which is roughly 10 times more expensive to fabricate than conventional light water reactor fuel. For a deep dive on that topic, see Decouple’s interview with Michael Seely below.
It has no construction experience, has never operated nuclear reactors, and is yet to produce commercial fuel. Yet it is priced as though the hardest engineering problems in one of nuclear’s least mature reactor classes are already solved.
The current AI-driven moment of nuclear euphoria creates the conditions where this is possible. In the absence of a sustained tempo of nuclear construction in the West, narratives trump empiric reality and reactor concepts that are genuinely interesting science projects get priced as commercial products well before the performance record justifies it.
The embedded assumption in X-Energy’s $11.9 billion valuation is that American startup dynamism can compress what has been for China, the world’s most capable nuclear industrial state, a 50-year institutional journey into a single first-of-a-kind construction project.
In sectors where design, capital, and IP are dominant, that assumption is often correct. China’s pebble-bed program, however, illustrates why even “advanced nuclear” is not one of those industries.
The German Origins and Chinese Technology Transfer of the HTGR
The pebble-bed concept originated with Rudolf Schulten at the Jülich Research Centre in Germany in the 1950s and 1960s. Germany built and operated the AVR and THTR-300 before political and economic pressure killed the HTGR program in the aftermath of Chernobyl.
China then absorbed 30 years of German engineering knowledge through formal technology transfer. This included a design review by Siemens/Interatom of the Chinese prototype HTGR, the HTR-10 in 1994. They have subsequently spent the next 3 decades building on it.
After 5 years of construction the 10MWt prototype reactor achieved criticality in 2000. Construction of the 80MWe commercial demonstration HTR-PM began in 2012, 38 years after basic research started at Tsinghua University.
This is the realistic development timeline of an immature reactor class, and it is the benchmark against which X-Energy’s claims should be evaluated.
The Technology Readiness Level and Capacity Factor problem
The pebble-bed high-temperature gas-cooled reactor is a precise example of the gap between scientific promise of “advanced nuclear” and the commercial maturity of conventional water cooled reactors. Approximately 650 commercial PWRs and BWRs have operated since the 1950s, accumulating 18,000 reactor-years of operational experience.
Against that baseline, exactly 5 pebble-bed HTGR cores have ever operated: 2 pilot-scale research reactors, one German and one Chinese and 3 commercial demonstration-scale cores. Those were Germany’s short lived THTR-300 at 120 MWe (1983-1989) and a pair of 80MWe HTR-PM modules at Shidaowan, which reached commercial operation only in 2023.
Commercial demonstration scale pebble-bed HTGRs have thus accumulated only 12 reactor-years of operation to date. X-Energy’s customers are being asked to assume that its reactor technology will automatically share the 90%+ capacity factors that the light water reactor fleet has obtained through 18,000 reactor years of learning and optimization.
Dow and Amazon need high availability and the Xe-100’s central marketing claim is that continuous online refuelling delivers exactly that: no scheduled outages, dispatchable baseload and therefore high capacity factors from day one.
The best way for a lay person to make sense of the pebble-bed online refuelling concept is a giant gum ball machine. The pebble-bed HTGR continuously recirculates hundreds of thousands of fuel spheres through the reactor core, approximately 2000 per day, through a burnup measurement station, and back into the top of the core. This process must function with exceptional reliability as a continuous industrial operation rather than a periodic maintenance event.
As a comparison, the only other commercial reactor type that refuels continuously on a daily basis is the CANDU, which shuffles approximately 15 fuel bundles per day through a well-understood mechanical sequence refined across a fleet of reactors over 50 years. CANDU online refuelling still ranks among the more demanding operational challenges Canadian operators face.
The HTR-PM experience suggests this is harder to sustain than the design predicts. These refuelling difficulties may contribute to the fact that the Chinese plant entered commercial service derated 20% below its design thermal rating and reached only around 20% capacity factor in its first 2 years, advancing to around 50% by 2025.
X-Energy’s fuel handling system has been modelled and tested at sub-scale. The Chinese had over 20 years of hard operational data from the HTR-10 informing the equivalent commercial demonstration system at Shidaowan.
The OPEX-CAPEX trade off
The 9 year construction record of the HTR-PM carries a specific warning for the X-Energy investment thesis. The business case for a low-pressure advanced reactor with expensive TRISO fuel, depends on offsetting high operating costs through faster, cheaper construction.
A pressurized water reactor must keep its coolant liquid at around 155 atmospheres, demanding thick-walled pressure vessels and elaborate emergency cooling systems that dominate capital costs and slow construction.
Despite its far lower power density and therefore relatively larger core, a helium-cooled pebble-bed reactor operates at lower pressure with fuel designed to retain fission products passively. The nuclear island should therefore in theory be simpler and faster to build.
China’s construction sector builds PWRs in under 6 years at 2-3 times the Western pace. The HTR-PM still took the 9 years, and a further 2 years of commissioning before entering commercial operation. The Chinese will no doubt improve on this schedule in subsequent builds however their FOAK timeline is a best case scenario for a start up building within the atrophied Western nuclear sector.
The bold claim made by X-Energy CEO Clay Sell in the present tense that “We make it easy to build nuclear power plants” will certainly be put to the test. Extraordinary claims require extraordinary evidence and that evidence has yet to be produced.
The Limits of American Startup Dynamism
X-Energy’s optimism relies on the assumption that American entrepreneurial dynamism will trump Chinese state development of advanced reactor technology.
This dynamism clearly works where iteration cycles are short, failure is recoverable, and the feedback loop between experiment and result is measured in months: shale drilling, rocketry and software are the cardinal examples.
SpaceX can destroy many prototypes on a single launch pad, absorb the data, and fly again within months. However, even an “advanced” nuclear reactor takes years to build with each reactor requiring its own seismically qualified civil works equivalent of a launch pad. In addition, its components must operate without failure for years and breaking things at the commercial reactor stage is not an option.
The iteration cycles, the failure intolerance, and the manufacturing precision of nuclear requirements impose longer timelines and slower iteration. This is what China’s HTGR program illustrates and no amount of venture or retail capital changes that.
But surely a peppy startup can move faster than a state run program in China?
The Western image of China as a cumbersome mass manufacturer of cheap goods rather than a technology leader is desperately out of date.
The Australian Strategic Policy Institute’s Critical Technology Tracker found China leading in just 3 of the technologies it tracked during 2003 to 2007. By 2024 that figure was 57 of 64, with nuclear explicitly among them. The country X-Energy is being benchmarked against, implicitly or otherwise, is not the China of 2003.
This is due to aggressive industrial policy and state capacity organized within a hybrid political economy. America’s nuclear reactor technology development dominance with the PWR/BWR similarly leaned heavily on robust state involvement to nurture the technology through its early development.
What China has built in pebble-bed HTGR technology over the last 5 decades is the capacity to cycle continuously from materials research through fabrication to operational deployment and back, accumulating compounding institutional learning at each pass.
X-Energy proposes to move from blueprints to commercial reactors at Dow’s Seadrift plant in under a decade, and according to Amazon’s commitment, envisions 5GW of capacity across the US by 2039.
X-Energy is a genuinely interesting technology company working on a compelling reactor concept. However, the entire history of pebble-bed HTGR development argues against the idea of leap-frogging the pilot and demonstration reactor stage and proceeding directly to a commercial build.
China ran the HTR-10 for over 2 decades before turning on its commercial scale demonstration reactor and is only now upsizing from its 2 module configuration to a fully commercial 6-pack, the HTR-PM 600 in the hopes of improving the small reactor’s challenging economics.
X-Energy has no equivalent to the HTR-10 or the HTR-PM, and $11.9 billion is an immodest price for a firm proposing to compress into a decade timeline what the world’s most capable nuclear industrial state, after 50 years of effort, has yet to fully accomplish.
So 11 billion is an immodest amount of money to develop a new reactor? I get you are doubtful, doubtful, wary. But what are you writing this for? Investor protection? You believe that 13 years is super fast for a reactor development? Much of the reactor can be tested without turning it on. If allowed to turn on, a reactor can be built and destroyed after a year. Then reconstructed. It is a carefull “stewardship” of resources that prevents this, not something inherently dangerous. With enough resources a space-X approach is possible.