Nuclear Milestone: China's HTR-PM Demonstrates Inherent Safety

Chinese researchers have confirmed the first of two units of the 200-MWe high-temperature modular pebble bed (HTR-PM) demonstration project at the Shidao Bay site in Rongcheng, Shandong Province, successfully achieved inherent safety during loss-of-cooling tests. The tests demonstrated, for the first time, a commercial-scale high-temperature reactor’s (HTR’s) ability to remove decay heat without active intervention, the researchers from the Institute of Nuclear and New Energy Technology (INET) at Tsinghua University in Beijing said in a July 2024 article published in the journal Joule.

An Inherent Safety Achievement

The confirmation is notable for the much-watched high-temperature gas-cooled reactor (HTGR) demonstration project, which began commercial operation on Dec. 6, 2023. Invented by German scientist Professor Rudolf Schulten, the HTR-PM evolved from experimental pebble-bed projects, such as the AVR, a 46-MWth test German HTR that operated between 1967 and 1988, and the 300-MWe Thorium Hoch Temperatur Reacktor (THTR), a commercial reactor that ran between 1985 and 1991. China has operated a test 10-MWth HTR at an INET site in a Beijing suburb since 2000.

“The modular HTR evolves from the classic HTR based on principles that restrict module power, power density, and core diameter,” the researchers explained. “The decay heat, which is the primary cause of core melting in a nuclear fission reactor, can be dissipated to the environment naturally by heat conduction, radiation, and natural convection without adopting emergency core cooling systems.”

The HTR-PM comprises two 200-MWth helium-cooled reactors driving a shared steam turbine. Each module comprises a pebble-bed reactor core that uses graphite as both the moderator and structural material and performs several passes through the core of fuel burnup (via the MEhrfach DUrchLauf cycle), with a recycling rate of 15 times of burnup. As a key safety feature, the HTR-PM’s fuel element embeds 12,000 tri-structural isotropic (TRISO)—coated particles in a spherical or prismatic matrix, with each TRISO particle fabricated by coating a kernel of uranium oxide with layers of pryocarbon and silicon carbide. These features “are able to prevent the leakage of fission products without exceeding a proven fuel temperature limit, for example, 1,620C,” the researchers wrote.

The HTR-PM design allows for a power density of about 3.2 MW/cubic meter—which is “about 1/30” that of a commercial pressurized water reactor (PWR), “guaranteeing that the decay heat can be efficiently removed by heat transport mechanisms, such as the conduction, radiation, and natural circulation, to the reactor cavity cooling system (RCCS), which is located outside the reactor pressure vessel (RPV),” they noted.

1. Loss-of-cooling tests conducted in mid-2023 on both reactors—each operating at a power level of 200 MWth—at China’s HTR-PM demonstration site at the Shandong Shidao Bay site confirmed that the reactors can be naturally cooled down, verifying inherent safety at a commercial scale for high-temperature reactors. Source: Zhang, Z., et al. (2024). Loss-of-cooling tests verify inherent safety in HTR-PM nuclear plant. Joule. Elsevier.

However, as the researchers underscored, the feasibility of realizing inherent safety—self-stabilization of reactor power at a very low level and passive removal of decay and the residual heat in the event of an accident—in an HTR had so far been limited to test reactors, such as Germany’s AVR and China’s HTM-10. The major “bottleneck” of decay-heat removal has been to manage the power level. Safety tests conducted at China’s HTR-PM demonstration project in August 2023 on Unit 1 and September 2023 on Unit 2 showed for the first time inherent safety at a commercial-scale HTR (Figure 1). “During the entirety of the tests, the reactor modules were naturally cooled down without emergency core cooling systems or any cooling system driven by power,” the researchers said.

A Leap for HTGR Technology

The findings are a substantial leap for HTGR technology and point to a potential advantage over conventionally used light water reactor (LWR) designs, which require dedicated safety systems. While HTGRs have had more than a half-century of development, enthusiasm for the advanced nuclear technology has been ticking up given their flexibility in industrial applications. In particular, their ability to provide high-temperature heat for a wide range of processes, from hydrogen production to chemical manufacturing, makes them an attractive option for industries looking to decarbonize and enhance energy efficiency. Along with China’s HTR demonstration, commercial HTGR ventures are being led by X-energy, which is spearheading a 320-MW Xe-100 demonstration in Texas with Dow and the U.S. Department of Energy, and Ultra Safe Nuclear Corp., which is developing a 5-MWe (15-MWth) Micro Modular Reactor (MMR), anticipating a first reactor could begin operating at Chalk River Laboratories in Ontario, Canada, by 2027.

HTGR technology, however, continues to grapple with market challenges, including economic uncertainties and a fledgling supply chain for high-assay low-enriched uranium (HALEU). In their article, the Chinese researchers noted the power generation cost of the HTR-PM “is still about 20% higher than that of commercial PWR plants.” Still, the researchers pointed to the design benefits, including its modularity, scalability, and application flexibility. Economics could be “managed by scheme cogeneration and further by the mass production of reactor modules,” they noted.

“We, along with the industry partners, believe that the cost-effectiveness will be achieved after the mass supply chains are established,” the researchers said. China has already initiated “new projects aimed at providing high temperature steam up to 500C and electricity to the petrochemical industry in China,” they added. “The reactor modules for the commercial plants are designed to adhere to the same standardized design.”

—Sonal Patel is a POWER senior editor (@sonalcpatel, @POWERmagazine).