How a high-temperature reactor (HTGR) works

Inside a 25-meter-tall steel vessel, a graphite cylinder 3 meters in diameter and 11 tall holds about 420,000 tennis-ball-sized fuel pebbles. There are no rods and no assemblies: the core is a heap of graphite balls resting on one another and creeping down very slowly, by gravity, toward the discharge cone at the bottom.

The graphite of the reflector and of the pebbles themselves moderates the neutrons. The control rods do not enter the bed: they slide in channels of the side reflector, and as backup there are other channels into which small boron carbide absorber spheres drop. The HTR-PM has two of these reactors, of 250 MW thermal each.

The coolant is helium at 70 bar: an inert gas, transparent to neutrons, that neither becomes activated nor changes phase. It enters at the top of the vessel at 250 °C, flows down through the gaps between the pebbles and leaves at the bottom at 750 °C, more than twice the temperature of the water in a PWR. A circulator of about 4 MW moves it at 96 kg/s in each module.

That temperature is the design's reason for being: it raises steam at 567 °C, like a modern coal plant, with 42% efficiency, and opens the door to industrial heat for hydrogen or petrochemicals. The hot helium travels through the inner pipe of a coaxial duct and the cold helium returns through the outer annulus, so the duct wall never sees 750 °C.

Each 6 cm pebble is graphite with about 12,000 TRISO particles dispersed in its central zone, wrapped in 5 mm of fuel-free graphite. Each particle is a half-millimeter uranium oxide kernel, enriched to 8.5%, covered by four layers: a porous carbon buffer that holds the fission gases, dense pyrolytic carbon, silicon carbide and pyrolytic carbon again.

A pebble holds barely 7 grams of uranium. The silicon carbide layer is the real pressure vessel: it retains fission products up to about 1,600 °C, far above any temperature the reactor can reach. It is the same technology as almost every advanced gas-cooled reactor, and of some molten-salt-cooled ones too.

Unlike a PWR, the steam generator does not sit around the vessel but has its own steel shell, beside the reactor and somewhat lower, joined by the coaxial duct. Helium enters at the center, flows down around a bundle of helical tubes and leaves cooled to 250 °C toward the circulator, mounted on the dome of the generator.

Inside the tubes the water enters at 205 °C and, in a single pass, with no recirculation or separators, leaves as superheated steam at 567 °C and 13.3 MPa. The two HTR-PM modules send their steam to a common header feeding a single 210 MWe turbine, smaller than that of any conventional nuclear plant and similar to a fossil one.

The reactor never stops to refuel. Pebbles leave one by one through the bottom cone, about 6,000 per day in each module, and a gamma-ray detector measures the burnup of each: if it still has usable uranium, it returns to the top of the core through a pneumatic tube driven by helium; if it has reached about 90 GWd per tonne, it is diverted to the spent fuel casks.

Each pebble passes through the core about fifteen times in three years. That way there is no excess reactivity to compensate with boron or poisons, burnup is very uniform and the plant has no refueling outages: it is enough to add about 400 fresh pebbles per day.

The power density of the bed is 3 MW per cubic meter, thirty times lower than in a PWR, and the enormous mass of graphite takes days to heat up. If the helium is lost and every system fails, decay heat leaves the vessel by conduction and radiation toward water panels on the cavity wall, circulating by natural convection. The fuel stays below 1,620 °C, under the silicon carbide limit: the reactor cannot melt, and needs neither pumps nor electricity to prove it.

The pioneers were Germany's AVR of 1967 and the THTR-300, closed in 1989, along with the prismatic Peach Bottom and Fort St. Vrain in the United States. Today the Japanese HTTR, experimental and with helium at 950 °C, and the Tsinghua-Huaneng HTR-PM at Shidaowan, in commercial operation since December 2023, are running. China is planning a six-module HTR-PM600, X-energy plans the pebble-bed Xe-100 in Texas, and other companies are adapting TRISO particles to prismatic designs and molten salt reactors.