Gas-cooled reactors are the second most common reactor technology used for commercial power application, due largely to the several carbon dioxide-cooled reactors deployed in the United Kingdom. All gas-cooled SMRs under development today use helium as the primary coolant. Both Germany and the United States previously built and operated helium-cooled test or demonstration reactors, and China and Japan each currently have small helium-cooled test reactors in operation. The key advantage of helium-cooled reactors is that the reactor can operate at much higher temperatures using a single-phase coolant, which is much simpler to manage. Typical gas-cooled reactors operate with an outlet temperature in the range of 700-800 °C, compared to 300-325 °C for LWRs. The advantage of the higher temperature is a higher efficiency conversion of the core heat to electricity and the ability to support a much broader range of industrial heat applications. The key drawback to gas-cooled reactors is that gases have a much lower heat capacity than liquids; therefore, the gas must be pumped at high velocity to remove the core heat. A related consequence is that the temperature differential across the core is very large, typically 500 °C, compared to 25-50 °C for a pressurized LWR. This temperature differential creates material challenges within the core and the secondary side of the plant.

The helium-cooled SMR designs under development generally fall into either a pebble bed or a prismatic configuration. In the case of the pebble bed, the fuel is dispersed in spheres about the size of a billiards ball. These spherical fuel elements stochastically migrate through the core and are continually removed and reinserted into the core for additional burnup. The prismatic configuration uses a more traditional rod geometry for the fuel and the rods are contained within monolithic blocks of graphite that are stacked to form the reactor core. In the case of prismatic configurations, the core is refueled in batch mode similar to fuel assembly-based LWRs.

Table 2.3 lists the four gas-cooled reactor SMR designs that currently have significant commercial support.

1.4.1 People’s Republic of China: HTR-PM design

The High Temperature Reactor Pebble-bed Module (HTR-PM) is a pebble-bed-type high-temperature, helium-cooled reactor. The fuel is UO2 enriched to 8.5% and contained in tristructural isotropic-type (TRISO) graphite-coated particles that are dispersed in 6 cm diameter graphite spheres. The 3 m diameter by 11 m tall core region represents a tall graphite ‘hopper’ containing 420 000 randomly packed spherical fuel

Figure 2.16 HTR-PM (China) — Institute of Nuclear and New Energy Technology (INET) © Institute of Nuclear and New Energy Technology (INET).

elements. The fuel elements migrate downward through the core as spheres are moved from the central discharge channel in the bottom reflector and optionally reinserted at the top of the core if maximum burnup has not been achieved. The graphite block reflector that defines the core region is contained within a 5.7 m diameter by 25 m tall steel pressure vessel. The helium coolant flows upward through the side reflector and then downward through the core region before flowing through a cross-duct to the helium/water steam generator contained in a separate steel pressure vessel. The steam generator is a once-through, counter-flow heat exchanger with multiple helical coil modules.

The HTR-PM is a successor to the HTR-10, a 10 MWt test reactor operated at the Tsinghua University. The HTR-10 was used to demonstrate the safety response of the HTR-PM, including its response to a loss of off-site power, a main helium blower failure, and a loss of main heat sink. The budget for construction of the first HTR-PM plant was approved in 2008 and the two-unit plant is being built in Rongcheng, Shandong Province, China. Construction was delayed after the destruction of the Fukushima Daiichi plant in Japan, but is now continuing with an expected completion in 2014. Key parameters and a representative graphic for the HTR-PM design are given in Figure 2.16. [14]