Constructing a high-temperature gas-cooled reactor
HTRs have great potential to help the world to decarbonize hard-to-abate sectors, but some areas still need to be fully addressed if they are to be broadly deployed
China’s High Temperature Gas Cooled Reactor - Pebble-bed Module (HTR-PM) demonstration plant at the Shidaowan plant in China’s Shandong province commenced operation at the end of 2021.
The “large-scale advanced pressurized water reactor and high-temperature gas-cooled reactor nuclear power plant” project to build the demonstration HTR-PM as well as the demonstration CAP1400 also at Shidaowan, was announced in January 2006 as one of 16 National Science and Technology Major Projects under the National Medium- and Long-Term Science and Technology Development Plan (2006-2020).
Construction of the HTR-PM began in December 2012. After almost 10 years construction and commissioning, the first of the unit’s twin reactors achieved criticality in September 2021 and the second reactor two months later, in November. The unit was connected to the grid on 20 December 2021.
The HTR-PM features two small reactors (each of 250 MWt) that drive a single 210 MWe steam turbine. It uses helium as coolant and graphite moderator. Each reactor is loaded with over 245,000 spherical fuel elements (‘pebbles’), each 60 mm in diameter and containing 7 g of fuel enriched to 8.5%. Helium at 250˚C enters from the bottom of the reactor and flows upwards in the side reflector channels to the top reflector level where it reverses direction and flows downwards through the pebble bed. Bypass flows are introduced into the fuel discharge tubes to cool the fuel elements there, and into the control rod channels. Helium is heated up in the active reactor core and then is mixed to the average outlet temperature of 750˚C and then flows to the steam generator.
Due to the presence of U-238, there is a very strong negative temperature coefficient of reactivity, which guarantees an inherently safe stabilization of the core temperature in case of power excursions.
The inherent safety features of the HTR-PM guarantees that under all conceivable accident scenarios the maximum fuel element temperature could never surpass the design limit temperature, even without the dedicated emergency systems. In the hypothetical case of an accident characterized by a total loss of coolant and active cooling, the core of the HTR-PM would not melt due to its low power density and geometry. The fuel temperature can never exceed 1600˚C in the HTR-PM. This ensures that accidents, such as core melting, or the release of radioactive fission products into the environment, cannot occur.
Interview
Lü Hua Quan, Chairman of the Nuclear Research Institute, Huaneng Company
What particular lessons and challenges were learned during construction and commissioning of the HTR-PM?
The HTR-PM is a project with a research attribute. There have been some challenges with the nuclear island systems and equipment, resulting in a long construction cycle and high construction cost. Resolving these will be necessary for the successful industrialization of high-temperature gas-cooled reactors.
Take the fuel handling system for example: the HTR-PM adopts a non-stop stacking and changing operation mode, and the fuel handling system must operate reliably. During commissioning there were many problems with the fuel handling system, resulting in delays. Design changes and optimizations will be made to the HTR-600 design to improve the reliability of the system.
Additionally, the experience gained will be applied to future construction to optimize equipment design and reduce project costs. The auxiliary system will be shared by multiple reactors to reduce the cost and complexity of the system. We will adopt modular design and construction methods to shorten the construction period. We will optimize the system and equipment safety classification according to the inherent safety of high-temperature reactors and we will provide technical support for the adjustment of emergency planning zone and planning restricted zone to improve the adaptability of the plant site.
What is the future development of high-temperature gas-cooled reactor (HTR) technology beyond power generation? Could we expect reactors dedicated solely to supplying process heat?
HTRs have the highest operating temperatures of all existing reactor types, and are also the only reactors that can provide very high-temperature process heat. In the near future, HTRs could be used as a new generation of advanced reactors and a supplement to China's nuclear power, for small and medium-sized modular nuclear power generating units.
With further research and development, HTRs could in the future provide a source of high-quality high-temperature process heat for various industries, in particular those that are required to limit their carbon emissions. This would increase the number of advantages of HTRs, which can be used not only for urban heating and the chemical industry, but also in coal gasification and liquefaction, seawater desalination, metallurgy, and in the production of energy resources including synthetic fuels, petrochemicals and hydrogen.
Under what conditions is there the potential for exporting HTRs?
HTRs have great potential to help the world to decarbonize hard-to-abate sectors, but some areas still need to be fully addressed if they are to be broadly deployed. These areas include advanced high-temperature materials, the regulatory framework, safeguards and waste management for new fuels, and economics.
Once addressed, HTRs can be widely used in the applications I mentioned above. The outstanding advantage of HTGR "going global" is that it is highly consistent with the market demands of countries and regions along the "the Belt and Road". HTGRs are particularly suitable for countries and regions where freshwater resources are scarce, such as Saudi Arabia, and can replace conventional energy in a wide range of industrial fields. Secondly, HTRs have outstanding advantages in adapting to the needs of different power grids. Most of the power grids of countries and regions along the "the Belt and Road" are not suited to nuclear power plants of more than 1000 MWe. Through multi-module combination, nuclear power units with installed capacities ranging from around 100 MWe to 1000 MWe can be built. These small modular reactors are particularly suitable for countries and regions with small and medium-sized power grids, or in locations where the supply is required close to the load centre.
