A land-based version of this SMR is known as RITM-200N and planned to be commissioned in Yakutia by 2028. The next SMR type is the molten salt reactor (MSR). A
crucial operational point lies in dispersing the fuel within the molten salt core. In this way, the core material itself can also be used as coolant. UF4
is the compatible chemical
form of uranium to be applied in this case. MSRs can be realised with a thermal as well as a fast neutron spectrum. The Danish developer Seaborg is designing a floating MSR with a capacity of 200 MWe per module. Their compact molten salt reactor (CMSR) has a thermal neutron spectrum and requires a moderator. The original design was based on HALEU and a sodium hydroxide moderator. To mitigate the timeline risk because of an insufficient HALEU supply, Seaborg changed their current design to more readily available low-enriched uranium (LEU) and accordingly changed the moderating material from sodium hydroxide to graphite. This example again stresses the need for a reliable HALEU supply chain. Seaborg is still willing to apply HALEU for future CMSR designs. Fuel based on metallic uranium (and various alloys) is
used for a subset of fast neutron SMRs. The most common reactor concept is the liquid metal-cooled fast reactor (LMFR). To maintain a fast neutron spectrum, the moderator has to be removed from the reactor design; therefore, water cooling is not an adequate option anymore. Using liquid metal as a coolant also allows higher operating temperatures and thus a higher thermal conversion efficiency. Compared to oxide fuel, metallic fuel has a better heat conductivity and a lower heat capacity. For example, the ARC-100 reactor, which is planned to be built in Canada, is fuelled by 13% enriched Uranium-Zirconium alloy. This sodium-cooled fast reactor (SFR) is based on the EBR-II (Experimental Breeder Reactor), which was operated for more than 30 years at the Argonne National Laboratory in the USA.
The high-temperature gas-cooled reactor (HTGR) is the
established design in the fourth SMR category. All high- temperature reactors (HTRs) utilise tristructural-isotropic (TRISO) particle fuel. TRISO particles usually consist of uranium dioxide (UO2
) or uranium oxycarbide (UCO) fuel
kernels, which are coated by chemical vapour deposition to produce TRISO-coated particles covered with four successive carbon and SiC-based layers. It is this durable and tightly specified fuel that largely guarantees the particularly high safety standard of HTRs. Near the end of 2023, China’s HTR-PM entered commercial operation and is therefore the first commercial high-temperature SMR. It is fuelled by 8.5% enriched spherical fuel elements (pebbles) containing about 12,000 TRISO particles each.
It is worth noting that besides gas-cooled HTRs, there
are also salt-cooled HTRs like the fluoride salt-cooled high-temperature reactor (FHR), for example the KP-FHR (140 MWe) promoted by Kairos Power based in California in the USA. Although it is salt-cooled, it has a typical solid HTR core and operates on TRISO fuel. Thus, it should not be confused with a MSR, where the core itself is in a liquid state.
Using experience as a fuel plant operator and designer
NUKEM developed TRISO spherical fuel elements in the 1960s which found their way to China and the HTR-PM. In the 1970s and 1980s, a TRISO fuel production plant was operated in Hanau, Germany and as part of a later cooperation agreement parts of the original fuel fabrication plant were shipped to Beijing, China, in 1995 and rebuilt at Tsinghua University. The laboratory production line at the Institute of Nuclear and New Energy Technology (INET) went into operation in 1998. Based on the experience gained from the INET plant, CNNC designed the fuel production plant of the HTR-PM. In addition to HTR TRISO fuel, uranium metal-based fuel
covers the remaining three SMR fuel types. The process for producing uranium metal-based fuels begins with AUC deconversion to UO2
and continues through UF4 to metallic
uranium. Since 1956, NUKEM has fabricated fuel assemblies from metallic uranium for the research reactor FR2 at Karlsruhe, Germany and the ECO reactor at Ispra, Italy. A superior AUC process, which has been well established for the mass production of UO2
Above left: Metallic uranium can be produced via calciothermic reduction of UF4
Above right:
HTR fuel elements are carbonised and annealed in two consecutive furnaces where they are significantly hardened
for standard light-water reactor
(LWR) fuel, was first introduced in 1978 at a new pilot plant. An industrial uranium metal AUC plant was designed and installed for BATAN, the national nuclear energy agency of Indonesia, and has supplied 20% enriched fuel elements for a Siemens material test reactor. A certain amount of uranium-bearing scrap – off-
specification material and the like – is normally generated at all stages of fuel element fabrication. Production processes should include the recycling of all of the uranium via dissolving, extractive purification and feeding the resulting uranyl nitrate (UN) solution back into the AUC process. This is done using the same equipment and ensures that no uranium is lost in the process. Design of industrial-scale fuel production plants with
throughputs of several tonnes of uranium per year for all common uranium-based SMR fuel types is possible. The prerequisite of a criticality-safe process keeping state-of- the-art radioprotection standards is available for HALEU material up to 20% enrichment. ■
www.neimagazine.com | May 2024 | 47
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