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To help inform policy decisions on these questions, the Union of Concerned Scientists (UCS) has evaluated certain claims about the principal types of NLWRs. In particular, this report compares several classes of NLWRs to LWRs with regard to safety and security, the risks of nuclear proliferation and nuclear terrorism, and “sustainability”—a term that in this context includes the often-claimed ability of some NLWRs to “recycle” nuclear waste and use mined uranium more efficiently. The report also considers the potential for certain NLWRs to operate in a once-through, “breed-and-burn” mode that would, in theory, make them more uranium-efficient without the need to recycle nuclear waste—a dangerous process that has significant nuclear proliferation and terrorism risks.

Non-Light-Water Reactor Technologies

UCS considered these principal classes of NLWRs:

Sodium-cooled fast reactors (SFRs): These reactors are known as “fast reactors” because, unlike LWRs or other reactors that use lower-energy (or “thermal”) neutrons, the liquid sodium coolant does not moderate (slow down) the high-energy (or “fast”) neutrons produced when nuclear fuel undergoes fission. The characteristics and design features of these reactors differ significantly from those of LWRs, stemming from the properties of fast neutrons and the chemical nature of liquid sodium.

High-temperature gas–cooled reactors (HTGRs): These reactors are cooled by a pressurized gas such as helium and operate at temperatures up to 800ºC, compared with around 300ºC for LWRs. HTGR designers developed a special fuel called TRISO (tristructural isotropic) to withstand this high operating temperature. HTGRs typically contain graphite as a moderator to slow down neutrons. There are two main variants of HTGR. A prismatic-block HTGR uses conventional nuclear fuel elements that are stationary; in a pebble-bed HTGR, moving fuel elements circulate continuously through the reactor core.

Molten salt–fueled reactors (MSRs): In contrast to conventional reactors that use fuel in a solid form, these use liquid fuel dissolved in a molten salt at a temperature of at least 650ºC. The fuel, which is pumped through the reactor, also serves as the coolant. MSRs can be either thermal reactors that use a moderator such as graphite or fast reactors without a moderator. All MSRs chemically treat the fuel to varying extents while the reactor operates to remove radio-active isotopes that affect reactor performance. Therefore, unlike other reactors, MSRs generally require on-site chemical plants to process their fuel. MSRs also need elaborate systems to capture and treat large volumes of highly radioactive gaseous byproducts.

The Fuels for Non-Light-Water Reactors

Today’s LWRs use uranium-based nuclear fuel containing less than 5 percent of the isotope uranium-235. This fuel is produced from natural (mined) uranium, which has a uranium-235 content of less than 1 percent, in a complex industrial process called uranium enrichment. Fuel enriched to less than 20 percent U-235 is called “low-enriched uranium” (LEU). Experts consider it a far less attractive material for nuclear weapons development than “highly enriched uranium” (HEU), with a U-235 content of at least 20 percent.

The fuel for most NLWRs differs from that of LWRs. . Some proposed NLWRs would use LEU enriched to between 10 and 20 percent uranium-235; this is known as “high-assay low enriched uranium” (HALEU).2 While HALEU is considered impractical for direct use in a nuclear weapon, it is more attractive for nuclear weapons development than the LEU used in LWRs. Other types of NLWRs would use plutonium separated from spent nuclear fuel through a chemical process called reprocessing. Still others would utilize the isotope uranium-233 obtained by irradiating the element thorium. Both plutonium and uranium-233 are highly attractive for use in nuclear weapons.

Typically, the chemical forms of NLWR fuels also differ from those of conventional LWR fuel, which is a ceramic material composed of uranium oxide. Fast reactors can use oxides, but they can also use fuels made of metal alloys or chemical compounds such as nitrides. The TRISO fuel in HTGRs consists of tiny kernels of uranium oxide (or other uranium compounds) surrounded by several layers of carbon-based materials. MSR fuels are complex mixtures of fluoride or chloride salt compounds.

The deployment of NLWRs also would require new industrial facilities and other infrastructure to produce and transport their different types of fuel, as well as to manage spent fuel and other nuclear wastes. These facilities may use new technologies that themselves would require significant R&D. They also may present different risks related to safety, security, and nuclear proliferation than do LWR fuel cycle facilities—important considerations for evaluating the whole system.

Non-Light-Water Reactors: Past and Present

In the mid-20th century, the Atomic Energy Commission (AEC)—the predecessor of today’s Department of Energy (DOE) and the NRC—devoted considerable time and resources to developing a variety of NLWR technologies, supporting demonstration plants at various scales at sites around the United States. Owners of several of these reactors abandoned them after the reactors experienced operational problems (for example, the Fort St. Vrain HTGR in Colorado) or even serious accidents (the Fermi-1 SFR in Michigan).

Despite these negative experiences, the DOE continued R&D on various types of NLWR and their fuel cycles. In the 1990s, the DOE initiated the Generation IV program, with the goal of “developing and demonstrating advanced nuclear energy systems that meet future needs for safe, sustainable, environmentally responsible, economical, proliferation-resistant, and physically secure energy.” Although Generation IV identified six families of advanced reactor technology, the DOE has given most of its subsequent support to SFRs and HTGRs.

Today, a number of NLWR projects at various stages of development are under way, funded by both public and private sources (Table 1). With support from Congress, the DOE is pursuing several new NLWR test and demonstration reactors. It is proceeding with the design and construction of the Versatile Test Reactor (VTR), an SFR that it hopes to begin operating in the 2026–2031 timeframe. The VTR would not generate electricity but would be used to test fuels and materials for developing other reactors. In October 2020, the DOE selected two NLWR designs for demonstrating commercial power generation by 2027: the Xe-100, a small pebble-bed HTGR that would generate about 76 megawatts of electricity (MWe), and the 345 MWe Natrium, an SFR that is essentially a larger version of the VTR with a power production unit. The DOE is also providing funding for two smaller-scale projects to demonstrate molten salt technologies. In addition, the DOE, the Department of Defense (DOD), and a private company, Oklo, Inc., are pursuing demonstrations of so-called micro-reactors—very small NLWRs with capacities from 1 MWe to 20 MWe—and project that these will begin operating in the next few years. A number of universities also have expressed interest in building small NLWRs for research.

Congress would need to provide sufficient and sustained funding for any of these projects to come to fruition. This is far from assured—for example, funding for the VTR to date has fallen far short of what the DOE has requested, all but guaranteeing the project will be delayed.

The Goals of New Reactor Development

If nuclear power is to play an expanded global role to help mitigate climate change, new reactor designs should be demonstrably safer and more secure—and more economical—than the existing reactor fleet. Today’s LWRs remain far too vulnerable to Fukushima-like accidents, and the uranium enrichment plants that provide their LEU fuel can be misused to produce HEU for nuclear weapons. However, developing new designs that are clearly superior to LWRs overall is a formidable challenge, as improvements in one respect can create or exacerbate problems in others. For example, increasing the physical size of a reactor core while keeping its power generation rate constant could make the reactor easier to cool in an accident, but it could also increase cost.

Moreover, the problems of nuclear power cannot be fixed through better reactor design alone. Also critical is the regulatory framework governing the licensing, construction, and operation of nuclear plants and their associated fuel cycle infrastructure. Inadequate licensing standards and oversight activities can compromise the safety of improved designs. A key consideration is the extent to which regulators require extra levels of safety—known as “defense-in-depth”—to compensate for uncertainties in new reactor designs for which there is little or no operating experience………

This is a condensed, online version of the executive summary. For all figures, references, and the full text, please download the PDF.         https://ucsusa.org/resources/advanced-isnt-always-better#read-online-content