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Assessing the Safety, Security, and Environmental Impacts of Non-Light-Water Nuclear Reactors, Union of Concerned Scientists, Edwin Lyman,  Mar 18, 2021

“Advanced” Isn’t Always Better

”………………..Assessments of NLWR Types

UCS has reviewed hundreds of documents in the available literature to assess the comparative risks and benefits of the three major categories of NLWR with respect to the three evaluation criteria (Table 2).

Sodium-Cooled Fast Reactors

Safety and Security Risk: SFRs have numerous safety problems that are not issues for LWRs. Sodium coolant can burn if exposed to air or water, and an SFR can experience rapid power increases that may be hard to control. It is even possible that an SFR core could explode like a small nuclear bomb under severe accident conditions. Of particular concern is the potential for a runaway power excursion: if the fuel overheats and the sodium coolant boils, an SFR’s power will typically increase rapidly rather than decrease, resulting in a positive feedback loop that could cause core damage if not quickly controlled.

Chernobyl Unit 4 in the former Soviet Union, although not a fast reactor, had a similar design flaw—known as a “positive void coefficient.” It was a major reason for the reactor’s catastrophic explosion in 1986. A positive void coefficient is decidedly not a passive safety feature—and it cannot be fully eliminated by design in commercial-scale SFRs. To mitigate these and other risks, fast reactors should have additional engineered safety systems that LWRs do not need, which increases capital cost.

Sustainability: Because of the properties of fast neutrons, fast reactors do offer, in theory, the potential to be more sustainable than LWRs by either using uranium more efficiently or reducing the quantity of TRU elements present in the reactor and its fuel cycle. This is the only clear advantage of fast reactors compared with LWRs. However once-through fast reactors such as the Natrium being developed by TerraPower, a company founded and supported by Bill Gates, would be less uranium-efficient than LWRs. To significantly increase sustainability, most fast reactors would require spent fuel reprocessing and recycling, and the reactors and associated fuel cycle facilities would need to operate continuously at extremely high levels of performance for many hundreds or even thousands of years. Neither government nor industry can guarantee that future generations will continue to operate and replace these facilities indefinitely. The enormous capital investment needed today to build such a system would only result in minor sustainability benefits over a reasonable timeframe.

Nuclear Proliferation/Terrorism: Historically, fast reactors have required plutonium or HEU-based fuels, both of which could be readily used in nuclear weapons and therefore entail unacceptable risks of nuclear proliferation and nuclear terrorism. Some SFR concepts being developed today utilize HALEU instead of plutonium and could operate on a once-through cycle. These reactors would pose lower proliferation and security risks than would plutonium-fueled fast reactors with reprocessing, but they would have many of the same safety risks as other SFRs. And, as pointed out, most once-through SFRs would actually be less sustainable than LWRs and thus unable to realize the SFR’s main benefit. For this reason, these once-through SFRs are likely to be “gateway” reactors that would eventually transition to SFRs with reprocessing and recycling. The only exceptions—if technically feasible—are once-through fast reactors operating in breed-and-burn mode. However, the only breed-and-burn reactor that has undergone significant R&D, TerraPower’s “traveling-wave reactor,” was recently suspended after more than a decade of work, suggesting that its technical challenges proved too great.

High-Temperature Gas-Cooled Reactors

Safety and Security Risk: HTGRs have some attractive safety features but also a number of drawbacks. Their safety is rooted in the integrity of TRISO fuel, which has been designed to function at the high normal operating temperature of an HTGR (up to 800ºC) and can retain radioactive fission products up to about 1,600ºC if a loss-of-coolant accident occurs. However, if the fuel heats up above that temperature—as it could in the Xe-100—its release of fission products speeds up significantly. So, while TRISO has some safety benefits, the fuel is far from meltdown-proof, as some claim. Indeed, a recent TRISO fuel irradiation test in the Advanced Test Reactor in Idaho had to be terminated prematurely when the fuel began to release fission products at a rate high enough to challenge off-site radiation dose limits.

The performance of TRISO fuel also depends critically on the ability to consistently manufacture fuel to exacting specifications, which has not been demonstrated. HTGRs are also vulnerable to accidents in which air or water leaks into the reactor; this is much less of a concern for LWRs. And the moving fuel in pebble-bed HTGRs introduces novel safety issues.

Despite these unknowns, HTGRs are being designed without the conventional leak-tight containments that LWRs have—potentially cancelling out any inherent safety benefits provided by the design and fuel. Given the uncertainties, much more testing and analysis are necessary to determine conclusively if HTGRs would be significantly safer than LWRs.

Sustainability: HTGRs are less sustainable than LWRs overall. They use uranium no more efficiently due to their use of HALEU, and they generate a much larger volume of highly radioactive waste. Although pebble-bed HTGRs are somewhat more flexible and uranium-efficient than prismatic-block HTGRs, the difference is not enough to overcome the penalty from using HALEU fuel.

Nuclear Proliferation/Terrorism: HTGRs raise additional proliferation issues compared with LWRs. Current HTGR designs use HALEU, which poses a greater security risk than the LEU grade used by LWRs, and TRISO fuel fabrication is more challenging to monitor than LWR fuel fabrication. Also, it is difficult to accurately account for nuclear material at pebble-bed HTGRs because fuel is continually fed into and removed from the reactor as it operates. On the other hand, it may be more difficult for a proliferator to reprocess TRISO spent fuel than LWR spent fuel to extract fissile material because the required chemical processes are less mature.

Molten Salt-Fueled Reactors

Safety and Security Risk: MSR advocates point to the fact that this type of reactor cannot melt down—the fuel is already molten. However, this simplistic argument belies the fact that MSR fuels pose unique safety issues. Not only is the hot liquid fuel highly corrosive, but it is also difficult to model its complex behavior as it flows through a reactor system. If cooling is interrupted, the fuel can heat up and destroy an MSR in a matter of minutes. Perhaps the most serious safety flaw is that, in contrast to solid-fueled reactors, MSRs routinely release large quantities of gaseous fission products, which must be trapped and stored. Some released gases quickly decay into troublesome radionuclides such as cesium-137— the highly radioactive isotope that caused persistent and extensive environmental contamination following the Chernobyl and Fukushima nuclear accidents.

Sustainability: A main argument for MSRs is that they are more flexible and can operate more sustainably than reactors using solid fuel. In theory, some MSRs would be able to use natural resources more efficiently than LWRs and generate lower amounts of long-lived nuclear waste. However, the actual sustainability improvements for a range of thermal and fast MSR designs are too small, even with optimistic performance assumptions, to justify their high safety and security risks.

Nuclear Proliferation/Terrorism: MSRs present unique challenges for nuclear security because it would be very difficult to account for nuclear material accurately as the liquid fuel flows through the reactor. In addition, some designs require on-site, continuously operating fuel reprocessing plants that could provide additional pathways for diverting or stealing nuclear-weapon-usable materials.

MSRs could also endanger global nuclear security by interfering with the worldwide network of radionuclide monitors put into place to verify compliance with the Comprehensive Nuclear Test Ban Treaty after it enters into force.5 MSRs release vast quantities of the same radioactive xenon isotopes that are signatures of clandestine nuclear explosions—an issue that MSR developers do not appear to have addressed. It is unclear whether it would be feasible or affordable to trap and store these isotopes at MSRs to the degree necessary to avoid degrading the effectiveness of the monitoring system to detect treaty violations.

Safely Commercializing NLWRs: Timelines and Costs

Can NLWRs be deployed quickly enough to play a significant role in reducing carbon emissions and avoiding the worst effects of climate change? The 2018 special report of the UN’s Intergovernmental Panel on Climate Change identified 85 energy supply pathways to 2050 capable of achieving the Paris Agreement target of limiting global mean temperature rise to 1.5°C. The median capacity of nuclear power in 2050 across those pathways is about 150 percent over the 2020 level. Taking into account planned retirements, this corresponds to the equivalent of at least two dozen 1,000 MWe reactors coming online globally each year between now and 2050— five times the recent global rate of new LWR construction. If the world must wait decades for NLWRs to be commercially available, they would have to be built even faster to fill the gap by 2050.

Some developers of NLWRs say that they will be able to meet this challenge by deploying their reactors commercially as soon as the late 2020s. However, such aggressive timelines are inconsistent with the recent experience of new reactors such as the Westinghouse AP1000, an evolutionary LWR. Although the AP1000 has some novel features, its designers leveraged many decades of LWR operating data. Even so, it took more than 30 years of research, development, and construction before the first AP1000—the Sanmen Unit 1 reactor in China—began to produce power in 2018.

How, then, could less-mature NLWR reactors be commercialized so much faster than the AP1000? At a minimum, commercial deployment in the 2020s would require bypassing two developmental stages that are critical for assuring safety and reliability: the demonstration of prototype reactors at reduced scale and at full scale. Prototype reactors are typically needed for demonstrating performance and conducting safety and fuel testing to address knowledge gaps in new reactor designs. Prototypes also may have additional safety features and instrumentation not included in the basic design, as well as limits on operation that would not apply to commercial units.

By a 2017 report, the DOE asserted that SFRs and HTGRs were mature enough for commercial demonstrations without the need for additional prototype testing. For either of these types, the DOE estimated it would cost approximately $4 billion and take 13 to 15 years to complete a first commercial demonstration unit, assuming that reactor construction and startup testing take seven years. After five years of operating the demonstration unit, additional commercial units could follow in the mid-2030s.

In contrast, for MSRs and other lower-maturity designs, the DOE report judged that both reduced-scale and full-scale prototypes (which the report referred to as “engineering” and “performance” demonstrations, respectively) would be needed before a commercial demonstration reactor could be built. These additional stages could add $2 billion to $4 billion to the cost and 20 years to the development timeline. The subsequent commercial demonstration would not begin until 2040; reactors would not be available for sale until the mid-2040s or even the 2050s.

In May 2020, after receiving $160 million in initial congressional funding for the new Advanced Reactor Demonstration Program (ARDP), the DOE issued a solicitation for two “advanced” commercial demonstration reactors. In October 2020, the DOE chose SFR and HTGR designs—as one might expect given its 2017 technology assessment. The DOE estimates that these projects will cost up to $3.2 billion each (with the vendors contributing 50 percent) for the reactors and their supporting fuel facilities. The department is requiring that the reactors be operational within seven years, a timeline—including NRC licensing, construction, fuel production, and startup testing—that it acknowledges is very aggressive.

However, even if this deadline can be met and the reactors work reliably, subsequent commercial units likely would not be ordered before the early 2030s. Moreover, it is far from certain that the two designs the DOE selected for the ARDP are mature enough for commercial demonstration. Past demonstrations of both SFRs and HTGRs have encountered safety and reliability problems. Additionally, for both reactor types, the DOE has chosen designs that differ significantly from past demonstration reactors.

In the 1990s, the NRC concluded that it would require information from representative prototype testing prior to licensing either of these reactor types—but no prototypes were ever built. More recently, in a letter to the NRC, the agency’s independent Advisory Committee on Reactor Safeguards reaffirmed the importance of prototypes in new reactor development. Nevertheless, the NRC—a far weaker regulator today—has apparently changed its position and may proceed with licensing the ARDP demonstration reactors without requiring prototype testing first. But by skipping prototype testing and proceeding directly to commercial units, these projects may run not only the risk of experincing unanticipated reliability problems, but also the risk of suffering serious accidents that could endanger public health and safety.

An additional challenge for NLWR demonstrations and subsequent commercial deployment is the availability of fuels for those reactors, which would differ significantly from the fuel that today’s LWRs use. Even a single small reactor could require a few tons of HALEU per year—far more than the 900 kilograms per year projected to be available over the next several years from a DOE-funded pilot enrichment plant that Centrus Energy Corporation is building in Piketon, Ohio. It is far from clear whether that pilot will succeed and can be scaled up in time to support the two NLWR demonstrations by 2027, not to mention the numerous other HALEU-fueled reactor projects that have been proposed……. https://ucsusa.org/resources/advanced-isnt-always-better#read-online-content