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     Current commercial nuclear power plants are massive, technically complicated, difficult for the public to understand. Recently there has been a lot of media about “advanced nuclear reactors”. Advanced nuclear is an ill-defined category that basically includes anything different from the commercial reactors operating now. Those all follow the same general formula. The subject of this post is an overview of advanced nuclear reactors.
     We begin our overview with a recap of the basics of nuclear power generation. Commercial nuclear power plants generate electricity via fission reactions, where atoms split apart, releasing energy as heat and radiation. Many neutrons are released during these splits, and they collide with other atoms and split them, creating a chain reaction.
     In nuclear power plants today, there are basically two absolutely critical pieces. The first critical part is the fuel, which is consumed to feeds the reactions. It is vital that the chain reactions happen in a controlled manner, or the system may suffer a nuclear meltdown. The second critical part of a nuclear plant is the cooling system, which keeps the whole reactor from overheating and causing problems. (There’s also the moderator and many other parts which will not be a part of our overview.)
     In the vast majority of nuclear power reactors on the electrical grid today, these two components follow the same general formula. The fuel is enriched uranium that’s packed into ceramic pellets, loaded into metal tubes, bundled into fuel assemblies and inserted into the reactor’s core. The cooling system pumps pressurized water around and through the core of the reactor to keep the temperature within operation bounds.
     But for a whole host of reasons, companies are beginning to work on making important changes to this tried-and-true formula. There are about seventy companies in the U.S. working on designs for advanced nuclear reactors. Six or seven are far enough along to be working with regulators, says Jessica Lovering. She is the cofounder and co-executive director of the Good Energy Collective which is a policy research organization that advocates for the use of nuclear energy.
     Many of these so-called advanced reactor technologies were invented and even demonstrated over 50 years ago. This happened before the nuclear industry converged on the standard water-cooled nuclear plant designs. However, now there’s renewed interest in getting alternative nuclear reactors up and running. New reactor designs could help improve safety, efficiency, and cost.
     Alternative coolants could improve on safety over water-based designs because they don’t always need to be kept at high pressures. Many coolants can also reach temperatures higher than current nuclear plant operating temperatures, which can allow reactors to run more efficiently.
     Molten salt is one leading contender for alternative coolants. It is used in designs from Kairos Power, Terrestrial Energy, and Moltex Energy. These designs can produce the same amount of power while using less fuel than conventional reactors. And they produce nuclear waste that is easier to manage.
     Other companies are looking at liquid metals, including sodium and lead, as coolants. There are a few sodium-cooled reactors operating today, mainly in Russia. That country is one of the pioneers in developing lead-cooled reactors. Reactors cooled by liquid metals share many of the potential safety benefits of molten-salt designs. Helium and other gases can also be used as a coolant for nuclear power reactors that require higher temperatures than water-cooled systems. X-energy is designing a high-temperature gas-cooled reactor that utilizes helium as a coolant.
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Part 3 of 3 Parts (Please read Parts 1 and 2 first)
     Few details are known publicly about the composition or agenda of the NCG or how it provides additional value relative to the constellation of existing deterrence consultative groups (including the DSC, EDSCG, and SCM). The S.K.’s hope is that it will serve as a kind of combined nuclear planning cell in peacetime and mechanism to consult on nuclear decisions in wartime. However, both of these alternatives are unlikely. The United States should carry out hypothetical nuclear planning with S.K. (to include target selection and attack plans, with approximate parameters). However, U.S. officials still carry an allergy to this kind of activity. Detailed planning might suffice to educate S.K.’s officials that nuclear weapons have significant limits in their ability to hold mobile, hardened, and urban targets at risk and cannot magically resolve any crisis. However, U.S. officials would hesitate to present this information on the grounds that it may be construed as a lack of resolve to use nuclear weapons. Proposals to develop a standing trilateral consultative mechanism with Japan have, unfortunately, failed to result in any concrete action.
     The U.S. commitment to consult with S.K. prior to any use of nuclear weapons is the least prominent, but the most promising, development. This step is qualitatively different to existing nuclear assurance mechanisms because it directly addresses S.K.’s concerns that it will not have input into a US president’s decision on nuclear weapon use. Though S.K. will never hold the launch keys to U.S. nuclear weapons, the opportunity to have input into the decision making is the first known adjustment to the nuclear authorization process in decades and distinctive among U.S. allies. U.S. officials should continue to emphasize and expand on the idea: to formalize it as a step in the nuclear authorization process; to establish a secure video, voice, and data channel to enable detailed consultations in a crisis; and to rehearse the link. Ideally, the activity would encourage the S.K.’s NSC and MND to begin their own planning process to compare nuclear and nonnuclear response options. These steps can also better prepare President Yoon for this kind of consultation.
     These are the twin failures of the Washington Declaration. First, to reassure S.K. citizens. Second, to reduce the relevance of nuclear weapons in the alliance. Nuclear assurance was doomed to fail. The summit is the swan song of nuclear assurance because there is little it could hope to accomplish without morphing into nuclear sharing, which is unlikely.
     Breaking the alliance’s addiction to nuclear weapons is politically and bureaucratically difficult but could be relatively straightforward. U.S. officials would renovate existing consultative deterrence mechanisms to ensure that they contain not only nuclear experts but also a range of types of U.S. officials from across the administration. They would emphasize U.S. and S.K.’s conventional capabilities, including in high-profile trips to the region. They would also be clear with S.K.’s public about the extremely limited role nuclear weapons play in their defense. At the same time, these moves would confront the nuclear zealots in S.K.

Ambient office = 104 nanosieverts per hour

Ambient outside = 93 nanosieverts per hour

Soil exposed to rain water = 94 nanosieverts per hour

Blueberry from Central Market = 103 nanosieverts per hour

Tap water = 86 nanosieverts per hour

Filter water = 73 nanosieverts per hour

Part 2 of 3 Parts (Please read Part 1 first)
     While U.S. officials value ambiguity in nuclear weapons policy to allow the president a range of options (and to avoid endless policy debates), the prevailing theory in S.K. is that this kind of ambiguity weakens deterrence and signals wavering commitment from the U.S. However, the U.S. would never make this commitment. The U.S. president reserves the right to decide on nuclear weapons deployment in a specific situation because they would like to consider an array of important factors specific to the crisis. They would almost certainly prefer to find an effective alternative to the use of nuclear weapons. They would also want to consider S.K.’s preferences which may disapprove of U.S. nuclear use on the peninsula, especially under another president. When he was inevitably rebuffed, S.K.’s President Yoon Suk Yeol apparently stated his preferred policy on behalf of the alliance at the White House. He wanted to bluff with U.S. nuclear weapons. Unfortunately, this was just the latest in a series of misstatements about alliance nuclear policy by President Yoon or senior members of his administration. Taken together, these actions signal increasingly severe friction in the alliance.
     Other plans range from forward deployment of U.S. nonstrategic nuclear weapons to the peninsula, offering S.K.’s officials a role in U.S. nuclear deployment planning, or the creation of a nuclear-sharing arrangement that may or may not resemble the one in NATO. The critical feature of all such proposals is that none of them would provide S.K.’s officials with additional information about when, where, and why the United States would use a nuclear weapon on the peninsula. None of these plans would dispel their anxieties.
     Toby Dalton is the co-director of the Nuclear Policy Program at the Carnegie Endowment for International Peace. He has presented a helpful taxonomy of the differing logics behind calls for additional nuclear integration within the alliance. Zealots believe that only a nuclear weapon can deter a nuclear weapon. They will never feel secure until a S.K.’s president of their own party has release authority over nuclear weapons. Bargainers may hold real anxieties over extended deterrence. However, they will exploit them in order to extract concessions from the U.S. Populists simply support nuclear proliferation or nuclear sharing as a kind of signal to appear hawkish on foreign policy. Zealots have been dominating the conversation. They have coopted the other groups to push for an indigenous nuclear weapons program or nuclear sharing. Few officials, in S.K. or in the U.S., have confronted this narrative.
     This fixation on nuclear weapons is to the detriment of other capabilities. This includes S.K.’s own exceptional conventional defense and deterrence posture. It is corrosive to the alliance. Further increasing the relevance of nuclear weapons cannot address anxieties, only fuel them. If U.S. officials cannot shift the alliance’s attention to nonnuclear deterrence and educate S.K.’s public opinion to support the shift, demand for nuclear sharing and/or nuclear proliferation will continue to increase.
     It was in this context that Yoon and U.S. President Joe Biden launched the Washington Declaration in April of 2023. This summit could be considered the swan song of nuclear assurance. It established a new Nuclear Consultative Group (NCG), expanded tabletop war games between the allies on nuclear weapons issues (including with U.S. Strategic Command), and announced a presidential commitment to make every effort to consult with S.K.’s president before using a nuclear weapon on the peninsula. This spring, S.K.’s officials visited the US SSBN base at King’s Bay and the SSBN USS Kentucky sailed to Busan.
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Ambient office = 114 nanosieverts per hour

Ambient outside = 159 nanosieverts per hour

Soil exposed to rain water = 159 nanosieverts per hour

Avocado from Central Market = 73 nanosieverts per hour

Tap water = 100 nanosieverts per hour

Filter water = 90 nanosieverts per hour

Part 1 of 3 Parts
     For the last twenty years, U.S. commitment to extend nuclear deterrence over South Korea (S.K.) has faced increasing pressure. During that time, seven North Korean (N.K.) nuclear tests and rapid advancements in a diverse array of advanced missile technologies have forced the alliance to adapt its posture and develop new capabilities and concepts to counter aggression and nuclear first use. For the most part, the alliance has risen to meet this challenge. Improvements in conventional capabilities have tipped the balance of power further south over the same period, providing additional capability not only to deny N.K. the benefits of aggression but also to deter and respond to nuclear use. This is especially true of S.K.’s stand-off strike systems.
     The alliance is meeting the military challenge. However, it is failing to manage the political challenge of extended deterrence. Over the last two decades, S.K.’s confidence in U.S. security guarantees has fallen sharply, despite this strong military position. Two trends have placed the alliance in a difficult situation. First, the U.S. commitment to defend S.K. with nuclear weapons has become cancerous, threatening to metastasize and infect other aspects of the alliance. Second, the Trump administration’s statements and actions forced S.K. to confront the problems of depending on an ally that is currently grappling with isolationist authoritarian tendencies.
     U.S. defense officials have considered the nuclear extended deterrence as a component of the framework of the nation’s alliance structure. The theory is that extended deterrence assures allies of their own security. This means that they need not consider developing their own nuclear arsenal. The problem is that, because the President of the United States retains sole authority over the use of nuclear weapons, no ally can be certain that the U.S. will use a nuclear weapon in and only in cases when they believe it is a necessity. On occasions when allies express anxiety about depending on U.S. extended nuclear deterrence, U.S. officials usually embark on a round of nuclear reassurance, displaying nuclear-capable systems and holding consultations designed to show allies that they are attentive and committed to their nuclear mission.
     Especially in S.K., nuclear assurance not only fails to address allied anxieties, but also fuels them. Existing nuclear assurance mechanisms can establish U.S. capability. However, it cannot prove the resolve of US leaders or provide S.K.’s officials with a reliable expectation about when, where, and why the United States might use a nuclear weapon on the Korean peninsula. Because there is nothing that U.S. officials can do to resolve either problem, extended nuclear deterrence is an unreliable foundation for the U.S-Korean alliance. Even worse, nuclear assurance mechanisms effectively raise the relevance of nuclear weapons, which contributes to a public and elite misperception that S.K.’s security depends on US nuclear weapons, even though this is less and less true as time passes because the alliance’s conventional capabilities continue to improve.
     S.K.’s officials and experts have presented a variety of proposals that they believe would assuage their concerns. The current idea is a request that the U.S. commit to use a nuclear weapon in response to N.K. nuclear first use, what we might call automaticity.
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