Calls for a world without nuclear weapons are common, even at the highest echelons of U.S. government. The United States unsuccessfully pitched this concept to the United Nations in 1946, and later served as a driving force behind international adoption of the 1968 Non-Proliferation Treaty (NPT), mandating the pursuit of nuclear disarmament. More recently, this call was echoed by President Barack Obama, as well as former Secretaries of State and Defense.
Despite these lofty ambitions, progress has remained modest. Under a series of bilateral treaties, culminating in the 2010 New Strategic Arms Reduction Treaty (New START), the United States and Russia have removed from deployment thousands of nuclear warheads, delivery vehicles, and launchers. However, these efforts have so far addressed only the low-hanging fruit of assembled weapons systems. While straightforward, this approach leaves intact the nuclear material that comprises the fundamental building block of a nuclear weapon. Elimination of this material is challenging. Thus, as disarmament proceeds from warheads and their accoutrements to these basic material ingredients, it will become more difficult.
While such concerns may appear distant, their implications are key to current policy debates. Dissatisfied with the slow pace of disarmament, last year 122 United Nations member states voted in favor of a Treaty on the Prohibition of Nuclear Weapons (TPNW), vexing American policymakers who consider its requirement of rapid, complete disarmament to be unrealistic. Opponents of this accelerated approach posit the difficulty of verification, the persistence of weapons design know-how, and the stabilizing effects of nuclear weaponry, but as the fundamental physical embodiment of nuclear armament, weapons material elimination stands at the crux of the matter. Both proponents of accelerated disarmament and those in favor of the conventional incremental approach would be wise to consider the challenges, feasibility, and permanence of the elimination of nuclear weapons materials.
Disposal, destruction, and disarmament
The most substantial steps towards global nuclear disarmament taken to date involve the dismantlement of assembled nuclear warheads. These consist, at their most basic level, of a package of conventional explosives that rapidly compress a mass of exotic material, so as to induce a nuclear chain reaction. Two solids, known as fissile materials, are used for this purpose: mined uranium that has been laboriously treated so as to enrich it in a specific isotope (highly enriched uranium or HEU), and plutonium that is synthesized during the irradiation of uranium-bearing fuel in a nuclear reactor. Warhead dismantlement involves separation of the components of these weapons (conventional explosives, fissile materials, etc.) and, in order to preclude reassembly, their subsequent destruction.
While conventional explosives, electronics, and the like are easily destroyed, fissile materials are another matter. Physical defacement has little effect on their potential for weaponization, as the expensive, time-consuming process of their synthesis is the limiting factor on weapons production, dwarfing the effort necessary to machine them into weapons components. They are composed of only a single chemical element (uranium or plutonium), so chemical processing is similarly ineffective, as no such treatment can change one element into another. Thus, it is these fissile materials, the foundation of nuclear weapon production, that represent the ultimate obstacle to achieving lasting, global nuclear disarmament.
While this may appear intractable, the past several decades have seen substantial progress in the elimination of HEU stockpiles. This material is distinguished from readily-available, non-weaponizable natural uranium by its isotopic enrichment. Thus, it can be deweaponized by mixing it with large quantities of natural uranium. This process, known as downblending, renders it no more a threat than the uranium found abundantly in Earth’s crust and oceans.
Elimination of plutonium is a more daunting task. The plutonium produced in nuclear reactors needs no isotopic alteration for use in a weapon, and non-weaponizable isotopes exist in such small quantities as to preclude downblending. Faced with the near-insurmountable cost and complexity of nuclear methods that completely destroy plutonium by converting it into another chemical element, a 1994 assessment by the US National Academy of Sciences (NAS) instead recommended methods that render plutonium more difficult to isolate and process. The NAS was inspired in this endeavor by the large quantities of plutonium contained in civilian stocks of spent nuclear power plant fuel, which are protected from weaponization by their dilution in a soup of highly-radioactive elements.
The study ultimately recommended two strategies for disposal: conversion to nuclear fuel by mixing with uranium dioxide and irradiation in a nuclear reactor, or intimate mixing with preexisting radioactive waste. Both were pursued by the United States as part of the 2000 Plutonium Management and Disposition Agreement (PMDA), a US-Russian reciprocal stockpile reduction scheme. Mixing with radioactive waste was quickly abandoned as a cost saving measure and due to a lack of suitably large waste inventories. Efforts to convert plutonium to nuclear fuel continue, yet construction of the necessary conversion facilities has been plagued by decadal delays and severalfold increases in the projected cost. Regardless, neither approach would drastically reduce the size of plutonium inventories. They would merely make it less easily obtainable. Even irradiation as fuel in a nuclear reactor would eliminate only a portion of incorporated plutonium, while additional plutonium would be bred by the uranium with which it is mixed.
Having judged the fuel conversion approach economically infeasible, the United States’ current plans involve burial of plutonium in a geological repository, eschewing any radiation barrier to extraction. Recovery would be as simple as mining the fissile material. Advanced mining methods might even allow this to be accomplished clandestinely, making it difficult for the international community to detect and prevent nuclear rearmament. This approach would leave the state possessing the geological repository with a route to acquiring plutonium that is potentially faster, cheaper, and less conspicuous than the conventional reactor-based route.
A world without nuclear materials
While all of the stockpile reduction efforts described above could superficially foster a world without nuclear weapons, they would not reduce global nuclear risk to the level extant prior to the advent of nuclear weaponry, as none serve to irreversibly eliminate plutonium stockpiles. That said, there does exist one nuclear reactor-based method which could effectively destroy plutonium by converting it to a mixture of non-weaponizable elements without simultaneous production of additional plutonium. This approach mirrors the fuel conversion strategy described above, but substitutes the uranium dioxide dilutant with a more inert material that does not breed plutonium when irradiated. Irradiation of this inert matrix fuel (IMF) in an advanced nuclear reactor could consume roughly 80% of the plutonium it contained.  The remaining plutonium could afterwards be recycled into fresh IMF fuel for further irradiation.
Yet even this approach involves uncertainty and risk. IMFs and the advanced nuclear reactors in which they would work best are largely unproven technologies that would require long development periods and massive investment. The extensive transport and handling of plutonium necessitated by this approach would make it vulnerable to loss or theft. Most importantly, reliance on the long-term operation of a large fleet of nuclear reactors to irradiate IMFs sets a ceiling on the extent to which the permanence of disarmament can be guaranteed. As long as such reactors operate, they could be loaded with uranium-bearing fuel and used by the possessor state to produce new plutonium in a matter of months. The means of fissile material destruction and production are thus coupled. The act of permanently and irreversibly destroying the world’s plutonium stockpiles would simultaneously serve to sustain the capability of nuclear-armed states to reverse the steps they have taken towards nuclear disarmament.
Clearly, there is more to nuclear disarmament than a binary distinction between the existence of weapons and their absence. Rather, the degree of disarmament is a complex function of various choices that influence the time, expense, and risk associated with reconstitution of a dismantled arsenal. Due to the persistence of fissile materials, no disarmament path can realistically render the production of nuclear weapons as great a challenge as it was prior to nuclear armament. Those favoring disarmament must therefore decide what end state is acceptable. For example, is burial of plutonium in a geological repository superior or inferior to complete destruction via irradiation of IMFs? The former could be accomplished relatively quickly, yet leaves the fissile material vulnerable to recovery in a matter of weeks. The latter would eventually yield complete elimination of this material but would, in the interim, require the operation of many nuclear reactors, any of which could be used to produce new plutonium.
Because fissile material disposal becomes a major factor only at the tail-end of a hypothetical disarmament process, these issues may seem irrelevant in the present day. However, given the technologically-demanding nature of disposal, the need for sustained international cooperation, and the decadal timescales projected for even modest fissile material elimination efforts, those interested in the pursuit of a world without nuclear weapons should think carefully about precisely what such a world entails.
 Bernard Baruch, “The Baruch Plan,” presented to the United Nations Atomic Energy Commission, 14 June, 1946; Treaty on the Non-Proliferation of Nuclear Weapons, 1 July 1968
 See Barack Obama, “Remarks by President Barack Obama,” Prague, 5 April 2009, https://obamawhitehouse.archives.gov/the-press-office/remarks-president-barack-obama-prague-delivered; George P. Schultz, William J. Perry, Henry A. Kissenger, Sam Nunn, “A World Free of Nuclear Weapons,” The Wall Street Journal, 4 January 2007, https://www.wsj.com/articles/SB116787515251566636
 Treaty Between the United States of America and the Russian Federation on Measures for the Further Reduction and Limitation of Strategic Offensive Arms, 8 April 2010
 Treaty on the Prohibition of Nuclear Weapons, 20 September 2017
 US National Academy of Sciences, Management and Disposition of Excess Weapons Plutonium (National Academy Press: Washington, D.C., 1994)
 Agreement Between the Government of the United States of America and the Government of the Russian Federation Concerning the Management and Disposition of Plutonium Designated as no Longer Required for Defense Purposes and Related Cooperation, 1 September 2000
 Aerospace Corporation, Plutonium Disposition Study Options Independent Assessment Phase 1 Report, 13 April 2015
 Per F. Peterson, “Long-term safeguards for plutonium in geologic repositories,” Science & Global Security vol. 6, pp. 1-29; Per F. Peterson, “Issues for detecting undeclared post-closure excavation at geologic repositories,” Science & Global Security vol. 8, pp. 1-39
 US National Academy of Sciences, Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options (National Academy Press: Washington, D.C., 1995), p. 47