This volume of the journal contains the three issues for 2024 combined into one. The seven articles included here cover three themes: nuclear warheads, nuclear reactors, and missiles, all of which are enduring and central concerns of nuclear disarmament science.
The first article is "Uniqueness, Reproducibility and Discrimination of Nuclear Warhead Gamma Signatures" by Christopher Fichtlscherer and Moritz Kütt. It explores a key question that underlies verification based on measurements of radiation signatures of fissile material components in nuclear weapons--are such signatures unique or can they be imitated sufficiently well by using alternative materials to undermine confidence in such measurements?
The analysis presents various radiation emission models--a 1 gram plutonium source, a roughly 3 kg weapon-grade plutonium pit, and a generic single-stage plutonium implosion nuclear weapon (based on the Frolov-Prilutskii-Sagdeev Model published in this journal in 1990: Steve Fetter, Valery A. Frolov, Marvin Miller, Robert Mozley, Oleg F. Prilutsky, Stanislav N. Rodionov, Roald Z. Sagdeev, "Detecting nuclear warheads," Science & Global Security, 1, no. 3-4, 1990). It also examines two thermonuclear (two-stage) nuclear warhead models similar to the U.S. W78 and W87 nuclear warheads (with and without their respective reentry vehicles). These emission signatures are then combined with models of warhead radiation measurement systems and compared with various possible substitutions involving replacing fissile materials in the warhead models with reactor-grade plutonium, combinations of different gamma emitting isotopes, and combinations of neutron sources with radioactive isotopes. The results suggest not all radiation signatures are unique and counterfeiting some signatures may be possible for detection systems that rely on broad gamma spectra. It is suggested that measuring the gamma spectrum at a higher resolution through the use of more energy bins may offer a solution.
There are three articles on nuclear reactors, spanning issues of modeling nuclear fuel cycles and plutonium production, the use of light-water reactors for plutonium and tritium production for weapons, and detecting plutonium production reactors at distance.
"Assessing Pakistan's Fissile Material Production" by Max Schalz, Erik Branger, Malte Göttsche, Sophie Grape, and Cecilia Gustavsson explores the significance of a possible natural uranium shortage constraining fissile material production for weapons in that country. This topic was last explored in this journal 15 years ago (Zia Mian, A. H. Nayyar, and R. Rajaraman, "Exploring Uranium Resource Constraints on Fissile Material Production in Pakistan," Science & Global Security, 17, no. 2, 2009). The article uses the Cyclus open-source fuel cycle code coupled with a quasi-Monte Carlo model to handle uncertainty propagation (Bicylcus) to model three possible fuel cycles for Pakistan to optimize fissile material (plutonium and highly enriched uranium, HEU, production) from the same initial uranium stock. It considers (i) a once-through model for the plutonium production reactors and enrichment program using natural uranium as fuel and feedstock; (ii) a model in which reprocessed uranium from production reactor spent fuel may be used as feedstock for the enrichment facility; and, (iii) a scenario in which the reprocessed uranium from the plutonium production reactors is blended with enriched uranium to obtain a natural uranium-equivalent mixture to be used as reactor fuel, while HEU production uses only natural uranium.
The results suggest that depending on the scenario by the end of 2022 Pakistan may have produced a plutonium stockpile of 370-660 kg and a HEU stockpile of 3-5.5 tons. These estimates are broadly consistent with those by the International Panel on Fissile Materials. The article also notes that if Pakistan produces tritium for nuclear weapons (to boost the yield of the fission component) at its plutonium production reactors this would affect the estimated rates of both uranium consumption and plutonium production.
The co-production of plutonium and tritium for weapons is taken up in the article "Estimating Potential Tritium and Plutonium Production in North Korea's Experimental Light Water Reactor" by Patrick Park and Alexander Glaser. It provides a detailed MCNP model for North Korea's 100 MWth Experimental Light Water Reactor (ELWR) which began operating in October 2023 and may be used for producing energy, tritium, and tritium plus plutonium. It seeks to assess if North Korea could produce sufficient tritium to support its arsenal, currently estimated to be on the order of 50 weapons. The tritium can provide the material needed for additional weapons or to replace depleted tritium in existing warheads (tritium decays at a rate of about 5.5% per year.)
The results suggest that for the case where the reactor core (initially enriched to 3.5% uranium-235) is discharged and reloaded after each cycle, North Korea's ELWR could produce each year about 48-82 grams of tritium. Assuming 10 grams of tritium per warhead, this production could supply 2-4 new boosted warheads each year, up to a maximum arsenal of 88-150 warheads. Production of weapon-grade plutonium in parallel with tritium production would require North Korea being able to reprocess ceramic fuels. If it could do so, the ELWR could produce up to 56-116 grams of tritium per year together with 15 kg of weapon-grade plutonium per year. The analysis finds that the ELWR fuel cladding material is a key factor in determining tritium production rates (the net tritium production rate for zircaloy cladding is about twice that for stainless steel) and the natural uranium demand and separative work for producing the enriched fuel for the reactor. In sum, as a light-water reactor using enriched fuel, the facility is not particularly suited for the production of weapon-grade plutonium unless more complex fueling and reprocessing schemes are pursued.
The third article on nuclear reactors is "Distances and detector sizes for neutrino monitoring: A survey of 64 historical plutonium production reactors" by Bernadette K. Cogswell, Patrick Huber and Rachel Carr. It provides a valuable new data set of nuclear reactors that have been used for weapon plutonium production, covering nine countries and 70 years of nuclear weapon programs. For each reactor, the article provides an estimate using satellite imagery of the distance from the reactor core to the facility boundary and to the nearest international border to offer possible standoff distances for locating a neutrino detector to monitor the respective reactor. For both distances, it provides an estimated neutrino detector size for monitoring each reactor, under the constraint that the detector be able to detect (at 95% confidence level) the neutrino flux from fissions within the reactor within the time it takes that reactor to produce 8 kg of plutonium--this is the International Atomic Energy Agency criterion for the amount of material needed to produce one first generation nuclear weapon.
The model allows for signal detection efficiency and neutrino backgrounds, including from other reactors.
The results suggest that a neutrino detector of less than one ton to a few hundred tons located at the reactor facility fence distance could monitor the reactor. Such a location would require cooperation by the host. For non-cooperative monitoring, where the detector must be at a national border distance, a detector would need to be above 1000 tons and in some cases 100,000 tons and possibly sited deep underground to shield against the neutrino background.
The final three articles in this volume cover missile issues, including the use of long-range precision-guided conventional missiles to attack intercontinental ballistic missiles in hardened silos, the viability of missile defenses in Europe against intermediate-range missiles, and the capabilities of hypersonic cruise missiles which are still under-development. "Assessing the lethality of conventional weapons against strategic missile silos in the United States, Russia, and China" by Ryan Snyder (Free PDF) offers a method for assessing the capability of conventional weapons to destroy strategic missiles in silos (underground vertical hollow reinforced concrete cylinders) in the United States, Russia, and China. It relies on a physical model for estimating the ground motions created by nuclear surface bursts and by earth-penetrating conventional explosions to calculate the maximum distance at which a silo-based missile could be destroyed by a conventional detonation. The analysis takes into account the presence of rattlespace (the gap between the missile's body and the silo wall) and a missile shock isolation system (shock absorbers) in the silo to dampen external shocks.
The model was applied to two current U.S. cruise missiles (the Tomahawk and the Joint Air-to Surface Standoff Missile) with earth-penetrating capabilities and ranges sufficient to reach strategic missile silos in Russia and China from launch sites in Europe, Asia, and off-shore. The circular error probable accuracy of these precision-guided munitions is less than 3 meters. On the assumption that conventional precision-guided missiles with similar capabilities may become available to Russia, China, and others, the model also was applied to determine the possible vulnerability of U.S. strategic missile silos. The results suggest that cruise missiles with accuracies of 2-3 meters may be as potentially lethal as U.S. strategic nuclear warheads against strategic missile silos. The article concludes that "long-range conventional weapons may be substituted for the silo targeting roles of nuclear weapons, allowing strategic counterforce capabilities to remain unaffected with far fewer deployed." It suggests further that conventional weapons must now be considered as a critical factor in assessments of the survivability of nuclear weapons, strategic stability, and in setting nuclear force levels.
"Analyzing the Utility of Arrow 3 for European Missile Defense Using Footprint Calculations" by Timur Kadyshev and Moritz Kütt takes up the challenge of estimating the capabilities of missile defense systems. It introduces a new open-source code (now available on GitHub) for calculating the trajectories of missiles and missile interceptors, and the location, size and shape of the areas that could in principle be protected by a missile defense system. These footprints depend on the properties of the attacking missiles, radar detection system, and the missile interceptor capabilities.
The new modeling program is applied to the Arrow missile defense system purchased by Germany. The article uses public information to establish the possible technical parameters of the Arrow system, in particular the Arrow 3 interceptor. These estimates are then used to assess the Arrow system's potential to defend German territory against possible missile threats from Russia. The results suggest that the Arrow 3 system purchased by Germany is unlikely to defend successfully against existing Russian short-range and medium-range missile systems capable of targeting Germany. Arrow 3 also may fail against Russian long range strategic missiles since these missiles carry decoys and other countermeasures against missile defense systems, and Russia could simply launch enough warheads to overwhelm Germany's limited number of Arrow interceptors. More generally, the new code opens the door to a much more inclusive, transparent, and technically informed debate about the utility and limits of missile defense systems around the world.
The final article in this volume is "Hypersonic Cruise Missiles" by David Wright and Cameron Tracy. It analyzes hypersonic cruise missiles; these are systems intended for powered flight at speeds faster than Mach 5 at relatively low altitudes and are under development by the United States, Russia, and China. They are an alternative to both hypersonic boost-glide vehicles and maneuverable reentry vehicles, that do not have powered flight. It models the US X-51A hypersonic cruise missile, which was designed to be launched from an aircraft and was flight tested over a decade ago. This analysis is then used to estimate the possible military capabilities of such systems, assuming capabilities that plausibly go beyond those of the X-51A but still allow it to be air-launched, looking in particular at mass, range, flight time, and maneuverability. These are key criteria for assessing the capabilities and desirability of such systems. The results suggest hypersonic cruise missiles have some advantages and some disadvantages compared to boost glide vehicles and maneuverable reentry vehicles. The analysis concludes that overall hypersonic cruise missile "do not appear to offer general advantages for either strike or surveillance missions compared to alternative systems."
