Summary
Against the backdrop of Russia’s threats to de-ratify the Comprehensive Test Ban Treaty in the coming weeks, Russia’s Burevestnik cruise missile, a nuclear weapon unlike any other insofar as it has been designed to be not only nuclear-armed but also nuclear-powered, has apparently undergone a new testing programme.
The idea of a nuclear-powered cruise missile is not new. The United States ran two open-loop propulsion reactor projects from 1956-1973: Project Pluto, which consisted of two reactors – Tory-IIA and Tory-IIC – for a supersonic nuclear ramjet; and Project Rover, which later became the NERVA program for propulsion in space, using hydrogen as the working fluid rather than atmospheric air. Tory was tested twice in the early 1960s, proving itself as a concept, and twenty reactors were built and tested for the NERVA/Rover program between 1959 and 1972. In both programs, the reactors were shown to degrade during use and produce radioactive particulate matter in the exhaust. Cheryl Rofer provides an excellent breakdown of those projects and their relation to Burevestnik here, and reading these is highly recommended for those that want to dig a little deeper into this issue.
In 2021, analysis from the Middlebury Institute of International Studies showed activity at the Novaya Zemlya site that indicated a test of the Russian Federation’s Burevestnik nuclear-powered cruise missile programme. A subsequent test in autumn 2023 has reignited international interest in this subject, and with all the excellent reporting already done on this activity, BASIC has produced this briefer to open up a speculative look at what the secretive missile is, how (and if) it works, and why even a successful test would be disastrous. We then take a look at the wider, strategic issues and try to answer a frankly puzzling question – why build such a thing?
What happened at Nyonoksa in 2019?
In 2019, a number of Rosatom engineers and scientists were killed in an accident recovering a Burevestnik missile that crashed off the coast of Nyonoksa in the Barents Sea. This missile had apparently been launched in 2017, with the recovery waiting two years, presumably for the seawater to cool the reactor and contain the high energy radiation from the core as it very slowly shut down. Civil nuclear facilities do something similar with spent fuel rods at nuclear reactors, depositing them in cooling ponds to allow the reactivity of the fuel to slowly drop away to manageable levels. Water transports the heat away from the “hot” rods, which is vitally important as temperature has an effect on the intensity of reactivity: sometimes positive, sometimes negative. Nuclear materials are extremely sensitive to their environments.
Without knowing the technical details of the missile-booster system it’s difficult to speculate on what caused the incident, but non-nuclear explosions can happen in fission reactors; the Chernobyl and Fukushima incidents are testament to that. It may be that seawater that entered the reactor corroded metallic components in the presence of an overheating, intensively radioactive reactor, generating significant trapped volumes of hydrogen, which could have ignited as it came in contact with the air. Or, it’s possible the booster stage was still attached and residual liquid propellant caused the explosion, damaging the reactor core and releasing the radiological material. This second option would fit with the Russian statements alluding to an ‘isotope power source for a liquid-fuelled rocket engine’. This assumes that ‘isotope power source’ here is a euphemism for a fission reactor, as radioisotope power sources are nowhere near powerful enough to keep a missile in the air. However, again, this is speculation.
Confusingly, a United States representative told a UN committee they concluded the explosion was ‘the result of a nuclear reaction’. This language is a little vague, and implies a criticality event. But it leaves room for a scenario in which the reactor underwent a rise in reactivity as seawater drained from the structure, or some other change in its equilibrium, raising its temperature to ignite residual rocket fuel or hydrogen. Water, temperature, and the geometry of nuclear materials all have an extreme impact upon the reactivity of a system.
What is known about Burevestnik?
The UK Chief of Defence Intelligence in 2021, James Hockenhull, stated pretty explicitly that Burevestnik is a subsonic nuclear-powered cruise missile with a global reach – there’s not much technical detail here, but subsonic is a helpful constraint for further speculation. Most other sources point to a ramjet design. In a nuclear ramjet, air enters at the intake at high speed (hence, ram-jet), is heated by the reactor, expands, and propels the missile forward. Despite being more efficient above Mach 2, ramjets can operate subsonically, so the twin assumptions in the statement ‘subsonic ramjet’ are not mutually exclusive.
The reactor that heats the air in the ramjet is a black box as far as open source material is concerned; there are no known design parameters. But to function as an engine, one can assume its primary function is to heat incoming air. Broadly speaking, there are two reactor configurations that could reasonably be considered here: ‘open-loop’, where the incoming air flows straight through the reactor to extract the heat and propel the missile, or ‘closed-loop’, where the reactor is isolated from the airflow by a heat exchanger that transfers the reactor’s heat to the air.
Both configurations bring their own challenges. Open-loop systems would have radioactive particulate matter in the exhaust and need to be large. The geometry of the reactor is extremely important, and ducts for airflow would increase the amount of fuel needed for critical mass to be achieved, thereby increasing system weight. Closed-loop systems would have less mass in the reactor, but this might be offset by the mass, volume and packaging issues of the heat exchangers, which would likely use liquid metal coolants like sodium and potassium.
Figure 1 – A generic open loop nuclear ramjet design. Air is drawn in, through the reactor, and out of the exhaust nozzle, providing thrust. A closed loop system would have the reactor offset, or otherwise separated from the airflow by a heat exchanger that takes heat from the reactor into the airflow chamber. Credit.
Some sources imply that Burevestnik could use a closed-loop 1-20 MW microreactor, but the reference for this claim is a US Department of Energy (DoE) microreactor explainer video and factsheet showing a concept reactor fitting on the back of a semi-trailer truck. Miniaturising a reactor from shipping container-sized to cruise missile-sized is no mean feat, especially a high temperature reactor such as the type needed for a nuclear ramjet.
The added complexity of introducing a heat exchanger into the design would make an open-loop concept more attractive to the engineers within the context of rapid testing and deployment, as well as the political pressure of Putin’s political unveiling of the weapon in 2018. Therefore, if it is an open-loop system, then the question becomes: what happens to the radioactive exhaust?
In an open-loop system, the air will flow through the reactor core before being ejected as exhaust, but the air itself won’t be made radioactive. The primary radiological concern is from degradation of reactor materials as a function of the heat, pressure, and intense radiation of operation. As these materials degrade, they may chip off and exit through the exhaust. These particles will be radioactive, but the amounts released depend on technical details that are not in the open source.
It seems reasonable to assume that even under optimum performance, some radioactive material will be released. But the testing so far has not seen anywhere close to optimum performance, with none of the previous thirteen tests showing signs of success. A reactor failure during flight would result in the missile losing power and crashing at sea, or less likely, over land. Cleanup of either crash scenario would be extremely perilous, complex and the contamination would be widespread. Even a crash into water could present a massive environmental hazard for the local wildlife, some of which may end up in Russia and their neighbours’ food supply.
Why is this kind of design so difficult to realise?
Reactors are extremely sensitive machines. There are some which can be operated quite safely by undergraduates, such as TRIGA reactors, but they are unsuitable for continuous, high-power operations. In the main, reactor designers seek to control precisely, either through design or operation, the geometry of the reactor, the fluids in contact with the reactor (as they contribute to its reactivity and longevity), and the temperature of the reactor – meltdowns occur when the fuel gets so hot that it melts, but doesn’t stop generating power.
a) Geometry
The reactor fuel (either monolithic blocks of uranium-oxide, or pin-style fuel assemblies) will be arranged precisely in the reactor to reach an optimum neutron flux in the core. For a cruise missile operating at 50-100m altitude, and following terrain at high speed, there will be a lot of inertial forces at play. The assemblies must be designed to handle these, as well as turbulence perturbations without significantly changing the neutron flux in the reactor, or deal with these perturbations in such a way that the reactor returns naturally to criticality. If the fuel geometry changes, reactivity could spike. And similarly, if they move too far away, reactivity could drop, causing the missile to crash.
b) Fluids
Assuming that the coolant for this reactor is the incoming air, changes in wind speed and heading will change the rate of cooling in the reactor. Consider a sudden gust of tailwind. This could reduce the incoming air speed, causing heat rejection to drop and the temperature to increase. This would probably be manageable, possibly by passive safety systems – we can picture the engine design being such that a reactor temperature increase would increase air outlet temperature and accelerate the missile back to its cruising speed.
At altitudes of 50-100m though, it won’t just be air that enters the ramjet intake, but dust, leaves, snow, rain, hail, and possibly even birds. Even if it dodged all the weather, flying animals, and detritus at that altitude, changes in humidity, temperature, and pressure will change the reactivity of the reactor – variables that will inevitably fluctuate on one to two day trip across 20,000 km of ocean and land. This is important, as water interacts with neutrons in such a way as to slow them down and make them more easily absorbed by the uranium in the fuel, sparking another fission event. If the missile flew through a rainstorm, or even just more humid conditions, water in the reactor ducts might increase the amount of fission events, raising its temperature and requiring precise control to avoid a criticality event.
c) Temperature
The average light water reactor operates below 400C (752F), and at these temperatures the reactor materials that clad and support the fuel elements can be metallic. This is advantageous as metals yield and fail in predictable, ductile ways. The temperatures touted for the air outlet temperature of Burevestnik are 1400-1600C (2552-2912F), which precludes most traditional reactor materials as they would melt. This analysis will not delve into the wealth of high-temperature fuel designs for power reactors, but suffice to say most structural elements are ceramic. Given these criteria, and the materials used in the Tory reactors, it is likely that Burevestnik uses structural ceramics. Yet ceramics are brittle, and fracture in fast and unpredictable ways.
We can assume that the 1400-1600C figure above is the temperature at the coolant-core interface. The centre of the fuel will be significantly hotter, only a matter of millimetres away. Couple this massive thermal gradient (and thus, stress) to the innate brittleness of ceramic materials, as well as the dynamic loading from wind, manoeuvres, and turbulence and one has a recipe for a lot of material degradation: degradation that could lead to radiological release in the missile’s wake.
How do you control all these factors without sacrificing stealth?
Burevestnik may have to manage all of these issues autonomously. In flight, on its way to devastate a target, it will not be sending telemetry to an operator who would issue commands, given that its advantage is stealth. This means that all the reactor control systems must be autonomous and onboard for some, if not all of its flight – not to mention capable of operating flawlessly in intense radiation.
And stealth is absolutely critical for this system – being able to evade early warning sensors is the only way this weapon could be used to hit targets intercontinentally. If it’s spotted on radar, adversary jets would be scrambled immediately and the low-flying, slow Burevestnik would be a sitting duck. The only purpose for it is as a first strike weapon, as there would be little utility in using it after an exchange of ICBMs, which makes it a deeply destabilising capability, should it be proven to work.
So why are Russian engineers pursuing this seemingly impossible system?
First of all, the main purpose of this programme seems to be as a signalling exercise to Russia’s adversaries that strategic surprise and defeating early warning systems is a key facet of its posture going forward, and secondly to the rest of the world that Russia remains a technological superpower able to develop and field technologies that others can’t. If nuclear deterrence is about capability as much as it is about communication and credibility, this latter aspect is likely to land poorly if the programme continues to see testing failures and lethal accidents involving Burevestnik.
With respect to the technical issues, clearly there is a belief within the engineering design team that nuclear ramjets can work, and with lightness being the key factor here given the mass of the reactor itself, an open-loop system is probably the most achievable – but also the most radiologically dangerous. It may be that the complexity of packaging an open-loop nuclear ramjet within a cruise missile envelope that also includes reliable control and ancillary systems for a potential multi-day flight is not currently feasible. And if reliability is an issue, then the deterrence value of Burevestnik is substantially diminished.
Despite all that, this puzzling programme limps on. But if successful tests result in major radioactivity releases on land or at sea, and unsuccessful tests kill those young scientists and support staff conducting them, then the bar for military nuclear safety is barely higher in Russia right now than it was for the Manhattan Project, for a weapon with substantially less utility.