A flying unshielded nuclear reactor would spew massive amounts of radiation, and that’s if it’s working correctly.
When President Donald Trump heard that Russia’s experimental nuclear-powered cruise missile had exploded, killing seven scientists and causing a major radiological incident less than 300 miles from the Finnish border, he fired off a boastful tweet. “We have similar, though more advanced, technology,” he said.
This is...not accurate. In the late 1950s and early 1960s, the United States pursued a less advanced version of a similar technology but abandoned the effort before ever launching an actual test vehicle. Nuclear-powered cruise missiles, the Pentagon concluded, are a bad idea.
But the concept still appeals to Vladimir Putin, who last year revealed his pursuit of an “unlimited-range” missile that Russia calls the 9M730 Burevestnik (Storm Petrel) and which NATO has dubbed the SSC-X-9 Skyfall. A missile powered by a small nuclear reactor could cruise about its target for days, giving it a wide range of potential targets it could strike upon command.
In 1957, the U.S. Air Force and the Atomic Energy Commission launched Project Pluto to build the Supersonic Low-Altitude Missile. The work proceeded at the Lawrence Radiation Laboratory (today, Lawrence Livermore National Laboratory), in Berkeley, California, under the supervision of Charles “Ted” Merkle, a hard-driving physicist. In 1959, Merkle reported to the Air Force on the feasibility of the project, noting a number of enormous technical challenges but also “some interesting and exciting possibilities to discuss.”
Like the makers of Skyfall, Merkle decided on a ramjet design. Powered into the sky atop a conventional rocket booster, the ramjet would compress incoming air in a uniquely shaped chamber, superheat it with a small nuclear reactor, and expel it as exhaust, propelling the missile almost three times faster than sound.
The biggest challenge: nuclear reactors are fragile things. Putting one in a cruise missile would require a design that could withstand three types of stress that no previous reactor had needed to endure.
“There are the stresses associated with the pressure drop through the 'reactor' and, as indicated earlier, this stress is of the order of hundreds of psi [pounds per square inch] when spread over the entire reactor,” Merkle wrote. “When concentrated at various support points, it contributes loads like thousands of psi. Next in order: to transfer heat from the fuel to the air stream, there must be a temperature drop in the fuel-bearing materials and, for typical ceramics and power densities that would be of interest for possible missile applications, stresses of many thousand psi result as a consequence of these temperature differences.”
Then there were the inertial stresses of flight. “Since in principle such ramjet power plants can operate from sea level to quite high altitudes, rather large ‘gust loadings’ must be anticipated,” he wrote.
Undaunted, the lab went to work creating a 500-megawatt reactor that could operate at 2,500 degrees Fahrenheit. Four years later, after much experimentation with different materials and the careful assembly of 500,000 small fuel rods, they had an engine called Tory-IIA.
On May 14, 1961, they tested it at an 8-square-mile facility in a desolate area of Nevada called Jackass Flats. But they wouldn’t be able to fly it, not yet, since it was potentially a nuclear bomb. Instead they used a flatbed rail car.
In a 1990 article for Air and Space Magazine, Gregg Herken writes that “the Tory-IIA ran for only a few seconds, and at merely a fraction of its rated power. But the test was deemed a complete success. Most importantly, the reactor did not catch fire, as some nervous Atomic Energy Commission officials had worried it would.”
But as Herken tells it, Washington was already beginning to cool to the idea of a nuclear-powered cruise missile. The biggest reason: the missile’s unshielded nuclear reactor would spew radiation along its flight path, potentially irradiating its own ground crew and everyone else between the launch pad and the target.
Anticipating this, Merkle downplayed the danger in his initial 1959 report, using language that sounds ripped directly from Dr. Strangelove. “One problem that bothers the design of reactors to be used near people is the necessity of confining all the fission products to the reactor fuel element,” he wrote. “A typical mission might produce some-what less than 100 grams of fission product. Of these it might be expected that some large percentage would naturally remain in fuel elements…Consequently the fission activity introduced locally into the atmosphere is minute compared with even the most minute atomic weapon.”
Edwin Lyman, senior scientist and acting director of the nuclear safety project at the Union of Concerned Scientists, offers some perspective. “I suppose that at a time when the nuclear weapon states were still engaged in atmospheric testing, there wasn't a whole lot of concerns about releasing additional radioactivity into the environment. Merkle's cavalier attitude seems in tune with the era. But such a system should be considered completely unacceptable today,” Lyman told Defense One in an email.
“One thing is that to characterize radiation releases in terms of ‘grams’ is misleading. Chernobyl released only a few hundred grams of iodine-131 yet it resulted in thousands of thyroid cancers among children.” He noted that the Pluto tests ejected not only radioactive gases but far more dangerous radioactive particle matter as well.
The team tested a modified version of the engine once more in 1964 and the project was canceled.
The high fallout, both politically and literally, mean that nuclear-powered cruise missiles remain a terrible idea, says Kingston Reif, the director for disarmament and threat reduction policy at the Arms Control Association. “If you think the current excessive U.S. plans to replace the U.S. nuclear arsenal are controversial, imagine the negative domestic and international reaction to a U.S. effort to renew R&D on nuclear cruise missile powered by an unshielded nuclear reactor,” said Reif. “Russia should abandon development of this grotesque, unnecessary and almost certainly unworkable weapon immediately.”
Added Lyman, “if the missile was shot down, the fuel would overheat and you'd have a 500-thermal-megawatt reactor meltdown — about one-sixth the size of a large power reactor — but without any containment. Also, the lack of radiation shielding would make it difficult, if not impossible, for emergency responders to approach it.”
That’s similar to the problem Russia is grappling with right now.