Worse than the Dirty Bomb? Mobile Nuclear Military Reactors

In January 2019, the Defense Department issued a call for information in support of the aptly titled Project Dilithium. It seeks to develop a tiny, readily transportable, yet virtually indestructible nuclear power reactor for use at forward operating bases, the military facilities that provide logistical and troop support to the front-lines of conflict zones.

To be sure, the type of reactor it is seeking could be a great military asset: all the benefits of nuclear energy with none of the risks. The costly and dangerous process of trucking diesel fuel to bases, sometimes through hostile territory, may eventually be a thing of the past. Unfortunately, the need to store and ship irradiated nuclear fuel in a war zone will introduce different problems. And the odds that a meltdown-proof reactor could be successfully developed any time soon are vanishingly small.

The Defense Department…is seeking a nuclear reactor capable of producing 1 to 10 megawatts of electricity. …The reactor, at a minimum, should be less than 40 tons total weight; small enough to be transported by truck, ship, and aircraft; able to run for at least three years without refueling; and capable of semi-autonomous operation… The reactor should have an “inherently safe design” that ensures “a meltdown is physically impossible in various complete failure scenarios;” cause “no net increase in risk to public safety … by contamination with breach of primary core;” and have “minimized consequences to nearby personnel in case of adversary attack.

 An Octrober 2018 report commissioned by the army’s Deputy Chief of Staff admits, quite reasonably, that exposed mobile nuclear plants would “not be expected to survive a direct kinetic attack.” If commanders need to expend significant resources to protect the reactors or their support systems from military strikes, such reactors could become burdens rather than assets.  Can one really invent a reactor robust enough to suffer such a strike without causing unacceptable consequences? …If a severe accident or sabotage attack were to induce more extreme conditions than the reactor was designed to withstand, all bets are off. How long would passive airflow keep nuclear fuel safely cool if, say, an adversary threw an insulating blanket over a small reactor? Or if the reactor were buried under a pile of debris?

Moreover, it is hard to imagine that a direct explosive breach of the reactor core would not result in dispersal of some radioactive contamination. An operating nuclear reactor is essentially a can filled with concentrated radioactive material, including some highly volatile radionuclides, under conditions of high pressure and/or temperature. Even a reactor as small as 1 megawatt-electric would contain a large quantity of highly radioactive, long-lived isotopes such as cesium-137—a potential dirty bomb far bigger than the medical radiation sources that have caused much concern among security experts. 

At best a release of radioactivity would be a costly disruption, and at worst it would cause immediate harm to personnel, render the base unusable for years, and alienate the host country. For any reactor and fuel design, extensive experimental and analytical work would be needed to understand how much radioactivity could actually escape after an attack and how far it would disperse. This is also true for spent fuel being stored or transported.

The 2018 report describes several existing reactor concepts that it thinks might meet its needs. One is the 2 megawatt-electric “Megapower” reactor being designed by Los Alamos National Laboratory. But a 2017 INL study of the design iden­­tified several major safety concerns, including vulnerabilities to seismic and flooding events. The study also found that the reactor lacked sufficient barriers to prevent fission product release in an accident. INL quickly developed two variants of the original Los Alamos design, but a subsequent review found that those shared many of the safety flaws of the original and introduced some new ones.

Building Mobile Nuclear Reactor LANL

The other designs are high-temperature gas-cooled reactors that use TRISO (“tristructural isotropic”) fuel, which was originally developed decades ago for use in reactors such as the now-decommissioned Fort St. Vrain plant in Colorado. TRISO fuel consists of small particles of uranium coated with layers of different materials designed to retain most fission products at temperatures up to 1,600 degrees Celsius.

TRISO fuel enthusiasts have long claimed that reactors utilizing it do not need containments because each particle essentially has its own. This would seem to make TRISO an ideal fuel for small, mobile reactors, which can’t be equipped with the large, leak-tight containment structures typical of commercial power reactors. The army report buys into the notion that these “encapsulated” nuclear fuels can “avoid the release of radioactive volatile elements” and prevent contamination of the surrounding area, either during normal operations or accidents.

TRISO fuel contained in pebble

TRISO fuel’s actual performance has been inconsistent, however, and much is still not known. The Energy Department has been carrying out a program for more than a decade to try to improve TRISO fuel, but final results are not expected for years. In addition, if the fuel temperature rises above 1,600 degrees Celsius, fission product release can rapidly increase, making it vulnerable to incendiary weapons that burn hotter, such as thermite. The Defense Department may have already realized that TRISO fuel is not as miraculous as it first thought.

The RFI also specifies that the reactor should be capable of being transported within seven days after shutdown, presumably with the irradiated nuclear fuel still inside. While this requirement is understandable—if forces need to retreat in a hurry, they would not want to leave the reactor behind—it is unrealistic to expect this could be met while ensuring safety. Typically, spent nuclear fuel is stored for many months to years after discharge from a reactor before regulators allow it to be shipped, to allow for both thermal cooling and decay of short-lived, intensely radioactive fission products. Moving a reactor and its irradiated fuel so soon after shutdown could be a risky business.

Finally, the proliferation risks of these reactors and their fuel is a concern. The original RFI stipulated that the reactor fuel had to be high-assay low-enriched uranium (HALEU), which is uranium enriched to levels above the 5 percent uranium-235 concentration of conventional power reactors, but still below the 20 percent that marks the lower limit for highly enriched uranium (HEU), which is usable in nuclear weapons….If the Defense Department goes forward with Project Dilithium, other nations, including US adversaries, may be prompted to start producing HALEU and building their own military power reactors.

Excerptsf rom Edwin Lyman The Pentagon wants to boldly go where no nuclear reactor has gone before. It won’t work, Feb. 22, 2019

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