On October 27, the Department of Defense announced that it wants to make a new variant of the B61 nuclear gravity bomb. This will be B61-13, the 13th such variant of the bomb design, and like all modern nuclear weapons, it will represent a repurposing of an older nuclear warhead, rather than a wholly new construction. As a gravity bomb, the B61-13 will be designed for release from a fighter or bomber, which would result in a thermonuclear blast and fallout plume that is devastating to anyone, civilian or military, in the affected area.
Atomic bombs are an almost 80-year-old technology, and thermonuclear bombs, which use an atomic warhead’s fission reaction to spark an explosive fusion reaction, are not much younger. The design for the first B61 variant began in 1963, which means the latest variant is continuing not just a 78-year legacy of nuclear gravity bombs, but a long legacy of this specific template for a gravity bomb. (A gravity bomb, by the way, is a bomb that falls to its target, sometimes though not always navigating as it descends.)
Of those B61 variants, five remain in service today (the B61-3, B61-4, B61-7, B61-11, and B61-12), with the B61-13 slated to replace the existing stockpile of B61-7s.
Some B61 bombs can be carried by fighter jets like the F-15E and the F-16. But the B61-7 and, presumably its replacement, the B61-13, is designed for nuclear-capable bombers only, meaning that the B61-13 will likely be carried by the B-21 Raider stealth bomber, and possibly the B-2 Spirit stealth bomber, if both are in service at the same time. (The iconic B-52 no longer carries gravity bombs, in part because modern anti-air missiles make the venerable bomber too vulnerable to being shot down when in bombing range. Instead, B-52s can carry existing and future air-launched nuclear cruise missiles.)
The B61-13 is intended to have the same yield as the B61-7, but with the modern safety, security, and accuracy features common to the B61-12 line currently in production. This includes the B61-12’s inertial guidance system, for greater accuracy, though there is only so much that specific accuracy matters when it comes to guiding a bomb that will produce blasts in the tens or hundreds of kilotons.
“The B61-13 represents a reasonable step to manage the challenges of a highly dynamic security environment,” said Assistant Secretary of Defense for Space Policy John Plumb in a release. “While it provides us with additional flexibility, production of the B61-13 will not increase the overall number of weapons in our nuclear stockpile.”
The single most concise way to describe a nuclear bomb is in terms of yield, or the TNT equivalent of explosive force that will be unleashed when it is detonated. The B61-3, -4, -7, and -12 variants all have dial-a-yields, meaning their explosive potential can be toggled before use, at the time the bomb is loaded onto the plane. For the B61-3, -4, and -12, this yield can be as low as 0.3 tons of TNT, or a fraction of the explosive force of the bombs dropped by the United States on Hiroshima (Little Boy, 15 kilotons) and Nagasaki (Fat Man, 20 kilotons) in August 1945. The maximum yield of the B61-4 and B61-12 is 50 kilotons, making every dialed-up bomb greater in explosive force than the only two nuclear weapons ever used in war.
The B61-12 was already designed to consolidate the four variants of B61 into a single upgraded universal design, replacing the 3s, 4s, 7s, and 10s. The B61-7 has a yield of 10 kilotons to 360 kilotons, and B61-10 has a yield of 0.3 tons to 80 kilotons. By replacing all of these weapons with the B61-12, that would cap the maximum yield of these specific gravity bombs at 50 kilotons. The B61-13 would have a yield of 10 kilotons to 360 kilotons.
Yields are an abstract way to talk about the effects of heat, pressure, and radioactivity on cities and people. NUKEMAP, by technology historian Alex Wellerstein, offers insight into how such blasts would play out in real life. A 50 kiloton warhead set off in lower Manhattan would kill an estimated 273,000 people, injure an estimated 471,000 more, and send a radioactive plume all the way to Hartford, Connecticut. A 360 kiloton bomb, in the same location, would kill an estimated 778,000, injure an estimated 1,045,000, and send a radioactive plume almost all the way to Lowell, Massachusetts.
While US cities would obviously not be the target of US nuclear bombs—and if they were hit by a nuclear weapon, it would likely be via intercontinental ballistic missile—it’s a useful context for understanding how the weapons, as designed, would work.
“The B61-13 will strengthen deterrence of adversaries and assurance of allies and partners by providing the President with additional options against certain harder and large-area military Targets,” reads a fact sheet shared as part of the announcement of the B61-13. The fact sheet also notes that the development of the B61-13 is “pending Congressional authorization and appropriation.”
The fact sheet and announcement both emphasize that there is no specific threat driving this development. It is, instead, a policy choice undertaken by the Biden Administration. Writing for the Federation of American Scientists, Hans Kristensen and Matt Korda argue that the B61-13 is announced as a way to replace the massive B83-1 (1,200 kiloton) gravity bomb with a larger weapon than the B61-12, but not one nearly as potent as the B83-1.
“The military doesn’t need an additional, more powerful gravity bomb,” write Kristensen and Korda. “In fact, Air Force officials privately say the military mission of nuclear gravity bombs is decreasing in importance because of the risk of putting bombers and their pilots in harm’s way over heavily defended targets – particularly as long-range missiles are becoming more capable.”
At present, the United States can deliver nuclear warheads through a range of means: submarine launched missiles, intercontinental ballistic missiles fired from silos, and nuclear bombs or missiles launched from planes. Taken together, these submarines, silos, and planes constitute the “nuclear triad,” a Cold War plan that spread risk and responsibility of nuclear launch across a range of means, ensuring that in the advent of the worst war humanity had ever seen, at least some nuclear weapons would be able to launch and share misery in retaliation. Deterrence, or the strategic concept of nuclear-armed nations avoiding war because of fear of nuclear retaliation, also hinges on the threat of some retaliatory nukes surviving a surprise first strike.
It is precisely because the scale and power of nuclear weapons constrains their use in all but the most existential of wars—to the point where none has so far been used in war since their devastating debut in August 1945—that the nature, design, and continued production of thermonuclear weapons is a policy question. The continued modernization of the US nuclear stockpile, which means refurbishing parts like plutonium pits and moving old warheads to newer casings, is a choice successive US presidential administrations continue to make, adapting the weapons of the past for an uncertain future.