As I mentioned a couple of days ago, Elaine Grossman had an excellent story on the B61 Mod 12 LEP. Elaine reported that NNSA was considering either increasing the size of the B61 or reducing the explosive yield to create “real estate” for new new safety and security features:

However, if the Pentagon could either increase the size of a given weapon system or reduce its explosive yield, additional safety and security features imagined for the replacement warhead might instead be incorporated into existing hardware as it is overhauled, the Air Force official said.

I’ve now been told that NNSA is considering precisely those two options: either to build a device that would require a physically larger casing or reducing the yield.

The policy question is whether Congress ought to view reducing yield as a “new” weapon that does not conform to the military characteristics of the existing stockpile. On the one hand, reducing yield would ceteris paribus reduce capability. But other things are seldom equal. The B61 Mod 12 will likely have all sorts of new components outside of the nuclear explosives package (fuses, spin rocket motors and the like) that will more than offset any loss in yield.

As yield declines, NNSA opens the US up to claims that it is pursuing “mini-nukes” and other “more usable” nuclear weapons that would lower the threshold for nuclear war. That’s a concern in this case because the lowest yield setting of the existing B61 tactical variants is in sub-kiloton territory. The B61 is said to have a variable yield between .3 kt (300 tons, most likely the yield of the unboosted fission primary) and a few hundred kilotons.

One option is, presumably, to re-use the W84 pits that are sitting in the strategic reserve and have a mechanical safing device. (Both the W84 and W85 were derived from the B61 3/4 and the B61 Mod 10 was made, in turn, from reused W85 pits.) The W84 has a minimum yield of 200 tons (.2 kt).

Mod Entered Stockpile Comment
0 1969 No command or enhanced electrical safety, strategic bomb all converted to Mods 6 and 9 by October 1992
1 1971 Strategic bomb; replaced MK 28; all converted to Mod 7 by Oct 1992
2 1975 Inertial command disable; tactical bomb; 10-345 KT yield; no IHE; converted to Mod 8
3 1979 In stockpile, includes IHE, command disable, weak link/strong link signal generator; tactical bomb replacing MKs 28, 43, 57
4 1979 Same as Mod 3
5 1977 Nonviolent command command disable, weak link/strong link signal generator; tactical bomb replacing MKs 28, 57; 10-345 KT yield; no IHE; all converted to Mod 8 by June 1993
6 canceled Upgraded B61-0; new PAL and IHE; IOC was to have been March 1991; included ENDS; cancelled Feb 1992
7 1985 Modified B61-1, in stockpile. Includes new PAL, IHE, backup fuzing, command disablement. High yield strategic SAC bomb replaced MK 28FI. Some Mod 7s were converted to EPWs.
8 canceled Upgraded Mod 0; IOC was March 1991; included IHE, ENDS; canceled Feb 1992
9 canceled Tactical bomb; Mod 0 conversion; included IHE and ENDS; canceled September 1991
10 1990 Yield between those of Mods 3 and 4 (0.3-80 KT); modified W-85 warhead in B61-4; in stockpile. uses IHE and ENDS
11 1996 EPW with a single yield in the hundreds of kilotons

Source: Comments, with the exception of the B61-11 are from Chuck Hansen, Swords of Armageddon, vol VI, Table 4-27. I am not 100 percent on the stockpile entry dates, but they should be within a year or so.

It would seem to me that there is no inherent problem with the idea of reducing yield, provided that the rationale is to create “real estate” for new security or reliability measures. In fact, that was precisely an exception outlined in the final Spratt-Furse language that prohibited the research and development of mini-nukes from 1994-2004:

Nothing in this section shall prohibit the Secretary of Energy from conducting, or providing for the conduct of, research and development necessary … (2) to modify an existing weapon for the purpose of addressing safety and reliability concerns …

It would seem to me that if Congress wanted to LEP the B61 at a reduced yield, while protecting our nonproliferation interests, they could fund the B61 Mod 12 LEP and reinstate Spratt-Furse.