Jeffrey LewisUranium Reliable Replacement Warhead (URRW)

I had a nice discussion with Bob Peurifoy today. He has kindly agreed to let me post the full text of his working paper on a notional uranium reliable replacement warhead (URRW).

Three comments, by way of introdcution:

  • Peurifoy’s first preference is to keep the current arsenal. If we must design new warheads, than he would like to see uranium considered.
  • Peurifoy is talking about creating uranium pits for thermonuclear weapons—not the retro-weapons that I wrongly assumed in my DefenseTech post. More on that after the text.
  • The deployment numbers are notional. Peurifoy hopes to start a much-needed discussion, not provide the last word.

With those caveats, here is the text:

Uranium Reliable Replacement Warhead (URRW)

Bob Peurifoy

In an e-mail to multiple addressees, dated June 29, 2005, I suggested that because of the Bush/Putin handshake, the RRW programs could take advantage of the use of uranium 235, rather than plutonium 239, in the redesigned primaries. The advantages of uranium pits could include the following:

  • Replacing plutonium pits with uranium pits will eliminate the need for a Modern Pit Facility and a refurbishment of TA-55.
  • Y-12 has expertise in the fabrication of uranium parts based on 60 years of experience. I suggest that Y-12 can be upgraded to handle the fabrication of uranium pits at a fraction of the cost estimated for a modern pit facility.
  • The half-life of uranium 235, due to radioactive decay, is 700 million years versus 25,000 years for plutonium 239. Therefore, the radioactive hazards associated with uranium pit fabrication would be reduced.
  • The radioactive hazards of weapon handling by DOE and military custodians could be reduced.
  • Plutonium is pyrophoric. Uranium is not.
  • With a 700 million year half-life, there should be no pit aging problems.
  • Given an accident and a uranium spill, decontamination could be less demanding.
  • The larger critical mass required by the use of uranium will result in thicker pit shells, thereby reducing machining problems during fabrication and resulting in higher yields and lower fabrication costs.
  • With the use of uranium, perhaps IHE will be less important.
  • The use of uranium pits will meet the NNSA objectives of a less expensive, easier-to-manufacture, longer-lasting, and less hazardous product.

The URRW designs will, of course, require some accommodations from the Air Force and Navy. Fortunately, with the Bush/Putin handshake setting limits on strategic weapons, the stockpile reductions will allow the design of larger and heavier weapons necessary to accommodate larger primaries based on uranium pits. For example, I’ll assume that the United States will reduce the stockpile to 1,820 strategic weapons by 2010. The new stockpile might be apportioned as follows:


The Navy will maintain 10 Trident submarines – 5 on the East Coast and 5 on the West Coast. For each of the two bases, 2 Trident submarines will be at sea, 2 will be undergoing maintenance and replenishment, and 1 will be a spare. All will be armed. Each of the 24 Trident missiles will carry 3 RBs for a total of 720 RBs containing URRW. These RBs will be heavier and have larger base diameters because the uranium pit primaries will be larger in diameter and heavier than the primaries they replace. I understand that NNSA has indicated that the Navy says this is okay. The third stage motor will be eliminated, thereby providing ample room to mount 3 larger- base-diameter URRW RBs on the clear deck. The 3 URRW RBs will each be heavier than each of the 8 W-88/Mark 5 RBs that they replace, but the total weight will be less by perhaps 40 percent. This lighter payload and the tare weight saved by removing the 3rd stage should provide the Trident weapon system with about the same maximum submarine-to-target range as is presently attained with 8 Mark 5 RBs.

The Navy will have to conduct a new bus and RB design, development, and test program and produce new RBs and buses. Of course, this is also true for all RRW designs.


Maintain 500 Minuteman III ICBMs with 1 URRW RV payload for each missile. The Minuteman III was originally deployed with 3 Mark 12s, and later upgraded to use 3 Mark 12 As. A single RV, the Mark 21, is now planned. The weight of a single URRW RV is less than the total weight of 3 Mark 12 As. There will be no base diameter constraint in using a single URRW RV. Again, NNSA indicates that the Air Force is agreeable.

As with the Navy Trident SLBM, the Air Force will need to conduct a design, development, and test program for at least the RV and then procure new RVs for deployment with the URRW.


The remaining weapons can be divided between bombs and ALCMs in some appropriate ratio. The total number of URRW air-delivered weapons will be 600. The nuclear-capable bomber force will contain 100 aircraft made up of B-52s, B-1s, and B-2s. I ignore tactical fighter/bombers such as the F-16 and F/A-18.

Each bomber will be configured to carry 6 weapons: bombs and/or ALCMs. For bomb carriage, the bomb-bays are sized to carry bombs larger in diameter and heavier than the replacement URRWs. Suspension systems will have to be reconfigured.

For the ALCMs, it may be necessary to redesign the air frame to carry the larger-diameter URRWs. Weight should not be a problem. Air Force weapon system compatibility testing will be necessary. Depending on ALCM diameter limits, a modified ALCM may be necessary that will require design, develoment, flight tests, and additional ALCM procurement by the Air Force.

In summary, the acceptance of the URRW configuration, coupled with the Bush/Putin agreement, will allow a restructuring of the U.S. strategic weapon inventory, with many advantages to be achieved.

The introduction of URRWs does not require a crash effort. Given the Bush/Putin handshake stockpile and a 45-year plutonium life, the start of the replacement of the current weapon stockpile need not begin until perhaps 2020 – 2025. If the plutonium pit life is 60 years, URRWs neeed not enter the stockpile before 2035.

As I mentioned, Peurifoy is talking about using uranium pits as the primary in a thermonuclear design.

1960s W53 with a uranium primary. New uranium
weapons would be substantially smaller.

A sidebar, written by John Fleck in the Albuquerque Journal, offers additional design details:

In modern thermonuclear weapons, the blast from the implosion “primary” first stage acts as a spark plug to ignite the even more powerful second stage hydrogen nuclear fusion that puts the “H” in “H-bomb.” It takes less plutonium to make such a bomb, which makes it ideal for the modern weapons. Weight is at a premium for military planners who want to pack as many warheads as possible atop a single missile. But uranium has also been used over the years in implosion weapons.

Uranium weapons testing in the 1940s and ‘50s lead to the development of the W53 warhead [pictured above] and B53 bomb, according to nuclear weapons experts Robert S. Norris and Thomas Cochran. Designed beginning in the 1950s and built during the 1960s, the twoweapons were extremely heavy, according to Norris and Cochran, of the Natural Resources Defense Council. They also packed a massive punch—a blast equivalent to 9 million tons of TNT. Earlier weapons also used uranium, according to Norris and Cochran, but they say the W53 and B53 are the only known weapons in which uranium was used to provide the primary blast in a thermonuclear weapon.

Bob Peurifoy, a retired Sandia National Laboratories scientist, thinks that now that fewer weapons are needed aboard each missile, uranium offers a viable alternative to plutonium.

According to a calculation Peurifoy provided to the Journal, as much as six times as much uranium and high explosives would be needed for his uranium bomb. With other changes to the weapon, he calculated a 13-inch diameter plutonium primary could be replaced by a uranium alternative about 20 inches in diameter. That would allow three uranium warheads to be carried by a Trident II submarine missile, according to Peurifoy. The missiles now carry eight plutonium warheads.

Here is a link to my past post on the URRW.


  1. Muskrat (History)

    “With a 700 million year half-life, there should be no pit aging problems.”

    Just hold your horses there, Mister Unilateral Disarmament. There is no fixed timeline for our NPT Article VI obligation for disarmament, and the USG has always fought ANY such timeline. Who’s to say that we won’t be making and using nukes 700 million years from now? When our mutant descendants are arguing with the Zontarans over the last burnable pound of hydrocarbons on the planet, do you want us to be helpless?

  2. Allen Thomson (History)

    “Replacing plutonium pits with uranium pits will eliminate the need for a Modern Pit Facility.”

    Well, not to be repetitious or anything, but there was a post to somewhat that effect here back in the distant past:

    During the mid-1990s, the JASONs were involved in studies of stockpile maintenance; some of the resulting reports are available online.

    During that time, I had an interesting lunch at the American Cafe in Tysons Corner with one of the involved JASONs, who said that the long-term problems with plutonium metallurgy/chemistry were still so worrisome that many of them were strongly in favor of moving to all-uranium primaries. That would mean an accompanying penalty in clunkiness, but with a lot fewer worries about whether the primary would yield sufficient yield after twenty years on the shelf. How to deal with tritium decay was a separate question.

    FWIW, I’d expect that the proven techniques that have gone into Pu pit design could lead to pretty svelte uranium versions that could be depended on to do the job without nuclear testing. Though, probably, with a fair amount of non-nuclear work with radiography etc. to make sure the implosion imploded as desired.

    Random question asking for comment: could you make a two-stage weapon in which the primary is a gun-assembly design? I can’t see why not, and a positive answer could be interesting in the non-proliferation context.

    [I don’t know, but Peurifoy confirmed the design would utilize an implosion primary. ACW]

  3. John Field (History)

    An important advantage of using a uranium pit is that the initial resting state of the pit can be much closer to critical because of the lower background neutron rate. This helps greatly with the hydrodynamics.

    We can also learn something by studying the context.

    The NRDC curves of yield vs mass at various technology points imply(but do not absolutely prove for a variety of reasons) that the potential achieved compression of the pit is nearly a factor of 2. Critical mass in spheical geometry scales as the square of this factor.

    Compressibilities of uranium and plutonium up in the millions of psi range should be relatively similar – as they are dominantly determined by the rearrangement of outer and F-shell electrons at high pressure. Uranium might even be a bit better in this regard.

    If a uranium pit were near critical initially(say perhaps 80% critical mass), a factor of two compression of the uranium would imply an implosion assembly of over 3 critical masses at the assembled density. This should result in an extremely efficient design. This is a design point which would be unaccessible with the large spontaneous neutron rate in Pu.

    Based on this, it seems possible but counterintuitive that a uranium bomb could actually be more efficient.

    I doubt that this is the case – if only because the military has chosen Pu – and I believe that the inconsistency is the published mass vs technology curves. It seems likely to me that the “high” technology points are probably not militarily usefully accessible points because the compression required necessitates unreasonable amounts of high explosive(e.g. read: warhead total mass) and quality hydrodynamics difficult to achieve.(e.g. 30+ million psi pressures)

    I suggest(without proof) that a more reasonable level of compression for (militarily useful) high technology weapons would be about 1.5:1 instead of 2:1. I think this would be more consistent with the debate above on uranium pits.

    I believe (without proof) that the actual lower bound on critical mass for weapons is about 4 kg Pu – as it was historically assumed to be.

    John Field

  4. Bob Massman (History)

    “With the use of uranium, perhaps IHE will be less important.”

    Perhaps, but don’t neglect the Human Reliability (“insider threat”) during the assembly, transportation and store phases. CHE presents much greater security issues than IHE.