Jeffrey LewisBoost Gas and the Reliable Replacement Warhead (RRW) Program

The Reliable Replacement Warhead program is either:

  1. A nonproliferation friendly alternative to the Robust Nuclear Earth Penetrator (RNEP), or
  2. A Trojan Horse bearing a return to nuclear testing.

This blog has suggested both possibilities

Whether the RRW is good or bad —from an arms control and nonproliferation standpoint—will depend on the actual research progam carried out by the labs—a highly technical question. So far, Goldilocks seems to have been running the public debate over the proper focus of the RRW … you know, not too hot, not to cold, just right.

Kudos, then, to Representatives Ellen Tauscher, John Spratt, Silvestre Reyes, and Ike Skelton for throwing down like physicists.

Prominent scientific advisory panels have noted that there are opportunities for further enhancing weapon performance margins, such as adjusting the boost gas components during regularly scheduled maintenance.

“What the hell is boost gas?” you ask, “And how does it increase performance margins?” Raymond Jeanloz, chair of the National Academies Committee on International Security and Arms Control, explains:

During weapon operation, a chemical explosion triggers the primary stage by compressing a core, or pit, containing plutonium-239 until it reaches critical density. The resulting fission yield is “boosted” by the presence of a deuterium­tritium mixture that is driven to fusion by the imploding Pu. The combined fission and fusion yield of the primary stage initiates the secondary, or main, stage, which may consist of fissionable materials, such as uranium-235, as well as fusionable materials. It is the secondary stage that provides the bulk of the weapon’s military yield, but the secondary requires a minimum energy from the primary to ignite. Thus, a vital index of the reliability of the nuclear package is the performance margin, which is the difference between the minimum yield obtained from the primary and the yield required to drive the secondary. One concern about aging is that the materials within the primary stage might deteriorate with time, so that the performance margin could vanish or even become negative.

Chuck Hansen—in the magisterial Swords of Armageddon—also provides a nice explanation of boost gas:

Enhancement of a fission reaction by thermonuclear neutrons. A term coined by Dr. Edward Teller in 1947 to describe a technique of improving the efficiency and explosive yield of fission devices by the injection of small amounts of tritium and deuterium gases into the hollow center of the fissile weapon core before the core is compressed and begins to fission.

When the gases are compressed inside the collapsing hollow shell of an implosion system and subsequently heated by fission in the shell of active materials, a fusion reaction occurs in the gas, releasing large quantities of highenergy neutrons.

If a large supply of neutrons is suddenly introduced into a fissioning system, while neutron multiplication is still occurring and before appreciable core expansion has taken place, the total number of nuclei fissions, and the resultant efficiency and energy yield, is greatly increased. Also called “gas boosting.”

A few years ago the JASON panel observed increasing the amount of boost gas in a warhead would improve the performance margins:

Enhancements of performance margins will add substantially to long-term stockpile confidence with or without underground tests. To cite one example, we can adjust the boost gas fill or shorten the time interval between fills. (This is discussed more fully in the classified text.)

JASONs Jeremiah Sullivan and Sid Drell (writing with Jim Goodby) have both suggested this would make an excellent focus of the RRW, without requiring new warhead designs or nuclear testing.

The phenomonology of gas boosting provides an interesting research agenda for labortories. As I understand the problem, a sphere of plutonium imploded by high explosives will result in some Pu mixing with the boost gas, which can reduce the yield of the nuclear weapon. The United States and Russia are both interested in the phenomenon, which is difficult to model. The Stockpile Stewardship Program included construction of a facility to investigate the problem.

Good times.


  1. Cheryl Rofer (History)

    Tritium hasn’t been produced in the US for some time, although TVA reactors are supposed to start producing it this summer.

    It’s probably been about a decade since the Savannah River reactors stopped production, so the US now has about half the stock of tritium (half-life 12 years) that it had before. That limits what can be done with boost gas.

    But a startup of production at the TVA reactors could change that. I wasn’t able to google up anything recent on that. Do you know anything more, Jeffrey?

  2. Jeffrey Lewis (History)

    The absorber rods have been irradiated and removed for extraction, although the Chattanooga Times Free Press reports some problems with the first batch (subscription only):

    During the recent outage at Watts Bar, TVA also removed the first of the 240 burnable tritium producing absorber rods from its Watts Bar reactor. TVA is irradiating the rods to help the federal government produce more tritium, a radioactive isotope of hydrogen that gives nuclear weapons their deadly blast.

    TVA reported in March that small amounts of tritium leaked into a monitoring well at Watts Bar during repair work at the plant.

    “We had some abnormal leakage from the assemblies in the core and we’re working with DOE to correct the problem,” Mr. Singer said recently.

    TVA spokesman Gil Francis said Monday that TVA is working with the Department of Energy to redesign the tritium bars inserted into the reactor to limit tritium releases within the plant.

    DOE will take the irradiated tritium bars from Watts Bar and turn them into bomb materials at the Savannah River complex in South Carolina.

    Federal standards require that tritium releases remain below 9.6 curies per tritium bar at Watts Bar. In the past year, TVA observed level of tritium of 2.8 curies per tritium bar, Mr. Francis said.

    “That’s well within the allowable limits, but is above what we expected,” he said.