Maintaining Nuclear Weapons Safe and Reliable Under a CTBT by Richard L. Garwin Senior Fellow for Science and Technology Council on Foreign Relations, New York and IBM Fellow Emeritus IBM Research Division P.O. Box 218 Yorktown Heights, NY 10598 Tel: (914) 945-2555 fax: (914) 945-4419 Email: RLG2 at watson.ibm.com Many papers at http://www.fas.org/rlg "The Comprehensive Test Ban Treaty and US National Security Interests" AAAS Annual Meeting San Francisco, CA February 16, 2001 ABSTRACT. I discuss nuclear weapons in general and U.S. thermonuclear weapons in particular. These have been proven by nuclear explosive development tests and production verification tests, and need to be maintained for several or many decades without further nuclear explosion testing, under the zero-threshold Comprehensive Test Ban Treaty (CTBT). The basis for stockpile stewardship has always been a strict surveillance program by which eleven nuclear weapons of each type are now removed annually from the stockpile and inspected; one is destructively dismantled by the nuclear weapon laboratories. Most of the 4000 or so parts of a nuclear weapon can be thoroughly tested without nuclear explosions and may not even be used in an underground nuclear test. The weapons can be maintained safe and reliable by remanufacture of the components-- the primary and secondary nuclear explosives. The Science-Based Stockpile Stewardship Program (SBSSP) provides increased understanding of the necessity for remanufacture, thereby permitting delay in remanufacture, in some cases, because inspection and analysis shows that a particular age-related defect is not significant. The role of advanced computation facilities (teraflop computers), new flash radiographic systems, and the National Ignition Facility will be evaluated for the Stockpile Stewardship Program. The surest way to lose confidence in the nuclear weapon stockpile is to orient the Stockpile Stewardship Program toward the introduction of new designs of the nuclear components. The goal of the SBSSP should be to maintain the weapons as reliable as they were during the days of nuclear testing. Historically, weapons are not most reliable when they first enter stockpile, because of infant mortality. Of the other four nuclear weapon states under the NPT, France and Britain appear to be emulating the United States in the SBSSP, while Russia and China are more likely to rely on scheduled remanufacture. Either way they will be able to maintain a stockpile of proven nuclear weapons safe and reliable for many decades or centuries. The essential point is not to make changes, even multiple relatively small changes, that will eventually lead to lack of confidence. 1046MNWS 021501MNWS Draft 3 Final 02/16/01 INTRODUCTION. Three important aspects of the analysis of cost and benefits of a Comprehensive Test Ban Treaty (CTBT) are: o Can U.S. nuclear weapons be maintained safe and reliable without nuclear explosion testing? o Is there a non-proliferation benefit to the United States in limiting nuclear weapons to those that might be developed without testing? o Can the CTBT be adequately verified? What kinds of nuclear weapons might be developed by experienced and by inexperienced states below the limit of detectability of the International Monitoring System and the U.S. unilateral capabilities? In this talk, I treat only the first(1) of these topics. WHAT ARE NUCLEAR WEAPONS? I have been involved with building and testing U.S. nuclear weapons from 1950 to the present day, so what I am about to tell you is limited by what can be said legally and prudently. All nuclear weapons are based on a fission chain reaction carried by neutrons. In a nuclear weapon, a supercritical mass of fissionable material needs to be assembled, such that a few fast neutrons introduced will cause fission. Since 2.5 to 3.5 neutrons are emitted per fission in U-235 or Pu-239, in an infinite medium each neutron disappearing gives rise to about two new net neutrons, and the neutron population (and energy density due to fissions that have occurred) grows exponentially as e&alpha.t. The "infinite-medium &alpha." corresponds to no loss of neutrons from the assembly. For a mass that is just (prompt) critical, the neutron population remains constant, and the fission energy density increases linearly with time rather than exponentially. The &alpha. for an infinite medium increases linearly with density, since the time between generations depends on the mean free path of the neutrons, which is inversely as density. Finally, for any geometrical configuration of fissionable material (even including a reflector of neutrons such as beryllium or uranium-238), the amount of material required to form a critical mass is inversely as the square of the density. THE "GUN-TYPE" NUCLEAR WEAPON. The nuclear weapon that destroyed Hiroshima August 5, 1945, was a gun-assembled U-235 bomb yielding some 13 kilotons (kt) of high-explosive equivalent energy-- 13 x 4 TJ. It weighed some 8000 lbs and is reported to have contained some 60 kg of highly enriched uranium. Just as important as a supercritical assembly to make a nuclear weapon is the sub-critical nature of the material up to the moment a chain reaction is desired. U-235 has such small spontaneous neutron emission (and the flux of neutrons from cosmic rays is sufficiently low), that a normal artillery piece with a projectile velocity of 300 m/s will assemble two sub-critical chunks of U-235 into a single supercritical mass in a time short enough that the probability of pre-initiation is acceptably small-- moving from the just-critical assembly to the maximum criticality in a time that might be 0.2 m/300 m/s = 7 ms. The Hiroshima design had a neutron generator to initiate the chain reaction at maximum criticality. The complete fission of one kg of material liberates the equivalent of 17 kt. THE IMPLOSION WEAPON. By placing the U-235 or plutonium at the center of an assembly of high explosive, in which a converging spherical detonation wave is launched, even solid metal can be compressed so that a subscritical mass can rapidly be made supercritical. With a detonation speed of 0.6 cm/microsecond, a 10 microsecond interval may be appropriate for this compression. A neutron generator may be optional for a gun-assembled nuclear weapon, if the projectile can be stopped so that the supercritical mass awaits a cosmic-ray neutron, but a neutron generator is essential for the implosion weapon, which will just bounce and rapidly become subcritical. But the gun is totally unsuitable for plutonium, which, with its nominal 6% Pu-240 content and a 6-kg mass in the Nagasaki bomb, emits about 0.36 neutrons per microsecond by virtue of spontaneous fission of the Pu-240. Because plutonium has both a larger fission cross-section and more neutrons emitted per fission, and density comparable to uranium, smaller amounts of Pu are needed than U-235. The 6-kg ball of the Nagasaki bomb gave a yield of almost 20 kt. EVOLUTION OF FISSION WEAPONS. Implosion is the assembly method of choice for U-235 as well as Pu. By 1951, with the aid of nuclear explosion tests, the U.S. had produced and stockpiled the "half-size" implosion weapon-- the Mk-7, weighing 1800 lbs and with a yield considerably larger than the 20 kt of the Nagasaki bomb. South Africa did build six gun-assembled U-235 weapons, without any explosive test, just as the United States did not test its gun before using it on Hiroshima. U.S. nuclear weapons (and perhaps those of the other four Nuclear Weapon States under the 1970 Non-Proliferation Treaty, NPT) now consist of hollow plutonium shells surrounded by high explosive. The shell can be accelerated and then abruptly arrests itself by symmetry, leading to greater compression than can be achieved in a solid sphere. Accordingly, the Pu content in a U.S. nuclear weapon is as little as 4 kg. BOOSTED FISSION WEAPONS. In 1951, the United States first tested "boosting." In the modern boosted fission weapon, deuterium and tritium gas are present in the hollow plutonium shell at the time of implosion. The d-t gas is compressed and then suddenly heated by the fission reaction, which provides thermonuclear reactions between d and t (at about 100 times the rate of that between d and d). Each d-t reaction yields a 14-MeV neutron, which has a high probability of causing fission in the surrounding Pu, which in turn contributes 150 MeV to the energy of the explosion. Thus, it is not the energy from thermonuclear boosting, but the additional flash of neutrons and fission that "boosts" the fission chain reaction to a higher level and multiplies the energy release. In U.S. nuclear weapons, the deuterium and tritium are kept in high-pressure steel gas "bottles" and provided to the hollow plutonium shell after the weapon is launched, but before the high explosive is fired. The plutonium shell is surrounded by a welded metal enclosure; the whole metal assembly is a "pit", as it was called in the earliest days of implosion weaponry. TWO-STAGE THERMONUCLEAR WEAPONS. In 1951, Edward Teller and Stanislaw Ulam at the Los Alamos Scientific Laboratory in New Mexico published a classified paper on "radiation implosion," (an officially declassified term) in which the energy of the x-rays in the thermal electromagnetic field is used to compress and prepare a secondary charge of thermonuclear fuel and to ignite it by bringing the compressed fuel to a high temperature. The boosted fission primary and the secondary are contained in a radiation case, of material of a high atomic number, such as uranium.(2) These are called "two-stage" thermonuclear weapons to distinguish them from other approaches in which substantial amounts of fusion fuel are in close proximity to the fission explosive. Some orders of magnitude may be of interest here. At the high temperatures resulting from a nuclear explosion, everything within the bomb is a plasma-- only the heaviest nuclei have even one electron attached. The thermal kinetic energy is thus 3/2 kT for each of the particles, and the particles are by number predominantly electrons. So there is about one mole of electrons per two grams of material. If 17 kt of energy were to be confined to 6 kg of Pu, the temperature (ignoring the black-body radiation field) would be 170 keV. But this same 70 TJ of energy is far from enough to heat the vacuum(!) to 170 keV. Indeed, the black body energy density in the electromagnetic field goes as T4, and is about 13 GJ/liter at T = 1 keV, or about 5 GJ/0.4 l at 1 keV. The initial volume of 6 kg of Pu at 15 g/cc is just 0.4 l. The yield of 70 TJ would heat this same volume to 11 keV, so that the matter temperature would also be held to 11 keV. In fact, the opacity even of Pu is sufficiently low at such temperatures that much of the black body radiation leaks out to fill the radiation case at a considerably lower temperature-- for instance, lower by the fourth root of the ratio of the volumes. Thus if the radiation case has a volume of 50 l, the ratio of volumes is some 125, and the temperature could rise to as much as 3 keV. Since a given energy density of radiation has almost as much pressure as the same energy density in matter, the pressure squeezing on the secondary charge corresponds in this case to some 70 TJ/50 l or about 1.4 TJ/l. High explosive provides something like 4 MJ/l, so the radiation implosion proceeds with a pressure some 0.3 million times that of high explosive. RECAPITULATION. The United States has roughly ten types of nuclear weapons in its stockpile of perhaps 12,000 warheads and bombs. All are radiation implosion weapons, with gas-boosted hollow plutonium primaries, and secondaries as described. The secondary fuel is normally solid lithium deuteride (LiD). The nuclear explosion is obtained when electrical firing pulses are applied with sufficient simultaneity to all of the electric detonators, which in turn detonate a booster pellet and then the main charge of high explosive. Initially, a spherical converging detonation wave was obtained from the multiple diverging detonation points by the use of "lenses" of various types-- initially of fast and slow explosives, later with "ring lenses" to reduce the enormous mass of the lens region, and still later with "air lenses" or other means to obtain a detonation wave of the appropriate shape. The d-t gas from the reservoirs or bottles enters the pit via an explosively operated valve. STOCKPILE STEWARDSHIP. A 1979 memo from the Director of the Los Alamos Scientific Laboratory to DOE headquarters,(3) states: "Will the nuclear device work? The reliability of a nuclear weapon (that has been tested) is determined primarily by the reliability of its non-nuclear components and not by its nuclear components. The non-nuclear components can be demonstrated to be statistically reliable to a desired level, normally 98% or better, by doing a sufficient number of non-nuclear local tests. This procedure has not been altered by the Threshold Test Ban Treaty and will not be altered by the Comprehensive Test Ban Treaty. The reliability of the nuclear performance of a weapon is a different matter. It is not a statistical quantity. After a few key nuclear tests are conducted, it is the judgment of the Laboratories, based upon 30 years of design experience, that the nuclear performance is guaranteed if the non-nuclear components function as desired and the nuclear components are maintained in their as-built condition. A very important conclusion, then, is that there has been no reduction in nuclear weapon reliability as a result of the TTBT and that there will be none under a CTBT if we utilize current nuclear systems which have been tested or utilize previously tested nuclear components as subsystems." In the United States, nuclear weapons are designed, developed, manufactured, and cared for by the Department of Energy. In a sense, they are loaned or transferred to the military to be ready for delivery as bombs, missile warheads, or (in the old days) nuclear-armed torpedoes, anti-aircraft rockets, atomic demolition charges, and the like. The Los Alamos National Laboratory and the Lawrence Livermore National Laboratory, both operated for DOE by the University of California, have responsibility for the nuclear components-- roughly those within the radiation case of a thermonuclear weapon. Sandia National Laboratory, with its main facilities at Albuquerque, NM, and a smaller site at Livermore, has responsibility for the non-nuclear components, such as batteries, arming, firing, and fusing, Prescribed Action Links or other use-control system, state-of-health monitors, and the like. Of the 4000 components of a typical bomb, all but a few are Sandia's responsibility. In analyzing the difference between an era of underground nuclear tests and the CTBT, it must be recognized that most of these non-nuclear components are not exercised at all during an underground nuclear test. For instance, the environmental sensing system that ensures that a nuclear weapon will not explode until it has been through the appropriate accelerations, coast, and the like, is clearly not applicable to an underground test. Such components for the most part are testable on the bench. Those that are destroyed by their activation (such as explosively fired valves or thermal batteries) are tested in large quantities, so that those that have not been tested are assured of performance as random selection from a lot that has been tested to ensure the desired confidence in reliability. Explosive detonators (in the first implosion weapons, a bridge wire of noble metal fired by a substantial pulse of electrical energy) were initially used in the almost 100 lenses of an early nuclear weapon. Many thousands of bridge wires had to be fired without fail in order to have confidence in adequate reliability that not a single one would miss when fired "for effect." The Stockpile Stewardship Program has two goals: (I) a safe and reliable stockpile of nuclear weapons of existing type, and (II) a viable community to resume weapon development and testing in case the Comprehensive Test Ban Treaty (CTBT) era ends. I have been involved in reviewing and assessing the program for the Department of Energy, primarily as a member of the JASON group of consultants to the government. Some of our reports are available as unclassified documents on the Web:(4) I believe the relevant question is whether nuclear weapons can be maintained as safe and reliable as they were before the CTBT was signed in 1996 (and even before a congressionally mandated moratorium on nuclear testing took effect in 1992). Although much of the emphasis, publicity, and budget go to the advanced facilities and capabilities involved in stockpile stewardship, such as ASCI (the Accelerated Strategic Computation Initiative, at the three weapon laboratories); DAHRT (the Dual-Axis Hydrodynamic Radiographic facility, at Los Alamos); and NIF (the National Ignition Facility, at Livermore); the core of the program is really an enhanced surveillance and in the capability to remanufacture the weapons in the stockpile, as needed. WHAT IS REQUIRED TO KEEP NUCLEAR WEAPONS SAFE AND RELIABLE? Under the Enhanced Surveillance Program, 11 copies of each of the ten types of nuclear weapons in the enduring stockpile are removed each year and radiographed and otherwise inspected. One of each is totally disassembled, and the pit and secondary cut open for inspection. Samples of the high explosive are removed and tested, and the plutonium pit is carefully inspected. Items of concern are noted and eventually may be categorized as "actionable", leading to some kind of remedy on the entire stockpile of such weapons. Problems might be as simple as a loose screw, excessive friction on a mechanical component such as the use-control system, or incipient corrosion on some metal part. Plutonium and uranium both react with hydrogen and with water. Plutonium pits were formerly manufactured at Rocky Flats, CO, now closed. A replacement small-scale pit manufacturing facility is being qualified at TA-55 (Los Alamos) which might produce on the order of 30-50 pits per year. The United States has experience with pits on the order of 30 years old, which show no signs of deterioration, and the general feeling from recent inspections and from aging experiments done with higher Pu-238 content (78 years half-life vs. 24,000 for Pu-239) is that current pits are not expected to deteriorate for 60-90 years or more. Note that the lack of a pit production facility is not a CTBT issue; it would be the same problem whether or not one could use nuclear explosion testing. Without being excessively tedious, I note the conclusion of the JASON studies (and laboratory assessments) that a secondary charge, if driven with the design radiation energy in the radiation case, will provide the overall energy output. Given the ability to test-fire detonators and to inspect them carefully in the disassembly process, and also to fire samples of the high explosive-- and even one per year of the explosive in an actual weapon-- the major uncertainty in the performance of a nuclear weapon comes in the details of the boosting process. This is affected by how much plutonium is mixed with the d-t gas by the implosion itself, and that is, in turn, affected by the surface finish and corrosion (if any) of the plutonium surrounding the d-t gas. To address this question, so-called subcritical experiments are being performed at the Nevada Test Site, specifically to monitor the ejecta produced when a plutonium plate or partial shell is subject to high explosive shock on the other side. What is actually being done in the Science-Based Stockpile Stewardship Program (SBSSP)? In addition to the normal "custodianship" sketched above, the SSP adopted by the Department of Energy and its laboratories is "science based." In part, this adds assurance that the impact of defects or artifacts discovered during the surveillance process will be fully understood. The experimental and analytical capabilities, in principle, could be used to assess defects of substantial magnitude, and might show that remedial action was not needed until they had grown further. By delaying replacement or refurbishment, there is in principle here the possibility of saving money. But, especially with small numbers of weapons in the stockpile, the acquisition of these capabilities might be more costly than early or routine replacement of the warheads. Specifically, DAHRT extends the conventional pulsed x-ray facility used for dynamic imaging of actual implosion systems (with the fissionable material replaced by a simulant-- for instance, depleted uranium). The first axis is operating in Los Alamos, and the second will be available in about a year-- imaging at right angles, and with the possibility of having four pulses spread over a two microsecond interval. These flash x-ray images were initially captured on film, but are now caught on an array of scintillation crystals-- electronic imaging. DAHRT is an excellent tool of exploring new weapon designs that have been developed by analysis and computation, as well as the assessment of the influence of flaws that may be discovered in the surveillance program. No nuclear weapon in the stockpile was designed with computational capabilities exceeding the 500 MHz processor now common in your desktop PC. Yet the ASCI program has already resulted in systems at each laboratory operating at the three tera-operation per second (TOPS) level. Such facilities are used in the SBSSP to try to understand the fundamentals of detonation of high explosives, and the details of hydrodynamics, neutron propagation, and the like. The need for such enormous computing capabilities is often stated to derive from the three-dimensional (3-D) nature of a flawed or aged nuclear weapon, in contrast with the strictly 2-D modeling that suffices for a two-stage radiation implosion-- which, after all, has an axis of symmetry. In 2-D, the dynamics can be analyzed by assigning every volume element a variable radius and axial position (R,Z), and thus modeling the system as elementary rings of material. Of course, at every R,Z there are material properties-- identity, density, temperature(s), velocities, etc. Designs were done on such a 2-D basis, even though turbulence, or instabilities at interfaces have no reason to respect the 2-D assumption. Furthermore, in a flawed system, there is no reason to believe that a flaw will automatically have azimuthal symmetry-- a nick or gap in the R-Z plane, for instance, must be modeled (at least locally) in 3-D. Straightforwardly, if a 1-D spherical implosion would be modeled now with 1000 radial points, and a 2-D system with 1000 x 1000, then a 3-D model would need an extra factor 1000-- a billion points. The National Ignition Facility at Livermore (NIF) has been in the news recently, with technical problems and budget and schedule difficulties. It consists of 192 laser beam lines, intended to focus more than 1 MJ of short-pulse laser light into a mm-size gold cylindrical radiation case (or Hohlraum) where they will equilibrate to produce soft x-rays with a black body radiation temperature on the order of 300 eV. Considerably lower in temperature than the black body radiation available from a nuclear primary, and hence much lower in energy density (remember T4!), NIF will nonetheless allow validation on the sub-mm scale of radiation flow and implosion calculations relevant to secondaries. At Sandia Albuquerque, the "Z-pinch" has produced more than 1 MJ of thermal x-rays, filling a larger Hohlraum than NIF with a black body temperature of more than 200 eV. Such results are obtained by imploding a cylindrical shell of hundreds of fine wires by the self-magnetic field of 20 megamp currents. Interesting results have been obtained not only on radiation flow but also by the use of magnetic pressure to drive flyer plates to produce shocks in materials of relevance to nuclear weapons. In other experiments on the Z machine, the magnetic pressure is used to provide isentropic compression of materials, such as deuterium, to explore their equation of state. THE CTBT AND THE STEWARDSHIP PROGRAM. My judgment is evident from the comments above-- the enhanced surveillance program and a remanufacturing capability, and people to carry it out, will serve to maintain the primary and secondary of nuclear weapons in good shape for many decades or even centuries. Atoms do not age, and a weapon rebuilt in the year 2100 (using perhaps rather archaic processes) will be just as good as when it was manufactured in 1985. Recall the 1979 letter from the director of Los Alamos. The facilities that get all the attention, in my view, serve primarily to attract, challenge, and validate the people who will be involved in the surveillance and remanufacturing efforts. But it would be totally incorrect to equate the ability to obtain "ignition" of d-t gas or ice in NIF with confidence in the U.S. stockpile of nuclear weapons. And it would be equally wrong to assess a failure to achieve ignition (for whatever reason) as impugning in any way the capability of our stockpiled weapons. In fact, new-design weapons entering the stockpile have sometimes had an infant mortality problem, so that existing and remanufactured weapons are likely to be more reliable, and we should have more confidence in them than if we were replacing them by weapons of new design. The important point is that nuclear explosive tests are of little use in maintaining a stockpile of weapons of existing type. CONCLUSION. The inability to test with nuclear explosion yield does not inhibit the United States from maintaining its stockpile of existing nuclear weapons safe and reliable. And the same is true for the other four nuclear states with fully developed and tested nuclear weapons in their stockpiles-- Russia, Britain, France, and China. ---------------- 1 In a paper for the American Geophysical Union meeting May 31, 2000, (at http://www.fas.org/rlg), I discuss the other two topics as well. 2 I contributed to the design of the first radiation implosion, the MIKE test of November 1, 1952, which yielded almost 11 megatons (MT) from a secondary charge of liquid deuterium surrounded by uranium. 3 Harold M. Agnew to J.K. Bratton, February 13, 1979. 4 At http://www.fas.org/rlg (search for "JASON" or "JSR").