An Expanding Role for AGEX Above-Ground Experiments for Nuclear Weapons Physics Philip D. Goldstone Procyon, the high-explosive pulsed-power generator at Los Alamos or the last fifty years U.S. ex- Imagine, twenty years from now, a bilities of others), if we are without pertise in nuclear-weapons de- military commander or a member of any fully integrated nuclear tests and Fsign and engineering was a nat- a stockpile-surveillance team notic- without ongoing development pro- ural by-product of an active develop- ing that one of the weapons being grams. It will be necessary, but not ment and testing program. With the stored has changed in appearance. sufficient, for the scientists and engi- end of the cold war, the development They will want to know, “is this still neers involved in this enterprise to of agreements to retire most of the safe, and would it work if needed?” do research; the research itself must U.S. and former Soviet weapon stock- They will call the Laboratory and stress their judgement in weapons piles, and plans to phase out U.S. nu- ask the experts regarding this science and engineering. Further- clear testing by 1996, the way the nu- weapon. Will they be able to rely on more, to prevent losing capability clear weapons program has operated the answer they get? A competent within a generation, these research is changing dramatically. However, technical capability—including the programs must be technically inter- nuclear weapons will remain a pres- honed expertise and judgement of esting to attract and train new scien- ence in the world for the foreseeable weapon designers and engineers—is tists and engineers. future, and the U.S. will retain a the principal element required for In the absence of underground smaller but very real nuclear capabili- the assurance and stewardship of testing, a high premium will be ty. We at the weapons laboratories whatever nuclear stockpile the nation placed on predictive computational will therefore have a vital responsi- retains. At Los Alamos, where over capability for design and engineering bility—that of long-term stewardship 80 percent of the weapons that are of weapons. If our computer-simula- of the stockpile, including continued expected to remain in the stockpile tion capabilities were perfect, then, assurance of reliability and safety. were designed, this responsibility is in principle, actual testing would not Since the weapons remaining in the particularly important. be required for us to be confident of stockpile will age, eventually we will Without nuclear testing we will the performance of an aging, modi- have to provide competent assess- simply not be able to maintain to- fied (for improved safety, for exam- ments of the need to modify or re- day's level of competent judgement. ple), or even redesigned device. place aging weapons or components. The challenge we now face is to con- Computational-simulation capabili- This role will continue indefinitely tinue to exercise our weapons R&D ties are simply insufficient for this into the future and beyond the career expertise so that we retain—as much challenge, despite many years of de- lifetime of many of today's weapon as possible—the ability to assess our velopment and the ongoing revolu- scientists. own systems (and the nuclear capa- tion in high-performance computing. 52 Los Alamos Science Number 21 1993 An Expanding Role for AGEX Therefore, part of the strategy for Second, and possibly more important, fidence in the U.S. stockpile. Explo- meeting the dilemma posed by a experiments provide ongoing valida- sives characterization as well as hy- cessation of nuclear testing is to tion and exercise of weapons capabil- drodynamic testing is discussed in develop higher-performance com- ity and judgement. A proper mix of “AGEX I—The Explosives Regime puting capabilities and to exploit experiments will even actively in- of Weapons Physics.” those capabilities by running more volve the fabrication, engineering, The second physical regime that accurate and more predictive codes. quality assurance, and fielding skills must be addressed by experiments is Although the weapons-design vital to weapons capability. Third, the “high-energy-density” (or AGEX codes may be viewed as an “archive” only a technically stimulating re- II) regime, which is typically where design physics is contained search program that combines experi- achieved only after the initial pro- for future designers to use, contin- ment with theory is capable of at- duction of nuclear energy. It in- ued comparison of code predictions tracting and retaining quality scien- volves temperatures from tens to against experiment—reality—is re- tists and engineers in the weapons thousands of electron-volts or eV (1 quired to provide validation of the effort. Finally, in its own right, ex- eV is equivalent to 11,600 kelvins) codes and the designer's judgement. perimental capability—the ability to and pressures greater than 10 The physics, materials behavior, and measure and interpret data, whether megabars (1 megabar equals 1 mil- engineering associated with nuclear that data derives from an under- lion atmospheres). Technical issues weapons is extremely complex. In a ground weapons test, an above- in the high-energy-density regime matter of moments, weapon compo- ground experiment, or from other include radiation flow and the inter- nents are brought from their normal sources such as nonproliferation ac- action of radiation with matter; radi- physical state to the most extreme tivities—is an essential component of ation hydrodynamics and hydrody- conditions found in the solar system. U.S. nuclear weapons stewardship. namics at extreme pressures; nuclear It is the act of continually testing The following two articles dis- cross sections and neutron interac- both the codes and the designers cuss the two physical regimes that tions; and the behavior of dense against experiment that develops and must be addressed in experiments plasmas. maintains expertise and that prevents related to weapons design. The “ex- Whereas the explosive regime can theory and reality from diverging. As plosives regime” (or AGEX I) in- be more or less directly accessed at long as a simulation code contains cludes the physics and chemistry of full scale, achieving the high-energy- implicit approximations and assump- explosives as well as the behavior of density regime over anything ap- tions, its value is intimately tied to matter subjected to the pressures, proaching the full spatial and time the judgement of those who use it and shocks, and temperatures that may scales of a weapon would imply an interpret its output. A divergence of be achieved with typical high-explo- extraordinary amount of energy, com- theory and reality can (and often sive configurations. For example, parable to that of a nuclear explo- does, in many human endeavors) re- the behavior of heavy-metal assem- sion. Although this is clearly neither sult in a false sense of confidence in blies compressed by high explosives achievable nor desirable in an above- computation or design judgement that is an important element of weapon ground experiment, various pieces of is potentially disastrous. This is the design and engineering. the physics can be accessed and stud- principal underlying reason for regu- Above-ground experiments direct- ied by using an appropriate set of larly performing nuclear tests, even if ly applicable to many (though not specialized facilities. The efforts to we could anticipate never having to all) important aspects of the high-ex- develop and use such facilities for modify a stockpiled weapon again. plosives regime of weapon design are the study of the high-energy-density Therefore, in the absence of under- possible without a nuclear explosion. regime are discussed in “AGEX II— ground testing, appropriate “above- The principal example is that of hy- The High-Energy-Density Regime of ground experiments” (AGEX) must drodynamic testing, including flash Weapons Physics.” be vigorously pursued. radiography of high-explosives-dri- The role of experiments is, first, to ven assemblies. The DARHT (Dual- provide the physics data and the un- Axis Radiographic Hydro Test) facil- derlying physical models to improve ity is the single most important new the predictive nature of the codes. AGEX capability for the future con- 1993 Number 21 Los Alamos Science 53 AGEX I the explosives regime of weapons physics Timothy R. Neal he history of explosives re- hitherto not contemplated. The such as triaminotrinitrobenzene search and above-ground ex- achievement of those goals left Los (TATB)—can be dropped from great Tperimentation for nuclear Alamos, at the end of World War II, heights and will shatter but not ex- weapons began with the Manhattan uniquely in possession of the most plode. If exposed to fire in an acci- Project. During the hectic, almost advanced explosive-fabrication tech- dent, TATB will burn, but it is ex- frantic, war days at Los Alamos, it nology on earth and a mission to tremely unlikely to undergo a transi- became clear that, if possible, the fis- make nuclear weapons safer and tion from burning to deflagration or sionable material in the weapon more efficient—a mission that has detonation. Even when exposed to should be plutonium. It was equally continued into the present. high temperature, extreme pressures, apparent that the critical mass of plu- For a long period of time, the or shocks, these materials resist ex- tonium needed to produce a nuclear work on weapons implosions has plosion. Thus, they can be handled explosion would have to be assem- utilized conventional plastic-bonded quite safely with simple precautions. bled in the weapon through a spheri- high explosives, which could be pre- In addition to safety, the stability cal implosion driven by powerful ex- cisely machined. Improvements and reliability of nuclear weapons in plosives (Figure 1).