Volume 23, Number 4 October 1994


Plutonium Disposition

Alex DeVolpi has made useful contributions to nuclear policy. His article (July 1994) on the supposed military inadequacy of high-burnup or high-even-isotope plutonium, and on how plutonium might be demilitarized by such simple expedients as mixing "weapons and separated reactor plutonium...in equal proportions," is not one of them.

His thesis, and his distinction between denatured and weapons-grade plutonium, are no more correct in 1994 than when published at book length 15 years ago (1). An ample (though discreet) explanation of why they are wrong, carefully reviewed by leading experts before publication as a Nature Review article (2), remains valid and unrebutted. In brief:

All plutonium isotopes are fast-fissionable. Even isotopes produce only minor changes in reactivity, neutron spectrum, and mean prompt-neutron lifetime. For example, increasing the 240+242 Pu content from 6% to 30% increases bare-sphere prompt-critical mass mc in the a phase by only ~2 kg. In fact, no known prompt-neutron absorber can make plutonium of any practical composition incapable of forming a prompt-critical mass -- for example, substituting 13B, the best known fast-neutron absorber, for 16O in reactor-grade crystal-density PuO_2 only doubles its m_c -- and of course any such absorber could be chemically separated. All the thermal and other effects of high even-isotope content can be readily handled by appropriate design.

Performance penalties (mc, yield, yield/mass, yield dispersion, shelf life, etc.) for using high even-isotope content range from considerable to insignificant, depending on design. With modest design sophistication already present in most military warheads, using components that are commercially available and techniques that are widely known, plutonium of any isotopic composition can produce quite powerful and predictable nuclear explosions. In essence, sufficiently rapid reactivity insertions can reliably overcome preinitiation caused by the spontaneous-fission neutron background.

High-technology designs can decrease yield dispersion to essentially nil. But even though combining unsophisticated designs with high-burnup plutonium can cause quite unpredictable yields, they are still likely to be kiloton-range (fissioning 1 kg yields 17 kT). Nor is their unpredictability inconsistent with a highly credible threat: what matters is not whether the designer can accurately predict the yield, but rather that the intended victim cannot. Moreover, blast area decreases only as the 2/3 power, and lethally irradiated area as the 1/3 power, of yield, so even large variations in yield normally produce results varying by less than is normally expected from differing circumstances of use, such as weather and topography.

Plutonium with high even-isotope content, though more awkward, is a perfectly serviceable explosive material. Designers prefer, and military designers with a free choice of materials use, low-burnup plutonium for the same reason that cabinetmakers prefer rosewood to pine; but pine works too. If pine can be innocently obtained in large quantities but rosewood cannot, those anxious to do woodworking may well prefer pine despite its suboptimality. Similarly, the extra fiscal and political cost of the unambiguously military dedicated facilities needed to make low-even-isotope plutonium would make ordinary reactor-grade plutonium more convenient for a clandestine weapons program, especially as it is made in supposedly civilian power reactors that hungry vendors and their captive export banks will gladly pay one to build.

The potentially greater difficulty of using high-burnup plutonium in boosted designs and in triggers for fusion secondaries is irrelevant, since anyone who can build such fancy designs can also work around suboptimal plutonium, and since, as the U.S. twice showed in 1945, even crude, kiloton-range pure-fission bombs can destroy cities and defeat empires.

Whether high-even-isotope plutonium can be directly substituted for the original low-even-isotope plutonium in a given military bomb is likewise irrelevant: Anyone with the skills to make or modify a military bomb would have little trouble either adapting its components or substituting suitable new ones. Either way, the result would be a formidable explosive.

The NAS report is therefore correct that all plutonium is "weapons usable." Characteristics, techniques, and outcomes may differ, but the differences, if any, are not militarily important or politically relevant.

Plutonium certainly cannot be protected by thoroughly mixing it with old myths. This one should be vitrified, buried deeply, and forgotten. Managing and ultimately disposing of 100 tons of plutonium is hard enough already.

Amory B. Lovins
Director of Research
Rocky Mountain Institute
1739 Snowmass Creek Road
Snowmass, Colorado 81654-9199

1.	A. DeVolpi, Proliferation, Plutonium and Policy, Pergamon (New York), 
2.	A.B. Lovins, ""Nuclear Weapons and Power-Reactor Plutonium," Nature 
	28 February 1980, pp. 817-823.

Trouble for Physics, and Fermi's Question

The real "Trouble for Physics" (July 1994) is the anti-religious, anti-life agenda of Art Hobson. Such an agenda is inappropriate for the role of physics in society, and insulting to physicists like myself.

As physicists primarily funded by tax payers, our main social objective ought to be making scientific discoveries accessible to the general public for intellectual growth. We ought not be forcing scientific and philosophical ideas on the public, nor trying to attack their religious beliefs. If we are so arrogant as to assert that we physicists have the final say in all matters of knowledge, then it is no wonder that the public perceives physicists as "mad scientists."

Second, we have an obligation to see to it that scientific discoveries are used in ethical ways. Too often, ethical debates that have raged for centuries are ignored as soon as we bring about new technological advances. Such issues are certainly controversial, but since our work has a direct impact on the life around us, we need to take responsibility for ethical use.

Regarding Art Hobson's commentary "Fermi's Question and the Human Condition" (April 1994), the glaring flaw in his hypothesis is that not only the billions of galaxies, stars and planets are necessary for extraterrestrial contact, but also human beings. It is sad that we treat people, especially those of the third world, as statistical hindrances to us having more VCRs, higher definition TVs, and faster home computers. Since we aren't willing to share our abundances with the poor, and we can't bear to see them die, there is a frightening immoral belief that we have an obligation to see that they never get born. Not only is this action unethical, but economic theory teaches us that no matter whether there are a hundred people or ten billion people for given amounts of resources, there will always exist the poor and the wealthy.

So let's keep to physics in our physics publications. I didn't become a physicist to assert my own political and social agenda in physics print. The APS ought not be getting into the business of non-physics-related political agendas.

T.A. Pelaia II
316 Highland Road
Apartment A303
Ithaca, New York 14850

Fermi's Question and the Human Condition

Your editorial (April 1994) raises the right questions, but there is an additional one to be added: With current ideas of morality, is there a viable strategy for avoiding the fate outlined? Perhaps we should be considering what modifications are required if we are to succeed in avoiding the demise of our technological civilization. Or is that like Surgeon General Elders found out when raising the question of considering drug legalization: whether or not it makes sense, it is unthinkable?

Elmer Eisner

Job Crisis: Statistics and Recommendations

Zachary Levine (July 1994) provides some useful data and makes some excellent recommendations. However, I question one statement that he makes: "Specialized education carries an implicit promise that a career using that training will exist."

Such an implicit promise is too much to ask of a department or of the physics establishment generally, and maybe not even a desirable goal. What is needed, I suggest, is full information made available to the student. If the student, armed with such information, decides to pursue a Ph.D. in physics, he or she should not be turned away by a department doing its social duty.

For a long time (maybe forever) there have been specialized fields in the arts and humanities where the job market is poor and students know it, but where no professional efforts to curtail enrollments or degrees have been undertaken--and where there are no implied promises of jobs in the field. At a different level, an upwardly aspiring laborer may enroll in a technical-college course to gain the skills of an electrician. The best this person can do is to try to learn everything possible about the job market. It isn't up to the college to ration its offerings or meter its enrollment. One can't help recalling the Soviet system where the pipelines were strictly regulated and jobs were guaranteed--not, I think, a good model.

At the same time, Levine is on target in stating that long, specialized training for nonexistent jobs is in no one's interest. However, let's keep in mind that this principle, valid for doctoral training, has no relevance to undergraduate training. The world can absorb--and really needs--lots more bachelor's degree physicists.

Kenneth W. Ford
729 Westview Avenue
Philadelphia, PA 19119

After the recent articles and letters about bleak employment prospects for physicists (the latest by Zachary H. Levine, July 1994), I must add my two cents' worth.

Suppose one knew for certain that the only job one could get after completing one's education, regardless of educational level, was a menial factory job. Under those conditions I'd be even more likely to pursue a Ph.D. than at present, because being a student might represent my only opportunity to be technically challenged. The point is that a doctorate doesn't have to improve one's job prospects in order for the education to be worthwhile.

Must job status be the only measure of success? Many unskilled workers make more money than Ph.D. physicists, but how many of us would trade our lives for theirs? The cultural benefits of education are not a myth. Being a graduate student is difficult, as is any lifestyle which leads to fulfillment. But how much better it is than a life which is too easy!

Levine's goals are laudable, but we mustn't lose sight of the emphasis on fundamental understanding which makes a physics degree special. Physics students should be encouraged to take more engineering courses rather than modify physics courses to include more engineering.

Let's be forthright and realistic about job opportunities for physicists, but don't curtail students' options. Let market forces determine supply and demand for physicists. The physics curriculum constitutes an awesome intellectual heritage which inspires, stimulates, and enriches even those (like myself) who leave the discipline. Don't sell it short.

Judith C. Powelson
Graduate student, Electrical Engineering
Campus box 440, University of Colorado
Boulder, Colorado, 80309-0440

Format of Forum Sessions at APS Meetings

This is a response to Alvin Saperstein's solicitation (July 1994) for opinions regarding the format of the APS meeting sessions for the Forum on Physics and Society. I believe that it is vital that we physicists have an organized forum at APS meetings to discuss societal issues, now more than ever. Although I sympathize with those (one of whom recently published an eloquent and passionate letter in Physics Today) who object to APS activities that are not technical in nature, I also feel that we physicists need to contribute, as an organized group, to the solutions of urgent non-technical problems facing America and the world. I cannot condone the segregation of our members' scientific interests from considerations of the social crises in the midst of which we pursue those scientific interests, much as I wish we could afford to. To promote such segregation in the APS is to tacitly deny that American and/or world social crises significantly affect the ability of American scientists to do science. Such denial is wrong, dangerous for science, and presupposes that professional scientific societies bear no responsibility for human welfare.

Ten minutes is often too brief for presenting complex issues to colleagues who are not, generally, experts in the subject at hand. I suggest putting some flexibility into the format. Allow each speaker anywhere from 10 minutes to 30 minutes (or even more) for their presentation and discussion time. The actual time should be agreed upon in advance of the session so that the chairman of the session can publish the start times of all the talks.

Regarding fears of our being accused of dilettantism by our colleagues: The arrogance of some of the physics community, born at the Trinity test and slowly (and very painfully) dying since the end of the Cold War, does not serve science nor society well. Let the accusations come! Those of us who can accomplish worthwhile things by combining our interest in physics with our interests outside of traditional physics (whether technical or non-technical) will not be perturbed by purist criticisms. Instead, we will do what good scientists have always done: Evaluate the criticism, correct our path if the criticism is valid, point out the errors of our critics when necessary.

We can expect the spectrum of quality of presentations to be the same as the spectrum at scientific presentations: There will be excellent, good, poor, and awful presentations. The long term viability of the sessions will depend on the overall ("average") quality of the presentations, as it should.

Jeffrey Marque, Staff Physicist
Beckman Instruments, Inc.
1050 Page Mill Road, Box 10200
Palo Alto, California 94303-0803

Asteroids for Impact Propulsion and Power

Small Earth-approaching asteroids have recently replaced the Soviet Union as a growing threat to be diverted with nuclear explosive and "star wars" technology (1). However, it may actually be desirable to divert non-threatening material on near-Earth asteroids towards Earth. Gravitational free-fall then amplifies its energy, and its impact with initially-slow craft at the edge of the atmosphere (around 100 km) could propel the latter into orbit, providing a potentially inexpensive way to put objects into orbit

. Silicate material extracted from a 1.2 AU circular-orbit asteroid can be brought to Earth e.g. by solar-thermal evaporation of 63% of the material acting as rocket-type propellant with average backward exhaust velocity (alpha)v_e, where v_e(= 3 km/s) is the ejection velocity from the surface and (alpha)(= 0.5) accounts for the deviation from a linear expansion of the vapor. For an original 120 tons, a 6-month trip would require a reusable solar concentrating reflector of diameter 16 m and mass 1.2 kg for aluminized mylar; icy material would require an even smaller reflector. The residual 44 tons then has a gravitationally boosted kinetic energy 31 times the input backward exhaust energy need to attain it. (For a 3 AU asteroid this factor becomes 4.0 with (alpha) = 0.8, a 21 m reflector and a residual 14 tons.)

Our 44 tons must next be spread out into packets directed towards a craft barely lifted, e.g. by ramjet or air-fuel explosive impulses, to the edge of the atmosphere. This directing can be done by small reusable homing or remote-control thrusters separating out at the last moment to avoid damage, or by surface-ablation thrust induced by external laser pulses; the craft could itself be steered into the path of the packets. To prevent craft damage, the packets would have to be rapidly expanded to low density just before impacting a "pusher plate" on the craft, e.g. by small masses expelled from the craft colliding with the packets at high relative velocity. The impacts would then launch a 34(1 + (epsilon))-ton craft into low-Earth orbit, where (epsilon)(> 0) is the elasticity.

Six packets should suffice for a craft which can tolerate high acceleration a. Shock absorbers are needed if a mass fraction F cannot tolerate high acceleration (Balzs, January 1991); F ~ 1/3 and a ~ 4 g would require 132 packets, a maximum velocity increment of 63 m/s per impact, shock-absorber stroke length of 38 m, 1.5 s between impacts and an overall impact-altitude spread of 20 km, which can be confined to the atmosphere. Any debris would then either escape Earth entirely or end up in the atmosphere, perhaps after following an elliptic path outside of it.

An uninterrupted packet supply would require ~ 6 asteroids and 66% evaporation, or e.g. one entire 3 x 10^5 ton asteroid first brought to translunar Earth orbit in 1.5 years with 86.5% evaporation and a 543 m 1.2 ton reflector.

Electric power could be generated by impacting orbiting magneto-hydrodynamic generators or reverse mass drivers from opposite directions. It could be used in space or beamed, directly or via relay, by cloud-penetrating microwave for environmentally-benign use on earth.

L.A.P. Balazs
Department of Physics
Purdue University
West Lafayette, Indiana 47907

1.	Ahrens and Harris, Nature 360, 429 (1992); Melosh and Nemchinov, 
	Nature 366, 21 (1993)