Can a Return to the Fusion Hybrid Help both the Fission and Fusion Programs?

Magnetic fusion has fallen on hard times recently. This article suggests that the jump from fusion research to a fusion economy is simply too large. Legislatures cannot plan over such a long time scale. The best way for the fusion project to prosper may be to re-orient itself toward a shorter-term goal, the fission/fusion hybrid. Underlying this proposed strategy are three basic assumptions: first, nuclear power will again become important; second, natural nuclear fuel is limited in supply, necessitating breeding; and third, reducing proliferation dangers is important in any nuclear option. This paper is a condensed summary of a thorough, fully referenced study written by the author [1].

In the 80's and early 90's, panel after panel studied the magnetic fusion program and proposed healthy increases in funding to support the building of a commercial fusion plant some time in the distant future. The statement by the Energy Policy Committee of IEEE (Institute of Electrical and Electronic Engineers) on the FY 1993 Department of Energy Request for Fusion spoke of 5% real growth per year. For this amount we could construct a demonstration reactor in 2025, followed by a commercial plant in 2040, assuming all the major nations of the world cooperate. However each year, what we have seen is more like a 5-10% decrease in the magnetic fusion budget. The cumulative effect is shown here, where the magnetic fusion budget, in 1997 dollars is plotted as a function of year.

The problem, as this author perceives it, is that the time scale for magnetic fusion development is very long compared to election cycles, political careers, recessions, wars, oil crises, etc. Lawmakers are unable to maintain interest in a project such as fusion, which has no immediate need and no immediate payoff. The figure was compiled while Congress and the Presidency were controlled by Democrats and Republicans in just about every possible permutation. If our elected leaders are so consistently sending this message, who are we to say they are wrong?

One proposed fix has been to internationalize the program with ITER (International Thermonuclear Experimental Reactor), a very large tokamak. The total construction cost was proposed at over $10 Billion with at least a $500 million dollar per year operating cost. To approve this project, not only must the American Congress agree, which is difficult, all of our foreign partners must agree as well. This introduces an even larger element of instability into the system. Any one can delay and possibly disrupt the project. An example is JET (Joint European Torus), a very successful tokamak project. At the outset, however, it was delayed for years and years as the European partners squabbled over where to build it. ITER multiplies these difficulties by a large factor. Over the years the project has had many ups and downs. As of this writing, Europe, Japan and Russia are tentatively planning to build a half size version without American participation. All of this investment is for what lawmakers must see as an extremely expensive power plant. When it comes time to actually appropriate money, this consensus will be greatly strained. This author sticks with his assertion, first made in April 1998 that for these reasons, ITER will never be built!

We suggest the fission/fusion hybrid as an alternate strategy to speed up the time for producing energy. The fission/fusion concept dates back to the earliest days of the fusion project when it was recognized that using fusion neutrons to breed nuclear fuel would vastly increase the energy from a fusion plant. It was last reviewed in the mid 80's by the National Academy of Science, but appears to have received almost no attention since then. They recommended proceeding with pure fusion instead. Since this is not happening, perhaps the issue should be re-examined.

The nuclear industry is certainly in the doldrums now, with no new orders for reactors and the price of mined uranium still fairly low. In a 21st century world with 10 billion people, all of who want a middle class life style, our basic thesis is that the nuclear industry will come back, probably in this country, and almost certainly in the world. Recently an entire issue of IEEE Spectrum was devoted to this issue--emphasizing technical advances in nuclear technology, including the possibility of passively safe nuclear power plants [2]. Unfortunately, nuclear power has its own energy supply problem. Estimates of the amount of naturally occurring ²³⁵U vary, depending on what recovery technology is assumed to be economical. Most estimate the energy content as about equal to that in the world's oil. Ultimately, breeding will be necessary.

Most of the focus has been on breeding ²³⁹Pu from ²³⁸U. Many knowledgeable experts have pointed out the dangers of a plutonium economy. An alternate nuclear cycle, namely breeding ²³³U from thorium could greatly reduce these dangers. Now is the time to investigate this cycle, so that when the nuclear industry receives orders for new plants, this option will be evaluated and available.

Fusion reactors could be very efficient producers of nuclear fuel. Tokamaks such as JET and TFTR have already generated about 1019 neutrons per second in a one-second pulse in a deuterium tritium (DT) plasma. JT 60 U has produced comparable hydrogen and deuterium plasmas. The Q (fusion power divided by beam power) is just under unity. Let us see what this would mean for a fission/fusion breeder. In a well-designed breeding blanket, each 14 MeV fusion neutron would generate approximately one ²³³U and one tritium atom. When this ²³³U burns in a fission reactor, it releases approximately 200 MeV. Thus in going from pure fusion to fission/fusion, one increases the total Q by approximately a factor of 14. This means that today's tokamaks, running cw could provide fuel for a 300 MW nuclear plant. Possibly the larger next generation fusion reactor could actually produce fuel economically

This paper therefore suggests a revitalized fusion program, to be accomplished by constructing such a small fission/fusion reactor. The size would be comparable to TFTR, but run steady state or at high duty cycle, and at high neutron flux. In short, it would run in a physics regime largely known. Research and development on maximizing the duty cycle in a DT plasma, and research on the associated nuclear technologies would be as important as the plasma physics research milestones. In such a fission/fusion research program, a double purpose would be accomplished. First, progress will be made on a much safer nuclear cycle, one which does not build up plutonium, but rather could build it down. Second, fusion research will be greatly enhanced. Furthermore, these objectives can both can be accomplished reasonably soon, in a decade or so, at which point fusion will contributing, albiet in a very small way to the nation's energy budget.

A single fusion breeder can supply many more satellite reactors than a single fission breeder reactor because fusion is neutron rich and energy poor, while fission is energy rich and neutron poor. In this sense at least, it is a perfect marriage. There will be relatively few fission/fusion plants (FFP's), but they will supply many nuclear power plants. Since there will be so few FFP's, they can be run, or closely monitored by the government in highly secured facilities. When introducing a new technology such as fusion, it would necessarily be less reliable and its down time would be greater. Since the FFP's are not the primary energy producers, the entire system could tolerate this much more easily.

The temper of the times certainly favors minimizing government involvement in energy production. Unfortunately, with any nuclear option, including fusion, this will not be possible. If the fuel is ²³⁵U mixed with ²³⁸U in a subcritical mixture, the government will be intimately involved in the isotope separation to make sure no ²³⁵U is clandestinely diverted. If ²³⁹Pu is the fuel, there will be an intrusive government presence at every power plant to prevent diversion. We have just discussed the necessary government involvement in breeding ²³³U. Even in the case of a fusion economy, any rouge nation could include ²³⁸U in the blanket, especially in a liquid or flowing blanket, breed plutonium, and produce atomic bombs. There exist real proliferation dangers to a fusion economy, which have thus far received very little attention.

The ²³²Th-²³³U cycle has important advantages in this respect. Only thorium enters the plant. There ²³³U is produced and mixed with ²³⁸U, so that only a subcritical mixture of ²³³U and ²³⁸U leaves. All of the material with bomb making potential (pure ²³³U in this case) would exist only in the heavily secured facility. Let us contrast this with a fission breeder economy. Since each fission breeder supplies only about one satellite reactor, about half of all power plants would have to be heavily guarded, as opposed to about 10% of them in a fission/fusion economy.

When this fuel mixture is used in conventional reactors, the ²³⁸U in it would generate small quantities of ²³⁹Pu. But this would be mixed into a highly radioactive waste and reprocessing would be difficult for hundreds of years. However this author certainly recognizes that there are long term problems with radioactive waste for any fission economy, and these are something the world will have to confront and solve.

Endorsing fission/fusion would obviously mean joining with the nuclear industry, which is weak and unpopular today. But are fission and fusion more naturally allies or competitors? Furthermore, can one add weakness to weakness and get strength? This author's contention is that the nuclear industry will and must come back and teaming with it will help both fission and fusion. He sees fission and fusion as allies, not competitors. Furthermore, no matter what we do, the rest of the world, especially China and India, will develop nuclear power. Whether we develop nuclear power in this country or not, there is a big export market out there for somebody; why not us? Also, by participating in the export market, this country will have a much greater voice in making nuclear power plants as safe and diversion resistant as possible.

Another factor that could bring the nuclear industry back is concern over global warming and green house gases. It seems likely that this problem will be regarded more and more seriously in the coming decades. If Congress ratifies the Kyoto Treaty, or a modified version of it, the United States will be obligated to reduce CO2 emissions by a very considerable amount.

If it makes sense to build a fission/fusion demonstration reactor the size of TFTR, the question is which type of fusion device to build. The only possible choice is a tokamak. This has certainly been the most successful fusion device worldwide for decades now. But, they have been built to such size that they can no longer be sustained by the reduced U.S. magnetic fusion budget. Accordingly, there is now an emphasis in the U.S. fusion project to go to other confinement schemes, to do more with less. However, just because a powerful politician or bureaucrat demands the laws of science bend to his will, that does not make it happen. Richard Feynman said it best in his report on the Challenge disaster, "Reality must take precedence over public relations, for nature cannot be fooled".

In this author's opinion, a turn away from the tokamak would be a calamity for the fusion project. Tokamaks were selected 30 years ago because they were the best way to confine a plasma. There were many alternate schemes then, and none could come even close to doing what tokamaks could. It is still true today, except that tokamaks have progressed even further. There is now a worldwide infrastructure supporting tokamak confinement, an infrastructure consisting of thousands of people working together for decades. No other confinement scheme has, or in the foreseeable future, will have anything close to this. This author gladly reiterates his offer to bet anyone that if the U.S. fusion project drops tokamaks in favor of other confinement schemes, in 15 years (from April 1998), these will not be where TFTR is today. Let us define these milestone as a) hQ>0.25, where h is the efficiency of the driver (the neutral beams in the tokamak case), b) 1019 neutrons per shot, and c) a reasonably clear technological approach to steady state or high duty cycle operation. Note that these other possible confinement systems will not only have to get over technical hurdles, but also political ones, which will not get easier in the coming decades. As larger and larger budgets are proposed for the new approaches (stellarators for instance), Congress will cut the program off just as it is doing today with tokamaks. To date, the author has had one taker; the offer continues.

The great advantage to the tokamak fusion hybrid, is that in a decade or so, it would contribute to the nations power supply in a small way. As Hans Bethe said [3], "It seems important to me to have an achievable goal in the not too distant future in order to encourage continued work, and continued progress, toward the large goal, in this case pure fusion." In the unforeseeable future, people may wish to, and be able to convert from a fission/fusion to a pure fusion economy. Unquestionable this is a decision for people who live at this time, people who are at least 50-100 years from even being born! However for the present, the sponsors of fusion energy have stuck with the project for almost 50 years without getting a single erg in return. Isn't it time that they got something for this tremendous support?

1) W. Manheimer, Back to the Future: The Historical, Scientific, Naval and Environmental Case for Fission/Fusion, Fusion Technology, 36, 1, 1999, also see NRL Memo 98-8151, April, 1998.

2) IEEE Spectrum, November, 1997.

3) H. Bethe, Physics Today, May, 1979.

Wallace Manheimer

Chevy Chase, MD

The author can be reached at:

Plasma Physics Division

Naval Research Laboratory

Washington, DC 20375

manheime@ccf.nrl.navy.mil