Induced nuclear reactions: processes – systems – and elements – Nuclear fusion – Including removal or use of impurities or reaction products
Reexamination Certificate
2002-02-14
2003-09-30
Behrend, Harvey E. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Nuclear fusion
Including removal or use of impurities or reaction products
C376S107000, C376S134000, C376S140000, C313S062000, C315S502000
Reexamination Certificate
active
06628740
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the field of plasma physics, and, in particular, to methods and apparati for confining plasma to enable nuclear fusion and for converting energy from fusion products into electricity.
BACKGROUND OF THE INVENTION
Fusion is the process by which two light nuclei combine to form a heavier one. The fusion process releases a tremendous amount of energy in the form of fast moving particles. Because atomic nuclei are positively charged—due to the protons contained therein—there is a repulsive electrostatic, or Coulomb, force between them. For two nuclei to fuse, this repulsive barrier must be overcome, which occurs when two nuclei are brought close enough together where the short-range nuclear forces become strong enough to overcome the Coulomb force and fuse the nuclei. The energy necessary for the nuclei to overcome the Coulomb barrier is provided by their thermal energies, which must be very high. For example, the fusion rate can be appreciable if the temperature is at least of the order of 10
4
eV—corresponding roughly to 100 million degrees Kelvin. The rate of a fusion reaction is a function of the temperature, and it is characterized by a quantity called reactivity. The reactivity of a D-T reaction, for example, has a broad peak between 30 keV and 100 keV.
Typical fusion reactions include:
D+D→He
3
(0.8 MeV)+
n
(2.5 MeV),
D+T→&agr;(3.6 MeV)+
n
(14.1 MeV),
D+He
3
→&agr;(3.7 MeV)+
p
(14.1 MeV),
and
p+
B
11
→3&agr;(8.7 MeV),
where D indicates deuterium, T indicates tritium, &agr; indicates a helium nucleus, n indicates a neutron, p indicates a proton, He indicates helium, and B
11
indicates Boron-11. The numbers in parentheses in each equation indicate the kinetic energy of the fusion products.
The first two reactions listed above—the D—D and D-T reactions—are neutronic, which means that most of the energy of their fusion products is carried by fast neutrons. The disadvantages of neutronic reactions are that (1) the flux of fast neutrons creates many problems, including structural damage of the reactor walls and high levels of radioactivity for most construction materials; and (2) the energy of fast neutrons is collected by converting their thermal energy to electric energy, which is very inefficient (less than 30%). The advantages of neutronic reactions are that (1) their reactivity peaks at a relatively low temperature; and (2) their losses due to radiation are relatively low because the atomic numbers of deuterium and tritium are 1.
The reactants in the other two equations—D-He
3
and p-B
11
are called advanced fuels. Instead of producing fast neutrons, as in the neutronic reactions, their fusion products are charged particles. One advantage of the advanced fuels is that they create much fewer neutrons and therefore suffer less from the disadvantages associated with them. In the case of D-He
3
, some fast neutrons are produced by secondary reactions, but these neutrons account for only about 10 per cent of the energy of the fusion products. The p-B
11
reaction is free of fast neutrons, although it does produce some slow neutrons that result from secondary reactions but create much fewer problems. Another advantage of the advanced fuels is that their fusion products comprise charged particles whose kinetic energy may be directly convertible to electricity. With an appropriate direct energy conversion process, the energy of advanced fuel fusion products may be collected with a high efficiency, possibly in excess of 90 percent.
The advanced fuels have disadvantages, too. For example, the atomic numbers of the advanced fuels are higher (2 for He
3
and 5 for B
11
). Therefore, their radiation losses are greater than in the neutronic reactions. Also, it is much more difficult to cause the advanced fuels to fuse. Their peak reactivities occur at much higher temperatures and do not reach as high as the reactivity for D-T. Causing a fusion reaction with the advanced fuels thus requires that they be brought to a higher energy state where their reactivity is significant. Accordingly, the advanced fuels must be contained for a longer time period wherein they can be brought to appropriate fusion conditions.
The containment time for a plasma is
&Dgr;
t=r
2
/D,
where r is a minimum plasma dimension and D is a diffusion coefficient. The classical value of the diffusion coefficient is
D
c
=&agr;
i
2
/&tgr;
ie
,
where &agr;
i
is the ion gyroradius and &tgr;
ie
is the ion-electron collision time. Diffusion according to the classical diffusion coefficient is called classical transport. The Bohm diffusion coefficient, attributed to short-wavelength instabilities, is
D
B
=({fraction (1/16)})&agr;
i
2
&OHgr;
i
,
where &OHgr;
i
is the ion gyrofrequency. Diffusion according to this relationship is called anomalous transport. For fusion conditions,
D
B
/D
c
=({fraction (1/16)})&OHgr;
i
&tgr;
ie
≡10
8
,
anomalous transport results in a much shorter containment time than does classical transport. This relation determines how large a plasma must be in a fusion reactor, by the requirement that the containment time for a given amount of plasma must be longer than the time for the plasma to have a nuclear fusion reaction. Therefore, classical transport condition is more desirable in a fusion reactor, allowing for smaller initial plasmas.
In early experiments with toroidal confinement of plasma, a containment time of
&Dgr;
t≡r
2
/D
B
was observed. Progress in the last 40 years has increased the containment time to
&Dgr;
t≡
1000
r
2
/D
B
.
One existing fusion reactor concept is the Tokamak. The magnetic field of a Tokamak
68
and a typical particle orbit
66
are illustrated in FIG.
5
. For the past 30 years, fusion efforts have been focussed on the Tokamak reactor using a D-T fuel. These efforts have culminated in the International Thermonuclear Experimental Reactor (ITER), illustrated in FIG.
7
. Recent experiments with Tokamaks suggest that classical transport,
&Dgr;
t≡r
2
/D
c
,
is possible, in which case the minimum plasma dimension can be reduced from meters to centimeters. These experiments involved the injection of energetic beams (50 to 100 keV), to heat the plasma to temperatures of 10 to 30 keV. See W. Heidbrink & G. J. Sadler, 34
Nuclear Fusion
535 (1994). The energetic beam ions in these experiments were observed to slow down and diffuse classically while the thermal plasma continued to diffuse anomalously fast. The reason for this is that the energetic beam ions have a large gyroradius and, as such, are insensitive to fluctuations with wavelengths shorter than the ion gyroradius (&lgr;<&agr;
i
). The short-wavelength fluctuations tend to average over a cycle and thus cancel. Electrons, however, have a much smaller gyroradius, so they respond to the fluctuations and transport anomalously.
Because of anomalous transport, the minimum dimension of the plasma must be at least 2.8 meters. Due to this dimension, the ITER was created 30 meters high and 30 meters in diameter. This is the smallest D-T Tokamak-type reactor that is feasible. For advanced fuels, such as D-He
3
and p-B
11
, the Tokamak-type reactor would have to be much larger because the time for a fuel ion to have a nuclear reaction is much longer. A Tokamak reactor using D-T fuel has the additional problem that most of the energy of the fusion products energy is carried by 14 MeV neutrons, which cause radiation damage and induce reactivity in almost all construction materials due to the neutron flux. In addition, the conversion of their energy into electricity must be by a thermal process, which is not more than 30% efficient.
Another proposed reactor configuration is a colliding beam reactor. In a colliding beam reactor, a background plasma is bombarded by beams of ions. The beams comprise ions with an energy that is much larger than the thermal plasma. Producing useful fusion reactions in this type of reactor has been infeasible because the background plasma s
Monkhorst Hendrik J.
Rostoker Norman
Behrend Harvey E.
Orrick Herrington & Sutcliffe LLP
The Regents of the University of California
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