|The slow reaction rate in the p-p chain and CNO cycle makes them in-practical for any application on Earth. The fusion reactions that seem most promising as terrestrial energy sources are listed in Figure 14-14a. These reactions occur at temperature about 100 times higher than that in the Sun's core. Reactions 5 and 6 are not thermo-nuclear reactions. They are used to produce the triton in reaction 1.
Figure 14-14a Thermonuclear Reactions [view large image]
|the Big Bang. The process lasted for about 15 minutes starting at temperature of 1010K (at the high end of the thermonuclear reactions in Figure 14-14a). Figure 14-14b lists the reactions in nucleosynthesis, many of them are identical to those in Figure 14-14a. Initiation of the process depended on the baryon to photon ratio, too many photons would drive the p + n D + reaction backward leaving no deuteron to proceed. Nucleosynthesis ended when temperature fell off with the cosmic expansion preventing elements heavier than beryllium to form. The calculation of mass abundances follows simple thermodynamic arguments and is
|insensitive to what happened before the process. Its agreement with observation presents a fairly reliable verification for the theory of Big Bang.
|However, long-lived pinched plasmas are extremely difficult to maintain. The plasma column is observed to break up rapidly. The reason for the disintegration of the column is the growth of instabilities. The column is unstable against various departures from cylindrical geometry. Small distortions are amplified rapidly and destroy the column in a very short time. The mechanisms of instability in plasma physics are nearly unlimited. Some instabilities are comparable to examples borrowed from fluid mechanics, as the Rayleigh Taylor's instability, which consists of superposing two fluids with the heaviest on top. Imagine for example a vessel in which you pour water and then carefully add oil over the top. The system is then in a state of meta-stable equilibrium. The slightest nudge will provoke a change with the heavier fluid dropping to the bottom, which corresponds to a stable equilibrium. Another type of instability are kink instabilities, which occur when a current parallel to the magnetic field cause twisting of the field lines, recalling the effect obtained if we twist a rope too much: the rope twists out and kinks. The sausage or neck instability causes a greater inwards pressure at the neck of a constriction. This serves to enhance the existing distortion.
Figure 14-14c Confinement of Plasma [view large image]
|In September 2006, Chinese researchers had, for the first time, managed to inject a plasma of ionized hydrogen into the Experimental Advanced Superconducting Tokamak (EAST, Figure 14-14d), and the plasma sustained currents of 250,000 amps for up to 3 seconds. But no attempt was made to introduce deuterium or tritium into the plasma, so no fusion has taken place. Eventually, the EAST team aims to hold the plasma for study for as long as 1000 seconds. Conventional experimental fusion machines use copper coils, or a combination of copper and superconducting coils, to trap the hot plasma. But copper coils heat up and need to be cooled down regularly, thus limiting operating time. EAST has only superconducting coils so it can be operated continuously. This US$25-million machine sets the stage for the multibillion-dollar
Figure 14-14d EAST
|ITER fusion experiment that is to be built in France, and starts operation in 2016.
|location of the reactor has been selected in Cadarache, southern France. The US$5.5 billion funding from ITER's six international partners could be in place by the winter of 2005, allowing construction to begin in 2006, and operation in 2016. ITER is designed to heat hydrogen to hundreds of millions of degrees centigrade, and then squeeze energy from the resulting plasma, while holding it stable for minutes at a time. It is based on the tokamak model, which up until today has only one machine that has begun to approach the "break-even point". It is believed that by building a tokamak with bigger size, it will allow the high-temperature high-pressure plasma to remain stable longer (~ 7-10 minutes) producing 500 megawatts of energy within the interval.
Figure 14-14e ITER
|By 2009 the ITER project faced with ballooning costs and growing delays, its seven partners are likely to build only a skeletal version of the device at first. This mini-ITER should be able to run in 2018. The full-scale version would not come alive until the end of 2025.
|The National Ignition Facility (NIF) was dedicated in 2009 for the production of nuclear energy by laser fusion. It tries to use 192 powerful laser beams (total 1.9 mega joules) to rise the pressure and temperature inside a hohlraum for triggering nuclear fusion in a pellet containing D-T gas (Figure 14-14f, also see text about their reaction below). The project promised to reach a break even point (called ignition) by September 2012. The target date has come and gone without meeting the much publicized goal. The problem seems to be related to leaking radiation and asymmetric implosion, which reduces the pressure and temperature inside the pellet (the problem could be solved by a spherical hohlraum instead of the cylindrical one?).
Figure 14-14f Laser Fusion
|Anyway, the research allocation for ignition will be reduced to 50% from 80%. The other 50% of the time will be used for experiments that mimic conditions inside nuclear weapons (to side step the moratorium on underground testing that began in 1992).
|The energy input is E1 ~ 1.9x106 j. However, due to several loss mechanisms, the actual energy delivered to the fusion target was much smaller than the 1.9x106 j pulse energy from the laser. That turns a different prospect on the experiment : "Preliminary analysis estimates an energy gain of more than 5 times the energy delivered to the capsule from the laser-produced radiation drive." as claimed by LLNL (Lawrence Livermore National Laboratory). Actually, the result is still under peer-review (as of August 8).
Figure 14-14g Laser Fusion, 2021
|Figure 14-14g shows the NIF facility for laser fusion.
Figure 14-14h Fusion Upstarts, 2014 [view large image]
|A smaller version of the colliding beam design adopts the D-D reaction. It runs like a stroke engine - injecting fuel / extracting energy in each cycle.
|fusion reactions (Figure 14-14i). Deuterium and tritium form helium under extremely high temperature, providing the energy:
D + T He + n + 17.6 Mev (See Figure 14-14j).
In principle, a mixture of D and T heated to very high temperature and in high density will start a chain fusion reaction, liberating an enormous amount of energy. But tritium is an unstable element; an ingenious method is to have it produced from lithium deuterate (Li6H2) in the
Figure 14-14i Thermo-nuclear
Figure 14-14j Fusion
|fission phase of the explosion -- thus one compound is used for both types of fuels ( D and T). In a thermonuclear bomb, the explosive process begins with the detonation of what is called the primary stage. This consists of a relatively small quantity of conventional explosives, its detonation brings together enough fissionable uranium or plutonium to create a fission chain reaction (sometimes magnified with a smaller fusion reaction), which in turn produces another fission explosion inside the temper in the secondary device and raises the temperature to several million degrees. When the temperature of the mixture reaches 10,000,000 oK, fusion reactions take place. The neutrons from the fusion reactions induced fission in the uranium-238 pieces (highly enriched with U-235) from the tamper and shield, which produced even more radiation and heat and the bomb exploded (See Figure 14-14k).
Specialized type of small fusion bombs designed to release neutrons rather than causing further fission reactions are called neutron bombs. This is accomplished by removing the U-238 tamper. Neutrons kill people, leaving the hardware and buildings intact. It is a "clean" bomb. The theorized cobalt bomb is, on the contrary, a radioactively "dirty" bomb having a cobalt tamper. This tamper is made of cobalt-59, which is transmuted into cobalt-60 by neutrons released from the fusion reactions. Cobalt-60 has a half-life of 5.26 years and produces energetic (and thus penetrating) gamma rays. The half-life of Cobalt-60 is just long enough so that airborne particles will settle and coat the earth's surface before significant decay has occurred, thus making it impractical to hide in shelters. This is the "doomsday machine" since it is capable of wiping out life on earth. The whole point about an automated doomsday response to nuclear attack is to let the world know about its existence, and to demonstrate the viability of the process perhaps in a
|small scale testing (to show that it's not a bluff). This is the ultimate meaning of deterrence.
|Steel, aluminum, etc.
|Outer layer of the bomb
|See Implosion type nuclear bomb
|Initial step of the thermo-nuclear explosion
|Confining X-rays for radiation implosion
|U, W, or Pb (not shown in diagram)
|Cavity in the casing for establishing the radiation implosion
|Uranium or tungsten
|To prevent premature heating
|Uranium, tungsten, lead, etc.
|Triggering the spark plug by implosion
|Another fission device to ignite the fusion fuel
|Fusion lasting for 10-9 sec to release a yield of ~ 50 MT
|0.4 - 400
|of course entirely classified. Because of the nuclear testing moratorium, researchers are trying to design new weapons without a test. It is known as RRW (Reliable Replacement Warhead). New design includes improving the plutonium pit, replacing the toxic materials with other heavier material (for the tamper to amplify the initial explosion), and substituting the volatile explosives (on the outer shell) with an insensitive type (Figures 14-14g). Such ideas have to pass the review of the generals, who have to change the software and hardware systems for delivering the bomb. RRW designers try to reassure the critics that the new warheads will be compatible with existing systems. Figure 14-14l shows the differences in the design of the old and new types. A 2007 study shows that existing warheads will last for at least another 50 years
Figure 14-14l H Bomb, Old and New
|making the new bomb seemingly less necessary. The US House of Representatives will vote on a bill that would eliminate funding for the RRW from the 2008 budget. But the RRW is not done yet. Other plans are working its way through Congress and the Senate.