Power from Hydrogen Fusion
Because of public concerns about nuclear fission in the United States, hydrogen fusion has been suggested as a way to take advantage of the properties of the atom in order to generate electricity. In theory, this is a great idea. Hydrogen fusion is more efficient in converting matter to energy than fission, and no radioactive waste is produced. But a workable hydrogen fusion reactor has not yet been developed.
FUSION IN THE SUN
Physicists believe that the sun converts hydrogen to helium by means of nuclear fusion. The term “fusion” means “combining.” Hydrogen fusion requires extremely high temperature. The powerful gravitation imposed by the sun’s huge mass keeps the core in a constantly compressed state. This compression keeps the core hot enough for hydrogen fusion to occur.
Solar hydrogen fusion is a multistep process. At first, two hydrogen nuclei (protons) are squeezed together, emitting a positron, also known as an anti-electron.
A positron has the same mass as an electron, but carries a unit positive charge rather than a unit negative charge. A neutrino is also emitted. Neutrinos are something like electrons with no electric charge and the ability to penetrate matter to an incredible extent. The fusing of two protons is attended by a loss of a unit positive charge; as a result, one of the protons becomes a neutron. This produces a nucleus of deuterium (H-2), a heavy isotope of hydrogen consisting of one proton and one neutron. The deuterium nucleus combines with another proton to form a nucleus of helium-3 (He-3), containing two protons and one neutron. As this happens, a burst of gamma radiation is emitted. Two He-3 nuclei, resulting from two separate iterations of the above-described process, then combine to form a nucleus of helium-4 (He-4), which has two protons and two neutrons. This is the isotope of helium we use to fill up lighter-than-air balloons. In this final phase, two protons are ejected. These can contribute to further fusion reactions.
In the solar fusion process (see figure ), the total mass of the matter produced is a little less than the total mass of all the ingredients. The “missing mass” is converted into energy according to the famous Einstein equation:
E=mc2
Figure 1. The hydrogen fusion process that takes place in the core of the sun.
where E is the energy in joules, m is the “missing mass” in kilograms, and c is the speed of light, equal to approximately 3 _ 108 meters per second. The sun produces a tremendous amount of energy in this way, because hydrogen nuclei are converted to helium nuclei continuously and in vast numbers. There is enough matter in the sun to keep its hydrogen fusion process going for millions of millennia yet to come.
Eventually the hydrogen fuel supply will run out, but not in your lifetime or mine!
FUSION IN BOMBS
In a hydrogen bomb, a different hydrogen fusion reaction takes place. This mode, if it can ever be controlled, may be used in a fusion reactor. Instead of simple hydrogen nuclei, which are protons, nuclei of heavy hydrogen merge. One nucleus is of deuterium (H-2), consisting of one proton and one neutron. The other nucleus is of tritium (H-3), which contains one proton and two neutrons. When these combine, the result is a nucleus of He-4, with the extra neutron ejected (see figure ).
Along with this, energy is liberated, just as is the case inside the sun. For this mode, called deuterium-tritium fusion or D-T fusion, deuterium and tritium fuel must be supplied. Ordinary hydrogen (H-1) won’t work. Several other fuel combinations can theoretically work for nuclear fusion, but the D-T mode has received the most attention.
Figure 2. The fusion
process that occurs in a
hydrogen bomb.
In the sun, the fusion process goes on continuously because of the heat produced by the crushing pressure of gravitation, and also because of the heat generated from the reactions themselves. In a hydrogen bomb, the necessary heat to start the reaction is supplied by a fission bomb, but the reaction burns itself out in a hurry. It’s impossible to get enough gravitation to start and maintain fusion indefinitely and in a controlled manner in a terrestrial sample of simple hydrogen, deuterium, or tritium. In order to make fusion work for the purpose of generating useful power, the fuel must somehow be confined.
PLASMA FUEL
When a sample of gas is heated to an extremely high temperature, the electrons are stripped away from the nuclei. The atoms therefore become ions. Instead of
“orbiting” the positively charged nucleus of a specific atom, an electron is “free” to move from atom to atom, or even to travel through space all by itself. When this ionization happens, the gas, which is normally a poor conductor of electric current, becomes a good conductor. A substance in this state is known as a plasma. Because a plasma differs from an ordinary gas, the plasma state has been called the fourth phase of matter (the other three being solid, liquid, and gas). In the prototypes of fusion reactors that most scientists favor, deuterium and tritium exist in the plasma state, heated to temperatures comparable to those in the cores of stars.
The behavior of a plasma can be dramatically affected by external electric or magnetic fields. An electric or magnetic field can cause a plasma to constrict, distort, bunch up, or spread out. If a plasma is surrounded by an external electric or magnetic field having certain properties, the plasma can be kept within a small, defined space, even if it becomes hot enough to sustain hydrogen fusion reactions. The external fields act on the plasma in much the same way as gravitation inside the sun keeps the hot core gases confined. The use of external magnetic fields to compress and hold a hot D-T plasma in place during a fusion reaction is known as magnetic confinement.
THE TOKAMAK
The most promising method of magnetic confinement makes use of an evacuated
toroidal (donut-shaped) enclosure called a tokamak. This term is an acronym derived from a Russian descriptive phrase that translates as “toroidal chamber and magnetic coil.” The plasma is contained inside the tokamak. Two sets of coils, called the toroidal field coils and the poloidal field coils, surround the toroidal enclosure (see figure 3). The coils carry electric currents that produce strong magnetic fields.
An electric current of up to five million amperes (5,000,000 A), provided by a large transformer, travels through the plasma around the toroid in a circular, endless loop.
This plasma current creates a magnetic field of its own. The magnetic fields from the currents in the coils and the plasma interact, confining the plasma, aligning it within the tokamak chamber, and forcing it toward the center of the chamber cross section, keeping it away from the walls. This is important, because the plasma must be heated to more than 100 million degrees Celsius (100,000,000ºC) in order for fusion to occur! If the superheated plasma were to contact the tokamak wall at any point, the chamber would rupture, air would leak in, the plasma would cool below the critical temperature, and the fusion reaction would cease. The helium product of the fusion process is removed from the chamber by divertors. |
Energía de la fusión del hidrógeno
Debido a preocupaciones públicas por la fisión nuclear en los Estados Unidos, se ha sugerido la fusión del hidrógeno como una manera de aprovechar las características del átomo para generar electricidad. En teoría, esto es una gran idea. La fusión del hidrógeno es más eficiente en convertir materia en energía que la fisión, y no se produce ningún desecho radioactivo. Pero un reactor de fusión hidrógeno realizable todavía no se ha desarrollado.
FUSIÓN EN EL SOL
Los físicos creen que el sol convierte el hidrógeno en helio por medio de la fusión nuclear. El término “fusión” significa “combinar.” La fusión del hidrógeno requiere una temperatura extremadamente alta. La poderosa gravitación impuesta por la masa enorme del sol mantiene al núcleo en un estado de compresión constante. Esta compresión mantiene al núcleo bastante caliente para que la fusión del hidrógeno ocurra.
La fusión solar del hidrógeno es un proceso de varias fases. Al principio, dos núcleos de hidrógeno (protones) se presionan fuertemente entre sí, emitiendo un positrón, también conocido como anti-electrón.
Un positrón tiene la misma masa que un electrón, pero lleva una unidad de carga positiva en vez de una unidad de carga negativa. Un neutrino también es emitido. Los neutrinos son como electrones sin carga eléctrica y con la capacidad de penetrar la materia en un grado increíble. La fusión de dos protones es acompañada por una pérdida de una unidad de carga positiva; consecuentemente, uno de los protones se convierte en un neutrón. Esto produce un núcleo de deuterio (H-2), un isótopo pesado del hidrógeno que consiste en un protón y un neutrón. El núcleo de deuterio se combina con otro protón para formar un núcleo de helio-3 (He-3), conteniendo dos protones y un neutrón. Mientras que sucede esto, se emite una explosión de radiación gamma. Dos núcleos He-3, resultados de las dos iteraciones separadas del proceso descrito antes, se combinan luego para formar un núcleo de helio-4 (He-4), que tiene dos protones y dos neutrones. Éste es el isótopo de helio que utilizamos para llenar los globos menos pesados que el aire. En esta fase final, se expulsan dos protones. Éstos pueden contribuir a más reacciones de fusión posteriores.
En el proceso solar de la fusión (véase figura), la masa total de la materia producida es un poco menos que la masa total de todos los ingredientes. La “masa faltante” se convierte en energía según la famosa ecuación de Einstein:
E=mc2
( Ver figura lateral ) Figura 1: El proceso de la fusión del hidrógeno que tiene lugar en en núcleo del sol.
donde E es la energía en julios, m es la “masa faltante” en kilogramos, y c es la velocidad de la luz, iguala al aproximadamente 3 108 metros por segundo. El sol produce una cantidad enorme de energía de esta manera, debido a que los núcleos de hidrógeno se convierten en núcleos de helio continuamente y en grandes cantidades. Hay bastante materia en el sol para mantener su proceso de fusión de hidrógeno por millones de milenios todavía por venir.
¡Eventualmente el suministro de combustible de hidrógeno se acabará, pero no en el curso de su vida o la mía!
FUSIÓN EN BOMBAS
En una bomba de hidrógeno, una reacción diferente de fusión del hidrógeno ocurre. Este modo, si puede alguna vez ser controlado, puede ser utilizado en un reactor de fusión. En vez de los simples núcleos de hidrógeno, que son protones, núcleos de hidrógeno pesado se combinan. Un núcleo es de deuterio (H-2), consistiendo en un protón y un neutrón. El otro núcleo es de tritio (H-3), que contiene un protón y dos neutrones. Cuando éstos se combinan, el resultado es un núcleo de He-4, con el neutrón adicional expulsado (véase figura). Junto con esto, se libera energía, al igual que en el caso dentro del sol. Para este modo, llamado fusión del deuterio-tritio o fusión D-T, combustible de deuterio y tritio debe ser suministrado. El hidrógeno ordinario (H-1) no trabajaría. Varias otras combinaciones de combustible podrían trabajar teóricamente para la fusión nuclear, pero el modo D-T ha recibido la mayor atención.
( Ver figura lateral ) - Figura 2: el proceso de fusión que ocurre en una bomba de hidrógeno.
En el sol, el proceso de fusión ocurre continuamente debido al calor producido por la enorme presión de la gravitación, y también debido al calor generado por las reacciones mismas. En una bomba de hidrógeno, el calor necesario para comenzar la reacción es suministrado por una bomba de fisión, pero la reacción se quema a sí misma rápidamente. Es imposible conseguir bastante gravitación para comenzar y mantener la fusión indefinidamente y de una manera controlada en una simple muestra terrestre de hidrógeno, de deuterio, o de tritio. Para hacer el trabajo de la fusión con el fin de generar energía útil, el combustible debe ser confinado de alguna manera.
COMBUSTIBLE DEL PLASMA
Cuando una muestra de gas se calienta a una temperatura extremadamente alta, los electrones se alejan de los núcleos. Los átomos por lo tanto se convierten en iones. En vez de “moverse en una órbita” alrededor de núcleo cargado positivamente de un átomo específico, un electrón está “libre” de moverse de un átomo al otro, o aún de desplazarse a través del espacio por sí mismo. Cuando esta ionización sucede, el gas, que es normalmente un conductor pobre de corriente eléctrica, se convierte en un buen conductor. Una sustancia en este estado se conoce como plasma. Debido a que un plasma se diferencia de un gas ordinario, el estado del plasma se conoce como la cuarta fase de la materia (las otras tres son sólido, líquido, y gas). En los prototipos de los reactores de fusión que la mayoría de los científicos apoyan, el deuterio y el tritio existen en estado de plasma, calentados a temperaturas comparables a las existentes en los núcleos de las estrellas.
El comportamiento de un plasma puede afectarse dramáticamente por campos eléctricos o magnéticos externos. Un campo eléctrico o magnético puede hacer que un plasma se contraiga, se distorsione, se agrupe hacia arriba, o se disperse. Si un plasma es rodeado por un campo eléctrico o magnético externo que tenga ciertas características, el plasma se puede mantener dentro de un espacio pequeño, definido, incluso si llega a ser bastante caliente para sostener reacciones de fusión del hidrógeno. Los campos externos actúan sobre el plasma de la misma forma que la gravitación dentro del sol mantiene los gases calientes del núcleo confinados. El uso de campos magnéticos externos para comprimir y sostener un plasma caliente de deuterio-titrio en un lugar durante una reacción de fusión se conoce como confinamiento magnético.
EL TOKAMAK
El método más prometedor de confinamiento magnético hace uso de un recinto evacuado toroidal (en forma de anillo) llamado tokamak. Este término es un acrónimo derivado de una frase descriptiva rusa que se traduce como “compartimiento toroidal y bobina magnética.” El plasma está contenido dentro del tokamak. Dos juegos de bobinas, llamados las bobinas toroidales de campo y las bobinas poloidales de campo, rodean el recinto toroidal (véase la figura 3 ). Las bobinas llevan corrientes eléctricas que producen fuertes campos magnéticos.
Una corriente eléctrica de hasta cinco millones de amperios (5.000.000 A), suministrada por un gran transformador, se desplaza a través del plasma alrededor del toroide en un lazo circular, sin fin. Esta corriente de plasma crea un campo magnético propio. Los campos magnéticos de las corrientes en las bobinas y el plasma obran recíprocamente, confinando el plasma, alineándolo dentro del compartimiento del tokamak, y forzándolo hacia el centro de la sección transversal de la cámara, manteniéndolo lejos de las paredes. ¡Esto es importante, porque el plasma se debe calentar a más de 100 millones de grados centígrados (100.000.000ºC) para que ocurra la fusión!. Si el plasma sobrecalentado fuera a entrar en contacto con la pared del tokamak en cualquier punto, el compartimiento se rompería, el aire se filtraría hacia adentro, el plasma se enfriaría debajo de la temperatura crítica, y la reacción de fusión cesaría. El producto del helio del proceso de fusión es quitado del compartimiento por derivadores.
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Figure 3. Functional diagram of a tokamak, showing plasma confinement coils and
two methods of heating the plasma - Diagrama funcional de un tokamak, mostrando las bobinas de confinameinto de plasma y dos métodos de calentamiento del plasma.
Figure 4: Simplified functional diagram of a hydrogen fusion power-generating
system, showing one reactor and one turbine - Diagrama funcional simplificado de un sistema de planta de generación de energía por fusión de hidrógeno, mostrando un reactor y una turbina.
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HEATING THE PLASMA
Several processes can be implemented in order to obtain the high plasma temperature necessary to sustain the fusion reaction:
•Ohmic heating Arises from the fact that the current, as it circulates in the plasma, encounters a finite resistance. This means that a certain amount of power is dissipated in the plasma, just as a wire gets hot when it carries high current.
•Self heating Takes place because the fusion reaction produces heat itself, and some of this heat is absorbed by the plasma.
•Neutral-beam injection Involves firing high-energy beams of neutral H-2 and H-3 atoms into the plasma. These heat the plasma when they collide
GETTING THE ELECTRICITY
The figure 4 is a simplified functional diagram of a hydrogen fusion power plant.
Except for the nature of the reaction, this type of power plant resembles a fission based generating system. The plasma chamber, where the fusion reaction takes place, is surrounded by a moderator, which consists of lithium blankets that absorb neutron radiation from the fusion reaction. The high-speed neutrons cause the moderator to heat up, and also “breed” additional tritium fuel from the lithium.
Heat from the moderator is transferred to a water boiler, also called a heat exchanger, by means of coolant. The coolant is pumped from the shell of the boiler back to the plasma chamber. The water in the boiler is converted to steam, which drives a turbine. After passing through the turbine, the steam is condensed and sent back to the boiler by a feed pump. The turbine rotates the shaft of an electric generator that is connected into the utility grid through a step-up transformer.
ADVANTAGES OF HYDROGEN FUSION FOR POWER
• The only material byproducts of hydrogen fusion are He-4, a harmless gas, and tritium that can serve as additional fuel.
• Deuterium fuel can be easily obtained from water. Lithium is abundant in the earth’s crust. Tritium can be bred in the reactor. These are the only material ingredients necessary to operate a D-T fusion reactor.
• A hydrogen fusion power plant will produce no greenhouse-gas emissions, CO gas, or particulate pollutants as do fossil-fuel power plants.
• A working fusion reactor will be safer than a fission reactor. Meltdown will not occur if the reactor is damaged, because terrestrial fusion reactions cannot be sustained without the continuous infusion of fuel and energy.
• Terrestrial fusion is not a chain reaction. It can’t get out of control and cause a fusion reactor to blow up. The reason a hydrogen bomb explodes is because abundant fuel is supplied and it is used up almost instantaneously, not because of a chain reaction. In a fusion reactor, the quantity of fuel will not be great enough to generate an explosion.
• The widespread deployment of fusion reactors, should they ever be perfected, will reduce or eliminate dependence on nonrenewable fuels for generating electricity.
LIMITATIONS OF HYDROGEN FUSION FOR POWER
• Although no radioactive waste is directly produced by D-T fusion, the emitted neutrons eventually make the reactor containment structure radioactive. This problem can be mitigated by using low-activation materials in the structure. Such materials become less radioactive from neutron bombardment than common containment materials such as steel.
Unfortunately, low-activation alloys tend to be expensive.
• Although no radioactive waste is directly produced by D-T fusion, some radioactive tritium will be released by the reactor during normal operation.
It has a half-life (that is, it loses half of its radioactivity) in 12 years.
• The widespread deployment of working fusion reactors is not expected to take place until at least the middle of the 21st century. Major technical and logistic hurdles remain. In addition, the public will have to be convinced that hydrogen fusion reactors are safe.
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CALENTAMIENTO DEL PLASMA
Varios procesos se pueden implementar para obtener la alta temperatura del plasma necesaria para sostener la reacción de fusión:
• La calefacción óhmica surge del hecho de que la corriente, cuando circula por el plasma, encuentra una resistencia finita. Esto significa que una cantidad determinada de energía será disipada en el plasma, de igual manera que un alambre se calienta cuando lleva gran intensidad de corriente.
• La auto calefacción ocurre porque la reacción de fusión produce calor por sí misma, y algo de este calor es absorbido por el plasma.
• La inyección de haz neutral implica disparar haces de gran energía de átomos neutrales H-2 y H-3 en el plasma. Éstos calientan el plasma cuando chocan
OBTENIENDO LA ELECTRICIDAD
La figura 4 es un diagrama funcional simplificado de una planta de energía de fusión de hidrógeno. A excepción de la naturaleza de la reacción, este tipo de central eléctrica se asemeja a un sistema de generación basado en fisión. La cámara de plasma, donde ocurre la reacción de fusión, es rodeada por un moderador, que consiste en planchas de litio que absorben la radiación de los neutrones de la reacción de fusión. Los neutrones de alta velocidad hacen que el moderador se caliente, y también “críe” combustible adicional de tritio a partir del litio.
El calor del moderador se transfiere a una caldera de agua, también llamada intercambiador de calor, por medio del líquido refrigerador. El líquido refrigerador se bombea del cuerpo de la caldera de nuevo al compartimiento del plasma. El agua en la caldera se convierte en vapor, que impulsa una turbina. Después de pasar a través de la turbina, el vapor es condensado y devuelto a la caldera por una bomba de alimentación. La turbina rota el eje de un generador eléctrico que está conectado en la red eléctrica a través de un transformador elevador de tensión.
VENTAJAS DE LA FUSIÓN DEL HIDRÓGENO PARA LA ENERGÍA
• Los únicos subproductos materiales de la fusión del hidrógeno son He-4, un gas inofensivo, y el tritio que puede servir como combustible adicional.
• El deuterio combustible se puede obtener fácilmente del agua. El litio es abundante en la corteza de tierra. El tritio se puede producir en el reactor. Éstos son los únicos ingredientes materiales necesarios para hacer funcionar un reactor de fusión D-T.
• Una planta de energía de fusión de hidrógeno no producirá ninguna emisión de gases de efecto invernadero, CO gaseoso, o agentes contaminadores de partículas como lo hacen las centrales eléctricas de combustible fósil.
• Un reactor de fusión en funcionamiento será más seguro que un reactor de fisión. La fusión del reactor no ocurrirá si se daña el mismo, porque las reacciones de fusión terrestres no se pueden sostener sin la incorporación continua de combustible y de energía.
• La fusión terrestre no es una reacción en cadena. No puede salir de control y hacer que un reactor de fusión explote. La razón por la que una bomba de hidrógeno estalla es porque se suministra combustible abundante y se utiliza todo casi instantáneamente, no debido a una reacción en cadena. En un reactor de fusión, la cantidad de combustible no será bastante grande para generar una explosión.
• El despliegue extenso de los reactores de fusión, en caso de que se perfeccionen, reducirá o eliminará la dependencia de los combustibles no renovables para generar electricidad.
LIMITACIONES DE LA FUSIÓN DEL HIDRÓGENO PARA LA ENERGÍA
• Aunque no se produzcan desechos radioactivos directamente por la fusión D-T, los neutrones emitidos eventualmente convierten la estructura de contención del reactor radiactiva. Este problema puede ser atenuado usando materiales de baja activación en la estructura. Tales materiales llegan a ser menos radiactivos a partir del bombardeo de neutrones que los materiales comunes de contención tales como el acero.
Desafortunadamente, las aleaciones de baja activación tienden a ser costosas.
• Aunque no se produzca ningún desecho radioactivo directamente por la fusión D-T, un poco de tritio radiactivo será liberado por el reactor durante la operación normal. Tiene una vida media (es decir, pierde la mitad de su radiactividad) en 12 años.
• No se espera que un despliegue extenso de reactores de fusión en funcionamiento ocurra por lo menos hasta mediados del siglo XXI. Siguen habiendo obstáculos técnicos y logísticos importantes. Además, el público tendrá que ser convencido de que los reactores de fusión de hidrógeno son seguros.
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