Fusion Power Plants

Well OK, then, here's a round description of a fusion power plant of the future based rather loosely on the one down the street from me.

[note: I'm an administrator, this is part fiction, part science, but definitely "hard" except where obvious]

In a fusion tokomak the components of deuterium are forced close enough to the components of the tritium using heat and pressure to let the strong nuclear force take over and fuse into helium, while releasing a lot of heat and neutrons. The heat is converted to electricity (either with a steam turbine or with a thermionic heat-electricity transference device) and the neutrons are captured in a lithium shell which breeds tritium to go back into the reactor. The helium is an unwanted byproduct and is siphoned off. Input energy on start up is quite high. Shielding from neutron radiation is required (see lithium blanket below). Tritium is needed for the start up, but deuterium is needed constantly in a relatively small quantity relative to the energy released. Deuterium is a frequent ion of the hydrogen gas scrounged from gas giants or cracked from seawater.

The fusion power plant itself consists of the following components; startup system, magnetic confinement tokamak, tritium collector, coolant circulation, thermionic heat collector (or steam plant if you prefer). Each of these systems is a complex system in its own right.

The start up system consists of a powerful energy storage device, such as a flywheel or a capacitor bank, coupled with an electricity delivery system. The system needs to provide initial power to the tokamak (on the order of 8-12MJ over a period of seconds). During regular operations, power is scavenged from the regular plant to maintain the charge in the energy storage system.

A Magnetic confinement tokamak is a doughnut-shaped vacuum chamber surrounded in three dimensions with superconducting magnets. Each of the magnets confines the plasma created at the center of the vacuum chamber in one of the three main axes. A fourth superconducting magnet induces an initial "flow" or current to the plasma. Since the magnets are superconducting, power is only needed to establish and adjust the fields. A computer adjusts the balance of forces in the magnets millions of times per second to keep the plasma contained and operating in the correct shape for maximum efficiency. The plasma is initially fueled by a small amount of deuterium and tritium. Microwave and radio frequency radiation is used to heat this to a plasma state, stripping the atoms of their protons and electrons, and releasing the neutrons (since they are neutral, the magnetic field does not contain them). Once temperature/pressure reaches a certain level, the physics takes over the atoms will "fuse" at a certain rate and continue as long as there are is deuterium and tritium and the conditions do not change.

The tokamak itself is a cylinder of high strength material (low activation candidate materials include vanadium and carbon fiber, but steel is the current material of choice) surrounded by wires and cables for power input, sensor signals, controls, and power collection equipment (thermionic collectors), all surrounded by shielding against neutron radiation reaching the crew compartments. If steam is used to make electricity, the primary coolant loop will also circle the tokamak. There is a system of vacuum pumps, a cryogenic cooling system for instrument probes and other components that uses liquid hydrogen, a helium collector system at the lower end of the tokamak, and a lot of power transmission equipment.

The inner surface of the shielding layer has a Tritium Collector system consisting of a neutron slowing layer of hydrogen, a layer of liquid lithium circulating in a grid surrounding the tokamak several layers deep, and a neutron reflector like graphite. A refining system separates the collected tritium from the lithium and introduces it into the reactor in a controlled flow. The lithium will last several weeks, but must be replenished eventually for the reaction to continue. Lithium is highly flammable, and tritium is dangerously radioactive, so these substances are the biggest hazards in the device aside from the neutron radiation being emitted when the plant is running. The lithium will shield some large percentage of the neutrons, but the remaining neutrons must either be stopped or bounced back into the reactor (and therefore the lithium shell). Graphite or beryllium will scatter the neutrons, some back, some not. A final layer of hydrogen, water or paraffin blocks will prevent any residual neutrons from breaking out to the inhabited part of the ship. This might be combined with the coolant system below.

Coolant circulation is provided by a cryogenic coolant system that would circulate inside the shielding then outside using liquid hydrogen as the coolant. The expansion of the hydrogen would be overcome by a series of compressors, whose waste heat would be converted to electricity through thermionic collection. The coolant tubing would need to go through radiation 'blinds' - a series of right angle turns - to ensure the shielding is not compromised by the coolant runs (although the hydrogen makes a good shielding, the heated hydrogen might not be dense enough to stop the required number of neutrons from getting through).

The Thermionic heat collector system uses the principle of thermionics to take the heat from the reactor and other ship's systems and convert it directly into electricity. This is a big handwave to take care of the problem of heat in space; it just can't be gotten rid of through the methods used on earth - radiation is the only means remaining and is not sufficient to shed the heat of a fusion power plant or the remaining hot steam after making electricity. Besides, I can't stand the idea that we would still be getting power from making steam, just like Fulton :). Thermionics (the Seebeck effect) converts heat into electricity, and the future materials and physics advances needed to convert heat to electricity with the needed efficiency are foreseeable (if not likely).

To convert heat to electricity will require a series of thin, connected ceramic sandwiches with a "hot" face towards the heat source and a "cold" face away from the heat. The difference across the device generated a flow of electrons due to the "Seebeck effect". These plates need power connectors and power infrastructure to bring the electricity generated to a storage place or to the consuming equipment. Around the tokamak these will likely be "stacked" five or six layers deep to absorb the heat output. They will also be around any other piece of equipment needing cooling, like pumps, compressors, transformers, etc.

Safety; the Tokamak cannot operate without containment, and that requires active control. The flow of deuterium and tritium must continue, and the temperature from the various heating sources must stay above a certain level. If any of these components are not present, the fusion reaction will stop and the plasma will simply collapse. This my itself is not a catastrophic event; the plasma is quite a low density material, so despite temperatures and pressures that are similar to that in the Sun, the collapse of the field releases little damaging heat to the surrounding infrastructure. Since the reaction is the source of the neutron, there will not be much radiation when the reaction is not running (residual activity from irradiated components like the pressure vessel - these are even less if non-activating materials like carbon fiber or vanadium can be used in the construction). In the event of a field collapse there will be a lot of electricity flowing in the system and electric arcs are common.

Lithium and hydrogen are both quite flammable. Release of either or both of these will be something the sensors and alarms would be tuned to detect. Fire in space is pretty easy to deal with if you can evacuate a chamber to vacuum, however. Cryogenic liquid spills can also be very dangerous and damaging. Liquid Hydrogen will be stored at 20 degrees Kelvin (-423F or -253C). This will 'burn' the skin off a person, liquefy the air and make the oxygen percentage too low to breathe in a close space, and of course be an explosion hazard once it has turned gaseous and mixed with oxygen. Some materials (i.e. plastics) will become brittle and non-functioning if L-Hyd is spilled on them. It's also stored at pressure, over 10,000 psi. Failure of the pressure container has obvious consequences.

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Peter Brenton, Administrative Officer
MIT Nuclear Science and Engineering Dept.