Fusion Power Plant
Knapp
ships and space stations work.
At 10:14 AM 9/15/2008, you wrote:
>
> That would be totally cool.
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
startup is quite high. Shielding from neutron radiation is required
(see lithium blanket below). Tritium is needed for the startup, 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 startup 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 forseeable
(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, liquify 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.
Pete
******************************************
Peter Brenton, Administrative Officer
MIT Nuclear Science and Engineering Dept.
Ph:(617) 253-3185 FAX:(617) 258-7437
--
Douglas E Knapp