Propulsion
Early submarines were driven by big diesel engines.
These engines provided plenty of power for turning the propellers to
drive the sub as well as a way to heat the sub and provide electricity. There
were however a few drawbacks, fuel enonomy and the ability to stay underwater
for long periods of time. In order to run a diesel engine, you of course
need diesel fuel and lots of it. This was a problem since you could
only carry so much fuel on a ship. Also diesel engines give off emissions.
This was not a problem as long as the submarine was on the surface.
When they dove, however, there was only a limited amount of space in
which you could pump the fumes before you had to surface and vent. You
also need an oxygen intake from the surface. Both of these problems
were solved, however, by nuclear power. The world's first nuclear powered
submarine was an American boat called the USS Nautilus, which was put to sea
in January 1955.
Nuclear powered submarines are driven by a process called fission. Fission is a process of creating energy by splitting atoms. Here is a brief description of what is going on. Fission uses a fairly common element called uranium. Uranium has several isotopes, the most common of which is U-238. The isotope of interest, however, is U-235. This is the only uranium isotope that can undergo induced fission. This means that if a neutron were to come into contact with a U-235 atom, the atom would readily absorb the neutron and would split into two smaller atoms. When this happens, there are enormous amounts of energy and heat released. This is illustrated here:
Nuclear powered submarines are driven by a process called fission. Fission is a process of creating energy by splitting atoms. Here is a brief description of what is going on. Fission uses a fairly common element called uranium. Uranium has several isotopes, the most common of which is U-238. The isotope of interest, however, is U-235. This is the only uranium isotope that can undergo induced fission. This means that if a neutron were to come into contact with a U-235 atom, the atom would readily absorb the neutron and would split into two smaller atoms. When this happens, there are enormous amounts of energy and heat released. This is illustrated here:
There is an absolutely astonishing amount of energy
released by this process, mostly in the form of heat and gamma radiation.
To put it in perspective, we can compare the energy available in gasoline
to that of uranium. If we take a U-235 enriched sample of uranium, such
as that available on a nuclear submarine, there would be about as much energy
in a 1 pound sample as there would be in about a million gallons of gas.
In order for a sample of uranium to be this productive, it usually has
at least 2-3 percent U-235.
Now that we have a basic understanding of the fission
process, we need to know how to harness all of the available energy to drive
the submarine. This is done by putting a nuclear power plant inside
the sub, often referred to as the propulsion plant. The propulsion plant
uses a nuclear reactor to fission U-235 to generate heat. There is
water surrounding the reactor and the heat from fission is transfered to
the water under very high pressure to keep it from boiling. The heated
water then goes from the reactor into the steam generator, still under very
high pressure. Then the water goes from the steam generator back to
the reactor to be heated all over again. This part of the propulsion
plant is usually called the primary system. The secondary system is
yet another system of pipes and water, which starts at the steam generators.
The heat from the water in the primary system is transferred to the
water in the secondary system and the water is allowed to boil and create
steam. The steam then drives turbines which are used to generate electricity
for the boat as well as to drive the propeller shaft, which is where the sub
gets its propulsion power from. After the turbines, the steam passes
through a condenser and is turned back into water and pumped back to the steam
generators. The primary and secondary systems are isolated from each
other and the water from each is never mixed. The water is also used
over and over again so there is no need for additional water to be added.
Here is a layout of the process just described.
This form of propulsion requires no oxygen and it gives
off no emissions or toxic fumes. This allows the sub to stay under
water for extended periods of time. In fact, a nuclear sub can stay
under indefinitely with no need to surface. The problem is the radiation
that is produced by the fission process. The radiation is contained
by surrounding the reactor with more than 100 tons of lead shielding in order
to protect the crew from exposure to radiation. This is really the
only drawback to nuclear powered submarines and the disposing of radioactive
products is still an area of active research.
