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PROPULSION
The Ohio class submarine is powered by a single S8G
pressurized water reactor (PWR) that is rated for 220 MW
thermal. Replacing the diesel/battery combination of
earlier submarines with nuclear power has several
advantages. The biggest one is that it provides a
much higher amount of stored energy, allowing the
submarine to stay at sea for a much longer time.
Another major advantage is noise reduction, as the nuclear
reaction process requires no moving parts unlike their
diesel engine predecessor.
HEAT ENGINE
Like all PWR systems, the S8G plant has a reactor coolant
system that uses water as a coolant and moderator.
This water is heated by the thermalization of fast
neutrons formed from fission. The heat/coolant then
flows to steam generators where the heat is then
transferred across metal tubes and into an entirely
different water in the steam system, similar to how a heat
exchanger works. The flowing coolant is meant to
take advantage of the most efficient form of heat
transfer: convection. This heat transfer provides
enough energy to cause a phase change from sub-cooled
water to super-heated steam that is used for various
functions within the engine room, mostly to drive turbines
for propulsion and electricity generation.
The most important advantage that PWRs have over boiling
water reactors (BWR) is safety. So long as the
reactor is designed with a negative temperature
coefficient of reactivity, it is nearly impossible to have
a meltdown so long as the core remains covered in liquid
water. The coolant is kept at very high pressure to
ensure it cannot boil. This is why the reactor
coolant system and the steam system are separated.
The 2nd law of thermodynamics prevents all 220 MWt from actually doing work due to conductive losses to the much lower ambient air. After all energy losses in the reactor coolant system are accounted for, the end result is a steam temperature of about 245° C, which corresponds to a maximum steam pressure of about 40 Bar on the following phase diagram. In more familiar terms, this is about 475° F at about 580 psi.
IMAGE SOURCE: MrReid.org
Accounting for additional losses in the steam system and
the fact the turbines are not 100% efficient, the result
is that not all of this pressure is used to spin the
turbines. The process of the turbines converting the
various energies in the steam to work can be found in more
detail here
in the EDS page. For now, we will just make
the claim that this conversion is done, which has the
result of rotating turbine rotors in order to rotate their
shafts. From the exhaust of the turbines, the steam
still has leftover pressure, so seawater is used as the
ultimate heat sink for the overall heat engine process,
which condenses the steam for reuse. This seawater
flows through condensers where the steam can impart its
thermal energy to the much colder ocean. This heat
transfer is so effective that the remaining pressure in
the steam is dropped to near perfect vacuum
conditions. The now liquid water is then pumped back
into the steam generator for reuse.
The tube bundle of a steam generator. While this is
larger than the submarine's on
account of belonging to a commercial power plant, it does
well to demonstrate
how much care is put into maximizing the heat transfer
surface area.
IMAGE SOURCE: Nuclear
Regulatory Commission Photo Gallery
THE PROPULSION TRAIN
With steam now driving the turbines, the shafts of the
turbines are now rotating. The speed of this
rotation is too fast to efficiently turn the
propeller. Also, the propeller is designed with low
speeds in mind to minimize cavitation. Thus, the
turbines are connected to the propeller shaft with a
couple of stages of reducing gears, similar to the image
below.
IMAGE SOURCE: Bright
Hub Engineering
An equation for tangential velocity in
rotational motion is this:
Where:
Tangential
velocity (constant)
Angular
velocity
Radius
of rotating object
Since there is no relative motion between
any two connected gears in this assembly, tangential
velocity remains constant throughout. So, in order
to decrease angular velocity (aka the speed of the
propeller shaft), the radius must increase, with the main
gear being the largest increase. This also has the
effect of increasing the torque on the propeller shaft as
evidenced by the following equation.
Where:
Torque
Radius
of rotating object
Force
on rotating object (constant)
Fun fact: This torque figure approaches 2
million ft-lbs. and the propulsion system delivers up to
60,000 shaft horsepower.
Backup forms of propulsion include a DC
electric motor that engages with the shaft via a clutch as
well as AC electric outboards. An example of such
outboards is shown on the frigate below.
IMAGE SOURCE: The
Drive
FLOW WITHOUT PUMPS
A ballistic missile submarine's typical mission doesn't require high speed operation, since it is out there only to hide nukes. A major feature in the S8G plant further enhances noise reduction, called natural circulation. While pumps do exist to circulate coolant during high power operation, at low power the pumps may be turned off and the system can continue operating. The well-known fact that hot fluids rise and cold fluids sink due to density differences is the main idea behind this. Basically, the hot water output from the reactor is located close to the bottom of the coolant loop. The input to the steam generator is located at a much higher location. Since heating the feed water to make steam causes the a heat transfer from the coolant, the coolant becomes relatively cool in the steam generator. This sets up a hot spot at the bottom and a cold spot at the top, places they don't want to be. Thus a thermal drive is created, allowing coolant to flow from the reactor to the steam generators with no moving parts.
IMAGE SOURCE: nuclear-power.net