There are a few small problems with this system, though, and we need to discuss them. The obvious one is the question of how to protect the men aboard from the harmful effects of the reactor's radiation. As we mentioned before, the early Soviet nuclear boats scrimped on shielding and became cancer incubators for the naval hospitals of that now-defunct nation. The answer, in a word, is shielding. The entire structure surrounding the reactor compartment is layered with a variety of different shielding materials.
Between the reactor compartment and the forward part of the boat is a huge tank of diesel fuel, which powers the big Fairbanks-Morse auxiliary engine in the machinery compartment. As it turns out, that fuel is extremely efficient at modulating or absorbing the various subatomic particles that could damage human tissues. In addition, the entire reactor is contained inside a reactor vessel that looks like an oversized cold capsule on end. Surrounding this vessel, as well as inside of it, is a system of layered shielding. While the materials actually used are classified, it is easy to deduce that lead (an excellent gamma ray absorber) and chemically treated plastics (based on fossil fuels) are probably used extensively.
In addition to its extensive shielding, the entire reactor plant has been overengineered. Since its earliest beginnings, the DNR has insisted that naval reactors be built with extremely high safety margins. While DNR will not comment, for example, upon just how much pressure all of the reactor plumbing can take, it is generally acknowledged that the entire reactor plant has been built several hundred percent more robustly than is required (400 percent to 600 percent has been mentioned). In addition, every system has at least one backup and usually an extra manual backup on top of that. The legacy of the Thresher loss is this fanatical obsession with safety.
Another area of extreme secrecy is the exact configuration and design of the reactor core itself. In fact, other than the technology used to reduce radiated noise, nothing on the Miami is as sensitive as the power plant core. This probably consists of a series of uranium fuel elements formed into plates to allow maximum heat transfer to the primary coolant loop. The fuel elements are probably mounted parallel to each other in a fuel assembly mounted atop a support structure in the base of the reactor vessel. The fuel used is highly enriched Uranium-235, probably 90 percent pure U-235 or better. For those who might wonder, the fuel used in commercial nuclear power generation plants runs about 2 percent to 5 percent pure, and the material used in nuclear weapons is about 98 percent pure. In between each fuel element is room for a control rod (also in the form of a plate and made of a neutron modulator), to control the rate of nuclear fission. Each rod is designed to drop automatically into place between two fuel elements in the event of a reactor problem, thus quenching the nuclear reaction. In addition, a procedure called scram allows the crew or the automated monitoring systems to shut down the reactor immediately, and restart it later if conditions allow.
Around the core circulates the coolant of the primary loop, which feeds the heated coolant into a steam generator. The steam generator directs its steam into a secondary cooling loop, which feeds a pair of high-pressure turbines in the machinery spaces, where the steam is condensed back into water and fed back into the steam generator. The turbines feed into a massive set of gears known as reduction gears, which turn the main propeller shaft. In addition, some of the steam is used to turn several smaller turbines that provide electrical power to the boat and its various pieces of machinery.
It may come as a surprise that other than the transit tunnel aft to the main machinery space, the reactor is not manned. The DNR limits the time a man can stay in proximity to the reactor, even how long he might stay in the transit tunnel. The actual control area for the reactor plant and the turbines, called Maneuvering, is located aft in the engine room. While it has never been shown to the press, it probably follows the convention of commercial power plants, with the controls laid out over a block diagram of the reactor/turbine system. This panel is manned at all times, even when the boat is in port and the reactor is shut down (noncritical).
The dominating feature of the machinery space is the deck, or more correctly, the mounting for all of the machinery. While it may seem solid enough, it is in fact a large platform or "raft," which is suspended on mounts on the inside of the hull. The mounts use at least one, probably two, sets of noise isolation mounts. These are like oversized shock absorbers designed to reduce the vibrations of the larger pieces of engine room machinery. The purpose of a raft is to take the noisiest things on the boat and isolate them from the hull, which radiates noise like a speaker into the water.
Mounted on the raft are the two main engines, the boat's electrical turbine generators, and the supporting pumps and equipment associated with moving the boat. Proceeding aft, you see the main propeller shaft leading back to the main packing seals in the stern. In addition there are a number of workbenches, as well as a limited machine shop capable of supporting many small-scale repairs. The size of the main gear, called a bull gear, would preclude repair, but virtually every other contingency in the space could be handled by the engineering team. These crew members, by the way, are recognizable by the different types of radiation monitoring devices they wear. Unlike the film badges worn by those who live and work forward of the reactor, these personnel wear a small dosimeter (which looks like a tiny flashlight), so that any dosage of radiation they receive can be assessed immediately.
To get the power plant started, the engineering officer of the watch orders the personnel at the reactor control panel to retract the control rods to a known position. This allows the core to heat up, causing the coolant to generate steam in the steam generator. From here the turbines are set turning, and so too the reduction gear train. There is a popular notion that the speed of the boat is increased by just retracting the control rods farther from the reactor core. This, in fact, is exactly the converse of what actually happens; the rods are simply retracted to a fixed point and held there. The engineers' main goal is to bring the reactor into equilibrium so that the basic amount of heat going into the primary coolant loop is constant. One can then control the speed of the boat by simply tapping more steam from the steam generator, thereby increasing the steam supply to the turbines. This results in cooling the primary coolant loop more, thus increasing the efficiency of the nuclear reaction, feeding more heat to the steam generator, and increasing the speed of the boat.
Conversely, stemming the flow of steam to the turbines not only slows down the spinning of the turbines, it also takes less heat from the primary coolant loop, and rapidly drops the efficiency of the nuclear reaction, "cooling" it down.
Life Support and Backup Systems
The auxiliary machinery space down on the third level aft of the torpedo room is arguably the most important compartment on Miami. Here is located all of the life support equipment, as well as the auxiliary power source. As you enter the space and head down the starboard aisle, you are given a quick introduction to "Clyde," the big auxiliary diesel engine. This is an old favorite of the chiefs onboard, because it is a direct link with the old World War II fleet boats. Built by Fairbanks-Morse, the design dates back to the 1930s and is a scaled-down version of the model used to power all of our submarines during the war. It is reliable and the crew loves it, therefore the name Clyde, as in, "… right turn, Clyde!"