Propulsion of a Nuclear Submarine Fission (Picture from "Thermodynamics", Cengel and Boles, 2002.) At the heart of the nuclear submarine is the nuclear reactor. This is where the sub gets all of its energy, electrical and mechanical. The heat needed to create all of this energy is produced by a fission reaction. This reaction involves the split of the uranium atom (isotope (U-235) into a cesium-140 atom, 3 nuetrons, a rubidium-93 atom, and .000000000032 J of energy. This equates to about 67300000 MJ of heat per kilogram of U-235. The fission reaction is basically a chain reaction. The 3 neutrons produced from the breakup of the U-235 atom go on to collide with and breakup other uranium atoms. The reaction can be slowed and sped up by lowering control rods into the core which impede these collisions. The initial supply of uranium can fuel the submarine for most, if not all, of its lifetime. A diagram of the typical submarine pressurized water reactor is illustrated below. The Pressurized Water Reactor (Picture from www.maritime.org/fleetsub/) The pressurized water reactor gets its rather descriptive name because of the pressurized primary loop. This loop must be kept under great pressure so that the extreme temperatures in the reactor core do not cause the water in the primary loop to boil and evaporate. The water in the primary loop is directly heated by passing through a pipe that runs through the reactor core. After passing through the core, the extremely hot water flows to the steam generator. In the steam generator (heat exchanger), water in a secondary loop is heated through a phase change to superheated vapor(steam). The water in the primary loop returns to the reactor core to regain the thermal energy it lost when heat was tranferred to the secondary loop. The steam in the secondary loop continues on to the main turbine where its tremendous pressure turns the turbine which creates work on the propellor shaft of the submarine. After the steam leaves the turbine it flows to a condenser where it is cooled back to water. This water then completes the secondary loop by returning back to the steam generator so it can be heated back to steam. So a nuclear reactor works the same as any conventional power plant except that it obtains its heat from a nuclear source. The following diagram shows the secondary loop from a thermodynamic perspective. In analyzing this loop, it could be assumed that the secondary reactor loop follows an ideal Rankine Cycle. The Rankine Cycle is the ideal cycle for vapor power plants but in reality vapor power plants deviate from this ideal cycle due to factors such as fluid friction in the pipes, heat loss from the steam to the surroundings, steam that may leak out, air leaking into the condenser, and friction of any mechanical parts. (Picture from "Thermodynamics", Cengel and Boles, 2002.) Each part of the loop can be isolated and values of all of the properties of the component can be computed using an energy equation derived from the first law of thermodynamics. The value of every physical characteristic (mass, pressure, volume, enthalpy, entropy, etc.) of the water/water vapor flowing into and out of these devices can be determined by using thermodynamic tables for water, the derived energy equation, and phase diagrams for water. Phase diagrams have been compiled for just about every Known substance and are unique for each substance. One of the common types of phase diagram is a pressure vs. volume diagram which is illustrated below. On this diagram, the substance exists as a liquid-vapor mixture under the curve, a liquid to the left of the curve, and a superheated vapor to the right of the curve. By the state postulate for thermodynamics, the state of a simple, compressible system is completely defined by two independent, intensive properties. (Picture from "Thermodynamics", Cengel and Boles, 2002.) So if have phase diagrams for water, thermodynamic tables for water and a knowledge of the first law of thermodynamics we can completely define all of the properties of the secondary loop of a pressurized water reactor. We begin with the steam generator. (Picture from "Thermodynamics", Cengel and Boles, 2002.) The steam generator tranfers heat from the primary reactor loop to the secondary reactor loop which changes the water in the secondary loop to steam. Since this is a steady flow device, we know: rate of mass flow in = rate of mass flow out. The first law of thermodynamics tells us that: The change in energy = heat - work (dE = Q-W). And since this is a steady flow device: The rate of net energy transfer in by heat, work, and mass = The rate of net energy transfer out by heat, work, and mass. All of this renders the energy balance equation for steady flow devices: Since Q = 0, W = 0, and kinetic energy = potential energy = 0 this equation simplifies to: So to define all of the properties of the steam generator we just need to find the mass flow rate (m dot) and enthalpy (h) for each of the inlets. (enthalpy = h = u + PV, where u = internal energy, P = pressure, and V = volume) The next device in the secondary loop is the turbine. The turbine is also a steady flow device and so the mass flow rate equation and energy equation is the same as for the steam generator. If the properties of the steam exiting the steam generator are already determined from the simplified equation above, then the properties of the steam entering the turbine are already known. Since heat tranfer in turbines is negligible and the work into the turbine is zero, the energy balance equation for steady flow devices can be reduced. By reducing the energy equation and placing work out on the left of the equation by itself, the work out of the turbine can be determined. This is the work that turns the propellor of the submarine. The simplified equation for this work is: Where h is enthalpy, V is velocity, g is the acceleration of gravity, and z is elevation. The next device in the loop is the condenser. Here the steam is cooled and the water is changed back to a liquid phase in preparation for its return to the steam generator. It seems like a waste to take this heat/energy out of the loop,however , this is a neccessary stage in the loop. Without this stage, the second law of thermodynamics would be violated. A heat engine must exchange heat with a high temperature source as well as a low temperature sink to keep operating. The heat/energy that is removed in the condenser is appropriately called waste energy. The condenser is simply the reverse of the heat exchanger/steam generator so the same equation that was used to determine the properties in the steam generator can be used for the condenser. This completes the secondary loop. If these equations were applied to an actual loop in a heat engine and all of the properties of the loop were determined, a complete quantification and understanding of the properties that govern the operation of the heat engine would be obtained. Moreover, this explains essentially how a nuclear reaction turns the screw (propellor) of a submarine. Home / Basic Anatomy of a Nuclear Submarine / Surfacing and Submerging / Controlled Surfaces: Planes and Rudder / Bibliography