When Rickover first began studying nuclear technology, he found a program in severe disarray. The Army’s Manhattan Project had accomplished its mission of completing a workable bomb before the end of the war. Many of the key scientists and engineers had left the program, eager to leave the security restrictions and poor working conditions behind.
Few people at the national labs that were left over from the bomb project seemed very interested in developing a usable power system. For the most part, they were scientists interested in cutting edge research as opposed to practical applications for a new energy source. They were interested in studying various coolants, materials and heat engines with the eventual goal of picking the “best” possible system.
Rickover looked the situation over, and recognized the need to choose a technical direction. All the research in the world would not result in the production of a single useful power source without eliminating most of the available options from consideration. As an experienced engineer, he was aware that aimless studies could go on indefinitely.
A key decision, which impacted all other practical engineering choices, was the selection of the fluid that would remove the fission heat from the reactor and take it somewhere where it could be converted to a more useful form. Several different coolants were under study.
General Electric had a program in place based on the use of sodium, a low melting point metal. The key attributes of sodium that make it a coolant candidate are:
- High heat capacity (useful for building small reactors)
- High temperature ability under low pressure (boiling point of 880 C at atmospheric pressure)
- Low neutron scattering ability (keeps neutron energy level high)
- Non corrosive for metal piping
Another suggestion was to use a gaseous coolant. Any one of several gases could be used, providing the following capabilities:
- High temperature capability even under low pressure
- Low neutron scattering ability
- Non corrosive environment for metal piping
- Possibility of use in a direct turbine cycle
A third coolant possibility was ordinary water, purified to eliminate corrosive ionic impurities. The key advantages for water include:
- High neutron scattering (gives possibility of using thermal neutrons to minimize the amount of uranium needed)
- High heat transfer capacity
- Familiar fluid for piping systems
Each of the potential reactor coolants have their disadvantages. Sodium and water react explosively, limiting the safety of using sodium for shipboard reactors. Sodium must also must be heated during shutdowns to keep it above its melting point to prevent pipes from becoming clogged.
Gaseous coolants have low densities and low heat transfer capabilities, leading to the need for larger reactors and more expansive heat transfer surfaces. Water, because of its relatively low boiling point, requires that the coolant system be kept under pressure in order for the fluid to reach temperatures useful for power generation.
Rickover’s team took a careful look at all three major coolant types. They determined that it would be a challenge to fit a gas cooled system into the projected size of the Nautilus, so they eliminated such a system from consideration.
The General Electric sodium cooled reactor effort was fairly well advanced. The company’s management did not want to abandon their project, seeing significant potential for commercial development. Even though there was some concern about the explosive hazard, Rickover’s team decided that the fluid could be used with an appropriate level of care in design and operation.
Sodium became the back-up coolant choice and work with the sodium cooled reactor continued through the production of a prototype and an initial ship, the Seawolf, which was the Navy’s second nuclear powered submarine.
The team’s primary choice, however, was pressurized water. Pressure vessel codes were well developed after many years worth of experience with steam and chemical plants, so the high pressure requirements were not viewed as a huge hurdle. The ready availability of replacement coolant and the familiarity of engineers with the design of pumps, bearings and valves in water systems were all viewed as strong pluses for the water reactor.
The pressurized water reactor could be sized to meet the required power, heat transfer and criticality considerations within the available space of Nautilus’ 27 foot 8 inch diameter reactor compartment. Though this compartment seems small by today’s submarine standards, when she was built, Nautilus was the largest submarine the U.S. had ever constructed.
The key selection criteria, however, seems to have been the fact that water reactors can be made quite small and use a small mass of uranium. At the time that the Nautilus design decisions were being made, uranium was assumed to be a very limited resource whose use in submarine engines would compete with other national priorities.
Rickover clearly desired to minimize the mass of uranium his engines required. Rickover had enough political savvy to understand that he would run into significant opposition to his plans for a large number of subs if they used more than their share of the precious metal.
Once the coolant was selected, a large set of variables was eliminated, allowing Rickover’s team to concentrate on overcoming the still extensive inventory of hurdles between nuclear theory and a working engine.