Ted (Theodore) Rockwell was one of the many talented nuclear engineers (in the early days they hadn’t received that label yet) who worked for Hyman Rickover during Rickover’s pioneering days. He was not only significant on the technical front, he was also prolific as an author, penning a number of books, mostly about Rickover and Rickover’s legacy. I had hoped to phone-interview him but he died in 2013, aged 90. His writings are sometimes over-exuberant but always atmospheric. For example, take this long explanation of why the STR (a submarine prototype based on the light water design now dominating the world) beat out the SIR (a competing submarine prototype that was much more radical, including strange “intermediate” neutrons and sodium cooling). Our current mythology says the SIR was flawed from the get-go and no history (including mine) gives it much thrift. Yet Rockwell makes it sound like a close call (you can conclude that just from Rockwell’s prose even if the technicals seem tough to understand).
The merits of a power plant rest on many things, of which thermal efficiency is only one. The SIR provides an excellent example of how these things tend to work in opposite directions. The reactor raised its incoming coolant temperature over ten times as much as the STR, and it offered an outlet temperature of 850 degrees Fahrenheit, compared to STR’s measly 500 degrees. This enabled the Seawolf’s steam plant to run with modern, high-temperature super-heated steam conditions, as opposed to the Nautilus’ low-temperature, low-pressure saturated steam. Definitely more satisfying for the design engineer. But what about the customer, the captain on the ship’s bridge? What does he get out of this?
Rockwell, Theodore. 1992. The Rickover Effect: How One Man Made a Difference. Naval Institute Press, Annapolis, pp. 207-208.
The higher-efficiency steam plant is lighter weight, which is advantageous, but this is more than offset by the heavy radiation shielding required by the activated sodium, which gives off high-energy gamma rays. The higher efficiency means that less uranium need be fissioned to produce the same shaft horsepower, but this is offset by the fact that the intermediate-energy neutrons produce a large number of captures without fission; that is, the fissionable U-235 isotope captures a neutron to make U-236, which does not then fission and is just a worthless by-product. Thus, although the SIR does its work with few fissions, it consumes just as much fissionable uranium, so there is no saving in fuel consumption, and its fuel inventory is larger.
Up to this point we have a standoff; as far as the naval customer is concerned. His SIR power plant is just as big and heavy, and it consumes just as much fuel, as a lower-efficiency STR. Sodium has only one real operational advantage: if the system can be kept absolutely oxygen-free, the corrosion and the transport of radioactive corrosion products throughout the system are essentially zero. This is obviously useful when major maintenance and overhaul are carried out. But sodium’s operational disadvantages are serious.
First, sodium as a reactor coolant becomes a thousand times more radioactive than water, and it remains too “hot” for maintenance for a week or so, compared to water, whose radioactivity decays another thousandfold to tolerable levels during the few minutes it takes to gain access to the compartment. Second, sodium metal burns in air and explodes in water, presenting a potential hazard during operation and a serious impediment to maintenance. And third, it freezes at about 200 degrees Fahrenheit, so that electric heaters must be fastened to all equipment and piping to keep it from freezing when the reactor is not operating. These electric heaters require a great deal of power, at the very time when the reactor is shut down and not producing power. All these inherent characteristics create enormous problems for operating and maintaining a fighting ship.
The crucial factor that doomed the SIR was the operational success of the STR.
