Memories. Ahhh Yes, Memories of My Life

Starting A Nuclear Reactor

Starting Up A Nuclear Reactor

         Today I thought I'd share with you how a Nuclear Reactor is started in very brief terms. Some of you might be interested in this, many of you might not be, but I'm sharing anyway.

         First things first. A Reactor Plant has more than one system, but right now we'll talk mainly about what's called the Primary Plant. There is a Secondary Plant that has to be mentioned briefly. Think of the Primary Plant as a producer of heat. That's done thanks to the Reactor Core. That heat is used by the Secondary Plant. The hot water in the Primary is sent to the primary side of a Steam Generator. It flows through tubes before returning to the core. The hot water in the tubes heats water on the outside of the tubes, producing steam. As you might be able to tell, the water in the Primary and Secondary never mix.

         This steam is used to spin turbines. On a submarine, we had 4 turbines. One set of turbines is connected to large generators (Ships Service Turbine Generators or SSTG's) which produce electricity. The other set is connected to the screw via a set of reduction gears. Yes, it's a screw, not a propeller. The screw operates most efficiently at low speeds (RPM) compared to the high speed of the turbines, which is why the reduction gears are needed. Now, back to the Primary Plant discussion.

         The core is quite small when you look at it. It would measure about 3-4 feet in height and width, so it's essentially square. I'm generalizing here for a couple of reasons. First, I've forgotten a lot these past 30 years. Two, I can't be too specific because if I did, I'd be breaking laws about sharing Confidential information. So we'll keep this discussion general in nature. The operation of the core is mainly controlled by Control Rods. These are made of Hafnium, their only job is to absorb neutrons, and the rods do that well. When the reactor is shutdown, the control rods are fully inserted and essentially rest on the bottom of the reactor.

         Before the reactor can be started up, many checks must be done. We (nuclear-trained personnel) would be busy all night before this startup because these checks would take 8-10 hours. Valve lineups were done on both the primary plant (not all primary plant valves were in the sealed Reactor Compartment), and secondary plant. Breaker positions for everything in both plants were done. Safety checks/pre-startup checks were performed on the reactor control portion of the system, as well as computing the ECP (Estimated Critical Position). Yes, the reactor has to reach criticality and actually becomes slightly supercritical to heat it up. Depending on the length of the shutdown and type of shutdown, the reactor was cooled to a certain point. I can't provide numbers because that's most likely Confidential data.

         Once all the checks have been performed and reviewed, the startup watch is stationed. In the small space called Maneuvering

         The Reactor Operator, who sat at the middle panel, would receive the order to commence reactor startup. He would then use the shim switch (the small 90 angle switch in the middle of his panel) to shim the rods up. The speed was very slow because if the rods were pulled at a high speed, the reactor would reach criticality before the instruments could detect it, then go super-critical, and we'd have an incident occur that would put Three Mile Island to shame. The primary instruments were monitored very carefully as the rods were pulled. As the ECP was neared, shimming would stop and start several times to verify reactor operation and response. Eventually the Reactor Operator would inform the Engineering Officer Of The Watch (EOOW) that the reactor was critical. At that point, rods were left where they were while readings were recorded. Once all the readings were recorded, the rods were again shimmed, and the reactor would be slightly super-critical. Very slightly, and I mean very slightly. This would allow the reactor to heat the water in the primary plant, and eventually turn the secondary water in the steam generator to steam. The Steam Generator would contain water in the lower portion of it, and steam in the upper portion. They weren't separated physically. Think of a covered pot on your stove.

         Once the Steam Generator pressure reached a set pressure, valves would be opened and steam admitted to the Engineroom. The pipes were heated, the system was heated, and slowly the Engineering plant would 'come to life'. What was a nice quiet space would now be noisy with the sounds of steam flowing through piping, turbines starting up, condensation occurring in the condensers, and pumps running to pump the condensed water back to the secondary side of the Steam Generators. Eventually, the Throttleman would be given the order to warm the main engines. Once warmed, the submarine is ready to go to sea. It's been quite a while since all those checks were started on the plant, and at this point the rest of the crew is onboard and the 'Maneuvering Watch' would be set so the boat could get underway. Yes, Nuclear Trained personnel had been on board for about 4 hours before getting underway, the duty section had performed all the checks before starting the plant.

         Since our duty sections usually had a small number of people in them, there were many times I'd stand six hours of watch (noon to six for example), get a couple of hours at most to relax/sleep (relax? No, I was doing some kind of work) before returning to the Engineroom and Maneuvering to relieve the man who had relieved me so he could perform the reactor pre-startup checks. I'd be on watch until midnight when he would once again relieve me so I could enjoy a bite of midrats before returning to Maneuvering to relieve him so he could finish his checks. Yes, there were times we were both up all night.

         Once the boat was underway, the plant was stable. The Electrical Operator (Right hand panel) would not change much on his panel, he would monitor the panel closely, but rarely adjusted anything. The Reactor Operator (again, sitting at the middle panel) would shim rods occasionally to control the primary plant temperature. The Throttleman (left hand panel would operate the throttles (those large cylindrical 'wheels') when ordered to change the boats speed. So he was probably the busiest of the three operators. And that my friends, is how a submarine nuclear reactor in started up, and operates when underway. There's more, a whole lot more, but this entry is already pretty lengthy. If you have questions, ask please and I'll answer as best I can without revealing Confidential Information.




Jim Dorrell