MEMOIRS and STORIES Journal entry: Sunday, July 15, 1990 (Age 22) – U.S.S. Nimitz – PSNS [Puget Sound Naval Shipyard] – 1214 AM Handwritten I never did finish what I wanted to talk about. The Nimitz has a reactor that is old technology. The two reactors are 550 MW each and more than 200 feet long and more than 40 feet in diameter. I mean we’re talking about ‘60s technology. Still, Navy reactor power is unique in how it works. The nuclear fuel fissions and the energy as heat is transferred to the water. The reactor is all metal with fuel rods and control rods control the fission production which results in the control of the heating of the coolant. We use pumps to circulate the coolant through the reactor. The coolant leaves the reactor and enters a S/G (steam generator). The heat from the coolant is transferred to a system of non-contaminated water. The coolant is radioactive and chemicals are added to both systems to minimize corrosion. Well, the heat is transferred from the coolant to the non-contaminated water (secondary water). The coolant, when the reactor is at power, is around 500 F. The heat from the coolant is transferred to the cooler secondary water via very thin steel in the shape of U-tubes. They are very thin to allow for better heat transfer and therefore raise plant efficiency. Also, if we didn’t control the secondary chemistry very strictly corrosion agents would quickly make a hole in the tubing and the radioactive coolant would contaminate the secondary. We have all kinds of detectors to prevent this from happening. We have detectors that will alarm in case we have that happen. The cooler secondary water flashes to steam and the steam drives a steam turbine which in turn turns a generator which in turn generates electricity for ship power. Our reactors are not perfectly safe so we have backup systems. Our reactors are designed to establish safe plant conditions. If, somehow we lost cooling of the reactor. Say, the coolant is gushing out through a hole in one of the pipes it flows through. Nothing naturally will cool the core except one safeguard. If cooling is not provided, the entire reactor and it components will heat-up. The danger to this is that the steel can crack at very high temperatures and melt. The metal which contains the fuel and the harmful gases would crack and the radioactivity would leak out. This is called meltdown and it isn’t good because basically the reactor isn’t safe anymore and the high radioactivity can be very harmful. So, we don’t ever want the reactor to meltdown; otherwise, we’re toast. So, we have a design in the reactor and it can’t fail. This is why. As the temperature rises the coolant becomes less dense. This has the effect of reducing fission therefore less heat is generated and the temperature drops. However, this will work only if there is still coolant. If no coolant is available we must stop the fission process, ourselves or by an automated system such as a scram. A scram is when all the control rods are dropped to the bottom of the core. The metal the control rods are made of prevent the fission process from taking place. This is where all the back-up systems come into play and they ensure that if regular means of shutting down the reactor fail then there will be enough backups to shutdown the reactor but much of it requires someone to be there to make sure everything is going OK. That’s were our training comes in. Well, that’s ‘50s technology. Nuclear technology is changing so drastically that the advances made make our plants seem antiquated. They have come a long way. Their goal to design the best reactor was simple. It must be economically sound. It must cost less to build. If it becomes uncontrollable no operator should be needed, or it must never become uncontrollable. Well, there are many designs but there is one I like in particular and I know it better than the others. The scary thing is I can understand it. I can see that it would work and I can see that it can be in operation now. People refer to the new designs as “next generation reactors”. Next generation reactors are designed to fit in between the beginning nuclear reactors and the probable fusion reactors. They are an intermediate reactor in which people hope the nuclear industry will standardize. I like the Modular High Temperature Gas-Cooled Reactor (MHTGR) which is being mainly developed by General Atomics. The reactor is smaller than ours. Its only 75 feet long and 20 some feet in diameter. It is rated at 135 MW. The design is how far reactor technology has come. The entire system is still made of metal.
Posted on: Fri, 26 Jul 2013 13:45:19 +0000
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