If you “let the pressurizer go solid,” it means that you have mismanaged the water level in it and allowed the shock-absorbing bubble to disappear — making the pressurizer become a “solid” block of water. This was the absolute worst thing that could happen in a cramped submarine power plant, and operators were trained to avoid it at any cost. It was not a bad lesson to bring to the power plant, but in the increased-power realm, worse things could happen. Much worse.
A remaining, nagging problem with nuclear reactors in general is the decay heat of fission. Each fission event releases an enormous amount of energy, 210 MeV, but only 187 MeV is immediately available. The remaining 23 MeV is released gradually, as fission fragments radioactively decay in a cascade of sub-events over the next few billion years.[224] The rate of energy release is exponential, which is engineer parlance meaning that at first the rate falls like a lead brick on your foot, but then it slows to a dead crawl.
The issue with decay heat is that it is quite easy to instantly shut the fission process down and stop the reactor, but there is always a coast-down period in which the machine is still making power at a greatly reduced and falling percentage. If the power before shutdown is not too great, then there is no problem with the coast-down, even if all reactor cooling systems are not working. The fuel will still be hot, but reactors are built to withstand overheating. An S2W reactor on an attack submarine built in the 1960s made 12 megawatts when running at full speed. Immediately after an emergency shutdown, that reactor was still producing 6.5 % of the 12 megawatts, or 780 kilowatts. That is not enough power to melt anything in the fuel matrix, which is made of high-temperature zirconium alloy and uranium oxide.
A typical PWR, on the other hand, can produce 1,216 megawatts of electricity. The efficiency of the steam-to-electricity power conversion process in a PWR plant is about 32 %, which means that the reactor is producing 3,800 megawatts of heat to make that 1,216 megawatts of electricity.[225] Upon a sudden shutdown, a PWR is still making 247 megawatts of heat; and in the confines of a reactor vessel, that is enough power to melt solid rock. It is therefore important to keep the cooling system running after the reactor has been stopped cold. If the coolant pumps are shut down for some reason or the pipes are blocked, then the ECCS takes over, spraying cool water into the vessel to soak up the heat that is still being produced.
The temperature in the fuel falls rapidly as the fission products decay away, and after one hour the power has dropped to 57 megawatts, which the system is probably able to withstand without external systems taking away the heat, but that one hour after shutdown is extremely critical. The fuel melts at an extremely high temperature, over 5,000° Fahrenheit, and the zirconium fuel tubes and structures come apart at over 3,000° Fahrenheit, but this level of temperature is achievable in an uncooled reactor core running at a hundred megawatts. After a day of sitting in shutdown mode, the high-power PWR is still making 15.2 megawatts, or more than enough to run a submarine at flank speed. After a week, the reactor has cooled down to only 7.6 megawatts. The only way around this problem with nuclear fission is to ensure a shutdown cooling system, particularly in that first hour after shutdown, using redundant, multiple devices. If one auxiliary reactor cooling device fails, then there are still other ways of cooling the fuel held in reserve. This is the application for which the ECCS was designed and installed on all power reactors.
Given these minor systemic flaws, the nuclear establishment complex of manufacturers, customers, and regulatory bureaucracy was confident that installed power plants were safe against the worst possible accident, a catastrophic steam explosion throwing fission products into the atmosphere. The thought was, if we built power plants to withstand the worst accident, then the resulting physical strength and over-engineered systems will prevent any minor accident.
Everything in the nuclear power world seemed safe and running smoothly right up until March 22, 1975, when things began to unravel at the Browns Ferry Nuclear Power Plant on the Tennessee River near Decatur, Alabama. At the time, Browns Ferry was the largest nuclear plant in the world, having three General Electric BWR reactors capable of generating 3.3 billion watts of electricity. It was owned and operated by the Tennessee Valley Authority, a government program created by congressional charter in 1933 under the Franklin D. Roosevelt administration.
It was 12:20 P.M., and Units 1 and 2 were running at 100 percent power, while Unit 3 was in the last phases of construction.[226] As a rule, rooms in a nuclear plant are airtight so that a negative pressure can be maintained in the reactor building using a very large blower. This prevents any radiation leakage that could get into the air outside the plant and spread into the surrounding territory, and every room must be airtight to prevent leak points. This rule applied to the spreading room, a large chamber underneath the control room and adjacent to the reactor building, used simply as a space in which electrical signal and control cables can meet and crisscross in an orderly way. In this room thousands of cables were neatly arranged and labeled on trays and in open conduits. This room had been the last one sealed, because cables from Unit 3 were still being installed, and any instrumentation change in Units 1 and 2 required an unsealing of the room.
Temporary seals around cables entering one of the four walls around the room were accomplished using a self-foaming polyurethane compound in an aerosol can, occasionally referred to as “great stuff.” When workers had to hack away at the seals to install a new cable, it was easy to then spray in some great stuff and watch it expand, seal the opening, and harden. Unfortunately, the hardened foam, consisting of extremely thin plastic bubbles, has an enormous surface area, and it burns like gasoline.
A technician tested his latest sealing job using a proven method: he lit a candle and held it up to the new foam. The seal was imperfect, and the flame was sucked into a small hole by the negative pressure in the spreading room. The foam caught fire, of course. Efforts to extinguish the blaze by beating it with a flashlight were unsuccessful. As the situation quickly became desperate, two men tried to smother the flames using rags, but the flames were spreading into places where a rag could not reach.
Ten minutes later, at 12:30 P.M., someone had dragged up a carbon dioxide fire extinguisher, and they emptied it into the fire in the spreading room. It looked like it had gone out, but one minute later it flamed up again, and this time it had crossed the concrete wall through a small hole and was now in the reactor building. A worker ran up to the guard at his post in the entrance to the reactor building and asked to have his fire extinguisher, remarking that there was a fire below. Honestly, it would not seem as if there was anything to burn in a nuke plant. Everything is concrete and steel, and there is enough water on-site to fill a lake. What burns? Paint? Thousands of pages of operating and procedures manuals? Obviously, the plastic foam plus tons of plastic wiring insulation can make quite a bonfire. The Public Safety Officer sitting nearby picked up his phone and called the control room. “The building’s on fire,” he began. It was 12:35 P.M., and the fire alarm started going off as the announcement came over the public address system. The fire was spreading down the cable trays, about 20 feet off the floor, stopping just short of coming through the sealed penetrations in the ceiling and into the control room. Smoke was accumulating in the rooms below, making it hard to see or breathe. Both reactors were still running at 100 percent power, oblivious to the developing problem.
224
Only 200 MeV are eventually recoverable. Of the 210 MeV released by fission, 10 MeV is in the form of escaping neutrinos from beta decay of fission products. Neutrinos have such a minuscule interaction cross section with matter, they can go clean through the Earth and out the other side without disturbing anything.
225
Much argument is made by anti-nuclear factions concerning the power conversion efficiency of only 32 %. No great effort is made to efficiently run the steam turbines in a nuclear plant, and 68 % of the power is lost in the cooling towers. The thermal pollution into the environment is therefore unusually large. A coal-fired power plant is typically 40 % efficient, but to achieve this, a great deal of effort is required using a lot of fancy, expensive, and crash-worthy plumbing. This difference is because of the expense of mining coal and transporting it to the power-plant site, and the environmental damage done by burning it. The amount of coal used must be minimized at any cost. Fuel cost and its transportation are no problem at a nuclear plant, and the steam system is made as simple as possible for the sake of reliability.
226
Unit 3 came online on August 18, 1976, and is licensed to operate until July 2, 2036. While under license to operate, it has achieved an impressive capacity factor of 99 percent.