This article is written by John, our new contributor to the blog.
Many, but not all, of the knee jerk environmentalists condemn nuclear energy without knowing, like almost all of us, anything about it.
The cry is for alternate forms of non polluting energy … the problem is that the criers don’t know what those alternatives might be. Sure, there are wind and solar and possibly some others, but the sun goes down every night when we turn on the lights and the TV and although it doesn’t seem like it, the wind only blows about half the time … these are the two most viable alternatives to burning fossil fuels … except for a gift given to us almost 70 years ago to learn how to use and which we here in America have closed our minds to for the last 40 years; a gift that could supply us with limitless, inexpensive, totally environmentally safe energy … forever … should be investigated.
Our new friend John has devoted his life to nuclear energy and knows of what he speaks … read what he has to say and then in the “comments” ask your questions … let’s get a dialogue going — Lee
As a retired nuclear engineer, I have been an observer of the development of nuclear power since the mid 1960’s. I have been disappointed that nuclear power hasn’t had the spectacular success that is surely possible for the technology. In the 1950’s and 1960’s, many ideas for building nuclear power plants were envisioned and in several cases some development work was done with the intent to commercialize several different designs. The light water reactors required the least development to commercialize and several companies in this country developed reactor designs and began selling them commercially. But with budget pressures from the Vietnam war and growing anti-nuclear sentiment growing in this country, the development work for nuclear power reactors was terminated by the government and no other reactor types became commercially available. All the operating power reactors in the US and practically all the power reactors in the western world are these light water designs from the 1960’s and 1970’s. The light water designs have had some success, and about 20% of US electrical productions is from these reactors, and in some countries is the predominate source of electricity (about 80% for France). They have been safer than all other significant technologies for producing electricity, with no deaths from nuclear accidents anywhere in the world from these light water reactor designs. Nevertheless, these designs have some shortcomings which have proved to be serious. The most significant problem is that the use of high pressure water as a coolant creates an inherent safety problem. If there is a break in the reactor system, the coolant will be lost as steam and the core will melt due to inadequate cooling to remove the decay heat from the fission products. In order to prevent radioactive fission products from escaping into the environment these reactors are built within very strong containment buildings to withstand the high pressure of steam escaping from the reactor. To reduce the probability of melting the core, these reactors are supplied with redundant emergency core coolant systems, and redundant power supplies to provide the power required for operation of the pumps and valves that are part of the emergency core coolant systems. The cost of providing the super strong containment building, emergency core cooling systems, power supplies, as well as the high cost of building the large, very thick steel reactor vessel and other primary coolant system components makes these plants expensive to build. In the US, plants of this design don’t compete well economically with natural gas-fired plants. Despite the efforts to ensure that no accident could occur that could affect the general public, the Fukushima accident demonstrated that it is still necessary to be able to evacuate people near these plants.
In addition to safety concerns, the current light water reactor designs also create concerns about nuclear weapons proliferation do to the need for enrichment plants to produce the enriched uranium fuel required and due to the production of substantial quantities of plutonium. Another concern is the production of nuclear waste with these reactors. In the US, we have a “throwaway” fuel cycle; we don’t reprocess spent fuel from the reactors to recover the remaining uranium and plutonium which could be used for fuel for the reactors. The whole spent fuel assemblies are treated as waste, including heavy transuranic elements, some of which have long half-lives. Plutonium 239 has a half-life of 24,000 years; thus, there is a design requirement for a spent nuclear fuel repository that nuclear waste must be isolated from the environment for tens of thousands of years. As a consequence of safety, proliferation, and nuclear waste storage concerns, the general public has not been comfortable with nuclear power, which has led to repressive government oversight of the industry.
Power reactor designs are possible that would be so inherently safe that they would not require emergency core coolant systems or emergency power supplies. The decay heat would be removed by passive systems. Using a coolant which is a liquid at high temperatures and atmospheric pressure, a loss of coolant can be made a practical impossibility. Containment buildings and primary coolant system components then wouldn’t need to be so strong since they would not have to withstand high pressures. The efficiency of converting heat to power could be increased from the 34% of light water reactors to 60%. Reactors can be designed so that the only nuclear waste that is radiologically significant consists of fission products with a half-life of 30 years or less, and the quantity of fission products produced could be 45% less than for the light water reactor design due to the higher efficiency. A nuclear waste repository would only need to provide nuclear waste with isolation from the environment for a few hundred years. With little energy to spread fission products away from the reactor, it might be possible to locate the plants in more densely populated areas yet have no need for evacuation plans. These reactors wouldn’t provide any practical route to nuclear weapons. I would like to describe one reactor design that would make all this possible which would use molten salts as the core and coolant.
One variation of a molten salt reactor, originally conceived at Oak Ridge National Laboratory, was called a Molten Salt Breeder Reactor. The core would consist of a molten mixture of the fluorides of thorium, lithium, beryllium, uranium 233, and possibly a few other things. It was designed to be a breeder reactor running on a thorium-uranium 233 cycle. The uranium 233 fissions to produce energy and the fissions emit enough neutrons to sustain the chain reaction as well as produce enough uranium 233 to replace what is destroyed by fission (neutrons are absorbed by thorium 232 which decays in two steps to uranium 233). The molten salt solution has a high heat capacity, remains a liquid without pressurization up to very high temperatures, and is non corrosive. Operating temperatures could be high enough to make it practical to drive a gas turbine, so the efficiency of plants of this design could be very high (current nuclear plants are able to convert about 34% of the heat they produce into electricity, combined cycle gas turbine plants have efficiencies near 60%). Because the system would require no pressurization, there is no need for the very thick steel of light water reactor vessels and other components. The containment building would not become pressurized in the event of an accident. Fission products are all well retained in the molten salt mixture except for the xenon and krypton noble gasses which are continuously removed from the reactor and stored for decay. Reactors would be designed to be inherently stable; if the temperature of the reactor increases the power output of the reactor declines. Nuclear explosions would not be possible. Plants of this design would be refueled as they operate by adding small amounts of thorium fluoride to replace the thorium consumed. Note that thorium is an element that is believed to be about four times as abundant on earth as uranium. Presently thorium has no significant uses. It exists in other ores that are mined, especially in rare earth mines, where it is discarded as waste. The amount discarded annually from rare earth mines is reportedly more than enough to supply the entire worlds’ energy needs for the year if consumed in this type of reactor. There is believed to be enough cheaply recoverable thorium in the world to satisfy the world’s energy needs for hundreds of thousands of years.
In this type of reactor the molten fuel solution is pumped between a reactor vessel in which the nuclear fission takes place and an external heat exchanger where the heat is removed by a non-radioactive secondary molten salt loop. The secondary loop provides the heat for a closed cycle gas turbine. Some processing of the fluid fuel mixture is done to remove some of the fission products and to control the concentration of uranium 233 in the fluid. Proliferation resistance is assured because uranium 232, which is unacceptable in a bomb, is produced as well as uranium 233 and the two isotopes can’t be easily separated. In the event the reactor core gets too hot, a freeze plug would melt and the solution in the core would drain into tanks provided with passive cooling sufficient to remove the decay heat. In thorium breeders, fewer non fissile heavy isotopes are produced than with a uranium-plutonium cycle; however, those that are produced can be left in solution. They will eventually fission. Only the fission products are considered nuclear waste.
Oak Ridge National Laboratory built and operated a prototype of a molten salt reactor called the Molten Salt Reactor Experiment. This reactor operated at 1200 degrees Fahrenheit from 1965 to 1969, first using uranium 235 and then using uranium 233. The reactor proved to be stable and easy to operate. The systems for controlling uranium 233 in the fuel solution worked well. The inert gas xenon and krypton fission products were effectively vented from the system and safely stored. Other fission products were well retained by the fluid mixture. The reactor operated the equivalent of one and a half years of full power operation. The loss of funding prevented the construction of a first molten salt breeder reactor based on this design. No insoluble problems with the design were noted. Today, China has an active program to develop molten salt breeder reactors. Perhaps the US should be developing these reactors!