The Keys to Lowering Reactor Costs: Inherent Safety

This post argues that the LFTR inherent safety features serve to lower nuclear cost. I am sure that IFR advocates would make the same case for the IFR. Discussions of nuclear safety seldom consider the impact of nuclear safety issues on reactor costs. Thew LFTR has fewer safety problems than the current Light Water Reactors used by the nuclear power industry. Switching to safer reactors means switching to different and lower cost safety measures. Hence a lowering of safety related reactor costs.

One of the Keys to lowering reactor cost is engineering high levels of inherent reactor safety into the reactor design. There are two ways to think about safety. The first way would be to examine reactor designs to identify inherent safety flaws. Once the flaws are identified, safety systems are designed to prevent those flaws from leading to serious accidents. The Light Water Reactor is an example of this approach. If the reactor looses its coolant water the nuclear fuel in the reactor core would overheat and eventually if it got hot enough it would melt. There are ways to prevent this. For example, inserting reactor control rods if the heat inside a reactor begins to rise. But sometimes a reactor operator might decide to do the wrong thing, and not insert the control rods. Human error is the biggest cause of accidents, so an inherently safe reactor has to be fool proof. That is no human error, no matter how serious can lead to an accident. The best way to do that is to build self regulation into the reactor design. If a reactor can regulate itself, then there is no need for a reactor operator. If you eliminate operators, you eliminate operator errors.

Simplicity is an important component of of reactor safety. The fewer parts there are in a reactor, the fewer parts there are to break. One way to build simplicity into a reactor is to rely on the laws of physics and chemistry as much as possible. For example the Westinghouse AP=1000 reactor relies on gravity to feed emergency cooling water into its core. Thus you do not need electricity to pump coolant water into the AP-1000 core, in the event it looses its normal coolant. This is what is called a passive safety feature. Operators do not need to turn on pumps, because as the core looses coolant water, an automatic process releases coolant water from an overhead tank. The water, powered only by gravity, flows into the reactor core, cooling it.

Of course even a gravity feed emergency cooling system costs money to build. Is there any way to save that money? Well if you could push the reactor fuel out of the reactor, you would not have to cool it inside the reactor. If for example your reactor was overheating because it had too much nuclear fuel in the reactor core, you could cool down the reactor by pushing some of the fuel out from the reactor core. Doing this would be difficult with a light water reactor, and doing it would more problems that would requite expensive solutions. So instead reactor designers take other approaches to controlling heat. One would be to increase coolant flow inside the reactor core. Increased coolant would remove heat from the core. Another method would be to insert reactor control rods into the reactor core. The reactor control rods would slow down the chain reaction. But this would lead to a decrease of reactor power output. Thus the reactor operator has a choice to make about how to deal with the increase in reactor heat. A wrong choice might lead to under production of power, or in the worse case it could lead to a serious accident. The history of nuclear accidents suggests that whenever you introduce the possibility of a human being making a bad choice you can count on that happening sooner or later. The best way to control accidents is to take the possibility of making bad choices away from the reactor operator.

The best way to remove bad choices from reactor operators is by designing reactors to operate in a stable fashion, to respond to increases in reactor temperature by automatically pushing fuel out of the reactor core. It would also be desirable if the reactor core got too hot to remove all of the reactor fuel to a place where no chain reaction would be taking place, and where its temperature could be easily controlled. Impossible, you say? Not at all!

So we want to lower reactor cost by building a reactor with these safety features:

* Simple reactor structure

* Continuous removal of radioactive gases from the reactor fuel

* A strong tendency to push nuclear fuel out of the core as reactor temperature rises

* The reactor is both stable and be self-controlling

* No externally operated controls are required

* Safety features are all passive, are triggered automatically before unsafe reactor conditions arise, and rely on the basic laws of physics and chemistry

* Safety is inherent and safety features cannot be altered by tampering, and are thus fool proof

* Ultimate reactor shut-down can accomplished by moving nuclear fuel from the reactor core

* An automatic, passive feature and automatically feature is used as a means of both triggering the transport and actually removing fuel material from the core if the reactor reaches an undesirable heat level

* The force to empty the core of nuclear fuel solely uses only the power of gravity

* core melt down could never be a problem

* Emergency core cooling would not be required

* Coolant leaks would not lead to core melt down

* Escaping radioisotopes would be either chemically bonded to materials that are solid at sub reactor core temperatures, or is not bonded, encapsulated by solid materials outside of reactor.

* Radioactive gases are continuously removed from the reactor fuel, so that no fuel accident will cause the escape of large amounts of radioactive gases from the reactor core.

* The chemistry of escaped isotopes would facilitate their local and relatively low cost containment.

* The chemical bonding and encapsulation of escaping radioisotopes would facilitate their post-accident clean-up and recovery.

* Even terrorist attacks using large amounts of explosives or direct attacks with large aircraft, would not lead to widespread dispersal of core radioisotopes. Radiation and radioactive materials would be contained locally.

You might believe that it is impossible to build a reactor for which safety is not an added on at extra expense feature. But not only is it possible, but such reactors have already been built and tested. Why do we still have reactors that are expensive to make safe?

Greater inherent safety is can be the outcome of simplified reactor design, and thus lower reactor construction costs. Chemical and physical features of reactor fuel that prevents its dispersal in the event of an accident, in turn can lead to a lowering of containment costs. Stable operating reactors, and the elimination of operator choice, lowers the number of operators needed for safe reactor operation. A smaller staff means that less space is required for staff housing, less staff housing lowers construction and maintenance expenses for staff housing. A smaller staff means smaller staff compensation expenses. Hence greater safety saves on many fronts. It saves in reactor expenses, site construction expenses, and in reactor operations expenses.

Again if we examine what we understand about the cost saving advantages of inherently safe reactor design stand out. We can observe that light water reactor technology, as well as LMFBR technology have some significant inherent safety problems. In order to build a satisfactory level of safety into the design of these reactors, expensive safety features have to be added to the reactor design. Two Generation IV reactor concepts demonstrate superior inherent reactor safety. They are again the Pebble Bed Reactor and the LFTR.

A Primer on Nuclear Safety: 1.2.3 Heat, Water and Uranium Dioxide

1.2.3 Hear, Water and Uranium Dioxide

Uranium commonly occurs in an oxide form. The uranium ore in the Oklo natural reactors was U3O8, or yellow cake. Quite obviously yellowcake can serve as a reactor fuel. And there are real advantages to using a uranium oxide compound, rather than pure uranium metal as a reactor fuel. Pure uranium burns. So uranium dioxide, for example, would be a safer fuel form, but it is still undesirable for UO2 to come into contact with water in the presence of heat because of a small potential for corrosion. UO2 melts at something like 2800 C, so it can get very hot without melting. It is a poor thermal conduction, so he inside of uranium dioxide fuel pellets may become very hot during reactor operations. Once the heat heat from UO2 comes in contact with a zirconium cladding it is quickly transfered to the coolant. Because of its poor heat transfer qualities, UO2 fuel pellets are usually small.

UO2 fuel pellets are formed by heating UO2 power until it fuses into a ceramic. Incased in zirconium cladding the UO2 ceramic has a number of desirable properties. It is unlikely to release fission products under all but very unusual circumstances.

Since UO2 is relatively stable from chemical and physical viewpoint something has to go very wrong with a reactor, before it poses a problem. Thus potentially dangerous radioactive materials are locked in a double barrier inside the fuel pellet. The first barrier is the Uranium Dioxide ceramic which locks the FP into a rock like structure. How good is the protection that the UO2 inner fuel pellet provides? Lets go back to the Oklo reactors for a check. The Oklo reactors formed in A rock like formation containing U3O8 ore. That ore was not entirely solid, in fact it had to be water permeable in order for the natural reactor to become critical. What happened to all of those radioactive materials that were produced in the 17 natural reactors? Surprisingly enough they stayed exactly where they were produced 1.7 billion years ago. Geologist checked it all, and came to the conclusion that the radioactive materials produced by the Oklo reactors had move dvery little during the last 1.7 billion years. The radio-active byproducts produced b the Oklo Reactors were the same byproducts produced in modern reactors. They were not moved by the ground water that repeatedly flowed through the OKlo ore body. Nor were they transported by other geological forces. Thus the UO2 pellet is a significant barrier to the release of radiation from a reactor. The zirconium cladding of the pellet forms a second barrier to isotope escape.

Thus very unusual conditions would be required to trigger an escape of radioactive materials from a UO2 fuel pellet.

A note on Naval reactor fuel:  I have been unable to determine the composition or the original  Nautilus reactor fuel formula. The Nautilus was able to travel 66,000 miles with the original core, suggesting a low level or enrichment. Later the range between refulings was extended considerably. It is not clear if the fuel was uranium in metal form, UO2, or a Uranium alloy. Recent naval reactors use a Zirconium-Uranium alloy with the Uranium enriched to 93% U-235.