Need for a new reactor design

For a number of reasons, scientists and engineers in this country and others are investigating new designs for nuclear reactors -- reactors that will produce energy from nuclear fission, but will avoid (perhaps even solve) some of the problems that have arisen in the current nuclear power industry:
  • Current reactors produce nuclear waste which must be stored and protected for thousands of years.

  • Current reactors involve certain risks of accidents.

  • Current reactors use only the rare isotope, 235U, and so can only meet world energy needs for a limited period of time.

Nuclear energy has to be re-considered because:

  • World supplies of fossil fuels are being depleted; new energy sources must take over within decades.

  • Fossil fuels contribute to global warming by producing carbon dioxide.


The fast breeder

During the 1960s and '70s considerable work was done in Europe and the U.S. on a type of reactor called the fast breeder. The breeder was intended to greatly increase the amount of energy available from uranium in the earth's crust, by indirectly obtaining energy from the more abundant isotope of uranium, 238U. The idea was that in the operation of a standard reactor (in this context the standard reactor was called a burner), while the 235U fissioned, there was also present in the fuel rods a large quantity of 238U. Many of the neutrons produced during the reactor's operation were absorbed by the 238U nuclei to produce plutonium (see site on reactions, Equs. 9, 10, 11). Plutonium itself is fissionable, and can potentially be used as source of energy. Thus, the reactor would be "breeding" fuel instead of just "burning" it.

Energy from 238U

In this way one can indirectly use the energy in 238U as well as 235U. If we remember the percentages of these isotopes found in nature (99.3% 238U and 0.7% 235U), we see that there is more than 100 times as much 238U in the earth as 235U. If the burner reactor could, theoretically, satisfy the world's energy needs for decades, the breeder reactor potentially could do so for thousands of years.


Reprocessing

The plan was to extract the plutonium from the fuel rods by chemical separation. This is called "reprocessing". It's not a simple matter, since the materials are all highly radioactive, so the separation has to be operated largely by remote control. But a number of nations (Britain and Germany among them) have set up reprocessing plants, in anticipation of some economic value, not yet realized, being attached to plutonium and other reactor products.


Fission by fast neutrons

The fission of plutonium, while superficially similar to that of 235U in a burner reactor, is actually a rather different process. Plutonium fissions when hit with fast neutrons. The neutrons emerging from fission are moving at high speeds, around 1/10 the speed of light. In a burner reactor, the neutrons are, by the action of the moderator, reduced to much lower speeds, at which they are readily absorbed by 235U. The nucleus becomes 236U, and this then fissions. In the case of plutonium, high speed neutrons are not absorbed, but, in a sense, crash into the plutonium nucleus, and cause it to fission immediately. Thus, the breeder reactor does not have a moderator to slow down neutrons between fissions. It operates with fast neutrons; hence the name, "fast breeder".


Heat transfer fluid

But the reactor needs a heat transfer fluid and cooling system -- some liquid to flow through the fuel rods, and carry away heat to a point outside the reactor core. There this heat can be used in a conventional way to generate electricity. In designing the breeder, engineers had to find a heat transfer substance in which the nuclei are relatively heavy, not like the hydrogen and oxygen nuclei in water.

The point of the heavy nucleus is the following: In a collision between a neutron and a light nucleus, like 1H, the 1H recoils and takes away part of the energy of the neutron. The neutron loses a substantial fraction of its energy in each of many collisions with 1H's, and so before its next fission its speed is very much reduced -- typically from 1/10 of the speed of light down to about 1/100,000 of the speed of light. These are the slow neutrons that cause fission in a water-cooled burner reactor. But in the breeder, the neutron emerging from fission will collide with a heavy nucleus (in the heat transfer liquid); the heavy nucleus recoils very little, and so the neutron loses very little of its energy. Even after many collisions with the nuclei of the liquid, the neutron's speed may be reduced only to around 1/100 of the speed of light. This is the speed of the "fast" neutrons that operate in the breeder reactor.


Liquid sodium

But most materials with heavy nuclei are metals (iron, aluminum, tin, lead, etc.), which are solid at room temperature. The decision was to use a metal, but keep it at a temperature above its melting point, and so one looked for an element with a low melting point. The breeders that were built mostly used liquid sodium; sodium's melting point is 97.5o C. The mass number of the sodium nucleus is 23 (11 protons and 12 neutrons); the mass of 23Na is 23 times the mass of the neutron.

The breeder did not gain wide use for various reasons, the main one being economic: at the time enough 235U was available at reasonable cost, so that development of a new reactor seemed unnecessary. The American research program on breeders was discontinued by President Carter.


Lead-bismuth coolant

Interest in a reactor of this type has been revived in recent years because of the factors given at the top of this site. The new version is called the heavy-metal reactor, referring to the heat transfer fluid. Instead of sodium, what is being considered now is lead, which also has a fairly low melting point (327o C), or a combination of lead and bismuth (Pb-Bi), with a melting point not much above 100o C). These nuclei are even heavier than sodium, and so neutron speeds in the reactor are not very much reduced by collisions.


Advantages of Pb-Bi

Pb-Bi has several advantages over sodium.

  • Use of sodium entails some risk, because sodium is chemically very reactive, and can explode on contact with air or water. Pb-Bi is not reactive.

  • The boiling point of Pb-Bi (1670o C) is higher than that of sodium (883o C), and this means the reactor can operate at a higher temperature than the sodium-cooled reactor. This makes the reactor more efficient in producing electricity.

    [Efficiency here means the following: A certain amount of energy is available from the fissioning nucleus. One wants to use this energy to produce electrical energy. But the laws of thermodynamics show that one cannot convert all the energy to electricity; some energy always ends up as waste heat -- heat that goes into the environment and is not used. Efficiency is defined as the percentage of the source energy that ends up as electrical energy. (Existing burner reactors typically have efficiencies around 30%.) Thermodynamics also implies that efficiency is higher when the operating temperature of the reactor is higher.]

  • The high boiling point of Pb-Bi also reduces the risk a loss-of-coolant accident. Since the Pb-Bi reactor operates at a temperature far below the boiling point of the coolant, it is unlikely, in the event the reactor overheats, that the coolant will suddenly and rapidly boil away. In contrast, the sodium-cooled reactor operates close to the boiling point of sodium.

  • Heavy elements are best at absorbing radioactivity; lead is commonly used for shielding from x-rays as well as radioactivity. One can imagine, therefore, that at the end of a reactor's useful life, it is turned off filled with molten lead. The lead solidifies, and the reactor is then automatically entombed in a lead blanket that protects the environment from its residual radiation.

Operation at normal pressures;
higher efficiency

This discussion points up two advantages of the heavy-metal reactor over the water-cooled reactor:
  • In the water-cooled reactor, as heat is transferred from the fuel rods, the water flowing through the pool must be able to go up to a fairly high temperature, about 300o C. This requires that it must be pressurized, so that it remains liquid above the normal boiling point (100o C). [See the following link, not a required part of this site, for background on this point.] But maintaining water at high pressure creates the risk of a malfunction in the associated plumbing system. This could lead to sudden evaporation and the possibility of a steam or a hydrogen explosion. Failure of pressurization is one of the possible causes of a loss-of-coolant accident at a nuclear power plant. In contrast, the Pb-Bi coolant is operating as a liquid at normal atmospheric pressure.

  • The heavy-metal reactor operates at a temperature of about 800o C, higher than the temperature of the water-cooled reactor. Hence, as discussed above, its efficiency will be higher.

Burning nuclear waste

One important reason for reconsidering the heavy-metal reactor at this time is its possible use in solving the problem of nuclear waste. Recall that nuclear waste consists of two types of nuclei: transuranic elements and fission fragments. In addition, large quantities of unfissioned 238U, remaining in spent fuel rods, must be stored along with this waste, according to current law. Fission fragments typically have half-lives less than 100 years, while some transuranics have much longer half-lives (239Pu, for example, 24,000 years). If transuranics and 238U are placed in a heavy-metal reactor, they will fission, and contribute to the production of electricity. At the same time, they will gradually be converted into fission fragments. The problem of storing relatively short-lived fission fragments is one of safeguarding a site for a period of several hundred years. The problem of storing transuranics is one of safeguarding a site for more than 100,000 years. The latter problem is one with which we have no experience, thus no confidence that we can deal with it successfully. The former problem is one which we may, perhaps, assume we have the will to solve.

The heavy-metal reactor, thus, may provide a way to utilize the energy in the world's 238U reserves, and simultaneously reduce the problem of radioactive waste to a level that appears more manageable than it has up till now. It should be stressed, however, that the heavy metal reactor is at the present time only in the research stage, and it is not known whether it, or some other new reactor design, will become a practical energy source.

KEY CONCEPTS

  • Need for a new reactor design
  • Breeding plutonium fuel
  • Fast fission
  • Reprocessing
  • Heat transfer by liquid sodium
  • Revival of the breeder as the heavy-metal reactor
  • Advantages of Pb-Bi
  • Reactor efficiency at high temperature
  • Burning nuclear waste