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WHAT HAPPENS IN A NUCLEAR REACTOR? Most nuclear reactors are used to power steam turbines to generate electricity. They take the heat energy from fission reactions to convert water into steam for this purpose. A few small reactors are used to generate radioactive isotopes for use in research or medicine. The most common type or reactor is a pressurized water reactor - a PWR.

Uranium provides the energy source for nuclear reactors. 1 ton of uranium has the equivalent energy of 20,000 tons of coal! Typical fuel pellet Fuel assembly in a representative boiling water reactor (about 4.3 meters [14 feet]) tall and each weighing about kilograms (700 pounds). NFI type 9x9 Fuel.

Water under high pressure is heated by the fission reaction. This water converts water to steam in a heat exchanger, and the steam drives a turbine to generate electricity. The advantage is that the water that contacts the reactor does not contact anything outside the containment structure.

It is similar to a coal fired power plant. The big difference is how the heat is produced. U235 is the fuel of choice in current nuclear reactors. In nature, most of the uranium is U238, with only 0.7% being U235. To use it in a reactor, it must be enriched to around 3.5% U235. For a nuclear weapon, one needs at least 90% U235. The important point is that a nuclear reactor will not explode. At the worst, the core would melt.

When a U235 nucleus captures a neutron, the total energy is distributed among the 236 nucleons in the nucleus. The nucleus becomes unstable and can fission into fragments. Typically, 2 or 3 neutrons (av. 2.5) are also released. This keeps the fission reaction going (chain reaction).

At criticality, the chain reaction system is exactly in balance so that the number of neutrons produced in fissions remain constant. This balance is achieved by using control rods. To raise or lower the power, the number of neutrons must be increased or decreased, and this is done by raising or lowering the control rods. The control rods are made of neutron absorbing material.

The ability to control a reactor is due to the presence of a certain number of delayed neutrons. Otherwise, there would be an instant rise and fall of neutron population. For neutrons to be absorbed by U235 and cause fission, they must be slowed down. Neutrons released from fission are fast (~10 9 cm/sec). U235 fission is caused by slow neutrons (105 cm/sec). The water in the reactor serves as a moderator to slow the neutrons down.

The fission fragments of U235 are distributed around masses 95 and 135.

U n --> Ba Kr n + energy U n --> Ba Kr n + energy U n --> Te Zr n + energy In these reactions, number of nucleons is conserved, but a small loss in mass occurs equivalent to the energy released. The fission products can decay further with some beta and gamma ray emissions. It is these emissions that make the waste products highly radioactive.

About 6% of the heat generated in the reactor core is due to decay of the fission products. This has to be taken into consideration when a reactor is shut down. It also has to be taken into consideration in the storage of spent fuel rods. They are usually stored under water for a couple of years until the short lived isotopes decay.

In addition to the fission decay products, neutron capture by one of the uranium isotopes can occur to produce transuranic elements (elements beyond uranium in the periodic table). Uranium 238 can absorb a neutron to become U-239. U-239 quickly emits a beta particle to become neptunium 239. Np-239 emits a beta particle to become Pu-239. Pu-239 is relatively stable. Pu-239 can absorb a neutron to become Pu-240, and Pu- 240 can absorb a neutron to become Pu-241. Pu-241 undergoes beta decay to americium 241.

Many of these transuranics are alpha emitters and have long half lives. The materials the reactor is constructed from can also absorb neutrons. Activation products include tritium, carbon 14, cobalt 60, iron 55, and nickel 63. These can make demolition a problem.

Fast neutron reactors are another alternative. They use the high speed neutrons produced directly from fission without a moderator. They have to use more highly enriched fuel - either U235 or Pu239. Advantages: 1.More neutrons are produced per fission than with slower neutron fission, so extra neutrons are available for transmutation 2.Can be used as a breeder reactor by surrounding the core with U238 3.Can convert transuranics to fissible products, so total waste is reduced.

4. The remaining waste has a maximum half life of 27 years rather than thousands of years. Disadvantages: 1.Design is more demanding, as the power density is higher 2.Water can’t be used as a coolant, as it is a moderator. Liquid metal coolants such as lead or a mixture of Na and K are used.

India has a program that will use FNBR’s to convert thorium to U233 for fuel. In the first stage, they will use PWR’s and natural uranium. In the second stage, the plutonium produced will go to a FNBR. The blanket around the core will contain thorium as well as U238, so more plutonium will also be produced. In the third stage, the U233 and Pu239 produced in the second stage will be used as fuel along with additonal thorium in a PWR.

MODERN REACTORS ARE BUILT INSIDE A CONTAINMENT STRUCTURE THAT IS MADE OF CONCRETE 3 FEET THICK AND IS REINFORCED WITH STEEL RODS 2.5 INCHES IN DIAMETER PLACED AT 1 FOOT INTERVALS. THIS COULD SUSTAIN AN AIRLINER CRASH. THIS WAS NOT THE CASE FOR CHERNOBYL.