核电专业英语第四章

Unit 4

UNIT 4—4.2

It is often convenient to consider an infinitely large reactor, as this enables us neglect neutron diffusion and leakage. The multiplication factor is then referred to as the infinite multiplication factor, and is expressed by important four factor formula: The relationship between the two multiplication factors is:

It was pointed out that for certain types of reactor enriched uranium is necessary to achieve criticality. The most important example is the pressurized water reactor which requires slightly enriched uranium with 2 to 3 percent of U. Some current British graphite moderated reactors use fuel of a similar composition. Uranium fueled fast reactors require highly enriched uranium with 25 to 50 percent of U.

The process of enriching uranium involves a partial separation of the U and U so that the product has a higher concentration of U than that of natural uranium. The waste, known as the tails, has a lower concentration. two processes are available for uranium enrichment on a commercial scale. In both of them; natural uranium is converted to gaseous compound uranium hexafluoride UF and two isotopes of uranium produce two gases of slightly different density, the UF being slightly more dense than the UF. Both processes make use of slight difference in density to achieve separation of the isotopes. In the gaseous diffusion process the UF gas diffuses through a series of semi-permeable membranes, and in the centrifuge process the UF gas is spun at high rotational speed in a centrifuge.

4.3 Conversion and Breeding

One important point to emerge from earlier section of this lesson is that in thermal reactors fueled with uranium, either natural or enriched, practically all the fission occurs in U. In fast reactors, which contain no moderator and in which neutron energies are much higher than in thermal reactor, U fission occurs to a small extent, but even in this type of reactor, it is U fission which predominates and sustains the chain reaction. It is nearly correct to say, therefore, that only the U in natural uranium contributes energy directly from its own fission.

Although U cannot itself be used as the fuel in a nuclear reactor, it does have a vital role to play as an isotope from which new fissile fuel can be created. In a uranium fueled reactor, a significant fraction of the neutrons produced by fission, possible 30~40 percent, are captured in U and produce U by an reaction. U is the start of a radioactive decay chain which produces NP, also radioactive and PU, which has a very long half-life and almost be regarded as a stable isotope. The processes involved are:

PU, as already pointed out, is fissile, its characteristics as far as fission is concerned are similar to those of U, and it can be used as the fuel in both fast and thermal reactors.

Another isotope which has characteristics similar to U is TH, the only naturally occurring isotope of the element thorium. This isotope can only undergo fission with neutrons of energy greater than about 1.4MeV, so it cannot sustain a chain reaction