Fuel utilization rate and nuclear waste types of breeder reactors

Theoretically, all uranium 238 in natural uranium can be converted into plutonium 239. At present, the commonly used once-through light water reactor can only use less than 1% of underground uranium resources, while breeder reactor can improve the utilization rate by more than 100 times. The high fuel utilization rate of breeder reactor greatly reduces the concern about underground storage fuel supply.

Since 1990s, nuclear waste has become the object of human attention. Broadly speaking, spent fuel is mainly composed of two parts. First, fission products, lighter than uranium; The second is transuranic elements (elements heavier than uranium), which are produced by uranium and other heavier elements absorbing neutrons but not fission. All transuranic elements belong to actinides in the periodic table.

The physical properties of fission products are quite different from those of transuranic elements. In particular, fission products themselves will not fission, so they cannot be used in nuclear weapons. Moreover, only seven kinds of long-lived fission products have a half-life of more than 100 years, which makes the geological storage or disposal of fission products easier than transuranic elements.

With the increasing concern about nuclear waste, breeder fuel cycle has attracted people's attention again, because breeder reactors can reduce the waste of actinides, especially plutonium and sub-actinides. The design of breeder reactor can make actinides fission like fuel, thus turning actinides into more fission products.

The spent fuel of light water reactor will undergo complex decay after being taken out. The half-life of fission products is not one order of magnitude higher than that of transuranic elements. If transuranic elements remain in spent fuel, most of the main radioactivity in spent fuel will be produced by transuranic elements after 1000~ 100000 years. Therefore, removing transuranic elements from waste can greatly reduce the long-term radioactivity in spent fuel.

Today's commercial light water reactors can proliferate some new fissile materials, mainly plutonium. Because commercial reactors are not designed as breeder reactors, they cannot convert enough uranium 238 into plutonium to replace the consumed uranium 235. However, one-third of the energy of commercial reactors comes from the fission of plutonium in fuel. On the basis of this plutonium consumption, the light water reactor only consumes a part of plutonium and actinides, producing non-fissile plutonium isotopes and a large number of gaseous actinides. After reprocessing, the reactor-grade plutonium used as mixed oxidation fuel is usually circulated only once in the light water reactor, which reduces the long-term discarded radioactive waste to a limited extent.