Figure 2.3 illustrates schematically the principal components of a typical nuclear fission reactor.

In this typical reactor the coolant (high-pressure C02 in the AGR case chosen for illustration) at high pressure is driven by the coolant circulator over the fuel element. In many reactors (including the AGR case illustrated) this consists of pellets of uranium in oxide form sealed in a can made of stainless steel. The can (or cladding) ensures retention of the fission products so they cannot enter the coolant stream. It also prevents the coolant from attacking the fuel, which would be possible with some combinations.

The fuel elements are embodied in a structure (the reactor core) that allows them to be surrounded by the moderator. In the AGR case, the fuel assemblies are stacked in vertical holes (channels) in the massive strncture of the graphite moderator. The whole is contained in a prestressed concrete pressure vessel re­taining the high-pressure carbon dioxide gas.

The coolant extracts the heat from the fuel elements. In many reactors, this heat is then used in a boiler or steam generator to convert water to steam. In the boiling-water reactor (B^WR the steam is generated directly in the reactor core. The steam is then passed through the turbine that drives the electrical genera­tor. The very low pressure exhaust steam from the turbine is passed to a con­denser where it is converted back into water and recirculated to the steam generator (or to the reactor in the case of the B^WR

As we saw in Chapter 1, the moderator may be a solid (e. g., graphite) or a liquid (e. g., heavy water). In light-water reactors, the coolant and moderator are both ordinary water. If the moderator is different from the coolant, it must either not react with the coolant or be separated from the coolant by a suitably inter­vening structure. In the heavy-water reactor, this structure is known as the ca — landria; it consists of a tank containing the heavy water penetrated by a series of tubes in which the fuel is mounted and through which the coolant passes.

The remaining main feature of the nuclear reactor core is the means of con­trolling neutron population, namely, the control rods. These consist of neutron­absorbing material such as boron or cadmium.

The number of neutrons produced per neutron absorbed is often referred to as the multiplication factor k. If k is the greater than unity, the neutron population increases; if k is exactly unity, the neutron population remains the same; and if k is less than unity, the population decreases. The rate of growth of the neutron

population depends on the neutron lifetime, i. e., that time between the creation of a neutron and its interaction with the fissile material to create further neutrons.

Most of the neutrons present in the reactor are the so-called prompt neu­trons. In thermal reactors they have a lifetime of typically 0.0001 to 0.001 s; in fast reactors their lifetime is even shorter. If the neutron population consisted of only these neutrons, it would grow very rapidly as soon as k slightly exceeded unity, and the reactor would be very difficult to control. This is because the time between successive generations is very short, and very rapid multiplication of the neutrons would be inevitable. For instance, for a neutron lifetime of only 0.005 s, the neutron population would increase (for k = 1.005) by over 20 times in 1/3 s, and this growth clearly could not be controlled easily.

Fortunately, at the steady state not all of the neutrons are of the prompt type; a small fraction (-0.7%) are of the delayed type, whose lifetime (as defined above) is typically 0.6 to 80 s. These delayed neutrons arise from the decay of fission products rather than directly from the fission process itself. Thus, at steady state only 993°% of the neutrons are of the prompt type and the popula­tion is “topped up” by delayed neutrons, whose number is just sufficient to maintain the steady state, i. e., k = 1.000.

The control system operates essentially on these delayed neutrons, and the response of the system is such that control rod movements over a time scale of 10-20 s can give adequate control over the chain reaction.

The system is designed so that the k value cannot exceed a critical value (1.007 for the example cited above) above which the k value for the prompt neutrons alone is greater than unity. If k were allowed to exceed this value, rapid growth of the prompt neutron population would occur and the system would be in what is known as the prompt critical condition. However, the de­sign of nuclear reactors is such that this condition is avoided.

The nuclear fission process results in intense radiation. The fission products also contribute substantially to the radiation field, and they continue to emit ra­diation after the fission reaction is closed down. Thus, it is very important to provide proper shielding around the reactor core. This shielding takes the form of a thick concrete biological shield. In the AGR plant illustrated in Figure 2.3 the prestressed concrete pressure vessel doubles as the biological shield.

Where necessary—as it is for water-cooled reactors—further protection is provided by housing the whole system inside a leak-tight containment building. We shall discuss the role of this containment building in possible nuclear reac­tor accidents in Chapters 5 and 6.

Figure 2.3 gives a generalized view of the components of one type of nu­clear reactor, and it should be realized that there are many possible permuta­tions of fuel type, coolant type, cladding, moderator, and steam generator. It would be tedious to describe every nuclear reactor type that has been built and practically impossible in any book of reasonable size to describe all those that have been conceived. Many of the early concepts for nuclear reactors departed from the format shown in Figure 2.3 in that they proposed to use the fuel in a fluid form, circulate it through the core, and pass it through heat exchangers ex­ternally before returning it to the core. The concepts included systems in which solutions of uranium salts were circulated through the core, slurries of fuel were made and circulated, or the fuel was circulated in fused-salt form or in solution in liquid metals. There was a tradition at Harwell[1] that in the early days it was possible to invent a reactor system in the bath in the morning and have a pro­ject by lunchtime. It took some years to realize that reactors that you have just thought of are simple, cheap, and reliable, whereas those you are actually working on are always complicated, expensive, and troublesome.

In the remainder of this chapter we shall concentrate on describing some of the main systems that have been implemented in practice and that form the basis of the development of nuclear power. These are the British Magnox and AGR (advanced gas-cooled reactors), the U. S. light-water reactors (BWR and PWR), the Canadian CANDU reactor, the Russian boiling-water graphite-moder­ated RBMK-type reactor, and the liquid-metal fast reactor.