There are a number of advantages in cooling a reactor core with a coolant that

vaporizes (boils) in the core itself.

1. The vapor produced can be fed directly to a turbine, and power can be gen­erated without an intermediate heat exchanger and/or vapor generator.

2. Boiling coolants are very efficient in heat transfer (see Section 3.3).

3. The evaporation process in the reactor core produces a mixture of vapor and liquid, which has a much lower neutron absorption than a liquid and at the same time maintains a very high heat transfer efficiency. As the proportion by volume of vapor in the coolant (commonly called the void fraction) in­creases, the neutron absorption decreases and there is an increase in the re­actor neutron population, or the reacitivity. If the coolant also acts as a moderator, the neutron population will decrease. Thus, reactors with boiling coolants that also serve as the moderator commonly have a decrease in neu­tron population with increasing void fraction, or a negative void coefficient. If the demand for steam from the reactor increases, therefore, the natural ten­dency of the reactor is to start to shut itself down, and the control system must be designed to accommodate this effect. In reactors of the pressure — tube type with separate moderators (e. g., graphite), there can be a positive void coefficient and the reactivity increases unless action is taken to offset the effect. It is noteworthy that when sodium boils in a fast reactor, where there is no moderator, an increase in reactivity is observed since there is a positive void coefficient in this case also.

The main disadvantages of boiling coolants are as follows:

1. The highly efficient boiling process can degenerate into an inefficient, essen­tially vapor-cooling process rather abruptly due to the phenomenon of dry­out or burnout, as described in Section 3.3.

2. Using vapor generated directly in the reactor core in the power generation system means that the latter system is somewhat radioactive, requires special design, and has increased maintenance and operating costs.

3. The rather complex behavior associated with the void coefficients, as de­scribed above, can also be a disadvantage.

Liquid-cooled reactors can inadvertently become boiling-liquid-cooled reac­tors in the event of a power excursion or a loss-of-coolant accident. We shall discuss this in detail in Chapter 4.