Why we don’t want a moderator to boil in a PWR?

Let me try to answer this question in a form that is easy to understand in accordance with my understandings. I do apologize if there is any inaccurate information here as I am such a green horn in this field.

First, the function of moderator itself is to slow down neutrons. Take a small stone and assume it as a neutron. Throw it through a steam or smoke and observe whether it slows down significantly or insignificantly? Take another identical stone and through it through water and see the difference. This is why you don’t want to have steam in your reactor for PWR. Arrangements of atoms is the key factor here. Solids followed by liquids have stronger bondings where each atom are align closely to each other compared to gas. This also explains why solids have higher heat transfer rate where heat is transferred by conduction while gas such as water vapor have lower heat transfer rate where heat is transferred by convection.

Second, among the huge advantages of using PWR is that you can insert control rods from the top and hence utilizing gravitational force to create a passive system. Allowing steam in PWR reactor will diminish this advantage as steam will take up the volume (increase in pressure) as it expands upwards and pushes the control rods out from the reactor.

Third, steam is a form of gas which obeys the ideal gas law. Temperature increases as pressure increases. Having steam will increase pressure and thus cause the temperature to rise and hard to control. But if we keep the fluid close to its boiling point, temperature could be controlled. The boiling point can be controlled by regulating the pressure and thus explains why a pressurizer is introduced in PWR.

The reason is that a liquid that is about to vaporize is called a saturated liquid. Once boiling starts, the temperature stops rising until the liquid is completely vaporized. That is, the temperature will remain constant during the entire phase-change process if the pressure is held constant. Looking back at PWR, we have a pressurizer to hold the pressure constant, keep the fluid as saturated liquid means that temperature can be controlled or held constant.

In addition, if we keep it at saturated liquid state we will have some bubbles at the bottom, slug and churn in the middle, and annular at the top. The benefit is turbulence flow which means faster heat transfer.

Fourth, steam is a for of gas which means that heat transfer is by convection while in fluid or solids, heat is transferred by conduction. These two methods of heat transfer differ significantly whereby heat transfer by conduction is far greater than by convection.

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