There’s good boiling in a reactor and then there is this.

The Idaho National Lab released a video a while ago that shows something called Departure From Nucleate Boiling (DNB). In high pressure, high temperature environments, water can go from nucleate boiling, which is good, to DNB, in an instant. DNB is about as bad a condition as you can have inside a pressurized water reactor (PWR).

In PWRs, reactor pressure is kept very high (2250 psia is pretty standard). This prevents the water being pumped through the core (thick black line in the below drawing) from boiling, despite being at approximately 600F. This hot water is then passed through a heat exchanger (we call them steam generators (S/G)), where that heat is transferred to the much lower pressure water on the secondary side (thin black line going into the S/G). This lower pressure water boils, making the steam (thin black line coming out of the S/G) we use to turn the turbine and make electricity.

However, even in a PWR, there might be regions of the core operating much closer to the boiling point. This can be due to variations in fuel loading, lower flow of cooling water in that area, or power distribution being uneven for various reasons. Every core has a postulated hot region, where the worst case conditions might exist.

In those hotter locations, some fuel assemblies will experience nucleate boiling. This is the incipient stage of boiling. When you boil a pot of water for spaghetti and you first start to see tiny bubbles form on the bottom and break away into the rest of the water, that is nucleate boiling. It’s called that because the bubbles form a tiny imperfections in the surface of the metal, called nucleation sites.

These steam bubbles are actually a great thing for moving heat. It takes a lot of energy to make water turn into steam. Each of those tiny steam bubbles is taking that energy with it, away from the heat source and into the coolant. When it collapses, all the energy is now part of the bulk coolant flow. Nucleate boiling is a very efficient way to transfer large amounts of heat.

The problem occurs when the amount of heat you are trying to move gets to be too large for this process to be successful. In a reactor, this can happen in certain transients, or from running over allowed power levels. When the heat flux in one of those hotter areas exceeds a value called critical heat flux (CHF), you move from the good nucleate boiling to DNB almost instantly.

(Some definitions. Heat flux is just an amount of heat over a given area. In US reactors, we would talk about KW/square foot of fuel surface. Actual heat flux (AHF) is the real amount of heat being applied over an area. CHF is the calculated amount of heat that if applied over an area would cause us to go from nucleate boiling to DNB. So, when AHF is > CHF, DNB happens. We look at this as a ratio, AHF/CHF. Design requirements ensure that this ration is always greater than 1. It is usually limited to >1.2 or so, just to give us lots of margin.)

When this happens, the entire surface area of a fuel rod can become surrounded by steam. Water turning into steam removes heat very well. Once it is all steam, it’s heat removal abilities are terrible. That fuel pin is still making enormous amounts of heat, which isn’t being removed. Fuel temperatures spike.

This can result in the failure of the fuel rod cladding. Cladding is what we call the tube the fuel pellets sit inside of to make a fuel rod. If the cladding bursts, fission products are released into the coolant and radiation levels rise rapidly. This can result in the plant being shutdown and a tedious search for the damaged fuel assembly, so it can be removed from the reactor.

In the video above, what you are seeing is a carefully controlled experiment using an electrically heated rod, not a real nuclear reactor. That being said, the physics are the same. As long as the amount of heat can be removed, everything is fine. As soon as the heat exceeds that critical heat flux value, all of the water in contact with that heat source flashes to steam. Heat is no longer being removed, and any water that happens to get lucky and touch that surface flashes instantly to steam as well.

To prevent this from happening, we operate our reactors within the designs approved by the NRC. We never operate over 100% power. If pressure gets too low, or the flow of the cooling water gets too low, the reactor automatically shuts down, reducing the amount of heat being produced and preventing DNB from being reached.

As always, this is open source. Some links below.

Click to access Thermodynamics-Volume2.pdf