Exceeding the saturation temperature of coolant on the clad for PWR

[nuclearsneke]

TL;DR Summary

tl;dr: is it likely for cladding temperautre to exceed the coolant saturation temperature in PWR reactor?

Hello there.

I have been conducting a thermal calculation of a fuel assembly for KLT-40S reactor (a small modular PWR-like reactor installed at the floating nuclear power plant "Akademik Lomonosov" with electric power of 35 MWe.) and obtained some interesting results. It seems that cladding temperature exceeds the saturation temperature of coolant by 30-60 degrees and it happens mostly in the center of the fuel rod's height.
I believe it could lead to boiling of coolant in these zones. But questions is: is it allowable for PWR reactor core to have some axial zones with boiling? And am I right about this case of boiling?

If you provided to your point of view some sources to refference at, it would be also a great piece of help. Thanks in advance.

 

[Astronuc]

Summary: tl;dr: is it likely for cladding temperautre to exceed the coolant saturation temperature in PWR reactor?

But questions is: is it allowable for PWR reactor core to have some axial zones with boiling? And am I right about this case of boiling?

It is permissible to have nucleate boiling in a PWR core, and some German plants have done so routinely. Some US plants may have some nucleate boiling, probably on the order of a few percent, and in the upper spans of the core. However, in general, to preclude high oxide thicknesses, most PWRs try to avoid nucleate boiling.

Edit/update - Departure from Nucleate Boiling (DNB) is strongly discouraged. DNB would damage the fuel, or otherwise, weaken the cladding.

 

[rpp]

There is some nucleate boiling (also called subcooled boiling) in most (if not all) PWR reactors. There isn't a lot, but it is present.

Having a wall temperature greater than the saturation temperature does not guarantee you will have nucleate boiling. The wall temperature has to be greater than the saturation temperature by some amount.

There are several correlations that can be used to calculate the wall temperature (Twall) that will produce nucleate boiling. These correlations include Jens-Lotte, Thom, and Chen. I did a quick search and came up with this webpage that shows the Thom and Chen correlations:

https://www.thermal-engineering.org/what-is-nucleate-boiling-definition/

Note that the different correlations will give different results, and sometimes the differences are significant. I suggest looking in multiple textbooks and making sure the correlations are valid over the pressure ranges you will be applying them.