# "Y plus" calculator value for a fuel bundle

#### Meb15aa

Hi everyone,
I am using a online y plus calculator to work out the y plus value for a fuel bundle.
http://www.pointwise.com/yplus/

In regards to the reference length, I have been told by peers to utilise the hydraulic diameter using 4A/P
where A is the cross sectional area, and P is the wetted perimeter.
Is this correct or is there any other better ways to undergo this.

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#### Astronuc

Staff Emeritus
In regards to the reference length, I have been told by peers to utilise the hydraulic diameter using 4A/P
where A is the cross sectional area, and P is the wetted perimeter.
Is this correct or is there any other better ways to undergo this.
For the calculator in question, if the reference length, L, is described by the hydraulic diameter, then yes, it is correct to use it as mentioned. Does the calculator documentation discuss or describe the 'reference length'? In other words, how is the 'reference length' defined?

CFD-online has the following discussion on turbulence scale length.

#### Astronuc

Staff Emeritus
the reference length is used to convert the fuel rod bundle into an equivalent pipe flow that is easier to analyse

https://www.nuclear-power.net/nuclear-engineering/fluid-dynamics/internal-flow/hydraulic-diameter-2/
So one wants to determine the reference length of an assembly as opposed to the subchannels in the lattice?

Bear in mind that PWR assemblies have uniform arrays, more or less, with spacer grids, and guide tubes may have some flow if not blocked by thimble plugs. Spacer grids in the active core are made of Zr-alloy and have mixing vanes, and some assemblies have mid-span mixing grids, which are shorter (in height) than the structural grids.

BWR fuel assemblies used to be more uniform, with one or two water rods which also served to fix the spacer grids. Nowadays, BWR rods have two groups of part-length rods (different lengths) that terminate in the core, and water channels that are round, square or cruciform.

""Y plus" calculator value for a fuel bundle"

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