Helium density in pellet-clad gap of nuclear fuel rod

Please let me know if you found any mistake , thanks.

The gap between nuclear fuel pellet and inner wall of the cladding of typical PWR fuel rods is filled with Helium gas. When performing neutronic calculation, the density of this gas is needed as an input for the calculation. We can estimate this Helium density by using the ideal gas equation, as described below.

The ideal gas equation [Ref.1]:

Now let’s do some benchmark calculation for Helium under Standard Temperature and Pressure (STP) condition, the data are:

• T = 273.15 K [Ref.2]
• p = 101325 Pa [Ref.2]
• M Helium = 4.002602E-3 kg/mol [Ref.3]
• R = 8.314472 J/(K.mol) [Ref.1]

Calculate..

The value according to Wikipedia [Ref.3] is 0.1786 g/L, which is in a very good agreement with our calculated value.

So all we need now are the value of Helium pressure and (average) temperature inside the pellet-clad gap. Some typical value of those data for PWR fuel rods are as follow:

• p = 3 MPa
• T = 650 K

Calculate..

So Helium density inside pellet-clad gap is roughly about 0.00222185 g/cc.

References:

1. http://en.wikipedia.org/wiki/Ideal_gas_law
2. http://en.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure
3. http://en.wikipedia.org/wiki/Helium

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