Meters would be the best way to record the length
Bit of an odd question. Power Plants are known to use water-powered turbines to generate electricity, but can also make use of nuclear fission.
For this case we have that by definition, the momentum is given by:

Where,
- <em>m: mass
</em>
- <em>v: speed
</em>
Therefore, replacing values we have:

From here, we clear the value of the speed:

Answer:
The magnitude of velocity is:

Wavelength = speed / frequency
(345 m/s) / (20,000 Hz) = 0.017 m