Great question, for a universe with no oxidizing agents strong enough to react with the sodium. (Note: that ain't our universe.) Anywayz...

"q = mCΔT" is always a good place to start when looking at absorption of heat from a substance in a single phase.

q = the heat absorbed or released from the substance

m = the mass of the substance

C = the specific heat of the substance

ΔT = the change in temperature of the substance

Rearranging to find mass of liquid sodium, we can plug in the numbers, make sure the units cancel out to leave only grams, then calculate. To get the units right, it helps to check what units are used in the specific heat, i.e., Joules, Kelvin, and moles.
(Sodium is 22.99 g/mol, a 10.0ºC temperature change is the same as a 10.0 K temperature change, and a megaJoule is a million Joules.)

m = q / {C x ΔT}

m = { 8.30 MJ x ( 10^{6} J / 1 MJ ) } / { ( 30.8 J / mol•K ) x ( 10.0 K ) }

x (22.99 g Na / 1 mol Na)

= 6.20 x 10^{5} grams of Na