[dredmorbius]

The principle cooling mechanism is vapourisation.

The heat of vapourisation for water, converting liquid to steam, is 40.66 kJ/mol, or 2257 J/g. It takes a lot of energy to boil off water.[1]

That compares with the latent heat of liquid water, the energy required to heat one unit of water by one degree, which is 4.2 J/g*K (where K is the delta temperature in Kelvin).

Raising the temperature of input by, say, 10 degrees, would only reduce the cooling factor by less than 2%.

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Notes:

1. Which is why live steam is so dangerous. Steam condensing will release a huge amount of thermal energy, making steam burns especially harmful.

[CobaltFire]

Backing this up, these high numbers are for typical cooling towers where they vent to atmosphere.

Once you get into closed loop PWRs the numbers get crazy. Actual figures for Naval Nuclear plants are classified, but as a nice round number example if you pressurize the secondary loop to 250psi you end up with about 1,725 kJ/kilogram to boil it.

Absolutely terrifying stuff.

[dredmorbius]

One of us seems to have our numbers wrong.

1,725 kJ/kg is 1,725 J/g, which is less than the enthalpy of vapourisation I'd quoted.

(Though yes, I understand your point: pressurisation raises boiling point, and so far as I'm aware, vapourisation energy.)

[CobaltFire]

Probably me. I wrote that on the train...