# Change in pressure or volume of molten salt

I am doing a project in which I fill a cylinder with a volume of $636cm^2$ (made of stainless steel) with molten salt: 60% Sodium Nitrate ($NaNO_3$) and 40% Potassium Nitrate($KNO_3$).

The container might reach (worst case scenario) -35 degrees Celsius in a cold winter night and in the summer (with ideal conditions) up to 700 degrees Celsius.

I'm trying to find out if I should add a relief valve (like in old water boilers) or just seal it tight. This of course depends o how much the pressure increases between those 2 states.

I know that molten salt melts at 221 degrees Celcius and starts freezing when it reaches 238 degrees Celcius.

I can't find any relevant data, so I don't know how I could find the result for this. Any ideas or suggestions?

EDIT: My worry is that it would blow up due to pressure or the lid would blow open...

• If you are worried about pressure building up, it may be more important to know what the container is made of. Anything that can withstand temperature range you posted can probably withstand the pressure change the occurs with the phase change at fixed volume (unless the phase change is concomitant with a massive density change). – Ben Norris Apr 11 '14 at 10:47
• Good point. I updated the question – Marshall Apr 11 '14 at 10:49

Venting initiates from blockage - anywhere. All your lines must be hot-traced.

Nitrate melts are powerful oxidizing agents. If containment fails onto organics (e.g., wood) you've got a catastrophe. Nitrate melts significantly shrink on cooling and solidifying, and they bond to polar surfaces (glass, metal). If you have a half-liter of solidified salt and heat the bottom of containment, the melt will expand against the plug of solid salt above it, possibly deforming or rupturing containment. The very hot and oxidizing liquid shoots out. Heat a stripe up the side to start.

Nitrate melts thermally decompose. Even if the average temp is OK, heating hot spots may give you oxidizing, corrosive, and toxic by inhalation nitrogen oxides.