Change in pressure or volume of molten salt
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. 

http://link.springer.com/article/10.1361%2F10599490417542 
"Corrosion of stainless steels and carbon steel by molten mixtures of commercial nitrate salts" 
http://prod.sandia.gov/techlib/access-control.cgi/2001/018518.pdf 
http://www.electrochem.org/dl/ma/197/pdfs/0209.pdf 

The articles say your hot end is out of the safety zone.  Welds.  Chloride.  Chloride corrosion, especially pitting, can be aggressive. Halide in general may destabilize nitrate.  Ammonium nitrate is rather tame.  Ammonium nitrate plus chloride contamination is Texas City, Texas 16 April 1947. 2300 tons.

http://www.energystoragenews.com/Molten%20Nitrate%20Salt%20for%20Solar%20Energy%20Storage.html 
1600 tonnes of molten nitrate eutectic. 

You might put it behind grout-filled cement block or cinder block with #5 rebar weaving vertical and horizontal, and into the foundation, when you build the wall.  Have a large area vent direction.