Triple-wall tube heat exchanger

A heat exchanger for generating steam, comprising:

• (a) a plurality of vertically arranged straight triple-wall tubes, each including:
• an inner tube for boiling water and permitting steam to flow upward, gaining heat;
• an intermediate layer of porous metal tightly abutting the inner tube wherein the intermediate layer comprises a plurality of metal fibers seamlessly interwoven;
• and
• an outer tube tightly abutting the intermediate layer;
• (b) a shell for holding liquid sodium therein and enclosing said triple-wall tubes, the liquid sodium flowing downward around and providing heat to said triple-wall tubes;
• (c) first, second, third and fourth tube sheets arranged downward in this order, wherein said tube sheets support the triple-wall tubes and tightly define a steam outlet plenum, an upper gas plenum, a sodium plenum, a lower gas plenum and a water inlet plenum, said plena stacked downward in this order within said shell, and
• (d) means for connecting the intermediate layers to a leak detector for detecting leakage in said triple-wall tubes;
• wherein:
• said first and fourth tube sheets are welded to said triple-wall tubes and seal said outer tubes, inner tubes and intermediate layers to each of said first and fourth tube sheets;
• top and bottom ends of said inner tubes are open to the steam outlet plenum and the water inlet plenum, respectively;
• said second and third tube sheets are welded to said outer tubes and seal said outer tubes;
• and
• each of said outer tubes has side holes communicating each of the gas plena to said intermediate layers.