wet thermal mass???

I am soon to be (retiring in May) moving back to Montana. Just bought 450 acres of mountain property about 35 miles out of Missoula. I will be building a monolithic dome home which is bacicly a concrete heat sink inside of polyurethane foam insulation...Typically I have built them with 4" of foam & 4-6" of concrete... the one I plan for mh home will have 6" foam & 6-8" concrete, it will also be about 3/4 underground. I intend to plumb the floor for radiant heating.. my question is has anyone used water for the thermal mass in their Rocket Mass Heater? I will have a green house next to the dome that will house the Heater. I have a 1600 gallon stainless tank that is about a 14' long cylinder. thinking I run a 6" stainless pipie with one end slightly sloping up from the bottm of the bell, a slight rise in the pipe goint towards rear of the tank then the U-bend with a slight rise back the length of the tank & out a stack...with coils inside the tank but outside of the Heater piping and will run through the slab of the house. Water in the tank would not go into the slab, only used as thermal mass... it will not be a closed tank so pressure from thetmal expansion is not an issue... would have a float valve on a supply line so evaporation will not be an issue.. has anything like this been done??? is it feasible?? am I nutz?

A non pressurized "buffer" water tank with heat exchanger coils inside it to heat the actual hydronic and potable systems is definitely the best way to go...

 Putting the whole tank inside of a big (insulated) bell chamber is another option, though for a mass that large a 10" J tube rocket, or 8" batch box might be the minimum necessary for heating it... Depending on how many gallons it holds.  

A bell chamber might also be easier to implement than welding a totally air tight duct inside of it...

 It could also be a bell chamber under the tank, with the bottom of the tank serving as the "lid" of the bell chamber...

The end of the tank could also sit directly over your riser, instead of the barrel, being that it is not pressurized.

I have given a lot of thought to this sort of system, but have not had the luck to come accross a suitable stainless tank I can afford yet.

the tank I have is 1600 gallons (about 14' long cylinder around 7' diameter){about 6 1/2 tons of thermal mass}  welding up a water tight system is not a problem ( I tig weld) ... I would foam much of  the outside of the water tank to help maintain heat so once heated it should not require a lot to keep the temp up...