I am designing a solar thermal space heating system that will use radiant heat for about 1500 square feet house. A contractor suggested using ICFs to build a large tank in the new basement construction to hols a large volume of water for the system. Has anyone heard of this being dome before as this seems like a good idea but I am no longer dealing with this contractor. Advice needed.

Yes, it has been done but you can also just make a frame of 2x6 which is lined with plywood and XPS and a liner material used for shower stalls or if you can find someone who sprays polyurea (truck bed liners) to create the seal. How big of a tank do you want to make?
Here is a pic from someone I know, a DIY project. It's not quite finished in this pic but it will be a lot easier than mixing all the concrete and getting into the forms.

 

When you say "large volume of water", that construction looks to be at least an order of magnitude or more smaller than what you would have to start with for a solar radiant system. When you are talking about large pools of very hot water residing in the middle of your house, you might want to get someone who has experience designing and building tanks or cisterns.

http://www.vermontthermalstorage.com/

Not too much going on at that site.

Or you could just purchase 2000 gallon pre-cast/delivered reinforced-concrete water tank(s) for about $1K each. These tanks can be plumbed together to form a larger capacity storage system, have a nice access hatch, provide for multiple plumbing fitting options, last forever with zero maintenance, and can be placed below ground level before you construct your footings, foundation, and slab floor. We use these tanks quite often for various applications including passive solar/hydronic floor heating heat storage. Often times a local concrete company will have engineered tanks available for purchase (for potable water and septic systems). We get our local southern OR tanks from here:

Riverside Ready Mix Inc

The below photos show these tanks (6000 gallon total capacity) being used for a residential potable water, irrigation, fire suppression system that is fed from a creek located on the property.  This client didn't want to use any energy to power this system.  The tanks are located above the residence to provide the required water pressure.  Creek water is pumped into the tanks from a creek that is below the residence using a hydraulic ram pump that uses water energy from the creek in lieu of any electrical power to accomplish the pumping.

As ICFHybrid indicated, having a large volume of heated water within your building envelope is something you will want to have properly engineered.  I can't imagine a place in the USA where this would not be an enforced construction requirement, but I continue to be amazed at what often passes muster in some states.

It is very difficult to have a solar storage system outside of a home in moderate climes. The heat losses are just too great. Most successful systems have the tank contained within the home to reduce and capture the heat that is lost.

They come in all sizes and I was showing a DIY tank. I have done 1m3 tanks and 50m3 tanks depending on how much solar you want to have. It makes no sense to put in a big tank when there is only 4 solar panels. You might be lucky to get the tank temp high enough to be be of any use. The ratios are important.

The one shown above is quite nice. I hope it is being insulated really well on the outside

The above tank photos were for one of our water work project applications as previously described. When we use these tanks for a building heat storage application, we insulate the tanks externally and locate them within the building envelope as previously described.