The white "powder" is granular absorbant (as opposed to cartridges) that chemically binds the CO2 the diver generates when metabilizing oxygen. The process is called scrubbing, the container that holds the absorbant is the scrubber canister. During the process heat and moisture is generated, giving the diver warm, moist gas to breath, rather than cold, dry gas in open circuit.
There are three main diving grade CO2 absorbants, Sofnolime, Sodasorb and Divesorb.
I use Sodasorb, a 37 lbs pail costs $99, and most rebreathers need between 5 and 6 lbs per scrubber fill. Duration is anywhere between 2 1/2 hrs and 5 hrs, depending on the size of the scrubber, the design of it and the test protocol.
Jean Michel Cousteau usually uses the Ambient Pressure Inspiration CCR, but has used and advertised SCRs from Dräger and OMG in the past.
His son Fabien uses the PRISM Topaz CCR, it is actually him in my avatar, I tool that picture when we dove together in 2004. There are a few more in my gallery.
The Inspiration costs between about $7,000 (Classic version) and $10,000 (fully optioned Vision model), the PRISM Topaz costs $7,800.
For Inspiration info:
http://www.ambientpressurediving.com/
http://www.silentdiving.com/
For PRISM Topaz info:
http://www.steammachines.com/
Most CCRs allow the diver to use the diluent gas (air to 40m, He mix deeper than 40m) as onboard bailout. Since CCR tanks tend to be quite small, only 2 or 3 ltr (13 to 21 cuft) it is standard procedure to carry a redundant gas supply for bailout. Depending on the dive profile (duration, deco obligation etc) that can be one or more tanks with appropriate gas(es) and regulators, sometimes with connectors for the rebreather or bouyancy compensator.
