Decay is a biological process. If the dead algae biomass is sterile, it will not decay. How keep it sterile? Store it amongst the canisters of spent nuclear fuel in a nuclear spent fuel storage site. That's a pretty deadly environment, I think. ;-)

Micro-organisms are hardly little things. And yes, some even live in nuclear reactor waste tanks, oddly enough.

Plus check out this guy:
http://en.wikipedia.org/wiki/Deinococcus_radiodurans

"You can never solve a problem on the level on which it was created."
Albert Einstein