Delay and decay is a method for the storage of low-level radioactive wastes until the radiation level is low enough to dispose of the wastes by normal methods - incineration, landfill, as sewerage (liquid wastes).

You could add "basement" to the answer to make it a cellar.

"Three general principles are employed in the management of radioactive wastes:

concentrate-and-contain
dilute-and-disperse
delay-and-decay.

The first two are also used in the management of non-radioactive wastes. The waste is either concentrated and then isolated, or it is diluted to acceptable levels and then discharged to the environment. Delay-and-decay however is unique to radioactive waste management; it means that the waste is stored and its radioactivity is allowed to decrease naturally through decay of the radioisotopes in it."
Source: http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Nuclear...

"Delay and Decay:

Medium activity radioactive waste and those with half-lives of less than a month may be stored. The storage room should be properly ventilated with an exhaust system conducted through a duct line to a roof top exit. The storage space should have lead shielding of appropriate thickness (10 HVL) to prevent radiation leakage. The radioactive waste should be stored for a minimum period of about 10 half lives when after decay only 0.1% of the initial activity remains. The waste is then monitored for the residual activity and if the dose limit is low it is disposed off as low activity solid or liquid waste. Most of the low and medium level radioactive hospital waste is of short half-life permitting this type of waste disposal."

Source: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3068798/

"Chamber" is a good expression for the spatial aspect associated with radiation.