Uranium-235 has a half-life of 710 million years. If it is determined that a certain amount of stored U-235 will be considered safe only when its radioactivity has dropped to 0.10 percent of the original level, approximately how much time must the U-235 be stored securely to be safe

Therefore the Uranium-235 must be stored safely for a time of 7075.71 million years.

Explanation:

Half life of Uranium-235 is given as 710 million years.

Uranium-235 is considered safe only when its radioactive level has dropped to 0.1 % of the original level.

The formula for half life of any element is

N  =      .... where N is the final amount after depletion

⇒ =     .... where it is given that N = 0.001  is the

                                               amount that will be considered safe with

                                               respect to radioactivity.

⇒           t   =     =  7075.71 million years.

Therefore the Uranium-235 must be stored safely for a time of 7075.71 million years.

Thus we can say that U-235 must be stored securely for a very, very long time after which it can be considered safe.