A $1000 \;MW$ fission reactor consumes half of its fuel in $5.00\; y$. How much $_{92}^{235} U$ (in $kg$) did it contain initially? Assume that the reactor operates $80 \%$ of the time, that all the energy generated arises from the fission of $_{92}^{235} U$ and that this nuclide is consumed only by the fission process.

Define the disintegration rate or radioactivity of a sample and obtain the relation $R = \lambda N$ and define its different units. 

$1\, Curie $ is equal to

If the half life of a radioactive sample is $10\, hours$, its mean life is ……….$hours$

Half life of radium is $1620$ years. How many radium nuclei decay in $5$ hours in $5\, gm$ radium? ( Atomic weight of radium $= 223$)