Which of the following is used as a moderator in nuclear reactors? 

1. Plutonium 

2. Cadmium 

3. Heavy water

4. Uranium 

The mass density of a nucleus varies with mass number A as 

1. $A^2$

2. A 

3. Constant 

4. $\frac{1}{A}$

A radioactive element has half-life period 800 yr. After 6400 yr, what fraction will remain? 

1. $\frac{1}{2}$

2. $\frac{1}{16}$ 

3. $\frac{1}{8}$ 

4.$\frac{1}{256}$

Energy released in the fission of a single ${}_{92}U_{235}$ nucleus is 200 MeV. The fission rate of a ${}_{92}U_{235}$ filled reactor operating at a power level of 5 W is 

1. $1.56\times {10}^{-10}<mspace\; width="1em"></mspace>s^{-1}$ 

2. $1.56\times {10}^{11}<mspace\; width="1em"></mspace>s^{-1}$

3. $1.56\times {10}^{-16}<mspace\; width="1em"></mspace>s^{-1}$   

4. $1.56\times {10}^{-17}<mspace\; width="1em"></mspace>s^{-1}$

Heavy water is used as a moderator in a nuclear reactor. The function of the moderator is 

1. to control energy released in the reactor

2. to absorb neutrons and stop the chain reaction

3. to cool the reactor

4. to slow down the neutrons to thermal energies

If a proton and anti-proton come close to each other and annihilate, how much energy will be released:

1. $1.5\times {10}^{-10}<mspace\; width="1em"></mspace>J$           

2. $3\times {10}^{-10}<mspace\; width="1em"></mspace>J$         

3. $4.5\times {10}^{-10}<mspace\; width="1em"></mspace>J$         

4. None of these