Since, efficiency of the reactor = 10%

 

 and power of the reactor = 1000MW = 10⁹ Watts

 

So, power generated due to the fission of U-235 in order to sustain the 1000MW

 

plant with 10% efficiency = 10¹⁰Watts

 

Now reactor is to function for 10years

 

So during these 10 years Energy generated due to the fission of U-235 is given

 

by E = P*t = 10¹⁰*10*365*24*3600 Joules

 

Energy liberated after fission of 1 nucleus of U-235 = E_f = 200Mev

 

or, E_f = 200*10⁶*1.6*10^-19 Joules

 

hence to produce energy E the no. of U-235 nuclei required N = E/E_f

 

or N = 10¹⁰*10*365*24*3600 /200*10⁶*1.6*10^-19  = 9855*10²⁵

 

Now 1 mole of U-235 = 235 g = 6.023*10²³ nuclei or atoms of U-235

 

So N number of U-235 = N/6.023*10²³  moles

 

or mass of N number of U-235 = [N/6.023*10²³] * 235 grams 

 

                                           = 9855*10²⁵*235/6.023*10²³

                                            = 3843180 g = 38431.80 kg

in mechanics we never reach velocities which are any where comparable to light

(even fastest rockets are slow in cmparision).so the rest mass energy almost remains equal when at rest or in motion .Actually you can take that in account here also but it cancels out both sides.in modern physics particles move in velocities of the order 10^6 ms.so, the mass in motion of such a partricle will be larger than at rest and which we can't neglect.