Quantum Physics

976 Chapter 30 Nuclear Energy and Elementary ParticlesMultiply N by the energy yield per nucleus, obtainingthe total disintegration energy:E NQ (2.56 10 24 nuclei) 208 5.32 10 26 MeVMeVnucleus(b) How many kilograms would provide for the world’sannual energy needs?Set the energy per kilogram, E kg , times the number ofkilograms, N kg , equal to the total annual energy consumption.Solve for N kg :E kg N kg E totN kg E totE kg 5 10 6 kg4 10 20 J(5.32 10 32 eV/kg)(1.60 10 19 J/eV)Remark The calculation implicitly assumes perfect conversion to usable power, which is never the case in real systems.There are enough known uranium deposits to provide the world’s current energy requirements for a few hundredyears.Exercise 30.2How long can one kilogram of U-235 keep a 100-watt lightbulb burning if all its released energy is converted to electricalenergy?Answer30 000 yr30.2 NUCLEAR REACTORSWe have seen that neutrons are emitted when 235 U undergoes fission. These neutronscan in turn trigger other nuclei to undergo fission, with the possibility of achain reaction (Active Fig. 30.2). Calculations show that if the chain reaction isn’tcontrolled, it will proceed too rapidly and possibly result in the sudden release ofan enormous amount of energy (an explosion), even from only 1 g of 235 U. If theenergy in 1 kg of 235 U were released, it would equal that released by the detonation23592 U9214156 Ba 36 KrNeutron138I5395138 9538 Sr 54 Xe 39 Y 23592 U9841 Nb13551 SbACTIVE FIGURE 30.2A nuclear chain reaction initiated by the capture of a neutron.Log into PhysicsNow at www.cp7e.com and go to Active Figure 30.2 to observe a chain reaction.