Thermodynamics

uranium-235 atom absorbs a neutron and splits during a fission process, itproduces a cesium-140 atom, a rubidium-93 atom, 3 neutrons, and 3.2 10 11 J of energy. In practical terms, the complete fission of 1 kg of uranium-235releases 6.73 10 10 kJ of heat, which is more than the heatreleased when 3000 tons of coal are burned. Therefore, for the same amountof fuel, a nuclear fission reaction releases several million times more energythan a chemical reaction. The safe disposal of used nuclear fuel, however,remains a concern.Nuclear energy by fusion is released when two small nuclei combine intoa larger one. The huge amount of energy radiated by the sun and the otherstars originates from such a fusion process that involves the combination oftwo hydrogen atoms into a helium atom. When two heavy hydrogen (deuterium)nuclei combine during a fusion process, they produce a helium-3atom, a free neutron, and 5.1 10 13 J of energy (Fig. 2–8).Fusion reactions are much more difficult to achieve in practice because ofthe strong repulsion between the positively charged nuclei, called theCoulomb repulsion. To overcome this repulsive force and to enable thetwo nuclei to fuse together, the energy level of the nuclei must be raised byheating them to about 100 million °C. But such high temperatures are foundonly in the stars or in exploding atomic bombs (the A-bomb). In fact, theuncontrolled fusion reaction in a hydrogen bomb (the H-bomb) is initiatedby a small atomic bomb. The uncontrolled fusion reaction was achieved inthe early 1950s, but all the efforts since then to achieve controlled fusion bymassive lasers, powerful magnetic fields, and electric currents to generatepower have failed.UraniumU-235nneutron(a) Fission of uraniumH-2H-2(b) Fusion of hydrogenChapter 2 | 573.2 × 10 –11 JCe-140nnnRb-93He-33 neutronsneutron5.1 × 10 –13 JFIGURE 2–8The fission of uranium and the fusionof hydrogen during nuclear reactions,and the release of nuclear energy.nEXAMPLE 2–1A Car Powered by Nuclear FuelAn average car consumes about 5 L of gasoline a day, and the capacity ofthe fuel tank of a car is about 50 L. Therefore, a car needs to be refueledonce every 10 days. Also, the density of gasoline ranges from 0.68 to 0.78kg/L, and its lower heating value is about 44,000 kJ/kg (that is, 44,000 kJof heat is released when 1 kg of gasoline is completely burned). Suppose allthe problems associated with the radioactivity and waste disposal of nuclearfuels are resolved, and a car is to be powered by U-235. If a new car comesequipped with 0.1-kg of the nuclear fuel U-235, determine if this car willever need refueling under average driving conditions (Fig. 2–9).Solution A car powered by nuclear energy comes equipped with nuclearfuel. It is to be determined if this car will ever need refueling.Assumptions 1 Gasoline is an incompressible substance with an average densityof 0.75 kg/L. 2 Nuclear fuel is completely converted to thermal energy.Analysis The mass of gasoline used per day by the car ism gasoline 1rV 2 gasoline 10.75 kg>L215 L>day2 3.75 kg>dayNuclearfuelFIGURE 2–9Schematic for Example 2–1.Noting that the heating value of gasoline is 44,000 kJ/kg, the energy suppliedto the car per day isE 1m gasoline 21Heating value2 13.75 kg>day2144,000 kJ>kg2 165,000 kJ>day