calciaiationd is 704 million years, and the radioactive decay of 1 kg of 235U re
ID: 152871 • Letter: C
Question
calciaiationd is 704 million years, and the radioactive decay of 1 kg of 235U releases 6.7 X 1013J.Radioactive material must be stored in a safe container or buried deep underground until its radiation output drops to a safe level. Generally, it is considered"safe"after 10 half-lives. (a) Assume that a nuclear power plant can convert 1. The atomic number of uranium-235 is 92, its half-life energy from 235U into electricity with an efficiency of 35 percent, the electrical transmission lines operate at 90 percent efficiency, and fluorescent lights operate at 22 percent efficiency () What is the overall efficiency of converting the energy of 235 U into fluorescent light? (2 points) How much energy from 1 kg of 235U is converted into fluorescent light? (2 points) (ii) (ii) Name one way in which you could improve the overall efficiency of this system. Explain how your suggestion would improve efficiency. (2 points) (b) What are the first and second laws of thermodynamics? (2 points) How long would it take for the radiation from a sample of 2SU to reach a safe level? (2 points) (c)Explanation / Answer
1. a) i) The required overall efficiency can be found out by multiplying the efficiencies of all the other parameters in the system = (0.35*0.9*0.22)*100 = 0.069*100 = 6.9 or 7% approximately. So the efficiency is 7%.
ii) Energy from 1kg of U235 that can be converted to fluorescent light = 6.7*1013*0.069 = 462.3 * 1010 J.
iii) One way in which we can improve the overall efficiency of the system is by using the waste heat or by decreasing the heat and sound lost as the energy is being transported from the plant to the light. Another method is with technical progress we can improve the efficiency that is by using a more efficient light bulb.
b) The first law of thermodynamics is also known as the Law of Conservation of Energy and it states that energy can neither be created nor destroyed. It can only be transformed from one form to another. This explains how it is possible to take away the energy from the fission of U235 and convert it to energy used in the fluorescent light. The second law of thermodynamics states that the entropy of an isolated system always increases. When energy is transformed, some of that energy is converted to a less usable form, usually heat and the capacity of that energy to do work also diminishes. This explains why no conversion from one energy form to another is exactly 100% efficient and less usable energy is available for application in such a fluorescent lighting.
c) It is considered safe after 10 half lives.
1half life = 704 million years.
So, the radiation from a sample of U235 would reach a safe level after = 10 half lives * 704 million years
= 7040 million years or 7.04 billion years.
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