We've reproduced below two of the better essays written by students for the first midterm exam: Essay 1 (on gasoline addiction) and Essay 2 (on nuclear reactors).  Read these to get an indication of what we were looking for.  Congratuations Catie and Liam!

 

 

Exam Essay Question 1:  Some people say that the United States is “addicted” to gasoline. Compare gasoline to alternative ways of powering an automobile.  Describe both the advantages and disadvantages of the alternatives you discuss, compared to a gasoline engine.

 

exam essay written by by Catie Cuan:

 

Gasoline used to be far less expensive that it is now. It is also easy to store, has 15 times more energy per gram than TNT and is familiar to the public. However, rising price of oil, wars in the Middle East, and only 20-25% efficiency for gas has led many to seek alternatives for powering our cars.

 

A recent development is the hybrid vehicle, which utilizes a combination to electric power and gasoline. These cars are 40-60% efficient (meaning they lose the remaining percent of the energy to heat), and are able to recharge when breaking (by running a generator, this is called “battery breaking”). Therefore the car uses less gasoline, emits less pollution, and has better mileage than a gas ca, especially at constant speed. However, hybrid vehicle owners must factor in the cost of replacing the batteries, which make the overall price tag comparable to a gas powered car. Hybrids still use gas, so the U.S. still has top purchase/manufacture oil.

 

Another alternative automobile is the electric car. These are run on batteries alone which are about 85-90% efficient. They emit no pollution( except perhaps for the power plant which manufactures the batteries and provides the electricity) and an owner can plug their car into a typical plug and pay about 10 cents for a kilowatt-hour. Some problems with electric cars include their short driving range, cost of replacing batteries, and the batteries’ weight. Some examples of such electric cars in production (development) are GEM cars, the Chevy Volt, and the Tesla Roadster.

 

Another alternative is a hydrogen car. Hydrogen has 3 times more energy per gram than gasoline, but 3 times less per gallon, meaning it is difficult to store. One more disadvantage to hydrogen cars include the fact that hydrogen must be extracted (from either H2O or fossil fuels) which takes energy, so hydrogen is generally thought of as a carrier rather than a source.

 

Solar powered cars are also being researched, though so far do not seem like a good alternative. Some advantages include the fact that the sun is renewable, plentiful, and a solar car would emit no pollution. However solar cells are incredibly expensive and only 15-40% percent efficient. For example, if you attach a 1 sqm solar cell to the top of a car, it would end up being 0.1-0.2 hp. A typical gasoline car operates at 100 hp. Solar power also doesn’t exist at night, so cars would need and energy storage mechanism. Companies are developing CIGS and solar cells which concentrate sunlight to make solar cells cheaper, CalSol is such a project.

 

Coal is the last alternative. It is cheap, plentiful, and has an existing infrastructure. But coal also emits high levels of pollution which leads to global warming and leaves an ash residue, it is only 35% efficient. Many agree that America should end it’s gas reliance soon.

 

 

 

Exam Essay Question 2: What is the purpose of a nuclear reactor?  How does it work?  What are the potential dangers?  Give historical examples.  Are there things that many people think are true about reactors that, in Muller’s analysis, are not?

 

exam essay written by Liam (William) Cabal:

 

The purpose of a nuclear reactor is to provide electricity by having radiation heat water, which produces steam which then turns turbines to produce electricity.  The heat comes from a sustained nuclear chain reaction where the neutrons from U-235 hit other uranium atoms and constantly sustain the reaction.  In the reactor, 97% U-238 and 3% U-235 is used, whereas a uranium nuclear bomb needs at least 80% U-235 to properly explode.  Since the reactor needs so much less U-235, there is little enrichment necessary as 99.3% of natural uranium is U-238 with 0.7% being U-235.  Additionally, nuclear reactors rely on only one of the 2 neutrons released to continue the constant reaction whereas nuclear bombs require both of them to produce the doubling effect needed for the explosion.  To sustain the reaction without producing the doubling, nuclear reactors use a moderator to slow down the neutrons to facilitate less reactions.  The moderator can be water, which is what the U.S. uses in its reactors; or heavy water, which is what Canada uses; or graphite. The water moderator also acts as a coolant to prevent overheating.  In addition to the rods (or pellets) of uranium used for the reaction, there are control rods of U-238 that can be used to stop the reaction if something were to go wrong.

 

One potential danger in using a nuclear reactor is that the core could overheat from a problem with the coolant, which would cause a meltdown where the nuclear material could melt through the floor into the earth (but not all the way through like the "China Syndrome" suggests).  Another potential danger is that the reaction could become uncontrolled (the control rods could malfunction) and an explosion could occur.  Some of these dangers were realized in two historical examples: Chernobyl and Three Mile Island.  The Chernobyl plant in the Ukraine had a problem and overheated causing an explosion that caused radiation to spread across the countryside.  If the linear hypothesis is correct, it caused an additional 24,000 cancers; if not, the number was less than 1000.  Three Mile Island in Pennsylvania had a meltdown and administrators chose to vent radiation into the air, which appears to have only increased radiation in the region negligibly. 

 

The public has some misconceptions about nuclear rectors.  The first is that if there is an explosion, it would be like a nuclear bomb, but because there is far less U-235 used, that is wrong.  The explosion would be more like a TNT explosion – bad, but not as much.  Another misconception is that there is no safe place to store the wast including the byproduct Pu-239, which is used for making bombs.  That is also wrong since scientists can guarantee the safety of stored wast in Yucca Mountain for 10,000 years.  Overall there are some dangers with nuclear power, but not enough not to use it.