Question 1

Otto Hahn and Fritz Strassmann discovered fission by bombarding uranium with neutrons. Their discovery was made possible by finding one of the fission products, barium (Ba, atomic no. = 56). What is a reasonable explanation for why they were unable to initially observe the other product, krypton (Kr, atomic no. = 36)

Accepted Answer

Krypton, being a noble gas, is both chemically unreactive and capable of diffusing out of the sample, making it difficult to detect through chemical means. Additionally, the specific isotope of krypton produced in the fission process might have had a short half-life, leading to its rapid decay into another element before it could be observed.

Question 2

Uranium-235 releases an average of 2.5 neutrons per fission, while plutonium-239 releases an average of 2.7 neutrons per fission. Which of these elements might you therefore expect to have the smaller critical mass?

Accepted Answer

Plutonium-239, releasing more neutrons per fission, would more easily sustain a chain reaction, thus requiring a smaller critical mass compared to uranium-235.

Question 3

Which of the following is a major advantage of nuclear-fission-based power plants?

Accepted Answer

A major advantage of nuclear-fission-based power plants is that the amount of energy per kilogram of fuel is greater than any other fuel source. Additionally, the reactors can be used to produce their own fuel, making them more self-sufficient and potentially less reliant on external sources for fuel. Therefore, the best choice is E, which combines both advantages. None of the other answer choices accurately describe a major advantage of nuclear-fission-based power plants.

Question 4

Which of the following statements about fission is true?

Accepted Answer

Fission is the splitting of a large nucleus into smaller fragments, and it is known to release a large amount of energy. This is because the difference in binding energy between the original nucleus and the resultant fragments is converted into energy according to Einstein's famous equation (E=mc^2).

Question 5

The isotope Cesium-137, which has a half-life of 30 years, is a product of nuclear power plants. How long will it take for this isotope to decay to about one-sixteenth its original amount?

Accepted Answer

To find out how long it will take for Cesium-137 to decay to one-sixteenth its original amount, we need to determine how many half-lives it takes for the amount to decrease by a factor of 16.

We can use the formula N = N0 (1/2)t/h, where N is the final amount, N0 is the initial amount, t is the time passed, and h is the half-life.

We want to find t when N/N0 = 1/16, so we can substitute these values into the formula:

1/16 = 1/2^(t/h)

Taking the logarithm base 2 of both sides:

-log2(16) = -(t/h)

4 = t/h

Therefore, it will take 4 half-lives for Cesium-137 to decay to one-sixteenth its original amount. Since the half-life is 30 years, the total time it will take is:

4 x 30 years = 120 years

Question 6

Why will nuclear fission probably not be used directly for powering automobiles?

Accepted Answer

All of the options listed are significant concerns when considering the use of nuclear fission directly in automobiles. Massive shielding would be necessary to protect passengers and the environment from radiation, managing radioactive waste from such a small and mobile source would be problematic, and it is more practical and efficient to use electricity generated by nuclear fission in stationary power plants to power electric vehicles.

Question 7

After a uranium fuel rod reaches the end of its fuel cycle (typically 3 years)most of its energy comes from the fissioning of plutonium because

Accepted Answer

During the fuel cycle, U-238 absorbs a neutron and becomes U-239 which undergoes beta decay to become plutonium-239. As the fuel rod ages, the amount of plutonium-239 in the rod increases, and eventually, most of the energy produced comes from the fissioning of plutonium-239.

Question 8

Why is carbon better than lead as a moderator in nuclear reactors?

Accepted Answer

Carbon is better than lead as a moderator in nuclear reactors because it slows down the neutrons, making them more effective in stimulating the nuclear chain reaction. Lead, on the other hand, slows down the neutrons making them less effective in stimulating the nuclear chain reaction, and it also absorbs neutrons rather than allowing them to rebound and maintain the nuclear chain reaction.

Question 9

Which of the following statements best describes a critical mass?

Accepted Answer

The answer of Which of the following statements best describes...

Question 10

Why does plutonium not occur in appreciable amounts in natural ore deposits?

Accepted Answer

Plutonium does occur naturally, but in very small amounts because its most stable isotopes have relatively short half-lives in geological terms, meaning any plutonium that was present when the Earth formed has largely decayed into other elements by now.

Question 11

What does Einstein's energy equation (E = mc2)say about the energy that is derived from nuclear fission reactions?

Accepted Answer

The energy released in nuclear fission reactions is indeed due to the conversion of the mass difference (missing mass) between the reactants and products into energy, as described by Einstein's equation E=mc^2. This mass difference is known as the mass defect, and its conversion into energy is what powers nuclear reactors and atomic bombs.

Question 12

Does the average distance that a neutron travels through fissionable material before escaping increase or decrease when two pieces of fissionable material are assembled into one piece? Does this assembly increase or decrease the probability of an explosion?

Accepted Answer

When two pieces of fissionable material are assembled into one piece, the average distance a neutron can travel before escaping increases because there is more material for it to traverse. This assembly also increases the probability of an explosion, as neutrons have a higher chance of causing further fissions due to the increased amount of material.

Question 13

If a nucleus of  Th absorbs a neutron and the resulting nucleus undergoes two successive beta decays (emitting electrons), what nucleus results?

Accepted Answer

The answer of If a nucleus of  Th absorbs...

Question 14

How do fission power plants work?

Accepted Answer

In a fission power plant, nuclear reactions are used to generate heat, which is used to boil water. The steam produced from the boiling water is then used to drive a turbine, which generates electricity. This is the most common method used in fission power plants.

Question 15

A neutron makes a better nuclear bullet than a proton or an electron because it

Accepted Answer

Neutrons carry no electrical charge, which allows them to penetrate the electrostatic defenses of an atom's nucleus more easily than charged particles like protons or electrons. This characteristic makes them more effective in initiating nuclear reactions.

Question 16

Which of the following is a major disadvantage of nuclear-fission-based power plants?

Accepted Answer

The byproducts of the fission process are radioactive and have very long half-lives, which can lead to hazardous waste disposal and potential environmental contamination. Additionally, nuclear power plants are complex and require strict safety measures to prevent accidents or contamination incidents, making them more challenging and expensive to build and maintain than other types of power plants.

Question 17

The original reactor built in 1942 was just "barely" critical because the natural uranium that was used contained less than 1% of the fissionable isotope U-235 (half-life 713 million years). What if, in 1942, the Earth had been 9 billion years old instead of 4.5 billion years old? Would this reactor have reached critical stage with natural uranium? Why?

Accepted Answer

The correct answer is D because the amount of U-235 in natural uranium would not change based on the age of the Earth, as it is a stable isotope with a long half-life. Even if the Earth were 9 billion years old instead of 4.5 billion years old, the natural uranium used in the reactor would still contain less than 1% U-235, which is not enough to reach critical stage. In fact, the percentage of U-235 would be much smaller due to the longer time that has passed since its creation.

Question 18

Why doesn't uranium ore spontaneously undergo a chain reaction?

Accepted Answer

Uranium ore is mostly comprised of the non-fissionable isotope U-238. In order for it to undergo a chain reaction, it needs to be mixed with a moderator to slow down the neutrons so that they are more likely to be absorbed and initiate fission. Weapons-grade uranium is enriched with a higher percentage of the fissionable isotope U-235, but this is not necessary for a chain reaction to occur. Uranium ore does not spontaneously undergo slow chain reaction as described in option C, as this would be a safety hazard in nuclear power plants. Lastly, option D is partly correct, as uranium that is intended for use in nuclear reactors needs to be enriched in U-235, but U-238 is not added to it.

Question 19

Which shape is likely to need more material for a critical mass, a cube or a sphere? Why?

Accepted Answer

The surface area to volume ratio of a cube is higher than a sphere, meaning that a cube needs proportionally more material to achieve a critical mass than a sphere does. Therefore, a cube is likely to need more material than a sphere.

Question 20

Which of the following statements about chain reactions is true?

Accepted Answer

In order for a chain reaction to occur, it is necessary for more neutrons to be produced than absorbed. This leads to a self-sustaining reaction where each fission event releases more neutrons that can cause further fission events.

Quiz 10: The Atomic Nucleus and Radioactivity