Phys 34 questions

What is the effect of placing small amounts of fissionable isotopes with large amounts of U-238?

Neutrons from the fission transmute U-238 to Pu-239, a fissionable fuel.

What isotope is produced when U-239 emits a beta particle?

Np-239

What are the four main components of a fission reactor?

Nuclear fuel (primarily U-238 + 3% U-235), control rods (made of neutron-absorbing material like cadmium or boron), a moderator (sometimes water), heat-extracting fluid (water, kept under high pressure to keep it at a high temperature without boiling, then transfers heat to a second, lower pressured water system)

Why doesn't a chain reaction normally occur in uranium mines?

Only a small fraction of natural uranium is the fissionable U-235, so most of the released neutrons do not trigger another fission. "Chain reactions would occur in naturally occurring uranium ore deposits if all uranium atoms fissioned so easily. Fission occurs mainly for the rare isotope U-235, which makes up only 0.7% of the uranium in pure uranium ore. In today's world, naturally occurring uranium is too "impure" to undergo a chain reaction spontaneously."

Is work required to pull a nucleon out of an atomic nucleus? Does the nucleon, once outside, have more energy than it did when it was inside the nucleus? In what form is this energy?

Yes, work is required; more energy; more mass per nucleon

Why does a neutron make a better nuclear bullet than a proton?

neutrons are better bullets because they have no charge and are not repelled by protons in the nucleus which they are aimed at. If it is to penetrate the nucleus, clearly a proton has the handicap of its positive charge. Like charge repel, all the more strongly that they are close. So the proton closing in on a nucleus will be scattered in another direction and hit the nucleus only if it is very energetic. The neutron has no charge and the same mass as the proton and it has strong interactions, so it is a better choice.

What is meant by the idea of a critical mass?

A critical mass of uranium is large enough so that neutrons ejected from one fissioning nucleus will hit another nucleus before exiting the mass. consider a large quantity of U-235 divided into two pieces, each unit being subcritical; neutrons in either piece readily reach a surface and escape before a sizeable chain reaction builds up. If the pieces are driven together, the total surface area decreases, and the combined mass is supercritical; the chain reaction builds up explosively.

What is the basic difference between the graphs in Figure 34.15 and Figure 34.16?

As a function of atomic number, Figure 34.15 shows the mass of a nucleus (how nuclear mass increases with increasing atomic number) while Figure 34.16 shows the mass per nucleon (how the average mass per nucleon in the nucleus varies from one end of the periodic table to the other).

What components are the safeguards to prevent a reactor from generating energy out of control?

Control rods

What isotopes of hydrogen fuse best at "moderate" temperatures?

Deuterium and tritium. The high-energy neutrons will escape from the plasma in the reactor vessel and heat a surrounding blanket of material to provide useful energy. The helium nuclei that remain behind will help to keep the plasma hot.

What celebrated equation shows the equivalence of mass and energy?

E = mc2

What are the three possible fates of neutrons in uranium metal?

Fission of a U-235 atom; escape from the metal into nonfissionable surroundings; be absorbed by a U-238 atom without causing fission.

What kind of nuclear power is responsible for sunshine? Give an example.

Fusion: proton + deuteron produces helium-3.

What were the two methods used to separate U-235 from U-238 in the Manhattan Project during World War II?

Gaseous diffusion and magnetic separation.

For helium to release energy, should it be fissioned or fused? What about uranium?

Helium should be fused; uranium should be fissioned. Fission only produces more energy than it consumes in large nuclei (common examples are Uranium & Plutonium, which have around 240 nucleons (nucleon = proton or neutron)). Fusion only produces more energy than it consumes in small nuclei (in stars, Hydrogen & its isotopes fusing into Helium)

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

High purity carbon slows neutrons down without absorbing many, so it is a good moderator. Reactors such as the magnox type were built to use natural uranium without enrichment using a graphite moderator, which shows how efficient it is. Lead would be completely useless as it would kill any chain reaction through absorbing neutrons.

From most abundant to least abundant, arrange the isotopes hydrogen, deuterium, and tritium.

Hydrogen (most abundant), deuterium, tritium (least abundant). Tritium is not found in nature on Earth due to its short 12-year half-life.

Why is a chain reaction more likely to occur in a big piece of uranium than in a small piece?

In a big piece, the ejected neutrons are more likely to strike another nucleus before exiting the piece. This is because of geometry. In a larger piece, more uranium atoms and a smaller surface area are presented to the neutrons. A smaller fraction of the free neutrons escape before triggering another fission, so the chain reaction builds up, releasing an enormous explosion.

What becomes of the "lost" mass per nucleon in fission and fusion reactions?

It becomes kinetic energy. The average mass of each nucleon in a uranium nucleus is greater than the average mass of each nucleon in any one of its nuclear-fission fragments. This decrease in mass is transformed into energy. Most of this enormous energy is kinetic energy. Hence, nuclear fission is an energy-releasing process.

Cite a major advantage of fusion power. Cite a major drawback.

It provides plentiful electricity, but it produces weapons-grade plutonium. It eliminates the release of the greenhouse gas CO2, but it produces radioactive waste. It eliminates megatons of sulfur oxides, but it risks large releases of radioactivity

What isotope is produced when Np-239 emits a beta particle?

Pu-239

If the graph in Figure 34.16 is seen as an energy valley, what can be said about the energy of nuclear transformations that progress toward iron?

The energy of nuclear transformations spontaneously goes downhill to the bottom of the valley, thereby releasing energy.

In which element is the mass per nucleon greatest? Least?

The greatest mass per nucleon occurs for hydrogen, whose lone central proton has no binding energy to pull its mass down. The mass per nucleon decreases progressively through the periodic table and is least for iron.

When a pair of hydrogen nuclei are fused to create helium, how does the mass of the resulting helium nucleus compare with the sum of the nuclear masses before fusion?

The mass of the helium nucleus is less than the sum of the masses of the hydrogen nuclei.

How does the mass per nucleon in uranium compare with the mass per nucleon in its fission fragments?

The mass per nucleon is greater in uranium. The average mass of each nucleon in a uranium nucleus is greater than the average mass of each nucleon in any one of its nuclear-fission fragments. Note that the mass per nucleon in the fission fragments is less than the mass per nucleon when the same set of nucleons is combined in the uranium nucleus.

Which ions of like charge and equal speed are least deflected in a mass spectrometer?

The more massive ions. In a mass spectrometer, the charged ions with identical speeds are directed into a magnetic field, where they deflect into circular arcs. The greater the inertia of the ion, the more it resists deflection and the larger the radius of its curved path. The magnetic force sweeps the heavier ions into larger arcs and the lighter ions into smaller arcs.

Is the mass of an atomic nucleus greater or less than the sum of the masses of the nucleons composing it? Why don't the nucleon masses add up to the total nuclear mass?

The nuclei are not converted completely to energy but rearranged and the mass per nucleon can change and be given off as energy. For the nucleons to stay together in a nucleus, they need to be in a lower potential energy than if they were alone. Otherwise, they would disperse. That lower energy is equivalent to lower mass for the whole nucleus (using E=mc^2). Note the binding energy of the nucleus.

Which will leak more neutrons: two separate pieces of uranium or the same pieces stuck together? Why?

The two separate pieces will leak more because smaller pieces have more surface area per volume.

In what way is a nuclear reactor similar to a conventional fossil-fuel plant?

They both boil water to make steam.

Name one benefit and one drawback to breeder reactors.

They generate more fuel than they use, but they are complex to operate safely.

Name two isotopes that can undergo fission when they absorb a neutron.

U-235, Pu-239. pg. 644, fig34.11: "Pu-239 or U-233, like U-235, undergoes fission when it captures a neutron."

What element reacts in a breeder reactor to breed nuclear fuel?

U-238

What isotope is produced when U-238 absorbs a neutron?

U-239