PHY 1020 – Chain Reactions, Nuclear Reactors, and Atomic Bombs (Chapter 5)

32 question-and-answer flashcards covering chain reactions, nuclear fission and fusion, bomb designs, reactor physics, notable accidents, enrichment, waste, and advanced technologies from Chapter 5 of PHY 1020.

What is a chain reaction?

A sequence of reactions in which one event induces subsequent events, leading to self-sustaining growth.

In nuclear fission of U-235, why does the number of fissions grow exponentially?

Because each fission releases on average two neutrons that can trigger two more fissions, leading to doubling (2^N) of events.

How many cell divisions are needed to reach roughly 10^11 cells in the human body?

About 37 doubling steps (log₂10^11 ≈ 36.5), or roughly 37 days if one division occurs per day.

Why is cancer described as a biological chain reaction?

It is uncontrolled cell growth where the body’s mechanisms to remove defective cells fail, allowing exponential division.

Why can’t exponential population growth continue indefinitely on Earth?

Environmental resources are limited; unchecked growth will exceed the planet’s carrying capacity within decades.

Give two non-nuclear examples of chain reactions.

Biological/computer viruses, rumors/urban legends, and lightning electron avalanches.

What is ‘critical mass’ in the context of nuclear bombs?

The smallest amount of fissile material required to sustain a neutron chain reaction.

Approximate critical mass values: uranium-235 vs plutonium-239?

~15 kg for U-235 and ~5 kg for Pu-239.

Name the uranium bomb dropped on Hiroshima and its yield.

“Little Boy,” about 15 kilotons of TNT.

Name the plutonium bomb dropped on Nagasaki and its yield.

“Fat Man,” about 20 kilotons of TNT.

Briefly describe the gun-type design used in Little Boy.

A hollow uranium bullet is fired into a uranium target, forming a super-critical mass that initiates fission.

Briefly describe the implosion design used in Fat Man.

Chemical explosives compress a plutonium core into a super-critical state, starting the chain reaction.

What is a thermonuclear (hydrogen) bomb?

A weapon that uses a fission bomb as a trigger to ignite fusion of light nuclei, producing vastly greater yields.

Compare the yields of the Hiroshima bomb and Tsar Bomba.

Hiroshima: 15 kt; Tsar Bomba: 50 Mt (over 3,000 times larger).

Why must natural uranium be enriched for reactors or weapons?

Natural ore has too little U-235; enrichment increases the U-235 concentration to sustain fission.

What chemical compound is used in gaseous uranium enrichment?

Uranium hexafluoride (UF₆).

Which U.S. site performed large-scale enrichment during the Manhattan Project?

The K-25 plant in Oak Ridge, Tennessee.

What is the key difference between neutrons in bombs versus reactors?

Bombs rely on fast neutrons for rapid, runaway reactions; reactors use slow (thermal) neutrons for controlled power generation.

What role do moderators play in a reactor? Name two common moderators.

They slow down neutrons by collisions; common moderators are (heavy) water and graphite.

What causes the blue glow seen in reactor pools?

Cherenkov radiation—light emitted when charged particles travel faster than light does in water.

Why can’t a commercial nuclear reactor explode like a nuclear bomb?

The geometry, fuel enrichment, and presence of moderators ensure neutrons are slow; if overheating occurs, the core melts before a fast chain reaction can form.

What primary failure led to the Three Mile Island accident?

Loss of water circulation caused by clogged/malfunctioning equipment, leading to overheating and partial core meltdown.

How did graphite-tipped control rods contribute to the Chernobyl disaster?

Inserting them displaced neutron-absorbing water with neutron-moderating graphite, briefly increasing reactivity and triggering an explosive power surge.

What natural disaster sequence disabled cooling at Fukushima?

A 9.0 earthquake followed by a massive tsunami flooded backup generators, stopping coolant pumps and causing hydrogen explosions.

What is a breeder reactor?

A reactor that produces more fissile material (e.g., Pu-239) than it consumes, effectively generating its own fuel.

Describe one design feature of proposed Gen IV lead-cooled reactors.

They use liquid lead for convection cooling, enhancing safety and thermal efficiency.

Why is nuclear waste management challenging?

Spent fuel remains radioactive for thousands of years, requiring secure, long-term storage (e.g., Yucca Mountain).

Define nuclear fusion.

The process where two light nuclei combine to form a heavier nucleus, releasing energy—occurs naturally in stars like the Sun.

What is a tokamak?

A torus-shaped device that uses strong magnetic fields to confine hot plasma for controlled thermonuclear fusion research.

How does inertial confinement fusion using lasers work?

Powerful laser pulses compress and heat a small fuel pellet so rapidly that fusion occurs before the target can disassemble.

What is muon-catalyzed (cold) fusion?

Fusion facilitated by muons replacing electrons, bringing nuclei closer; not yet practical due to high muon production costs and sticking losses.