Nuclear Fission and Fusion · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Splitting heavy, joining light

Science · Matter & Materials · ages 15-16
Name ______________________   Date ____________

  1. Which description gives the jobs of control rods and the moderator in a thermal reactor?

    • Rods slow neutrons; the moderator absorbs them
    • Rods absorb neutrons; the moderator slows them
    • Rods cool the fuel; the moderator produces new neutrons

  2. A uranium-235 nucleus absorbs a neutron and undergoes fission. What happens?

    • A heavy nucleus splits, releasing energy and neutrons
    • Light nuclei join, releasing energy and further neutrons
    • Electrons leave an atom, splitting it into smaller nuclei

  3. What joins in an energy-releasing fusion reaction using hydrogen fuel?

    • Two whole atoms joined only by sharing their electrons
    • A heavy nucleus and the electrons that orbit around it
    • Two light nuclei such as hydrogen nuclei

  4. Control rods are inserted farther into a running thermal reactor. With other conditions fixed, what happens to fission power?

    • It increases as more neutrons are absorbed by the rods
    • It decreases as fewer fissions occur each second
    • It stays unchanged because rods do not affect neutrons

  5. Why does increasing the temperature of hydrogen fusion fuel help its nuclei approach closely enough to react?

    • Neutral nuclei repel one another until electrons are removed
    • Positive nuclei repel one another electrically
    • Nuclei must first be split by incoming neutrons to join together

  6. Deuterium-tritium fusion makes stable helium and fast neutrons. Why does stable helium not prove that the reactor will have no radioactive waste?

    • Neutrons can make reactor materials radioactive
    • Every helium nucleus becomes a long-lived radioactive isotope
    • Fusion neutrons turn all surrounding matter into uranium fuel

  7. An experiment releases energy by fusion. What further requirement matters before a plant can provide a steady net electricity supply?

    • Record that the fuel gets hot, without measuring any energy leaving the device
    • Detect one fusion event, without checking how much energy the whole plant uses
    • Sustain suitable fuel conditions and produce more electricity than the whole plant uses

  8. Once control rods stop a reactor's fission chain, radioactive decay stops too, so cooling is no longer needed.

    Circle one:   True   False

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Answer key

For grown-ups. Fold this page away before handing over the rest.

Splitting heavy, joining light W1-mt_jkO89KodSb-s1

  1. Rods absorb neutrons; the moderator slows them · Control rods absorb neutrons to regulate the chain. The moderator slows neutrons in a thermal reactor.
  2. A heavy nucleus splits, releasing energy and neutrons · This neutron-induced fission splits a heavy nucleus into lighter nuclei, releasing energy and neutrons.
  3. Two light nuclei such as hydrogen nuclei · Nuclear fusion joins light nuclei. Sharing electrons between atoms describes a different process.
  4. It decreases as fewer fissions occur each second · The rods absorb more neutrons. Fewer remain to cause fission, so fission power decreases under the stated conditions.
  5. Positive nuclei repel one another electrically · Hydrogen nuclei are positive and repel electrically. Higher temperature makes close encounters more likely.
  6. Neutrons can make reactor materials radioactive · Stable helium is only one product. Fast neutrons can make reactor materials radioactive, and tritium fuel is itself radioactive.
  7. Sustain suitable fuel conditions and produce more electricity than the whole plant uses · Energy released in fusion fuel is not the same as net electricity from a whole plant. Sustained operation and all plant electricity use must be considered.
  8. False · Stopping the chain does not stop radioactive decay. Decay continues to produce heat, so cooling remains necessary.
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