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238 U + n 239 U + γ .

Uranium-239 then β decays:

239 U 239 Np + β + v e ( t 1/2 = 23 min) .

Neptunium-239 also β decays:

239 Np 239 Pu + β + v e ( t 1/2 = 2 . 4 d ).

Plutonium-239 builds up in reactor fuel at a rate that depends on the probability of neutron capture by 238 U size 12{ {} rSup { size 8{"238"} } U} {} (all reactor fuel contains more 238 U size 12{ {} rSup { size 8{"238"} } U} {} than 235 U size 12{ {} rSup { size 8{"235"} } U} {} ). Reactors designed specifically to make plutonium are called breeder reactors    . They seem to be inherently more hazardous than conventional reactors, but it remains unknown whether their hazards can be made economically acceptable. The four reactors at Chernobyl, including the one that was destroyed, were built to breed plutonium and produce electricity. These reactors had a design that was significantly different from the pressurized water reactor illustrated above.

Plutonium-239 has advantages over 235 U size 12{ {} rSup { size 8{"235"} } U} {} as a reactor fuel — it produces more neutrons per fission on average, and it is easier for a thermal neutron to cause it to fission. It is also chemically different from uranium, so it is inherently easier to separate from uranium ore. This means 239 Pu size 12{ {} rSup { size 8{"239"} } ital "Pu"} {} has a particularly small critical mass, an advantage for nuclear weapons.

Phet explorations: nuclear fission

Start a chain reaction, or introduce non-radioactive isotopes to prevent one. Control energy production in a nuclear reactor!

Nuclear Fission

Test prep for ap courses

Which of the following statements about nuclear fission is true?

  1. No new elements can be produced in a fission reaction.
  2. Energy released in fission reactions is generally less than that from fusion reactions.
  3. In a fission reaction, two light nuclei are combined into a heavier one.
  4. Fission reactions can be explained on the basis of the conservation of mass-energy.

(d)

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What is the energy obtained when 10 g of mass is converted to energy with an efficiency of 70%?

  1. 3.93 × 10 27 MeV
  2. 3.93 × 10 30 MeV
  3. 5.23 × 10 27 MeV
  4. 5.23 × 10 30 MeV
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In a neutron-induced fission reaction of 239 Pu, which of the following is produced along with 96 Sr and four neutrons?

  1. 56 139 Ba
  2. 56 140 Ba
  3. 54 139 Xe
  4. 54 140 Xe

(b)

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When 235 U is bombarded with one neutron, the following fission reaction occurs: 92 235 U + n 56 141 Ba + y 92 Kr + x n .

  1. Find the values for x and y.
  2. Assuming that the mass of 235 U is 235.04 u, the mass of 141 Ba is 140.91 u, the mass of 92 Kr is 91.93 u, and the mass of n is 1.01 u, a student calculates the energy released in the fission reaction as 2.689 × 10 −8 , but forgets to write the unit. Find the correct unit and convert the answer to MeV.
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Section summary

  • Nuclear fission is a reaction in which a nucleus is split.
  • Fission releases energy when heavy nuclei are split into medium-mass nuclei.
  • Self-sustained fission is possible, because neutron-induced fission also produces neutrons that can induce other fissions, n + A X FF 1 + FF 2 + xn , where FF 1 size 12{"FF" rSub { size 8{1} } } {} and FF 2 size 12{"FF" rSub { size 8{2} } } {} are the two daughter nuclei, or fission fragments, and x is the number of neutrons produced.
  • A minimum mass, called the critical mass, should be present to achieve criticality.
  • More than a critical mass can produce supercriticality.
  • The production of new or different isotopes (especially 239 Pu ) by nuclear transformation is called breeding, and reactors designed for this purpose are called breeder reactors.
Practice Key Terms 9

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Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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