Problem set 6

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Fission and Nuclear Reactors

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The mass number of a given nucleus increases by one unit when a neutron is captured. Table 6.1 gives information about mass excess for three important isotope pairs. Calculate the difference in average binding energy between the pairs of nuclide and discuss why the 238U/239U pair differs so greatly from the other two pairs in regards to utilization of nuclear energy.

Table 6.1: Mass excess
Nuclide 1 Mass Excess (keV)
Nuclide 2
Mass excess (keV)


Find the answers to the following questions:

  1. A specific thermal fission of 239Pu gives 99Y + 2 neutrons + X. Which nuclei is X?
  2. Calculate the Q-value for the above fission from a mass table.
  3. Use a mass table to calculate the residual heat, that is the energy developed from disintegration after fission has taken place.
  4. A part of this energy is of no importance for the safety of the reactor, explain why.

1.0 g 239Pu is irradiated with a neutron flux of 1.0 [math]\cdot[/math]1014 n cm-2 s-1. Assume that all the fissions have the same Q-value as the one calculated under 2.b.

  1. Calculate the developed effect from fission in plutonium during the irradiation.
  2. How much 240Pu is formed after 100 days of irradiation?

4: In a thorium breeder reactor, fissile 233U is formed from fertile 232Th.

  1. Write down the nuclear reaction and how the disintegration occurs.
  2. A specific thermal fission of 233U gives 99Y + 2 neutrons + X. Which nuclei is X?
  3. Assume one ton thorium is irradiated with a neutron flux of 1.0*1014n cm-2 s-1. How long does it take to generate 100 grams of 233U?
  4. What is the disintegration rate of this uranium?