6.40P Explain how the chain reaction is controlled in a nuclear reactor,…

The idea

In a reactor, fuel rods contain uranium-235 and fission releases fast neutrons. A moderator, such as water or graphite, slows these neutrons to low speeds because slow neutrons are more likely to be absorbed by uranium-235 and cause further fission. Control rods, made of a material such as boron that absorbs neutrons, are inserted or withdrawn to change how many neutrons are absorbed. Pushing the rods further in absorbs more neutrons, so fewer cause fission and the rate falls; pulling them out lets more neutrons cause fission and the rate rises. Together the moderator and control rods keep the average number of further fissions per fission at about one, giving a steady controlled chain reaction.

What earns the marks

• Explains that a moderator slows fast neutrons so they are more likely to cause further fission.
• States that control rods absorb neutrons and so reduce the number available to cause fission.
• Describes that inserting control rods further lowers the fission rate and withdrawing them raises it.
• Explains that moderator and control rods together keep the average number of further fissions per fission at about one.

Common mistakes and how to put them right

• Claims the moderator absorbs neutrons.

  Put it right: State that the moderator slows neutrons while control rods absorb them.

• Says control rods speed up neutrons.

  Put it right: Describe control rods as absorbing neutrons to reduce the fission rate.

• Believes inserting control rods increases the reaction rate.

  Put it right: State that inserting them absorbs more neutrons and lowers the rate.

Exam tips

• Keep the roles separate: moderator slows neutrons, control rods absorb neutrons.
• Explain the direction of change: inserting rods lowers the rate, withdrawing them raises it.
• Link both components to keeping the average further fissions per fission at about one.