1. 1
   In one possible neutron-induced fission of uranium-235, a slow neutron is absorbed and the nucleus splits into xenon-140 and strontium-94, together with x free neutrons: $^{1}_{0}\mathrm{n} + ^{235}_{92}\mathrm{U} \rightarrow\ ^{140}_{54}\mathrm{Xe} + ^{94}_{38}\mathrm{Sr} + x\,^{1}_{0}\mathrm{n}$. The masses are: uranium-235, 235.0439 u; xenon-140, 139.9216 u; strontium-94, 93.9154 u; neutron, 1.00866 u.

   (a)

   What is the value of x?

   [1 mark]

   • A1
   • B2
   • C3
   • D4

   (b)

   In which form does most of the energy released in this fission first appear?

   [1 mark]

   • AKinetic energy of the free neutrons
   • BEnergy of gamma photons
   • CRest mass of the extra neutrons produced
   • DKinetic energy of the xenon and strontium nuclei

   (c)

   Calculate the energy released in this fission, in MeV.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In a pressurised water reactor, fuel rods of enriched uranium sit in water under high pressure. The water acts as both moderator and coolant, and is pumped in a closed primary circuit through a heat exchanger (steam generator), where it boils water in a separate secondary circuit that drives the turbines. Rods containing boron can be raised or lowered between the fuel rods. The reactor core is surrounded by a thick steel pressure vessel and a concrete building.

   (a)

   What is the main purpose of the moderator?

   [1 mark]

   • ATo absorb neutrons so that the reaction rate is reduced
   • BTo transfer thermal energy to the turbines
   • CTo slow neutrons down so that they are more likely to cause fission of uranium-235
   • DTo absorb gamma radiation from the core

   (b)

   What happens when the boron rods are lowered further into the core?

   [1 mark]

   • AMore neutrons are absorbed, so the rate of fission decreases
   • BMore neutrons are slowed down, so the power increases
   • CThe temperature of the coolant increases
   • DFission products are removed from the fuel

   (c)

   Outline why the heat exchanger is used instead of sending the water from the reactor core directly to the turbines.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In a reactor operating at steady power, each fission releases on average 2.4 neutrons. Of these, on average 0.6 are absorbed by the control rods, 0.5 are absorbed by uranium-238 and structural materials without causing fission, and 0.3 escape from the core. The rest cause further fissions of uranium-235. Each generation of neutrons lasts a fraction of a second.

   (a)

   Deduce whether this reactor is operating with a steady chain reaction.

   [3 marks]

   (b)

   To increase the power, the operators withdraw the control rods slightly so that they absorb only 0.4 neutrons per fission. Determine the factor by which the rate of fission increases after 10 generations, and explain why the rods must later be returned to a position where the reaction is steady.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   When fuel is removed from a reactor, it contains fission products including iodine-131 (half-life 8 days), strontium-90 (half-life 29 years) and caesium-137 (half-life 30 years), as well as plutonium-239 (half-life 24 000 years), formed when uranium-238 absorbs neutrons. The spent fuel is first placed in deep pools of water at the reactor site. A government proposes to keep it there for 10 years and then seal it in steel and copper containers inside a repository several hundred metres underground in stable rock.

   (a)

   Explain why spent fuel is highly radioactive and why it must be kept cool and shielded for a long time.

   [6 marks]

   (b)

   Evaluate the government's proposal for managing the spent fuel.

   [6 marks]

   Total for question 4: 12 marks

End of questions