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The Nuclear Fuel Cycle: Open vs. Closed Cycle and Uranium Enrichment, Study notes of Physics

University of CambridgePhysics

An overview of the nuclear fuel cycle, comparing open and closed fuel cycles, and discussing uranium enrichment processes. It covers fuel types, the nuclear fuel process, and the economic and environmental considerations of each cycle. Uranium ore purification and enrichment methods are also discussed.

Typology: Study notes

2010/2011

Uploaded on 09/07/2011

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THE NUCLEAR FUEL CYCLE
Fuel Types
• Natural Uranium metal
• Enriched Uranium oxide
• Mixed Uranium and Plutonium oxide (MOX)
• Plutonium oxide (fast reactors)
Open Fuel Cycle
• The Uranium is mined & purified.
• U235 content is enriched
• Uranium is converted to UO2 and formed into fuel elements.
• The fuel elements are left in the reactor for the desired 'burn up'
• Spent fuel is removed from the reactor to local cooling ponds.
• After a period of local cooling they are transferred to long term dry storage.
• No attempt is made to recover the unused Uranium or the Plutonium.
Closed Fuel Cycle
• The Uranium is mined & purified.
• U235 content is enriched
• Uranium is converted to UO2 and formed into fuel elements.
• The fuel elements are left in the reactor for the desired 'burn up'
• Spent fuel is removed from the reactor to local cooling ponds.
• After a period of local cooling they are transferred to reprocessing.
• The unused Uranium and the Plutonium are recovered for reuse and the
fission product stored.
Comparison of Cycles
• The closed cycle was essential for early designs of reactor because the
integrity of the cladding could not be guaranteed.
• However, oxide fuel cladding will allow long term storage so the question is
now one of economics.
• When the decisions to build the large reprocessing plants at Sellafield and
Cap la Hague (France) were made in the early 1980s it was predicted that the
price of Uranium would rise to $100 per pound by 1990.
• In fact the price is now about $10. Hence the economics of reprocessing must
now be questioned.
• However once plant s had been built the comparison must be on operating
costs.
• Some environmentalists argue against reprocessing on the grounds that it
produces waste and results in stockpiles of Plutonium.
• The waste argument is not proven because reprocessing actually
concentrates the highly active fission products and it is a well proven process.
• There are to date no full scale disposal facilities for the direct disposal of
spent fuel.
• The Plutonium produced from civil power reactors is largely made up of the
240 and 241 isotopes which cannot readily be made into weapons but it can
be used as a reactor fuel.
• As supplies of gas and oil run out in the next century and environmental
pressures increase the cost of power generation from coal, the price of
Uranium will rise.
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THE NUCLEAR FUEL CYCLE

Fuel Types

  • Natural Uranium metal
  • Enriched Uranium oxide
  • Mixed Uranium and Plutonium oxide (MOX)
  • Plutonium oxide (fast reactors) Open Fuel Cycle
  • The Uranium is mined & purified.
  • U 235 content is enriched
  • Uranium is converted to UO 2 and formed into fuel elements.
  • The fuel elements are left in the reactor for the desired 'burn up'
  • Spent fuel is removed from the reactor to local cooling ponds.
  • After a period of local cooling they are transferred to long term dry storage.
  • No attempt is made to recover the unused Uranium or the Plutonium. Closed Fuel Cycle
  • The Uranium is mined & purified.
  • U 235 content is enriched
  • Uranium is converted to UO 2 and formed into fuel elements.
  • The fuel elements are left in the reactor for the desired 'burn up'
  • Spent fuel is removed from the reactor to local cooling ponds.
  • After a period of local cooling they are transferred to reprocessing.
  • The unused Uranium and the Plutonium are recovered for reuse and the fission product stored. Comparison of Cycles
  • The closed cycle was essential for early designs of reactor because the integrity of the cladding could not be guaranteed.
  • However, oxide fuel cladding will allow long term storage so the question is now one of economics.
  • When the decisions to build the large reprocessing plants at Sellafield and Cap la Hague (France) were made in the early 1980s it was predicted that the price of Uranium would rise to $100 per pound by 1990.
  • In fact the price is now about $10. Hence the economics of reprocessing must now be questioned.
  • However once plant s had been built the comparison must be on operating costs.
  • Some environmentalists argue against reprocessing on the grounds that it produces waste and results in stockpiles of Plutonium.
  • The waste argument is not proven because reprocessing actually concentrates the highly active fission products and it is a well proven process.
  • There are to date no full scale disposal facilities for the direct disposal of spent fuel.
  • The Plutonium produced from civil power reactors is largely made up of the 240 and 241 isotopes which cannot readily be made into weapons but it can be used as a reactor fuel.
  • As supplies of gas and oil run out in the next century and environmental pressures increase the cost of power generation from coal, the price of Uranium will rise.
  • This will re-awaken interest in Plutonium as a reactor fuel and make the recovery of waste Uranium an economic proposition again. Uranium Ore Purification
  • The techniques used are standard mineral processing and chemical engineering methods and will not be discussed further.
  • If the fuel is to be used as natural Uranium metal the ore is smelted again using fairly conventional methods and the metal cast in to rods which are then machined and put into Magnox cans.
  • If the U 235 content is to be enriched the Uranium is first converted into UF 6 (HEX) Enrichment
  • Because an isotopic separation is required it must be by physical rather than by chemical means.
  • All established techniques make use of the difference in density between U 235 and U238. (as HEX)
  • Other techniques have been tried and one based on Laser technology has shown some promise but the engineering would be very complex and expensive so it is still at the laboratory stage.
  • The original process was based on gaseous diffusion using a counter-current cascade as shown in Fig 4
  • Many stages are needed to give a reasonable degree of enrichment.
  • To produce a typical PWR fuel at 3% U 235 form natural Uranium at 0.7% and a tails composition of 0.2% would require 1272 stages.
  • As each stage need a compressor and a cooler the energy costs of this process are enormous.
  • Figure 5 shows a typical stage arrangement.
  • The alternative process uses high speed gas centrifuges, again relying on density difference.
  • A typical centrifuge arrangement is shown in Figure 6.
  • Because it is possible to get a much greater degree of separation per stage only 25 centrifuge stages are needed per 1000 diffusion stages and the power consumption is about one tenth.
  • The main problems are engineering, the centrifugal forces are several thousand g needing very expensive materials and very accurate design.
  • The centrifuge process has not totally replaced the diffusion process partly because of various complex cross subsidies which keeps it going in the USA the Russian Federation and France.
  • A great deal of research was carried out in the 1980’s into the use of laser enrichment techniques using uranium metal vapour and specially turned lasers which ionised only the U235. Whilst the technique was proved in concept it is not economically competitive with centrifuge systems

Figure 1 Open Fuel Cycle for nuclear reactors Figure 2 Closed fuel cycle for nuclear reactors Conversion HEX To Oxide Fuel Fabrication REACTOR Reactor spent fuel store Conversion To HEX Isotope separation Ultimate spent fuel Repository UOC Conversion HEX To Oxide Fuel Fabrication REACTOR Reactor spent fuel store Conversion To HEX Isotope separation UOC (^) Reprocessing Plant Conversion Uranyl Nitrate to HEX Plutonium to MOX High level waste to vitrifcation Medium level waste to encapsulation Low level to treatment/discharge