UN1001: REACTOR CHEMISTRY AND CORROSION Activity Transport

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UN1001REACTOR CHEMISTRY AND CORROSIONActivity
Transport

• By
• D.H. Lister W.G. Cook
• Department of Chemical Engineering
• University of New Brunswick

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• Ideally, in a nuclear plant, there would be no
  radioactivity outside of the reactor core.
• In practice this is not the case.
• Activity transport, as the name implies, is the
  transport of radioactive materials from the
  reactor core to other components of the heat
  transport system.

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• There are three main sources of radioactivity
  that account for radiation field build-up in
  components outside the core
• Corrosion products
• Fission products
• Tritium (CANDUs)
• Note reactors also have fields associated with
  N-16 produced by positron decay of O-17 but this
  is only an issue while at power since its
  half-life is fairly short ( 7 seconds)

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• Active components can lead to difficult
  maintenance issues during shut-downs and can
  significantly increase the associated maintenance
  costs due to
• Increased individual and collective doses to
  maintenance personnel
• Increased shielding requirements

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• Control of activity transport can be achieved
  through several techniques including
• Diligent fuel inspection and monitoring for
  failed fuel elements
• Keep corrosion product transport to a minimum by
  proper reactor coolant chemistry control.
• In CANDUs, tritium is accepted as a necessary
  byproduct due to the use of heavy water.
• Of these, chemistry control is the primary in
  service method to keep out-of-core fields as low
  a reasonably achievable (ALARA).
• Circuit purification is also used and has been
  shown to perform well, eg. Point Lepreau

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• Mechanism of Corrosion Product Transport
• Early testing and operational experience showed
  that under certain chemistry conditions (low
  alkalinity), the fuel elements became coated with
  crud.
• This increased the pressure drop through the core
  as well as significantly increased the observed
  radiation fields around the primary
  heat-transport circuit the steam generators in
  particular.
• It was found that the fuel deposits were due to
  the temperature dependence of the primary
  corrosion product, magnetite Fe3O4

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Tremaine Leblanc Fe3O4 Solubility Data
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Sweeton Baes
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• Thus, operating with low alkalinity results in
  fuel deposits.
• The fuel deposits are in a constant state of
  flux, dissolving, precipitating being released by
  erosion etc, such that for the duration of time a
  corrosion product stays in the core, it may
  become activated by the high neutron flux.
• The activated corrosion products then erode off
  the fuel as particulate or my re-dissolve as
  ionic species and be carried to other parts of
  the reactor.

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• The activated corrosion products then incorporate
  themselves into the native oxides formed in the
  other parts of the plant.
• Radioactive isotopes of particular concern or
  interest in terms of activity transport are
• Cr-51 half-life 27.7 days
• Fe-59 half-life 44.6 days
• Sb-124 half-life 60.2 days
• Co-58 half-life 70.8 days
• Mn-54 half-life 312.5 days
• Co-60 half-life 1924 days

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• Of these, Co-60 is of the most concern due to its
  long half-life and the fact that the Co2 ion is
  easily incorporated into both the inner and outer
  oxide layers typically formed in these reactor
  coolant systems by displace the native Fe2 or
  Ni2 species.
• Operating with higher alkalinity, ie. at the high
  end of the pH specification for a particular
  plant, leads to clean cores and fuel elements and
  significantly reduces activity transport.

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• This however, is one of the primary causes of
  another corrosion problem that we have seen in
  this course in CANDU reactors
• Flow-accelerated corrosion (FAC) of the outlet
  feeder pipes!

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• Modern techniques to control activity include the
  use of a chemical additive to the coolant such as
  zinc.
• The Zn2 ion has a much higher site-preference
  energy in the spinel oxides than does Co2, thus
  if Zn is present in the reactor at low levels (10
  50 ppb?) the Co is displaced from the oxide and
  may be picked up in the purification circuit.