Pebble Bed Reactors for Once Trough Nuclear Transmutation

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Pebble Bed Reactors for Once Trough Nuclear
Transmutation

• Pablo T. León, J.M.Martínez-Val, A.Abánades,
  D.Saphier.
• E.T.S.I.Industriales, U.P.M.

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Contents

• Advantages of Pebble Bed Fuel in Once Through
  Transmutation Scenarios.
• High Burn-up.
• Description of the nuclear spent fuel.
• Once through strategy Pu242 accumulation.
• Pebble Bed transmutator.
• Conclusions

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Advantages of PB Fuels

• The Triso Coated Fuel Particles can withstand
  very high burn-ups.

747 MW-days/kg gt95 239Pu gt65 all Pu Transmuted
Thermal Spectrum
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Description of Spent Fuel

• Description of Actinides composition for
  transmutation.
• Isotopic Composition of Actinides in the LWR
  Discharged Fuel, after 40MWd/kg burn-up and 15y
  cooling.

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Description of Spent Fuel

• The fuel cycle for a LWR park is defined

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Description of Spent Fuel

• The effective ingestion committed dose of Natural
  Uranium is 19.7 Sv/kg (ICRP 68.)
• The dose for ICRP 72 is 30.8 Sv/kg, due to the
  210Po dose increment (56.3 increment.)
• All the radiotoxicities results are evaluated
  taking into account that for 1 ton of Natural
  Uranium, 2.83 kg of TRUs are generated in the LWR
  reactor (ICRP 68.)
• The radiotoxicity values given in next figures
  are normalized to the radiotoxicity of 1 ton of
  Natural Uranium.

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Description of Spent Fuel

• The analysis have identified the Pu isotopes (and
  direct daughters) as the principle contributors
  to the effective committed dose.
• In a thermal reactor, the behavior of Pu isotopes
  is as follows

?c
?c
?c
?c
?c

Am Cm
Pu238
Pu239
Pu240
Pu241
Pu242
?f
?f
?f
?f
?f
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Once Through Strategy

• The Once Through Strategy limit the maximum
  burn-up to 700 MWd/kg, approximately. This is the
  nominal burn-up taken for calculations.
• The amount of TRUs mass transmuted to obtain
  700MWd/kg is defined by the approximate equation
• Burn-up (MWd/kg) 975.9?(1-RF)
• Where RF is the Actinides residual fraction.
• The result is, for 700 MWd/kg, a 28.27 of the
  actinides mass not transmuted.
• The isotopic composition of this TRUs mass not
  transmuted is fundamental for final ingestion
  effective committed dose calculations.

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Once Through Strategy

• As it has been demostrated in previous
  calculations, the Am and Cm isotopes have a high
  radiotoxicity level.
• Cm isotopes (Cm242 to Cm248) decay by ?
  disintegration to Pu isotopes, so the final
  radiotoxicity evolution with time is high
  (specially for Cm244.)
• Am isotopes (Am243 to Am241) behaves differently
  than Cm isotopes. Am243 and Am241 decay by ? to
  Np. Np239 decays to Pu239 by ?-, and Np237
  decays by ?. Am242 decays primarily (83) to
  Cm242 by ?- disintegration, and the rest to
  Pu242.
• If the 28.27 of actinides remnant in the fuel
  are Am and Cm isotopes, the reduction in
  radiotoxicity after 700 MWd/kg is not very
  important as compared to the non-transmutation
  scenarios.

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Once Through Strategy

• One of the isotopes with a lower radiotoxicity
  level is Pu242. The half life of this isotope is
  T1/2 3.7E5 s, and by ? disintegration, it
  decays to U238, the isotope that starts the
  nuclear fuel cycle.
• If the Pu capture chain during transmutation
  (86.24 Actinides mass) can be broken in Pu242
  isotope, then the final radiotoxicity of the
  28.27 of the actinides not transmuted after 700
  MWd/kg BUP will be much lower, with a Pu242 mass
  accumulation.

?c
?c
?c
?c
?c

Am Cm
Pu238
Pu239
Pu240
Pu241
Pu242
?f
?f
?f
?f
?f
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• Theoretical Analysis

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Once Through Strategy

• To obtain a final accumulation of Pu242 after
  transmutation, a thermal spectrum is necessary.
• If a thermal spectrum is used for transmutation,
  the neutron flux level is going to be defined
  primarily by Pu239 fission cross section, for a
  given transmutator power density.
• The spectral index to be studied is then the
  ratio of Pu242 capture cross section and the
  Pu239 fission cross section . The smaller the
  spectral index, the higher the Pu242 accumulation
  in the spent fuel.

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Once Through Strategy

• The minimum of the spectral index is at 0.3 eV
  neutron energy.

0.3 eV
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Pebble Bed Transmutator

• Pebble Bed fuel can be designed to obtain
  different neutron spectra in the fuel region.
• An analysis has been done to predict the
  possibilities of pebble bed fuel to obtain
  neutron spectrum with a minimum spectral index.

Pebble External Diameter 6 cm.

• Active Core Diameter Rff(Mf)
• 50 mass TRISO
• 50 Graphite for compactation

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Pebble Bed Transmutator

• The smaller the amount of mass charged in the
  pebble, the smaller the radius of the active zone
  (50, 50)
• Small active zones give a more thermal neutron
  spectrum.
• To optimize the spectrum for a minimum 242Pu
  capture, different masses charged per pebble have
  been analyzed (MCNPX.)

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Pebble Bed Transmutator

• The thermal spectrum used for transmutation of
  actinides to a maximum burn-up (700 MWd/kg)
  without reprocessing permits the burn-up of the
  fuel in two steps.
• Initially, as a critical reactor
• Need further safety studies.
• When kefflt1, the burn-up of the fuel have to
  continue in a subcritical reactor.

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Pebble Bed Transmutator

• For the critical reactor burn-up analysis, the
  following reactor geometry have been adopted.
• The active length of the reactor is higher than
  the active diameter for LOCA cooling of the
  reactor (radiation is enhanced.)

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Pebble Bed Transmutator

• Some preliminary analysis have been carried out
  for three different active radius (corresponding
  to 0.25, 0.5 and 1 gr charged per pebble.)

• Initially, an infinite array of cells, each one
  containing a pebble, have been studied.
• The active zone can be taken as homogeneous or
  heterogeneous. Both neutron spectra have been
  studied.

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Pebble Bed Transmutator

• For the optimum case (Mf0.5 gr), the
  heterogeneous and homogeneous calculation have
  different results of capture and fission
  actinides cross sections.
• The heterogeneous active zone, with TRISO
  geometry description, have to be taken into
  account in the MCNPX calculations.

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Pebble Bed Transmutator

• The neutron spectra for the reactor have been
  also calculated for the clean fuel composition.
• A total mean spectrum have been analyzed, and the
  results are in next figure.

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Pebble Bed Transmutator

• The final burn-up values of the critical phase
  have not been calculated, because of some
  problems in lumped fission products cross
  sections and fission products characterization
  (important differences in maximum BUPs for
  keffcritical 1.)
• Once the problem is solved, as a second step, we
  will start the subcritical analysis of the
  reactor.

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Pebble Bed Transmutator

• Some thermal-hydraulic analysis have been carried
  out for the critical phase, with FLUENT.
• The reactor parameters for thermall-hydraulic
  calculations are defined in next table.

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Pebble Bed Transmutator

• The axial and radial power distribution in the
  reactor have been calculated, with MCNPX.

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H2 Prod
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Conclusions

• Pebble Bed Reactors present good characteristics
  to be used in a Once-Trough Strategy
  transmutation.
• The Pu242 accumulation in the final Actinides
  mass not transmutted after maximum BUP (28.27
  for 700MWd/kg) minimize final radiotoxicity for a
  once-through strategy.
• Critical and subcritical transmutation steps are
  studied.
• Heterogeneous analysis of the active zone of the
  fuel is necessary.
• An additional advantage is that it is a
  non-proliferation strategy (Pu239 BUP.)
• High temperature operation can permit H2
  production during transmutation.