AVKOK Neutrondetection based monitoring of void effects in boiling water reactors

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AVKOKNeutron-detection based monitoring of void
effects in boiling water reactors

• John Loberg

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Parameter Study

• All simulations are performed at Vattenfall
  Bränsle, using Casmo4 Extended version.

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Background

• Experimental in-core determination of the void.
• Validation of codes, safety.
• Ratio of thermal and fast neutrons depends on the
  void
• Increased void More fast neutrons,
  less thermal
• The ratio decreases!
• Decreased void Less fast neutrons, more
  thermal
• The ratio increases!
• The thermal neutron flux is already monitored in
  reactors
• today by fission chambers based on 235U (Power
  Range Monitors).
• The fast neutron flux could be monitored by
  fission chambers
• based upon 238U.

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Thermal neutrons
Fast (MeV) neutrons
0.625 eV
1.353 MeV
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• BWR detectors F3

• 4 nodes with PRM detectors
• 37 detectors in each node

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3
2
1
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Range of detector
Thermal diffusion length L 2,7
cm Slowing-down length 5
cm Migration length
L
f
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There is a correlation!
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Calculations in Casmo 4E
First 1x1 cases

• Atrium 10B
• Areva
• 10x10
• Forsmark 3
• Input file specifies
• Exact geometry
• Material compositions of each fuel pin
• BWR or PWR
• Temperatures, void

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CASMO

• Calculates resonance and macroscopic cross
  sections in 70 energy groups for the problem,
  i.e. geometry, temperatures, densities etc., from
  neutron data libraries
• Divides every fuel pin in to microregions and
  calculates individual spectra for all microgroups
• Microscopic calculation is then condensed to 2D
  macrogroup calculation in 40 energy groups
• ..which gives the neutron spectra for the energy
  condensation of cross sectios for the transport
  calculation performed in 8 energy groups
• Output is cross sections and group constants in 2
  energy groups for nodal codes like POLCA and
  SIMULATE

D1,D2,S1, S2
s1 s70
Sn1 ...Sn40
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Infinite lattice calculation ?
Mirror Boundary condition
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Neutron flux distribution, void 0-90
MeV neutron flux
Thermal neutron flux
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Not only the void has influence on the neutron
flux

• Burnable absorbers
• Control rods
• Channel bow
• Burn up
• And more

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Control rod
Reference
Channel bow
Burnable absorbers
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Results
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Channel bow
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Channel bow (2 mm)
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Void change and burn up 0-50 MWd/KgU
Axial
Radial
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Some (humble) observations so far

• Even though different factors offset the ratio up
  or down, the slope of the ratio remains fairly
  constant, 2 , in the void range 0-90.
• E.g. if the void changes 10 the prediction will
  be (10 0.2)
• A small change in position of the box will have
  significant influence on the thermal neutron
  flux, but not on the fast flux, hence it may be
  possible to detect channel bow.

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Present work

• MxN ¼ -core Forsmark 3
• Time consuming
• Also 3x3 from this ¼ -core for closer study of
  crd and depletion effects

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Future

• Investigate more ! Geometry changes, power
  measurements, void determination, depletion
• Build a prototype detector for validation of
  results