Experimental and Monte Carlo methods for
burn up assessment of spent fuel elements
A. Borella, R. Carchon, K. Van der Meer
SCK•CEN, EHS/SPS
Nuclear Science and Technology Studies group
aborella@sckcen.be
Copyright © 2010
SCK•CEN
International Conference on Management of Spent Fuel from Nuclear Power Reactors
IAEA, 2 June 2010
Overview
 Burnup measurements on spent fuel
 Introduction and motivation
 Focus on Fork detector
 Fork detector simulations with MCNPX
 Geometry, composition, particles
 Simulations carried out so far
 Count rate
 Importance function (Geometrical sensitivity)
 Positioning sensitivity
 Conclusion
 Outlook
Introduction
 Importance of burnup determination for spent fuel
assembly
 Criticality Safety (Burnup credit) related to
 Storage
 Transport
 Reprocessing (e.g. minimum BU extremities)
 Disposal
 Methods to determine burnup
 Calculations, supported by in-core measurements
 Destructive methods
 Non Destructive methods (NDA)
Burn up project
 Project “Burn up measurement method for spent fuel
Assemblies”
 Ideally one would like to determine a 3D burnup map of the
spent fuel element
 axial and radial burn up profiles
 verify the absence of local "hot spots" with increased reactivity
 Study state-of-the practice burn up measurement methods
 Evaluation of existing methods
 Identify new methods for further investigation
 Legal requirements consequences for the existing
measurement systems (France, Germany, Sweden, Finland
and the US)
 Further investigation of a limited number of measurement
systems with respect to expected performance
Monte Carlo simulations
 Fork detector in use for mid-point burnup and
extremity burnup/scanning in Belgian NPP.
Can we improve the Fork?
We need to understand it to see its weaknesses and
strengths
 Monte Carlo modeling of the detector for simulation
with MCNPX
 Measurements with Spent Fuel not always possible
 MCNPX is a very powerful tool that allows to carry out
parametric studies
 Simulating a detector is usually cheaper than building a
detector
Fork detector
 It measures the neutron and the gross
gamma emission from a fuel assembly
 Two watertight polyethylene arms wrapped
with cadmium containing one ionization
chamber and one fission chamber each
(other versions exists)
 The detector surrounds the fuel element and
measurements are carried out at different
heights
 Determination of BU based on correlation
 NE = a BUb
 NE neutron emission (from buildup of Cm)
 ‘a’ depends on the measurement condition and fuel
(type, enrichment)
 ‘b’ ~ 3  4 and depends on fuel (type, enrichment)
Fork detector
 Main features
 High sensitivity to inner pins (neutron)
 Calibration approach (‘a’ and ‘b’ determined by
measuring well known fuel elements)
 Depletion/Evolution code (e.g. Origen) could be
used to determine the correlation between BU
and NE
 Axial scanning
 Burnup can be determined with ~5% uncertainty
 Irradiation history important for cooling times <3y
Monte Carlo simulations - Geometry
 3D Geometry
 MCNPX input – horizontal and vertical cross section
FUEL
Cadmium
Sleeve
FORK
ARM
Cadmium
Sleeve
FUEL
FORK
ARM
FORK
ARM
FORK
ARM
FORK
BODY
Monte Carlo simulations
 Particles: Neutrons (244Cm spontaneous fission)
 Composition from Origen-ARP
 17×17 PWR, BU=44 GWd/tU over 4 y, CT=1000d
 Measurement conditions considered
 Wet (with/without boron)
 Dry (interesting for the feasibility of measurement in dry
conditions)
 Simulations carried out
 Expected count rate in different conditions
 Importance function (sensitivity to fuel geometry)
 Radial+Axial
 Impact of boron amount
 Impact of detector-fuel displacement
Monte Carlo simulations
 Comparison expected count rate
wet
2270 ppm
w/o boron
wet
dry
no boron
Quantity
Cd
no Cd
Cd
no Cd
Cd
no Cd
Neutron Counts
(counts per source neutron)
0.18
0.35
0.33
1.00
0.11
0.12
Total Neutron Fluence
(n/cm2 per source neutron)
0.49
0.52
0.90
1.00
0.71
0.71
Monte Carlo simulations - Results
 Importance function (sensitivity to fuel geometry)
0.0
-200
0.2
0.4
0.6
0.8
Axial importance function
1.0
-150
-100
-50
d /cm
0
50
100
150
200
 Where, in the fuel geometry, do the neutron counts come from?
 Axial Importance Function (Sensitivity)
can be well described by a gaussian
profile
 A localized sensitivity useful for local
assessment of burnup (Axial Scanning)
 In wet conditions
 90% counts within +-12 cm
 In dry conditions/wet no B no Cd
 90% counts within +-15 cm
 Arm size ~9 cm
 Detector diameter 2.54 cm
Monte Carlo simulations - Results
 Radial importance function
1E-3
17
0.10
16
0.20
15
0.30
0.40
column
14
0.50
13
0.60
12
0.70
11
0.80
10
0.90
1.00
9
8
7
6
5
4
3
2
1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
row row
of pins
 How sensitive is the Fork to the
different pins
 A flat sensitivity useful for the
assessment of the mean (radial)
burnup
 Maximum value reached for pins
close to detector
 Calculcated results in agreement
with experiments from P.M. Rinard,
Los Alamos Report, 1984, LA10068-MS
 Like the axial importance function it
depends on measurements
conditions (Wet/Dry, B, Cd)
 Projection along row pin of radial
importance function for an easy
comparison
Monte Carlo simulations - Results
1.0
0.8
0.6
0.4
0.2
2270 ppm B, Cd sleeve
2270 ppm B, no Cd sleeve
0 ppm B, Cd sleeve
0 ppm B, no Cd sleeve
DRY
Projection of the importance function
Projection of the importance function
WET
1.0
0.8
0.6
0.4
0.2
dry, Cd sleeve
dry, no Cd sleeve
0.0
0.0
1 2 3 4 5 6 7 8 9 1011121314151617
1 2 3 4 5 6 7 8 9 1011121314151617
row number
row number
Monte Carlo simulations - Results
 Sensitivity to positioning (wet)
 vs X positioning within 1 cm required
 vs Y Limited but averaging of the response is required
Count rate relative to X=0
1.05
X
Y
1.00
0.95
0.90
-4
-3
-2
-1
0
1
2
Displacement along X / cm
3
4
Monte Carlo simulations - Results
 Sensitivity to positioning (wet)
 vs X positioning within 1 cm required
 vs Y Limited but averaging of the response is required
Count rate relative to Y=0
1.05
X
Y
1.00
0.95
0.90
0.0
0.5
1.0
Displacement along Y / cm
1.5
Conclusions
 MCNPX was a helpful to carry out different kind of
simulations
 Study of count rate, geometrical and position
sensitivity were carried out for the Fork detector
 Dry conditions
 Count rate decreases by about 30 %
 The axial importance function would be increased by
about 35% in dry conditions. Therefore, a worse spatial
resolution for a fuel axial scanning is expected.
 Impact of Cd sleeve is limited
 Wet conditions
 Reducing the boron amount results in a higher sensitivity
to inner pins
 Relative positioning Fuel-Fork is important
Outlook
 Fork




Simulations on gamma rays
Threshold detector for n
Simulations with BWR fuel
Additional parametric analysis on importance function
 Investigations of other techniques
 SINRD (Self Interrogation Neutron Resonance
Densitometry)
 Differential Die Away time
 X-ray fluorescence
 Origen-ARP to investigate correlation between
neutron emission and burnup for different fuel
types/enrichements
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