1. TARGET SYSTEM NEUTRONICS
STUDY FOR THE NxGENS LPSS
Carl Willis,*
Guenter Muhrer†
*Nuclear Engineering Program,
The Ohio State University
†
Los Alamos Neutron Science Center
July 30, 2007

2. 07/28/07 2
Background
• The LANSCE Materials Test Station (MTS) will provide a
high flux of fast neutrons for irradiating advanced nuclear
fuels and other samples.
– Tungsten target / tungsten reflected spallation source
• The MTS may also afford an opportunity to test the
LPSS concept for neutron scattering studies.
– Proton pulses on target are ~650 us at ~70 Hz
• Our aim has been to understand the performance of
various backscattering moderator configurations for the
proposed LPSS flight path, using MCNPX modeling.

3. 07/28/07 3
Outline
• Basic geometry and neutronics overview
• Neutronic significance of beam mask, Cd “decoupler”
• Parametric studies of water moderator performance
– Flux vs. thickness
– Effects of a beryllium reflector / premoderator
– Flux vs. longitudinal proximity of moderator to target
– Impact on fluxes in MTS fuel
• Parametric studies of LH2 moderator performance
– Flux vs. thickness
– Effects of a Be reflector / premoderator
– Ortho-para composition effects
– Impact on fluxes in MTS fuel
• Neutron pulse shapes
• Conclusions and questions

4. 07/28/07 4
Green: Tungsten Yellow: Aluminum
Blue: Coolant (D2O) Orange: Fuel
Red: Moderator Fluid (water or LH2)
Target Station Geometry

5. 07/28/07 5
Be Reflector (rust color) and
Cd Decoupler (light green)
Target Station Geometry (ctd.)

6. 07/28/07 6
High-energy neutrons are
produced in the spallation
target and irradiate
surrounding fuel and
material samples.
Shown: flux > 1 eV
Blue: ≤ 2E+13 n cm-2
s-1
mA-1
Red: ≥ 2E+15 n cm-2
s-1
mA-1
(Logarithmic shading scale)
Neutronics Overview (1)

7. 07/28/07 7
Low-energy neutrons are
produced in the moderator
via the slowing-down of
spallation neutrons, and are
scattered out into the flight
path.
Shown: Be-reflected, Cd-
decoupled water moderator
Flux < 0.5 eV
Blue: ≤ 1E+10 n cm-2
s-1
mA-1
Red: ≥ 1E+14 n cm-2
s-1
mA-1
(Logarithmic shading scale)
Neutronics Overview (2)

8. 07/28/07 8
Energy-dependent
flux is tallied in a 1-
cm-long central
region of two fuel
pins: an upstream
pin (1) and a
downstream pin (2).
Evaluating Impact of LPSS
Moderators on MTS Fuel

9. 07/28/07 9
NEUTRON FLUX, 5.2 cm Water Moderator
(Neutron Beamline Point Detector)
1.E+04
1.E+05
1.E+06
1.E+07
1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02
ENERGY (eV)
FLUXPERUNITLETHARGY(n/cm2
/s/mA)
With Cd Liner
Without Cd Liner
Adding Cd liner around moderator
decreases flux in the flight path by 0.65%.

10. 07/28/07 10
NEUTRON FLUX
(Upstream Fuel Pin)
1.E+09
1.E+10
1.E+11
1.E+12
1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03
ENERGY (eV)
FLUXPERUNITLETHARGY(n/cm
2
/s/mA)
With Cd Liner
Without Cd Liner
Adding Cd liner around moderator
results in a 48% reduction in flux < 1eV.

11. 07/28/07 11
NEUTRON FLUX < 400 meV
(Neutron Beamline Point Detector)
0.0E+00
5.0E+06
1.0E+07
1.5E+07
2.0E+07
2.5E+07
3.0E+07
4 6 8 10 12 14 16 18 20
WATER MODERATOR THICKNESS (cm)
FLUX(n/cm
2
/s/mA)
Optimal thickness for water moderator:
~ 9 cm

12. 07/28/07 12
NEUTRON FLUX < 400 meV
(Water Moderator)
1.0E+07
1.5E+07
2.0E+07
2.5E+07
3.0E+07
3.5E+07
4 6 8 10 12 14 16 18 20
Moderator Thickness
Flux,n/cm2
/s/mA
No Reflector
4 cm Be Reflector
10 cm Be Reflector
15 cm Be Reflector

13. 07/28/07 13

14. 07/28/07 14
NEUTRON FLUX
(Downstream Fuel Pin)
1.E+08
1.E+09
1.E+10
1.E+11
1.E+12
1.E+13
1.E+14
1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08
ENERGY (eV)
FLUXPERUNITLETHARGY(n/cm2
/s/mA)
No Reflector
4 cm Be Reflector
9.2 cm Water Moderator
Adding 4 cm Be Reflector Causes ...
A Total Flux Increase of 0%
A Flux < 1 keV Increase of 40%

15. 07/28/07 15
NEUTRON FLUX < 400 meV
(Water Moderator)
0.0E+00
5.0E+06
1.0E+07
1.5E+07
2.0E+07
2.5E+07
3.0E+07
0 1 2 3 4 5 6 7 8 9 10
Moderator -z Offset (cm)
Flux,n/cm2
/s/mA

16. 07/28/07 16
Water Moderator Summary
• For a bare water moderator, highest flight-path flux < 0.4
eV is achieved with a thickness of 9 cm.
• For a water moderator reflected / premoderated by 4 cm
Be, highest flux is achieved with a thickness of 7.5 cm.
2.00×107
4.80×107
PCS (Lujan), measured1
PCS (Lujan), MCNPX1
3.19×107
7.7 cm Water / 4 cm Be
1.95×107
9.2 cm Bare Water
Flight Path Flux, < 0.4 eV @ 27 m
(n cm-2
s-1
mA-1
)
Moderator
• Muhrer G. Personal communication.

17. 07/28/07 17
Other Conclusions
• Cadmium Decoupler
For a 5.2 cm water moderator:
– Upstream fuel pin flux < 1 keV reduced by 48%
– Flight path flux reduced by 0.68%
– The decoupler significantly diminishes the low energy flux
impact in the fuel, while having minimal impact on LPSS
function.
• Beam Mask
– No influence on neutron production or moderator function
was noted.
• Moderator Offset
– Moving the moderator upstream results in a flux penalty of
about 1.75 × 106
n cm-2
s-1
mA-1
per centimeter of offset.

18. 07/28/07 18
NEUTRON FLUX < 5 meV
(Neutron Beamline Point Detector)
8.0E+05
9.0E+05
1.0E+06
1.1E+06
1.2E+06
1.3E+06
1.4E+06
1.5E+06
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
MODERATOR COMPOSITON (Fraction Ortho H)
FLUX(n/cm
2
/s/mA)
5.2 cm LH2 Moderator

19. 07/28/07 19
NEUTRON FLUX < 5 meV
(Neutron Beamline Point Detector)
0.0E+00
5.0E+05
1.0E+06
1.5E+06
2.0E+06
2.5E+06
3.0E+06
3.5E+06
4.0E+06
4.5E+06
5.0E+06
4 6 8 10 12 14 16
MODERATOR THICKNESS (cm)
FLUX(n/cm2
/s/mA)
75% Ortho LH2
50% Ortho LH2
100% Para LH2

20. 07/28/07 20
20-50 meV: Large drop
in cross-section for
parahydrogen
Ortho Hydrogen
Para Hydrogen

21. 07/28/07 21
NEUTRON FLUX
(Neutron Beamline Point Detector)
1.E+04
1.E+05
1.E+06
1.E+07
1.E-04 1.E-03 1.E-02 1.E-01 1.E+00
ENERGY (eV)
FLUXPERUNITLETHARGY(n/cm
2
/s/mA)
100% para LH2
100% ortho LH2
5.2 cm LH2 Moderator

22. 07/28/07 22
NEUTRON FLUX
(Neutron Beamline Point Detector)
1.E+04
1.E+05
1.E+06
1.E+07
1.E-04 1.E-03 1.E-02 1.E-01 1.E+00
ENERGY (eV)
FLUXPERUNITLETHARGY(n/cm
2
/s/mA)
100% para LH2
25% para LH2
15.2 cm LH2 Moderator

23. 07/28/07 23
NEUTRON FLUX < 5 meV
(Neutron Beamline Point Detector)
0.E+00
1.E+06
2.E+06
3.E+06
4.E+06
5.E+06
6.E+06
4 6 8 10 12 14 16
MODERATOR THICKNESS (cm)
FLUX(n/cm
2
/s/mA)
No Reflector
4 cm Be Reflector
Liquid H2 Moderator, 50% ortho

24. 07/28/07 24
LH2 Moderator Summary
• A thick pure parahydrogen moderator gives highest flux.
– Absorption is minimized
– Not realistic: neutrons may drive the conversion between
parahydrogen and orthohydrogen.
• For moderators thinner than 11 cm, pure parahydrogen
is inferior to mixtures containing some orthohydrogen.
– High inelastic scattering cross-section of orthohydrogen lowers
mean free path and improves slowing-down in thin moderators.
• Reflection and premoderation with Be greatly increases
performance.
– Flux improvement is more pronounced with LH2 than with water
moderators.

25. 07/28/07 25
LH2 Moderator Summary (ctd.)
• A Be reflected / premoderated thick pure parahydrogen
moderator would be best but is not likely to be realistic.
* 9.1×106
* 8.6×106
SPEAR (Lujan), measured1,2
SPEAR (Lujan), MCNPX1,2
2.45×107
Be Reflected, 7.7 cm, 50% o
1.67×107
Bare, 12.2 cm, 50% o
Flight Path Flux, < 0.4 eV @ 27 m
(n cm-2
s-1
mA-1
)
Moderator
* Calculated for 27 m from a flight path of 8.64 m via inverse square law.
• Muhrer G. et al., Nuc. Inst. Methods A 527 (2004) 531-542
• Ino T. et al., Nuc. Inst. Methods A 525 (2004) 496-510

26. 07/28/07 26
NEUTRON FLUX
(Upstream Fuel Pin)
1.E+08
1.E+09
1.E+10
1.E+11
1.E+12
1.E+13
1.E+14
1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09
ENERGY (eV)
FLUXPERUNITLETHARGY(n/cm
2
/s/mA)
Base Case
9.2 cm LH2 Moderator
9.2 cm Water Moderator

27. 07/28/07 27
Pulse Shapes
• The time-dependent nature of the flight-path flux is
explained by slowing-down and storage of neutrons in
the moderator and reflector materials.
• One theoretical prediction of the neutron pulse shape
which attempts to account for both these effects is the
Ikeda-Carpenter Model.

28. 07/28/07 28
Ikeda-Carpenter Model
Φ(v,t) = α (αt)2
exp(-αt) / 2
+ A exp(-βt)
-A exp[(-αt) (1+ (α-β)t + α(α-β)2
t2
/ (2β))]
Where
α = Σs v (inverse of the slowing-down time constant)
β = inverse of the storage time constant
A = R α3
β / (α-β)3
(R is a scaling parameter)

29. 07/28/07 29
NEUTRON FLUX <5 meV
(Neutron Beamline Point Detector)
Storage Time = 74.4 µs
1.E+05
1.E+06
1.E+07
1.E+08
1.E+09
1.E+10
1.E+11
0.0E+00 1.0E-04 2.0E-04 3.0E-04 4.0E-04 5.0E-04 6.0E-04 7.0E-04 8.0E-04 9.0E-04 1.0E-03
Time (s)
FLUXperTIME(n/cm
2
/s
2
/mA)
Data
Expon. (Fit)
5.2 cm LH2 Moderator, 50% ortho

30. 07/28/07 30
NEUTRON FLUX, λ = 4 Angstrom
(Neutron Beamline Point Detector)
1.E+05
1.E+06
1.E+07
1.E+08
1.E+09
1.E+10
0.0E+00 5.0E-05 1.0E-04 1.5E-04 2.0E-04 2.5E-04 3.0E-04 3.5E-04 4.0E-04 4.5E-04 5.0E-04
Time (s)
FLUXperTIME(n/cm
2
/s
2
/mA)
Data
Ikeda-Carpenter
Expon. (Fit)
Storage Time = 71.2 µs
5.2 cm LH2 Moderator, 50% ortho

31. 07/28/07 31
NEUTRON FLUX, λ = 2 Angstrom
(Neutron Beamline Point Detector)
1.E+07
1.E+08
1.E+09
1.E+10
1.E+11
0.0E+00 5.0E-05 1.0E-04 1.5E-04 2.0E-04 2.5E-04
Time (s)
FLUXperTIME(n/cm2
/s2
/mA)
Data
Ikeda-Carpenter
Expon. (Fit)
Storage Time = 69.7 µs
5.2 cm LH2 Moderator, 50% ortho

32. 07/28/07 32
NEUTRON FLUX, λ = 1 Angstrom
(Neutron Beamline Point Detector)
1.E+05
1.E+06
1.E+07
1.E+08
1.E+09
1.E+10
1.E+11
0.0E+00 2.0E-05 4.0E-05 6.0E-05 8.0E-05 1.0E-04 1.2E-04 1.4E-04 1.6E-04 1.8E-04 2.0E-04
Time (s)
FLUXperTIME(n/cm
2
/s
2
/mA)
Data
Ikeda-Carpenter
5.2 cm LH2 Moderator, 50% ortho

33. 07/28/07 33
NEUTRON FLUX, λ = 0.5 Angstrom
(Neutron Beamline Point Detector)
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
1.E+09
1.E+10
1.E+11
0.0E+00 2.0E-05 4.0E-05 6.0E-05 8.0E-05 1.0E-04 1.2E-04 1.4E-04 1.6E-04 1.8E-04 2.0E-04
Time (s)
FLUXperTIME(n/cm
2
/s
2
/mA)
Data
Ikeda-Carpenter
5.2 cm LH2 Moderator, 50% ortho

34. 07/28/07 34
NEUTRON FLUX, λ = 4 Angstrom
(Neutron Beamline Point Detector)
1.E+06
1.E+07
1.E+08
1.E+09
1.E+10
0.0E+00 5.0E-05 1.0E-04 1.5E-04 2.0E-04 2.5E-04 3.0E-04 3.5E-04 4.0E-04 4.5E-04 5.0E-04
Time (s)
FLUXperTIME(n/cm
2
/s
2
/mA)
Data
Ikeda-Carpenter
Expon. (Fit)
Storage Time = 77.1 µs
9.2 cm Water Moderator

35. 07/28/07 35
NEUTRON FLUX, λ = 2 Angstrom
(Neutron Beamline Point Detector)
1.E+06
1.E+07
1.E+08
1.E+09
1.E+10
1.E+11
0.0E+00 1.0E-04 2.0E-04 3.0E-04 4.0E-04 5.0E-04 6.0E-04 7.0E-04
Time (s)
FLUXperTIME(n/cm
2
/s
2
/mA)
Data
Ikeda-Carpenter
Expon. (Fit)
Storage Time = 77.4 µs
9.2 cm Water Moderator

36. 07/28/07 36
NEUTRON FLUX, λ = 1 Angstrom
(Neutron Beamline Point Detector)
1.E+07
1.E+08
1.E+09
1.E+10
1.E+11
0.0E+00 5.0E-05 1.0E-04 1.5E-04 2.0E-04 2.5E-04 3.0E-04 3.5E-04 4.0E-04 4.5E-04 5.0E-04
Time (s)
FLUXperTIME(n/cm
2
/s
2
/mA)
Data
Ikeda-Carpenter
Expon. (Fit)
Storage Time = 78.5 µs
9.2 cm Water Moderator

37. 07/28/07 37
NEUTRON FLUX, λ = 0.5 Angstrom
(Neutron Beamline Point Detector)
1.E+05
1.E+06
1.E+07
1.E+08
1.E+09
1.E+10
1.E+11
0.0E+00 5.0E-05 1.0E-04 1.5E-04 2.0E-04 2.5E-04 3.0E-04 3.5E-04 4.0E-04 4.5E-04 5.0E-04
Time (s)
FLUXperTIME(n/cm
2
/s
2
/mA)
Data
Ikeda-Carpenter
9.2 cm Water Moderator

38. 07/28/07 38
Conclusions
• A thick pure parahydrogen moderator gives highest flux.
– Absorption is minimized
– Not realistic: neutrons may drive the conversion between
parahydrogen and orthohydrogen.
• For moderators thinner than 11 cm, pure parahydrogen
is inferior to mixtures containing some orthohydrogen.
– High inelastic scattering cross-section of orthohydrogen lowers
mean free path and improves slowing-down in thin moderators.
• Reflection and premoderation with Be greatly increases
performance.
– Flux improvement is more pronounced with LH2 than with water
moderators.

39. 07/28/07 39
Acknowledgements
I’d like to thank Guenter Muhrer for his mentorship.
My work was financially supported through the generosity
of the U.S. Department of Energy Nuclear Engineering
Fellowship.
Thanks to Eric Pitcher, Erich Schneider, Luc Daemen,
Bernice Williams, John Seal, and others who have
provided assistance.