Shielding Calculations in Support of the Spoliation Neutron
Source (SNS) Proton Beam Transport System
Jeffrey O. Johnson, Franz X. Gallmeier, and Irina Popova
Oak Ridge National Laboratory, P. O. Box 2008, MS 6363
Oak Ridge, Tennessee 37831-6363 U.SA.
Abstract. Determining the bulk shielding requirements for accelerator environments is generally an easy task compared
to analyzing the radiation transport through the complex shield configurations and penetrations typically associated with
the detailed Title n design efforts of a facility. Shielding calculations for penetrations in the SNS accelerator
environment are presented based on hybrid Monte Carlo and discrete ordinates particle transport methods. This
methodology relies on coupling tools that map boundary surface leakage information from the Monte Carlo calculations
to boundary sources for one-, two-, and three-dimensional discrete ordinates calculations. The paper will briefly
introduce the coupling tools for coupling MCNPX to the one-, two-, and three-dimensional discrete ordinates codes in
the DOORS code suite. The paper will briefly present typical applications of these tools in the design of complex shield
configurations and penetrations in the SNS proton beam transport system.
INTRODUCTION
The Department of Energy is on schedule in the
design and construction of the Spallation Neutron
Source (SNS) facility at Oak Ridge National
Laboratory with commissioning operations planned for
FY06. The SNS initially will consist of an accelerator
system capable of delivering an approximately 0.5
microsecond pulse of 1 GeV protons, at a 60 Hz
frequency, with 1 MW of beam power, into a single
target station. The SNS will eventually be upgraded to
a 2 MW facility with two target stations (a 60 Hz
station and a 10 Hz station). The proton beam is
accelerated in a linac, consisting of a drift tube linac
(DTL), a coupled cavity linac (CCL) and a superconducting linac (SCL), bunched in an accumulator
ring, and directed to a target station.
This paper briefly discusses some of the shielding
calculations performed for the design of the SNS
proton beam transport system based on a hybrid Monte
Carlo and discrete ordinates particle transport method.
This methodology relies on a newly developed high
energy neutron cross-section library and new coupling
tools that map boundary surface leakage information
from the Monte Carlo calculations to boundary sources
for one-, two-, and three-dimensional discrete
ordinates calculations.
BACKGROUND
Determining the bulk shielding requirements for
accelerator environments is generally an easy task
compared to analyzing the radiation transport through
the complex shield configurations and penetrations
typically associated with the detailed Title II design
efforts of a facility. Although there exist a couple of
simple hand calculation formulas1'2 developed over the
years from the expertise gained at accelerator facilities,
these are limited to simple duct arrangements through
shields. Monte Carlo transport codes are able to model
radiation fields in complex geometries. However, they
consume a significant amount of resources, and may
give solutions with poor statistics even when applying
variance reduction methods.
In the design of the SNS proton beam transport
system, hybrid Monte Carlo and discrete ordinates
methods were applied to many of the complex
shielding tasks. In almost all cases the front-end of the
multi-step calculations was performed by MCNPX3,
modeling the proton beam interactions with the
accelerator
components,
beam
dumps,
or
building/tunnel structural elements. At some distance
from the primary interaction area, the high-energy
protons and secondary charged particles have
interacted or ranged out, and neutrons and photons
become the dominant radiation. At this distance
neutron and photon boundary leakage terms are scored
and passed as boundary sources to the discrete
ordinates suite in the DOORS code system4 to
continue the radiation transport through the bulk
shields and/or penetrations.
If necessary, further
coupling between successive discrete ordinates
calculations allows the analyst to treat multiple legged
duct arrangements and mazes.
The successful application of the discrete ordinates
methods to the SNS accelerator shielding design was
made possible by the development of two new tools.
One is a series of coupling tools that serve as the
interface between the different transport codes by
generating boundary sources, and the second is the
new 75-neutron-group/22-photon-group HILO2k
CP642, High Intensity and High Brightness Hadron Beams: 20th ICFA Advanced Beam Dynamics Workshop on
High Intensity and High Brightness Hadron Beams, edited by W. Chou, Y. Mori, D. Neuffer, and J.-F. Ostiguy
© 2002 American Institute of Physics 0-7354-0097-0/02/$ 19.00
183
cross-section library5 with P5 Legendre polynomial
expansion of the scattering
cross sections and an upper
5
cross-section
neutron
energy oflibrary
2 GeV. with P5 Legendre polynomial
aligned cylinder axes. The code performs a spatial and
angular transformation to equate the angular fluxes at
aligned
cylinder
axes. The
a spatial
each
radial,
azimuthal,
andcode
axialperforms
boundary
mesh and
point
angular
transformation
to
equate
the
angular
fluxes
of the "perturbed problem to the angular flux
at atthe
each radial,
axial boundary
point
nearest
meshazimuthal,
point andand
angular
directionmesh
of the
base
of
the
“perturbed
problem
to
the
angular
flux
at
the
problem.
expansion of the scattering cross sections and an upper
neutron energy of 2 GeV.
Coupling Tools
nearest mesh point and angular direction of the base
problem.
Coupling
Tools
The coupling tools
utilize the
event-wise Monte
Carlo boundary
crossing
information
into energy bins,
The coupling tools utilize the event-wise Monte
angular
bins,
and
spatial
bins
(if
applicable)
of bins,
the
Carlo boundary crossing information into energy
corresponding
discrete-ordinates
mesh,
angular
angular bins, and spatial bins (if applicable)6 of the
quadrature,
and multi-group
energy structure
This
corresponding
discrete-ordinates
mesh, .angular
6
procedure
mapsandthemulti-group
Monte Carlo
surface
crossing
quadrature,
energy
structure
. This
information
a boundary
to be
procedureinto
maps
the Montesource
Carlo definition
surface crossing
usedinformation
by the discrete
ordinates
codes.
into a boundary source definition to be
Geometry Models
Geometry
Models
Detailed geometry
models
for use in the MCNPX
codeDetailed
were developed
the linac,
beam
geometry for
models
for usehigh-energy
in the MCNPX
transport
line,forthe
ring, and
code were(HEBT)
developed
theaccumulator
linac, high-energy
beamthe
ring
to target
building
(RTBT) line
the the
SNS
transport
(HEBT)
line,transfer
the accumulator
ring,ofand
proton
beam
transport
system.
The
linac
model
ring to target building transfer (RTBT) line of the SNS
integrated
the details
for the
drift tube
proton beam
transport
system.
Thelinac
linac(DTL),
modelthe
coupled
cavity
linac for
(CCL)
andtube
the linac
super-conducting
integrated
the details
the drift
(DTL), the
coupled
cavity
(CCL)
and the
linac
(SCL)
intolinac
a single
model.
Thesuper-conducting
MCNPX models
linac highly
(SCL) into
a single model.
MCNPX
were
representative
of theThe
design
and models
included
were
highly
representative
of
the
design
and
included
the details of the proton beam transport
line
the details of
the proton
beam transport
line
components
(magnets,
collimators,
etc.), tunnels,
componentsand(magnets,
collimators,
tunnels,
buildings,
earth berms.
Auxiliaryetc.),
models
of the
buildings, and
earth berms.
Auxiliary models
the
penetrations
(oxygen
deficiency/helium
(ODH)ofvents,
penetrations
(oxygen
deficiency/helium
(ODH)
vents,
klystron wave-guides, etc.), personnel and truck
klystron beam
wave-guides,
personnel
and truck
egresses,
dumps, etc.),
and shield
wall mazes
were
egresses, beam dumps, and shield wall mazes were
modeled in both MCNPX and DORT geometries for
modeled in both MCNPX and DORT geometries for
subsequent
analyses using the coupling methodology
subsequent analyses using the coupling methodology
described
above.
An example MCNPX model of the
described above. An example MCNPX model of the
CCL
is
shown
in
Figure
CCL is shown in Figure 1.1.
used by the discrete ordinates codes.
The
first coupling tool is Monte Carlo to ANISN
(MTA),The
which
boundary
source
fortotheANISN
onefirstprepares
couplinga tool
is Monte
Carlo
4
(MTA), which
prepares a boundary
source
the onedimensional
discrete-ordinates
transport
codeforANISN
4
discrete-ordinates
code ANISN
fromdimensional
surface crossing
informationtransport
written primarily
by
from surface
information
writtenfor
primarily
by
MCNPX.
MTAcrossing
can prepare
sources
planar,
MCNPX.
can problems.
prepare ANISN
sources allows
for planar,
cylindrical
andMTA
spherical
two
cylindrical
and spherical
problems.
ANISN
allows
types
of sources,
a distributed
volume
source
thattwo
is
types
of
sources,
a
distributed
volume
source
restricted to isotropic angular distributions, and a that
shellis
restricted
isotropic angular
shell
source
that istoimplemented
as adistributions,
directional and
fluxa step
source that
implemented
a directional
fluxThe
step
condition
at a isspecified
rightasmesh
boundary.
condition at a specified right mesh boundary. The
second source type enables any angular distribution
second source type enables any angular distribution
within the discrete angular quadrature set. Both source
within the discrete angular quadrature set. Both source
types
are are
supported
by by
MTA.
types
supported
MTA.
TheThe
second
tool,
MTD
(Monte
second
tool,
MTD
(MonteCarlo
CarlototoDORT),
DORT),
prepares
a
boundary
source
for
the
two-dimensional
prepares a boundary source for
the
two-dimensional
4
discrete
ordinates
discrete
ordinatescode
codeDORT
DORT4from
from the
the surface
surface
crossing
information
written
by
the
Monte
Carlo
crossing information written by the Monte Carlocode
code
MCNPX.
At At
thethe
current
time,
MTD
MCNPX.
current
time,
MTDis islimited
limitedtotothe
the
preparation
of surface
sources
forforthethe(r,z)-cylindricalpreparation
of surface
sources
(r,z)-cylindricalgeometry
option
meaning
that
sources
geometry
option
meaning
that
sourcesare
areallowed
allowedon
on
bottom
surfaces
radialsurface
surface
the the
top top
andand
bottom
surfaces
andand
ononthetheradial
at the
outer
boundary
problem,but
butalso
alsoatatany
any
at the
outer
boundary
of of
thethe
problem,
internal
right
mesh
boundary
constant-zplanes
planesoror
internal
right
mesh
boundary
of ofconstant-z
constant-r
cylinder
radii.
constant-r
cylinder
radii.
third
tool,
MTT(Monte
(MonteCarlo
Carlototo TORT),
TORT),
TheThe
third
tool,
MTT
which
completes
suite
MonteCarlo
Carlototodiscretediscretewhich
completes
thethe
suite
of of
Monte
ordinates
coupling
tools,
preparesa asimilar
similarboundary
boundary
ordinates
coupling
tools,
prepares
source
for
the
three-dimensional
discrete
ordinates
source for the4 three-dimensional discrete ordinates
code TORT
. MTT is similar to the MTD code except
4
code TORT . MTT is similar to the MTD code except
it has only been tested in Cartesian (x,y,z) coordinates.
it has only been tested in Cartesian (x,y,z) coordinates.
MTT prepares surface sources on all six sides at the
MTT
prepares
surface
sources
on problem.
all six sides at the
user
prescribed
boundaries
in the
user prescribed boundaries in the problem.
FIGURE 1.
1. A
A section
FIGURE
section of
of the
the CCL
CCL model
modelshowing
showingthethe
complex cavity structure.
complex cavity structure.
Representative Applications
Representative
Applications
The methodology and models described above
Theapplied
methodology
andof models
describeddesign
above
were
to a number
Title II shielding
were
applied
to a number
Title
II shielding
design
problems
associated
with ofthe
SNS
proton beam
problems
associated
with theto SNS
proton bulk
beam
transport system.
In addition
the standard
transport
system.
In
addition
to
the
standard
bulk
shielding design calculations, analyses were performed
shielding
design
calculations,
analyses
were
performed
to design the various penetrations through the
toshielding
design including,
the various
through
ODHpenetrations
vents, HVAC
ducts,the
shielding
ODH vents,
HVAC waveducts,
personnel including,
and truck egresses,
and klystron
personnel
and methodology
truck egresses,
guides. The
was and
alsoklystron
appliedwaveto
Similar coupling codes are available in the DOORS
Similar
are available DORT
in the DOORS
packagecoupling
to couplecodes
two-dimensional
to threepackage
to
couple
two-dimensional
DORT
threedimensional TORT problems, or two TORT to
problems.
4
4
dimensional
TORTcodes
problems,
or two TORT
problems.
The coupling
are TORSED
and
TORSET
,
4
4
Therespectively.
coupling codes
are TORSED
andanalysis
TORSET
,
Of particular
value for the
of the
respectively.
Of particular
value for
the analysishas
of been
the
proton beam
transport system
penetrations
proton
beam (DORT
transportto system
has been
the DTD
DORT) penetrations
code7 that couples
two
arbitrarily
the cylindrically
DTD (DORTsymmetrical
to DORT) problems
code7 thatwith
couples
two
cylindrically symmetrical problems with arbitrarily
guides. The methodology was also applied to
184
determine the shielding requirements for the DTL
determine the shielding requirements for the DTL
sections in the SNS Front End Building, the HEBT
sections in the SNS Front End Building, the HEBT
shield wall maze to allow work in the ring during linac
shield wall maze to allow work in the ring during linac
tuning, the design of the linac tune, ring injection, and
tuning, the design of the linac tune, ring injection, and
ring extraction beam dumps, and the required RTBT
ring extraction beam dumps, and the required RTBT
shielding for backstreaming from the mercury target.
shielding for backstreaming from the mercury target.
In
conjunction with these analyses, the radiation
In conjunction with these analyses, the radiation
environment
during operation
operationand
andresidual
residualactivation
activation
environment during
after
shutdown
was
determined
for
normal
operational
after shutdown was determined for normal operational
line losses
losses as
as defined
defined by
bythe
theSNS
SNSaccelerator
acceleratorphysics
physics
line
8
8 . Furthermore, credible accident scenarios were
group
group . Furthermore, credible accident scenarios were
examined toto determine
determinethe
theadequacy
adequacyofofthe
theshielding.
shielding.
examined
A
typical
result
is
shown
in
Figure
2.
Detailed
A typical result is shown in Figure 2. Detailed
descriptions
of
some
of
these
calculations
can bebe
descriptions of some of these calculations can
found
in
References
9
and
10.
found in References 9 and 10.
ACKNOWLEDGMENTS
ACKNOWLEDGMENTS
This work was supported by the U.S. Department
This work was supported by the U.S. Department
of Energy through the Spallation Neutron Source
of Energy through the Spallation Neutron Source
(SNS) Project. SNS is managed by UT-Battelle, LLC,
(SNS) Project. SNS is managed by UT-Battelle, LLC,
under
contract DE-AC05-OOOR22725 for the U.S.
under contract DE-AC05-00OR22725 for the U.S.
Department
Energy.
Department ofofEnergy.
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low
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with the
the design
design changes
changes and
and
provide
provide timely
timelyfeedback
feedback totothe
theSNS
SNSdesign
designengineers.
engineers.
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modifications are
are constantly
constantlybeing
beingaddressed
addressed
to
to ascertain
ascertain the
the impact
impact the
themodifications
modificationshave
haveononthe
the
radiological
radiological safety
safety ofof the
the SNS.
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forthe
thedetails
details
of
the
advanced
Title
II
design
that
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