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Neutronics & RP IssuesNeutronics & RP Issues
nToF Review 14nToF Review 14thth February 2008 February 2008CERN AB/ATB/EET CERN AB/ATB/EET n_TOF Teamn_TOF Team
nToF Review 2008 - Neutronics & RadioProtection 2February 14th 2008
OverviewOverview Experience from the Existing Target
Activation measurements Comparison with FLUKA Consequences for design choices
New Target Design Critical Design Questions concerning:
Target support material Additional target alloy materials Target cooling Area ventilation
Impacts to be Studied for Neutronics Radio Protection Issues
activation and residual dose rates handling radioactive waste concerns air activation and dose to the public
nToF Review 2008 - Neutronics & RadioProtection 3February 14th 2008
Geometry Implementation the simulation includes a detailed layout and design, for
both the target and the tunnel up to the experimental area New Design Options
quick flexibility to change design parameters and estimate respective influences
Detailed Estimates concerning neutron fluences (physics) energy deposition (engineering design, cooling) isotope production (radioactive waste, air activation) residual dose rates (handling, waste)
Accuracy well benchmarked code in all required fields
FLUKA CalculationsFLUKA Calculations
nToF Review 2008 - Neutronics & RadioProtection 4February 14th 2008
Geometry DetailsGeometry Details
nToF Review 2008 - Neutronics & RadioProtection 5February 14th 2008
Target in the PitTarget in the Pit
TargetEarthPit filled(concrete)
Beam Pipe
Concrete
Marble
Beam
nToF Review 2008 - Neutronics & RadioProtection 6February 14th 2008
XX
ZZ
YY
Beam EntranceBeam Entrance
Beam ExitBeam Exit
n_ToF ExperimentalArea
Target outside the Pit (arb. location)Target outside the Pit (arb. location) Important for 2-Step
calculations e.g., used for inter-
comparison with final activation measurements
nToF Review 2008 - Neutronics & RadioProtection 7February 14th 2008
Chemical Composition accurately known for the used lead (e.g., 19ppm Bi) for steel: first estimated based on preliminary dose rate
measurement and finally evaluated during the target removal
Irradiation History beam intensity and irradiation time profile are accurately known
Geometry implemented in a very detailed way
MC Calculation extensive calculations (computer cluster)
FLUKA Models – Activation/Residual DR well benchmarked for low/medium-mass materials at CERF recent comparison for high mass isotopes show a very good overall
agreement
Important Input ParametersImportant Input Parameters
nToF Review 2008 - Neutronics & RadioProtection 8February 14th 2008
Neutron Fluence - BenchmarkNeutron Fluence - Benchmark20% difference between 1 and 1E5 eV ???
Performance ReportCERN-INTC-2002-037, January 2003
CERN-SL-2002-053 ECT
nToF Review 2008 - Neutronics & RadioProtection 9February 14th 2008
• Preparing for Lead target dismount
• Discovery that the water layer is 6 cm thick instead of 5!!!
• New FLUKA simulations with + 6 cm water (red) compared with
+ 5 cm (black)
Neutron Fluence - BenchmarkNeutron Fluence - Benchmark
-> Perfect Agreement
nToF Review 2008 - Neutronics & RadioProtection 10February 14th 2008
Experimental Area: Neutron FluenceExperimental Area: Neutron Fluence
The energy resolution is dominated by the 5cm of water with the resolution experiencing a peak and a tail at low energies the peak being determined by the water moderation (width ~= 2cm) the tail is due to the interface lead/water
The resolution inside lead has delta-lambda of about 30cm with an absolute position lambda equal to 5.7m
Anything more than 5cm of water produces the same resolution
nToF Review 2008 - Neutronics & RadioProtection 11February 14th 2008
Inspection & Inspection & MeasurementsMeasurements
nToF Review 2008 - Neutronics & RadioProtection 12February 14th 2008
First Dose Rate SurveyFirst Dose Rate Survey Pit survey with dose
rate meter attached to a cord and reading the maximum recorded value
Target survey with manual reading and measurements for a predefined set of locations
First comparison with FLUKA showed a significant disagreement
10.3 m
Decay tube
Rectangular section
pool
Circular section
Square section
nToF Review 2008 - Neutronics & RadioProtection 13February 14th 2008
Important Findings & ChangesImportant Findings & Changes Pit & Target
update of geometry (container, support, 30cm, steel faces) Pit
new survey with special dose rate meter and laser controlled distance
negligible contribution to residual dose rates coming from contamination
Target detailed survey with special dose rate meter chemical composition stainless steel – cobalt content
important influence on residual dose rate distribution (up to a factor of 25 in the possible concentration range)
a cobalt content of 0.1% results a very good agreement (this concentration value is confirmed by existing steels at CERN)
nToF Review 2008 - Neutronics & RadioProtection 14February 14th 2008
Inside the pit: using a laser attached to the crane to control the position of the remote detector (attached to the hook)
Around the target: same method, starting at 3meters distance & going towards the target surface.(fully remote, thus possibility to wait & get enough statistics while performing continuous measurements)
Detailed Dose Rate SurveyDetailed Dose Rate Survey
nToF Review 2008 - Neutronics & RadioProtection 15February 14th 2008
New FLUKA Comparison after Detailed Pit Survey Measurements 01.11.2007
0.1
1
10
100
1000
10000
0200040006000800010000
Distance from the Top (Access Gallery) / mm
Res
idu
al D
ose
Rat
e / S
v/h
Detailed Measurements
FLUKA New Simulations
First FLUKA Simulations
First Measurements
FLUKASimulationUpper Shaft
Lower Shaft
Target
Decay-Tube
Containerr
Target
Decay-Tube
Lower Shaft Upper Shaft
nToF Review 2008 - Neutronics & RadioProtection 16February 14th 2008
Residual Dose Rates ComparisonResidual Dose Rates Comparison
nToF Review 2008 - Neutronics & RadioProtection 17February 14th 2008
Residual Dose Rate Scan - Entry Face New FLUKA Comparison for Different Cobalt Contents
10
100
1000
10000
400 900 1400 1900 2400 2900
Distance from Target Face / mm
Res
idu
al D
ose
Rat
es /
Sv/
h
Measurement / uSv/h
FLUKA Co1 (0.01%)
FLUKA Co2 (0.05%)
FLUKA Co3 (0.075%)
FLUKA Co4 (0.1%)
Target
EntryFace
ExitFace
FLUKA SIMULATION
nToF Review 2008 - Neutronics & RadioProtection 18February 14th 2008
Steels used at CERNSteels used at CERN Cast No E33408, Nippon Steel,
Inspection Certificate(F.Bertinelli, used for LHC)
Density 7.252 g/cm3
CERN store 44.57.10.420.4SCEM: 44.57.10.420.4, INOX RNDS.304L
Density 7.908 g/cm3
EA: ICP-AES (AES=Atomic Emission Spectrometry) EMPA: WD-XRF (wavelength-dispersive X-ray fluoresence spectrometry) EIG: XRF
In addition, direct measurements on the target steel support will be performed(measurement device is ordered and soon to arrive at CERN)
nToF Review 2008 - Neutronics & RadioProtection 19February 14th 2008
Dependency on Cobalt ContentDependency on Cobalt ContentResidual Dose Rate (Sv/h) as a function of the stainless steel Cobalt content
(representative for location in front of the target support)
0.11
0.52
0.77
1.00
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1.20
0 0.02 0.04 0.06 0.08 0.1 0.12
Cobalt Content within Stainless Steel (%)
Re
sid
ua
l Do
se
Ra
te (
Sv
/h)
Using a stainless steel typewith low Co59 content will be important for the new target design
nToF Review 2008 - Neutronics & RadioProtection 20February 14th 2008
Impact on Impact on DesignDesign
nToF Review 2008 - Neutronics & RadioProtection 21February 14th 2008
Peak Energy Density - Dilution Increase in beam size
Target materials Additional target alloy materials (Sb, Ag, PbO)
influence in neutron production (Neutronics) impact on isotope production and residual dose rates
Target support choice of material (Al, SS, Special)
impact on residual dose rates additional means to reduce residual dose rates
Cooling Installation of cooling system
residual dose rates and accessibility Activation of water
handling and radioactive waste Area ventilation
installation of ducts and influence on prompt dose rates upstairs prompt dose rates and area classification
Critical Design QuestionsCritical Design Questions
nToF Review 2008 - Neutronics & RadioProtection 22February 14th 2008
Increasing the Beam SizeIncreasing the Beam Size
Factor of ~10
nToF Review 2008 - Neutronics & RadioProtection 23February 14th 2008
Residual Residual Dose RatesDose Rates
nToF Review 2008 - Neutronics & RadioProtection 24February 14th 2008
Possible constraint during installation of the piping system
Same constraint possibly also during target removal
Lowest plug will remain in place
Final technical solution might require short interventions to manipulate the connection flanges
Very low dose rates for both short and long operation scenarios
Connection of Cooling SystemConnection of Cooling System6m + 5m 10y + 1y Sv/hSv/h
nToF Review 2008 - Neutronics & RadioProtection 25February 14th 2008
Calculation Methods “One-Step” simulation looking at residual dose rate
distributions when the target is in its lower pit position “Two-Step” calculations resulting in 3D residual dose rate
maps around the target only (without surroundings) For both: different operation and cooling times
Support Materials Aluminum Stainless Steel with 0.1% / 0.03% / 0.01% Cobalt
Target materials Standard ‘very pure’ lead Addition of Sb (3%)
Additional “Shielding” Borated polyethylene plates (10cm, side and entry face)
with 6% natural boron, i.e., about 1% 10B
Target Support Material ChoiceTarget Support Material Choice
nToF Review 2008 - Neutronics & RadioProtection 26February 14th 2008
Target Support MaterialTarget Support Material
Aluminum Container Steel Container6m + 5m
Sv/hSv/h
nToF Review 2008 - Neutronics & RadioProtection 27February 14th 2008
Target Support MaterialTarget Support Material
Aluminum Container Steel Container10y + 1y
Sv/hSv/h
nToF Review 2008 - Neutronics & RadioProtection 28February 14th 2008
Target Support Material (Steel/0.1%Co)Target Support Material (Steel/0.1%Co)
“Shielded” with Boron6m + 5m
Standard Container Sv/hSv/h
nToF Review 2008 - Neutronics & RadioProtection 29February 14th 2008
10y + 1yStandard Container
Target Support Material (Steel/0.1%Co)Target Support Material (Steel/0.1%Co)
Sv/hSv/h
“Shielded” with Boron
nToF Review 2008 - Neutronics & RadioProtection 30February 14th 2008
Target MaterialTarget Material
Lead with 3% Sb6m + 5m
Standard Lead Sv/hSv/h
nToF Review 2008 - Neutronics & RadioProtection 31February 14th 2008
10y + 1yStandard Lead
Target MaterialTarget Material
Lead with 3% Sb Sv/hSv/h
nToF Review 2008 - Neutronics & RadioProtection 32February 14th 2008
Comparison (20cm distance)Comparison (20cm distance)
0
10
20
30
40
50
60
70
80
6 months Op. + 5 months Cool. 10 years Op. + 1 year Cool.
Operation and Cooling Time
Re
sid
ua
l D
os
e R
ate
s /
mS
v/h
Alu container - Pb
Alu container - Pb + Sb
Steel container - Pb
Steel container - Pb + Sb + Boron
Steel container - Pb + Boron
Steel container - Pb + Sb
x
“BoronShielding”
“BoronShielding”
“Sb Alloy”
“Sb Alloy”
nToF Review 2008 - Neutronics & RadioProtection 33February 14th 2008
Target Only – Two-Step CalculationTarget Only – Two-Step Calculation
Lateral Cut - Centre Longitudinal Cut - Centre10y + 1y
~20 mSv/h~2 mSv/h
(Standard Lead + Stainless Steal Container with 0.1% Cobalt)
Sv/hSv/h
nToF Review 2008 - Neutronics & RadioProtection 34February 14th 2008
Target Only – Two-Step CalculationTarget Only – Two-Step CalculationStd. Pb + SS with 0.1%Co 10y + 1y
~20 mSv/h~2 mSv/h
Pb + 3%Sb + SS with 0.01%CoSv/hSv/h
nToF Review 2008 - Neutronics & RadioProtection 35February 14th 2008
Base Material Choice Aluminum would be best, however stringent constraints in terms of
structural disadvantages corrosion issues poses problems for radioactive waste treatment
Stainless steel is favorable, especially in case special steels with low (~0.03%) cobalt content can be found
Additional ‘Shielding’ The basic principle is to get into ‘competition’ with 59Co in terms of low-
energy neutron capture, by following one of the following approaches borated poly-ethylene (6% natural boron)
-> studied in terms of “proof of principle” possibility to use borcarbid plates addition of boron in the cooling circuit
Addition of Sb into the Lead Augmentation of residual dose rates for short cooling times due to
important production of 124Sb
Material ChoiceMaterial Choice
nToF Review 2008 - Neutronics & RadioProtection 36February 14th 2008
NeutronicsNeutronics
nToF Review 2008 - Neutronics & RadioProtection 37February 14th 2008
Influence of Target Alloy MaterialsInfluence of Target Alloy Materials
New Target: + 3% Sb, 0.01% Ag
nToF Review 2008 - Neutronics & RadioProtection 38February 14th 2008
Influence of Target Alloy MaterialsInfluence of Target Alloy Materials
New Target: + 3% Sb, 0.01% Ag
nToF Review 2008 - Neutronics & RadioProtection 39February 14th 2008
Neutronics – Boron ‘Shielding’Neutronics – Boron ‘Shielding’
nToF Review 2008 - Neutronics & RadioProtection 40February 14th 2008
Neutronics – Different ConfigurationsNeutronics – Different Configurations
nToF Review 2008 - Neutronics & RadioProtection 41February 14th 2008
Neutronics – Different ConfigurationsNeutronics – Different Configurations
nToF Review 2008 - Neutronics & RadioProtection 42February 14th 2008
Ventilation Ventilation IssuesIssues
nToF Review 2008 - Neutronics & RadioProtection 43February 14th 2008
Critical Group:Border Guards
Critical GroupsCritical Groups
Target
ExperimentalArea
Decay Tube
nToF Review 2008 - Neutronics & RadioProtection 44February 14th 2008
FLUKA simulations to calculate the isotope production yield (39 different isotopes considered)
Exposure of personnel (access to nToF area) Dose conversion coefficients based on the
Swiss and French legislation Dose to the public (outside CERN)
Definition of critical groups (border guards) Calculation of dose conversion coefficients (P. Vojtyla) based on
environmental models Different ventilation scenarios
Existing situation Continuous ventilation (laminar flow) Enclosed area and flush before access (full mixing)
Best solution Enclosed area, continuous filtering during operation and flush before
access, leading to annual doses below 0.1Sv
Annual Dose CalculationAnnual Dose Calculation
nToF Review 2008 - Neutronics & RadioProtection 45February 14th 2008
A full 3D simulation being too time consuming we decided for a two-folded approach combining in a first step detailed simulation followed by a second analytical calculation Calculating the prompt equivalent dose rate in the tunnel
below the expected location of the ventilation duct (~40m downstream)
Using analytical ‘over the thumb’ formulas to calculate the attenuation for a given duct
Considered parameters position: 40m downstream duct is 5m long 40cm diameter straight line dose reduction as ‘line of sight’ in front of the duct and
directly behind
Installation & Streaming through DuctsInstallation & Streaming through Ducts
nToF Review 2008 - Neutronics & RadioProtection 46February 14th 2008
Lateral to the ventilation duct a maximum prompt dose rate in the order of 100mSv/h can be expected
Assuming a ventilation duct with 5m length and 40cm diameter one expects a reduction by about three orders of magnitude
Resulting at the top in a prompt residual dose rate of about 100Sv/h
Streaming CalculationStreaming Calculation Sv/h
X
X
nToF Review 2008 - Neutronics & RadioProtection 47February 14th 2008
Radioactive Radioactive WasteWaste
nToF Review 2008 - Neutronics & RadioProtection 48February 14th 2008
Comparison between the existing target (4 years of operation & three years of cooling) with the new design (10 years of operation & one year of cooling)
Total activity and specific activity increase, however in acceptable margins increased operation, shorter cooling time significant reduction in total mass (factor of ~4)
Alpha emitters are largely below ATA levels
Activation of Target, Support & WaterActivation of Target, Support & Water
nToF Review 2008 - Neutronics & RadioProtection 49February 14th 2008
ConclusionsConclusions
nToF Review 2008 - Neutronics & RadioProtection 50February 14th 2008
ConclusionsConclusions Careful Evaluation of FLUKA Simulations
The detailed measurements performed during the target removal and inspection interventions allowed for a careful benchmark of the simulations
Peak Energy Density - Dilution The increase in beam size reduces the peak energy density by about a
factor of 10 Target materials
Additional target alloy materials (Sb, Ag, PbO) No significant influence in neutron production No significant impact on isotope production in terms of waste disposal Significant increase in residual dose rates for short cooling times (< one
year), however no “show-stopper” in terms of handling and possible advantage for long cooling times (competition reaction with 59Co)
Target support choice of material (Al, SS, Special) and additional means to reduce
residual dose rates Low-Cobalt stainless steel combined with a possible implementation of a
“Boron-Shielding” (e.g., Borcabide plates) would be an optimum solution
nToF Review 2008 - Neutronics & RadioProtection 51February 14th 2008
ConclusionsConclusions Neutronics
Neutron fluxes were verified for all studied design options and no ‘show-stopper’ was found
Ensuring an optimum water layer thickness of 5cm Cooling
Installation of cooling system Residual dose rates were checked and accessibility is guaranteed
at all times Area ventilation
Acceptable levels of prompt dose rates are introduced at the location of the ventilation station
Envisaged ventilation system minimizes dose to the public to an absolute minimum
Radioactive Waste Target is optimized in terms of mass (reduction by a factor
of 4) and the reduced amount of cooling water eases future disposal
nToF Review 2008 - Neutronics & RadioProtection 52February 14th 2008
Thank Thank YouYou
Thank Thank YouYou
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