neutronics & rp issues

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


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


Geometry DetailsGeometry Details


Target in the PitTarget in the Pit

TargetEarthPit filled(concrete)

Beam Pipe

Concrete

Marble

Beam


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


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


Neutron Fluence - BenchmarkNeutron Fluence - Benchmark20% difference between 1 and 1E5 eV ???

Performance ReportCERN-INTC-2002-037, January 2003

CERN-SL-2002-053 ECT


• 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


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


Inspection & Inspection & MeasurementsMeasurements


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


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)


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


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


Residual Dose Rates ComparisonResidual Dose Rates Comparison


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


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)


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


Impact on Impact on DesignDesign


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


Increasing the Beam SizeIncreasing the Beam Size

Factor of ~10


Residual Residual Dose RatesDose Rates


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


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


Target Support MaterialTarget Support Material

Aluminum Container Steel Container6m + 5m

Sv/hSv/h


Target Support MaterialTarget Support Material

Aluminum Container Steel Container10y + 1y

Sv/hSv/h


Target Support Material (Steel/0.1%Co)Target Support Material (Steel/0.1%Co)

“Shielded” with Boron6m + 5m

Standard Container Sv/hSv/h


10y + 1yStandard Container

Target Support Material (Steel/0.1%Co)Target Support Material (Steel/0.1%Co)

Sv/hSv/h

“Shielded” with Boron


Target MaterialTarget Material

Lead with 3% Sb6m + 5m

Standard Lead Sv/hSv/h


10y + 1yStandard Lead

Target MaterialTarget Material

Lead with 3% Sb Sv/hSv/h


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”


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


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


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


NeutronicsNeutronics


Influence of Target Alloy MaterialsInfluence of Target Alloy Materials

New Target: + 3% Sb, 0.01% Ag


Neutronics – Boron ‘Shielding’Neutronics – Boron ‘Shielding’


Neutronics – Different ConfigurationsNeutronics – Different Configurations


Ventilation Ventilation IssuesIssues


Critical Group:Border Guards

Critical GroupsCritical Groups

Target

ExperimentalArea

Decay Tube


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


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


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


Radioactive Radioactive WasteWaste


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


ConclusionsConclusions


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


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


Thank Thank YouYou

Thank Thank YouYou

neutronics & rp issues

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