Author: Diane Reitzner x3465 15388N

Shielding Design for the LBNE Decay Pipe
S.D. Reitzner, Fermilab, Batavia, IL 60510, U.S.A
V.S. Pronskikh, V.V. Kashikhin, N.V. Mokhov, Fermilab, Batavia, IL 60510, U.S.A.

SATIF-12

Introduction
The proposed Long Baseline Neutrino Experiment (LBNE) beamline is planned to deliver high intensity neutrino beam to a detector approximately 1300 km away in South Dakota.
Operation of LBNE will generate radionuclides in the surrounding soil which can leach into the groundwater resources.
Sufficient shielding on the decay pipe is needed to maintain the radionuclide concentration in the groundwater resources below State and Federal environmental regulatory limits on drinking water.
Use MARS15 to calculate radionuclide concentrations in soil immediately outside of the decay pipe shield.
Apply conservative assumptions in calculation to minimize risk of underestimating radionuclide concentrations in ground water.

LBNE Beamline Layout

Regulatory Limits on Radionuclide
Concentration
3H and 22Na are the radionuclides of concern
All other radionuclides generated are either too short lived, not mobile or are produced in significantly less quantities than 3H and 22Na to not warrant inclusion in the concentration calculation.

Decay Pipe Shield Cross section
4m diameter decay pipe
20 cm air gap between inner an outer steel pipes for cooling purposes.
Square cross section

Federal Limits:

Ci = Concentration of ith nuclide
Ci,max = Derived Concentration Standard
C3H,max = 20 pCi/cm3
C22Na,max = 0.4 pCi/cm3

State of Illinois Standard:
No degradation of the waters of the State
To maintain radionuclide concentrations below detectable limits, use as target:

Detectable Limits:
3H: 1 pCi/cm3
22Na: 0.04 pCi/cm3

(cm-3s-1)

MARS Target Hall and Decay Pipe Model
F

Star Density Calculation
Extract out average star density (Sav) over a volume which contains 99.9% of the stars in the soil.
Impractical to do multiple runs through very thick shielding to calculate the radionuclide concentrations.
Use radial star density distributions to extract attenuation factor for concrete and soil.
Extrapolate star density results for shielding thicker than 3m.

Radial Star Density Distribution

Full Layout

Target Hall Detail

Star Density Distribution

200 m Decay pipe
Detailed shielding in target hall
Simplified focusing horns
Air filled decay pipe
Decay pipe cross section is circular instead of square for ease of calculation
Shield thickness fixed at 3 m
Soil scoring region 2 m thick
Decay pipe shield and soil scoring region subdivided into 10 longitudinal subsections

Nuclide Production
Calculate the fraction of 3H and 22Na produced per star in soil (Ki).
K3H = (2.9±0.3)×10-2
K22Na = (2.6±0.2)×10-2

Radionuclide Concentrations
Use average star density in the volume containing 99.9% of the stars and the nuclide production per star as calculated from MARS to estimate the radionuclide concentrations.
Total integration time is 30 years for a 2.3MW 120GeV proton beam at a 65% duty factor.

The concentration ratio (Ci/Ci,max) for 22Na and 3H as a function of shield thickness. 22Na dominates due to the smaller value for the Derived Concentration Standard. These results are extrapolations from the results for 3m of concrete shielding.

Minimum Shield Thickness Definition
Need to minimize the chance of detecting radionuclides in the ground water during the lifetime of the LBNE beamline while remaining within the project’s budget.
Uncertainties in operation and input parameters may underestimate the amount of radionuclides produced during operation.
Add the 1σ error to the final result to accommodate the uncertainties in the input parameters.
Assume operation at the maximum possible beam power for the maximum lifetime of the facility even though the beamline will be initially operated at a lower beam power.
The circular shield cross section in the MARS model will overestimate the star densities compared to the planned square cross section. The square shielding cross section will reduce the average star density in by 88%.

Results
Minimum shield thickness is 559±10 cm
Corrected minimum shield thickness for all 20 m long subsections are less than the conservative average minimum shield thickness.

The sum of concentration ratios as a function of decay pipe shield thickness. The minimum shield thickness is located where the sum of the concentration ratios matches the condition:

The correction factor shielding cross section was determined by using MARS to calculate the star density in a cylindrical scoring region surrounding the decay pipe but excluding the area that would lie inside the square decay pipe shield. The ratio of the star densities in the scoring region for the circular shield and that for the square shield was found to be 0.88

The minimum shield thickness as a function of z-position in the decay pipe. The black boxes are the results scaled by the shield cross section correction and the integration time correction for 10 years at 700kW followed by 20 years at 2.3MW. The blue line shows the average minimum shield thickness plus 1σ with no correction factors applied.