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July 10, 2000

Environmental Radiation Studies for the Collimation Section of the NLC Beam Delivery System S. H. Rokni, J. C. Liu, S. Roesler. July 10, 2000. Outline. Soil and Groundwater Activation around the Collimation Section of the NLC Beam Delivery System Tunnel.

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July 10, 2000

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  1. Environmental Radiation Studies for the Collimation Section of the NLC Beam Delivery SystemS. H. Rokni, J. C. Liu, S. Roesler July 10, 2000

  2. Outline • Soil and Groundwater Activation around the Collimation • Section of the NLC Beam Delivery System Tunnel • Three approaches for calculating induced radioactivity in soil with FLUKA • Groundwater activation 2. Air Activation in the Collimation Section of the NLC Beam Delivery System Tunnel • Particle fluence in the tunnel of the collimation section, activation cross sections • and radionuclide production • Worker exposure during access • Dose to public

  3. Radiation Studies for the NLC Soil and Groundwater Activation around the Collimation Section NLC Beam Delivery System Tunnel

  4. Beam Delivery System • Collimation Section • 300 meter long • Intercepts 0.1% (10 kW) of the total beam power Positron Injector Positron Main Linac Beam Delivery System • Radiological hazards • Prompt radiation • Soil and groundwater activation • Air activation • Induced activity of beam line components Electron Main Linac Electron Injector

  5. Radioactivity in Soil Production of radioactive nuclide in the soil is mainly due to neutron-induced reactions:

  6. FLUKA Simulation Energy cut-offs • Neutrons down to thermal energy • Electrons, positrons and photons set at 5 MeV • Biasing • Leading particle biasing • Reduction of the interaction length of photons • Region importance biasing • Magnetic field

  7. Cross sectional view of the NLC Tunnel Concrete Soil

  8. FLUKA Geometry • Cylindrically symmetric • 30 cm thick concrete wall • 100 cm thick soil layer • 6 collimation sections

  9. FLUKA Geometry Absorber Spoiler 500 GeV e 10 kW Quad • 2 spoilers (vertical/horizontal) aperture: 0.0061 cm, length: 0.357 cm • 2 absorbers (vertical/horizontal) aperture: 0.05 cm, length: 50 cm • 2 quadrupoles (focusing/defocusing) aperture: 0.7 cm, length: 100 cm

  10. Source Terms

  11. FLUKA Simulations Neutron fluence inside and around the tunnel Neutron Fluence (cm-2)

  12. FLUKA Calculations Three approaches were considered in estimating the induced soil activity: 1. Direct calculation of residual nuclei Activation due to high energy hadron interactions, and due to low energy neutrons for most elements Lack of a complete fragmentation model could underestimate the yield of radionuclides with A and Z far from the medium to heavy mass targets 2. Calculation of the star density Inelastic interactions with E > 50 MeV Need atoms per star conversion factors 3. Calculation of particle fluence spectra Fold the fluence with the measured cross sections Measured cross sections on neutron-induced reactions are scarce Used evaluation of proton-induced reaction cross sections by Tesch*. * K.Tesch, -DESY, D3-86, January 1997.

  13. FLUKA Simulations Neutron stars around the tunnel Star Density (cm-3) • All stars: 1.10 (0.2%) per primary electron • Neutron stars: 1.05 (0.2%) per primary electron

  14. Tritium Production in Soil 16O 28Si 28Si 16O

  15. 3H production from16O • Kruger et al., Phys. Rev. C27 (1973)

  16. Radionuclide Concentration in Soil(atoms/cm3/primary electron) Comparison of the results from the three approaches in the small region of soil (thickness = 10 cm , length = 50 cm) * * Using atoms per star conversion factors from H. Vincke & G. R. Stevenson, CERN/TIS-RP/IR/99-20

  17. Soil Activation around the BDS Tunnel Used the atoms/star factors obtained from the direct production of radionuclides*(10 kW)for the small region to calculate the soil activation around the entire collimation section (1 m x 300 m). Operation Parameters: • 1.25x1011 electrons per second • 250 days of continuous operation per year • 10 years of operation for NLC * A. Fasso et al., 9th ICRS, 1999 and H. Vincke & G. R. Stevenson, CERN/TIS-RP/IR/99-20.

  18. Soil Activation Results • There are no limits for the activation of soil in the U.S. regulations. • Natural activity concentration in soil: 0.3-1 Bq/gm

  19. Groundwater Activation Water content of soil is assumed to be 30% (by volume). The following leaching factors are used: 3H : 100 % 22Na : 15 % Average concentration in groundwater at shut-down after 10 years of NLC operation: 3H : 0.5 Bq/cm3 22Na : 0.1 Bq/cm3 The regulatory limits in drinking water are: 0.74 Bq/cm3for 3H and 0.37 Bq/cm3for 22Na.

  20. Radiation Studies for the NLC Air Activation inside the the Collimation Section NLC Beam Delivery System Tunnel

  21. Air Activation Air activation in the BDS is mainly due to interaction of photons and neutrons with nitrogen, oxygen and argon. Inside the tunnel • Workers Limit: • DOE (2000 DAC-hr) 2000 DAC-hours (1 year) correspond to 50 mSv for a breathing rate of 2400 m³ per year Two radiation safety concerns: Outside Environment • Maximum Exposed Individual (MEI) Limits: • 40CFR61 HMEI < 100 mSv/y • Monitoring if HMEI > 1 mSv/y

  22. Air Activation Calculations Neutron fluence (cm-2) Photon fluence (cm-2) Calculate average fluence of g,n,p,p in air: 14N (75.6%), 16O(23.1%), 40Ar(1.3%) Fold the fluences with reaction cross sections to estimate the production of 3H,7Be,11C,13N,15O,41Ar.

  23. Air Activation Cross Sections The following reactions were considered : x GDR, measurements (data eval. by A. Fasso) g-spallation on 16O (data compiled by K. Tesch et al.) * + g-spallation on 14N (16O) assumed n-spallation (data and FLUKA eval. by M. Huhtinen) # 40Ar (n,g) 41Ar , thermal neutron capture (data compiled by M. Huhtinen)

  24. Fluence Spectra and Cross Sections Particle fluence spectra 16O 11C 14N 13N 16O 7Be

  25. Radionuclide Concentration in Air Average concentration of radionuclides in the air of the NLC BDS tunnel (10-14 atoms/cm3/e) • Radionuclide production by photons dominates due to large fluence. 40Ar (n,g) 41Ar: atoms/cm3 /e, 0.0018 atoms/e

  26. Worker Exposure • 1.25x1011electrons per second (0.1% of 10 MW, 500 GeV) • No ventilation during operation Activity Concentration Ac after one month of operation

  27. Effluent Dose to Public • Used the CAP88 codewith the SLAC specific parameters for average wind speed, annual precipitation, and conservative release assumptions (no stack, zero velocity for air plume rise) • 12 releases per year Results for a Maximum Exposed Individual at 50 m Limit: HMEI < 100 mSv/y Continuous monitoring: HMEI > 1 mSv/y

  28. Summary - Soil Activation There are uncertainties associated with each of the three methods used to calculate the induced activity in soil: • Predictions of 3H, 7Be and 22Na production with FLUKA could be subject to uncertainties due to the lack of a fragmentation model. • Star-to-isotope conversion factors are not available for electron accelerators, the factors may depend on the specific setup (experiment, geometry, Monte Carlo code etc.) used to determine them. • Experimental cross sections for neutron-induced reactions producing 3H and 7Be and 22Na are not available and the use of proton-induced cross sections instead could be a poor approximation. • Needed: • Measured cross sections for (~100 MeV )neutron-induced reactions • Activation measurements

  29. Summary - Soil Activation 10 years after NLC shutdown (assuming 10 years of operation) the average induced activity concentration in the soil along the BDS tunnel is lower than the natural activity of the soil. Immediately at NLC shutdown the induced activity concentration in groundwater is comparable to the U.S. drinking water limits.

  30. Summary - Air Activation • There are two main sources of air activation in the BDS tunnel: • Activation by photonuclear interactions (mainly GDR reactions) which is due to the high photon fluence as compared to the hadron fluence. • Activation by thermal neutron capture on 41Ar that isdue to the large thermal neutron capture cross section. • Due to the low density of air the only feasible approach is to fold calculated photon and hadron spectra with measured or calculated cross sections. • The GDR cross sections and the thermal neutroncapture cross sections are relatively well known so that more reliable predictions are possible.

  31. Summary-Air Activation Exposure of workers that will access the tunnel after one-month of operation is not a major concern (dominated by 13N, 15O) and can be easily mitigated by a cooling time (1 hour) and/or ventilation before access. Assuming that all radioactivity in the air at the end of an one-month operation is released to an environment that is the same as that of SLAC, the annual dose to the public is minor. The hazard can be mitigated by a waiting period of one hour prior to release.

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