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Neutronics & RP Issues

Neutronics & RP Issues. nToF Review 14 th February 2008 CERN AB/ATB/EET n_TOF Team. Overview. Experience from the Existing Target Activation measurements Comparison with FLUKA Consequences for design choices New Target Design Critical Design Questions concerning:

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Neutronics & RP Issues

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  1. Neutronics & RP Issues nToF Review 14th February 2008CERN AB/ATB/EET n_TOF Team

  2. Overview • 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

  3. FLUKA Calculations • 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 nToF Review 2008 - Neutronics & RadioProtection

  4. Geometry Details nToF Review 2008 - Neutronics & RadioProtection

  5. Target in the Pit Earth Target Pit filled(concrete) Beam Marble Beam Pipe Concrete nToF Review 2008 - Neutronics & RadioProtection

  6. Target outside the Pit (arb. location) • Important for 2-Step calculations • e.g., used for inter-comparison with final activation measurements Beam Entrance Beam Exit Y X n_ToF Experimental Area Z nToF Review 2008 - Neutronics & RadioProtection

  7. Important Input Parameters • 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 nToF Review 2008 - Neutronics & RadioProtection

  8. Neutron Fluence - Benchmark 20% difference between 1 and 1E5 eV ??? Performance Report CERN-INTC-2002-037, January 2003 CERN-SL-2002-053 ECT nToF Review 2008 - Neutronics & RadioProtection

  9. Neutron Fluence - Benchmark • 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) -> Perfect Agreement nToF Review 2008 - Neutronics & RadioProtection

  10. Experimental 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

  11. Inspection & Measurements nToF Review 2008 - Neutronics & RadioProtection

  12. Rectangular section Circular section 10.3 m Square section pool Decay tube First 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 nToF Review 2008 - Neutronics & RadioProtection

  13. Important 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

  14. Detailed Dose Rate Survey 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 3 meters distance & going towards the target surface. (fully remote, thus possibility to wait & get enough statistics while performing continuous measurements) nToF Review 2008 - Neutronics & RadioProtection

  15. New FLUKA Comparison after Detailed Pit Survey Measurements 01.11.2007 10000 FLUKA Simulation Upper Shaft 1000 Lower Shaft Sv/h Decay-Tube m Target 100 Residual Dose Rate / Detailed Measurements 10 FLUKA New Simulations First FLUKA Simulations First Measurements 1 Target Upper Shaft Container Lower Shaft r Decay-Tube 0.1 10000 8000 6000 4000 2000 0 Distance from the Top (Access Gallery) / mm nToF Review 2008 - Neutronics & RadioProtection

  16. Residual Dose Rates Comparison nToF Review 2008 - Neutronics & RadioProtection

  17. nToF Review 2008 - Neutronics & RadioProtection

  18. 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 Steels used at CERN In addition, direct measurements on the target steel support will be performed(measurement device is ordered and soon to arrive at CERN) EA: ICP-AES (AES=Atomic Emission Spectrometry) EMPA: WD-XRF (wavelength-dispersive X-ray fluoresence spectrometry) EIG: XRF nToF Review 2008 - Neutronics & RadioProtection

  19. Dependency on Cobalt Content Using a stainless steel typewith low Co59 content will be important for the new target design nToF Review 2008 - Neutronics & RadioProtection

  20. Impact on Design nToF Review 2008 - Neutronics & RadioProtection

  21. Critical Design Questions • 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 nToF Review 2008 - Neutronics & RadioProtection

  22. Increasing the Beam Size Factor of ~10 nToF Review 2008 - Neutronics & RadioProtection

  23. Residual Dose Rates nToF Review 2008 - Neutronics & RadioProtection

  24. Connection of Cooling System • 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 6m + 5m 10y + 1y mSv/h mSv/h nToF Review 2008 - Neutronics & RadioProtection

  25. Target Support Material Choice • 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 nToF Review 2008 - Neutronics & RadioProtection

  26. Target Support Material 6m + 5m Aluminum Container Steel Container mSv/h mSv/h nToF Review 2008 - Neutronics & RadioProtection

  27. Target Support Material 10y + 1y Aluminum Container Steel Container mSv/h mSv/h nToF Review 2008 - Neutronics & RadioProtection

  28. Target Support Material (Steel/0.1%Co) 6m + 5m Standard Container “Shielded” with Boron mSv/h mSv/h nToF Review 2008 - Neutronics & RadioProtection

  29. Target Support Material (Steel/0.1%Co) 10y + 1y Standard Container “Shielded” with Boron mSv/h mSv/h nToF Review 2008 - Neutronics & RadioProtection

  30. Target Material 6m + 5m Standard Lead Lead with 3% Sb mSv/h mSv/h nToF Review 2008 - Neutronics & RadioProtection

  31. Target Material 10y + 1y Standard Lead Lead with 3% Sb mSv/h mSv/h nToF Review 2008 - Neutronics & RadioProtection

  32. Comparison (20cm distance) x “BoronShielding” “Sb Alloy” “BoronShielding” “Sb Alloy” nToF Review 2008 - Neutronics & RadioProtection

  33. Target Only – Two-Step Calculation (Standard Lead + Stainless Steal Container with 0.1% Cobalt) 10y + 1y Lateral Cut - Centre Longitudinal Cut - Centre mSv/h mSv/h ~20 mSv/h ~2 mSv/h nToF Review 2008 - Neutronics & RadioProtection

  34. Target Only – Two-Step Calculation 10y + 1y Std. Pb + SS with 0.1%Co Pb + 3%Sb + SS with 0.01%Co mSv/h mSv/h ~20 mSv/h ~2 mSv/h nToF Review 2008 - Neutronics & RadioProtection

  35. Material Choice • 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 nToF Review 2008 - Neutronics & RadioProtection

  36. Neutronics nToF Review 2008 - Neutronics & RadioProtection

  37. Influence of Target Alloy Materials New Target: + 3% Sb, 0.01% Ag nToF Review 2008 - Neutronics & RadioProtection

  38. Influence of Target Alloy Materials New Target: + 3% Sb, 0.01% Ag nToF Review 2008 - Neutronics & RadioProtection

  39. Neutronics – Boron ‘Shielding’ nToF Review 2008 - Neutronics & RadioProtection

  40. Neutronics – Different Configurations nToF Review 2008 - Neutronics & RadioProtection

  41. Neutronics – Different Configurations nToF Review 2008 - Neutronics & RadioProtection

  42. Ventilation Issues nToF Review 2008 - Neutronics & RadioProtection

  43. Critical Groups Critical Group: Border Guards ExperimentalArea Decay Tube Target nToF Review 2008 - Neutronics & RadioProtection

  44. Annual Dose Calculation • FLUKA simulations to calculate the isotope production yield (39 different isotopes considered) • Exposure of personnel (access to nToF area) • Dose conversion coefficientsbased 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 0.1mSv nToF Review 2008 - Neutronics & RadioProtection

  45. Installation & Streaming through Ducts • 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 nToF Review 2008 - Neutronics & RadioProtection

  46. Streaming Calculation Sv/h • 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 100mSv/h X X nToF Review 2008 - Neutronics & RadioProtection

  47. Radioactive Waste nToF Review 2008 - Neutronics & RadioProtection

  48. Activation of Target, Support & Water • 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 nToF Review 2008 - Neutronics & RadioProtection

  49. Conclusions nToF Review 2008 - Neutronics & RadioProtection

  50. Conclusions • 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

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