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Neutronics and Radio Protection Considerations for the n_TOF Target Design at CERN

This 2008 n_TOF review discusses challenges and insights from existing target designs, focusing on neutronics and radio protection. It outlines activation measurements, comparisons with FLUKA simulations, and implications for new target designs including materials, cooling, and ventilation. The report emphasizes thorough geometry implementation and dose rate surveys, critically analyzing the roles of chemical composition and irradiation history on residual dose rates. These findings aim to enhance safety protocols and design choices in ongoing and future neutron transport experiments at CERN.

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Neutronics and Radio Protection Considerations for the n_TOF Target Design at CERN

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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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