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Clearance Protocol for Lead at Accelerator Facilities

This presentation discusses the progress made in developing a lead clearance protocol at SLAC National Accelerator Laboratory, including the measurement methods and criteria used. It also explores the sensitivity of different instruments and the challenges associated with clearance of lead.

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Clearance Protocol for Lead at Accelerator Facilities

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  1. 1 Clearance Protocol for Lead at Accelerator Facilities James Liu, Maranda Cimeno, Ted Liang, Ludovic Nicolas and Henry Tran Radiation Protection Department SLAC National Accelerator Laboratory, CA, USA ARIA2017, Lund, Sweden, May 22-24, 2017

  2. Progress of clearance of common metals at SLAC • Development of leadclearance protocol: • Induced activity characteristics from FLUKA calculations: • Dominated by Tl-204 (a hard-to-measure, pure-beta-emitting radionuclide) • Development of clearance measurement method: • Clearance criteria: measurements are indistinguishable from background (IFB) • Have to Use a large-area GM survey meter • Detection threshold (DT) is lower than ANSI N13.12-2013 Screening Level (1 Bq/g for Tl-204) • Conclusions 2 Outline

  3. 3 DOE Order 458.1 “Radiation Protection of the Public and the Environment”, 2011 (chg 3, 2013): dose criterion of 10 Sv/y US Regulations and Standards for Clearance • ANSI N13.12 “Surface and Volume Radioactivity Standards for Clearance” (2013): Dose-risk-dependent Screening Level (SL in Bq/g)

  4. SLAC Clearance Protocol for Common Metals SLAC RPD-010 “SLAC Material Release Program Manual” 2011 Radionuclides with long half-lives are of interest Hard-to-measure radionuclides (55Fe), which emit only low-energy X rays or beta rays Proxy radionuclides (22Na, 54Mn, 60Co), which emit high-energy and high-intensity gamma rays ANSI N13.12 SL: 55Fe: 1000 Bq/g 22Na, 54Mn, 60Co: 0.1 Bq/g From FLUKA calculations, SLAC technical bases show that: • Evaluation: ∑i(Ai / SLi)  1 • Measurements with IFB criterion: ∑i(DTi / SLi)  1

  5. 5 DOE-STD-6004 (2016): 7 Chapters SLAC, JLAB, ORNL, SNL, ORNL, and US DOE • Purpose and Scope • Introduction • Material Clearance Protocols • Documentation and Reporting Requirements • DOE Independent Verification, Stakeholder Communication • References • Common Acronyms and Definitions

  6. 6 DOE-STD-6004 (2016): 5 Appendices • Rationale for Endorsement of ANSI N13.12-2013 SLs as DOE Pre-Approved Authorized Limits • Process Knowledge for Volumetric Activation in Accelerator Facilities • Technical Basis for Volumetric Activation at Electron Accelerator Facilities • Technical Basis for Volumetric Activation at Proton Accelerator Facilities • Technical Basis for Measurement Methods of Volumetric Radioactivity and Determination of Detection Capabilities Appendices C and D: “proxy” radionuclides in materials are identified for effective measurements in the field

  7. SLAC Instruments for Common Metal Clearance Measurements Ludlum Model 2241 meter with 1”x1” NaI probe (DT ~ 0.03 Bg/g; 30 nSv/h) & small-area GM pancake probe Field Gamma Spectrometer Portal Gate Monitor

  8. 8 Excess Metal at SLAC (before clearance)

  9. 9 Excess Metal at SLAC (after clearance) First DOE Lab to release metals 6-2011 to 4-2017: 157 tons Al, 271 tons Cu, 3441 tons steel, 493 truck loads, $2.05M

  10. 10 How About Clearance for Lead? • Common shielding material: may become expensive, mixed waste (radioactive and hazardous) 82 tons (10 m3) of lead stored at SLAC • FLUKA shows that Tl-204 (pure beta emitter) dominates and no proxy radionuclides • FLUKA calculation gives sensitivity of small-area GM probe (8 cpm per Bq/g of Tl-204 in lead) • Developed IFB clearance measurement method using a large-area GM probe with a detection threshold (DT) that is quantified to be < ANSI SL of 1 Bq/g

  11. 11 Key Radionuclides from Activated Lead for Clearance Reactions leading to induced radionuclides (p,n) (,n) (,np) (,p) (,np)

  12. An experiment: 3 days irradiation and 21 days decay at SLAC FACET 0.3 µSv/h on contact (gammas from Tl-202, Bi-205, Bi-206) 12 Agreed with FLUKA, which also shows that Tl-204 (pure beta emitter) would dominate after 90 days Gamma Spectrum for Activated Lead Tl-201 Hg-197 Tl-202 Bi-205 Bi-206 Tl-204

  13. Radionuclides and activities normalized to 1 W for irradiation of 1 and 10 years at decay from 1 day to 1 year • PRESICIO, PHOTONUC, RADDECAY, DCYSCORE, RESNUCLE IRRPROF, DCYTIMES 13 FLUKA Calculations of Lead Activation

  14. 1 year irradiation • 1 day decay • Number after isotope ID is slope (attenuation length of reaction in lead in g/cm2) 14 Activity versus Lead Layer Tl-202 53 Bi-205 18 Pt-193 44 Tl-204 51 Os-185 Bi-206 17 Bi-207 22

  15. Dose Risk (Activity / SL) for 1 & 10 year irradiation 15 • Tl-204 (pure beta emitter) has highest activity and dose risk in most cases • No easy-to-detect “proxy” gamma emitter after a few months decay Radionuclides in Inner 1-cm-thick Lead Layer Tl-204 Tl-202 Bi-207 Tl-204 Bi-206 Bi-205 Bi-206 Bi-205 Tl-202 Bi-207 Os-185 Os-185 Pt-193

  16. Tl-204 beta spectrum (Emax of 0.764 MeV) FLUKA-generated beta spectrum agrees with ICRP38 16 Tl-204 Beta Spectrum from FLUKA Simulation of GM probe sensitivity • FLUKA built-in isotope source BEAM, ISOTOPE, HI-PROPE (Tl-204, Z=81, A=204) • Beta decay RADDECAY • Score transport of beta decays DCYSCORE

  17. Tl-204 inside lead sheet • Tl-204 betas emitted isotopically inside lead • Lead thickness 0.05 cm (range of Tl-204 betas) and radius 5 or 8 cm 17 • GM probe (2.5 cm radius) • Steel wire thickness 0.03 cm • Wire spacing 0.125 cm • 0.5 or 1 cm to lead • FLUKA scoring “USRBDX”: one-way current passing from lead region to GM gas • Lead to Air • Air to GM Mica Window • Window to Ne-Cl Gas FLUKA Calculations of GM Probe Sensitivity for Tl-204 in Lead 15.5 cm2 GM probe Steel wire mesh (32x32 wires covering 35% of window area; wire diameter = 475 keV beta range) is critical.

  18. 18 Sensitivity of GM Probe (active area 15.5 cm2) GM at 1 cm from thin Pb (5 cm radius)

  19. 19 Benchmark of FLUKA Calculations of Small-Area GM Sensitivity with Three Tc-99 Beta Sources FLUKA-generated Tc-99 beta spectrum (Emax 0.294 MeV) GM window 5 cm diameter 5 µm Tc-99 source Tc-99 AH-1333

  20. Ludlum Model 44-94 • four LND 7311 GM probes • 4 x 15.5 = 62 cm² active area • x4 sensitivity, x4 background • x½ detection threshold (DT) 20 Need to Use Large-Area GM Meters for Lead Clearance Measurements Lead Clearance On-contact Measurements Ludlum 44-9 GM Probe 15.5 cm2 active area

  21. 21 Static, 1-min measurement both sides of lead brick ANSI N13.12 SL of 1 Bq/g Detection Thresholds of GM Meters

  22. DOE Standard DOE-STD-6004 (2016) was developed for clearance of common metals (e.g., Al, Fe and Cu) in accelerator facilities • Unlike common metals, there are no proxy radionuclides for lead clearance measurements in electron accelerators • Tl-204 (pure beta emitter, Emax = 764 keV, 3.78 yr) dominates activity and risk in lead in most cases 22 Sensitivity of a small-area (15.5 cm2) GM probe for Tl-204 in lead was calculated with FLUKA to be 8 cpm per Bq/g. With an average background of 120 cpm, detection threshold of a large-area GM meter (4x15.5 cm2) can be 0.8 Bq/g for Tl-204 inside lead, which is lower than ANSI N13.12 SL of 1 Bq/g. Lead clearance measurement protocol (e.g., static 1-min measurements with large-area GM probe on contact with a lead brick under maximum background of 180 cpm) is being developed to meet IFB release criterion. Conclusions

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