Summary of Shielding and Activation analysis for the European Spallation Source Linac Tunnel

Transcription

1 Summary of Shielding and Activation analysis for the European Spallation Source Linac Tunnel SHORT SUMMARY OF FINAL RESULTS by Lali Tchelidze March 21, 2013 Several different approaches were taken to estimate the amount of berm shielding needed for the ESS accelerator. Sullivan s Guide to radiation and radioactivity levels near high energy particle accelerators was used to calculate required berm thickness for point and line beam loss scenarios. Also, MARS simulations were run to perform the same task. FLUKA was also used for 2.5 GeV to verify simulation results. A short summary of the outcome is summarized in the two tables below. The calculations are based on certain assumptions, which are highlighted in Table captions. Sullivan s point loss model Sullivan s line loss model MARS point loss model (steel target) FLUKA point loss target (Cu) 10 msv/y (2000 h) 5 usv/h Recommended minimum berm thickness (m) 1 msv/y 0.25 msv/y (2000 h) (2000 h) 0.5 usv/h usv/h 0.1 msv/y (2000 h) 0.05 usv/h General Public (24/7) 3.4 nsv/h Table Calculated and simulated minimum required berm thicknesses for several different annual dose limits and occupancy time, for 2.5 GeV only. Calculations based on Sullivan s handbook use a source term given in the book, while simulations are performed for 1 m long and 20 radius target. MARS used a stainless steel target and FLUKA used a copper target. Michal Jarosz provided FLUKA results. All answers include a safety factor of two. Beam Energy (MeV) 10 msv/y (2000 h) 5 usv/h Recommended minimum berm thickness (m) 1 msv/y 0.25 msv/y (2000 h) (2000 h) 0.5 usv/h usv/h 0.1 msv/y (2000 h) 0.05 usv/h General Public (24/7) 3.4 nsv/h Table Minimum required berm thicknesses for several different annual dose limits and occupancy times, for end of DTL (80 MeV), end of SPK (200 MeV), end of MB (628 MeV), middle (1.5 GeV) and end of HB (2.5 GeV). Based on MARS simulations. The more conservative results, among those for various target dimensions were exercised. All the results include a safety factor of two. 1

2 1. INTRODUCTION For a high intensity and energy accelerator facility, like ESS, the shielding and activation analysis are important to meet the environmental and safety requirements. ESS linear accelerator will produce 2.5 GeV protons and will operate at an average 5 MW beam power. Due to beam losses, linac components, air, concrete and soil around the accelerator will get activated and radiation fields will be present inside and outside the linac tunnel. An earth berm shielding design is proposed to meet the dose limit requirements. Tunnel air activation and radiation analysis around the klystron stubs are also performed. In this report, the following four items are addressed specifically: - Linac tunnel berm shielding: minimum soil thickness on top of the linac tunnel for radiation shielding is calculated at the following locations along the accelerator: end of the drift tube linac DTL (80 MeV), end of the spokes SPK section (200 MeV), end of the medium- beta MB section (628 MeV), middle of the high- beta HB section (1.5 GeV) and end of the HB section (2.5 GeV). - Fence distribution around the linac: minimum distances to fences are calculated from the linac buildings to limit access for general public. - Linac air activation: various radioisotope production rates are estimated in the tunnel air. - Radiation levels around linac stubs: first assessment of the prompt dose rates in the stubs during normal operations is performed. 2. GENERAL ASSUMPTIONS The following assumptions are valid for all the shielding results presented in this report: - Tunnel location is fixed so that the top of the concrete ceiling is at the ground level (see Figure 1). - Tunnel cross- section dimensions are 3.3 m x 5.4 m (Figure 1). - Size of the tunnel ceiling (concrete) is fixed to 70. Tunnel walls (concrete) are 50 wide. - Beam axis is in the middle of the tunnel. - In case of uncontrolled full beam loss, the machine will be shut down within few pulses (at most 1 sec). 2

3 Figure 1 ESS linac tunnel cross section (September 2012). 3. SHIELDING CRITERIA/DOSE LIMITS Linac earth berm shielding analysis are based on the following dose limits: - During normal operations The accelerator shielding should reduce the dose rate on the accessible outside surfaces of the shield to less than 0.03 msv/year for general public and/or less than 10 msv/year for radiation workers [1]. - During postulated catastrophic accidents The shielding should reduce the dose rate on the accessible outside surfaces of the shield to less than 0.05 msv/occurrence for general public and/or less than 20 msv/occurrence for radiation workers [1]. These dose limits are summarized in the table below: Mode Dose limit to public (acc. Dose limit to rad. workers (acc. related only) related only) Normal Operations 0.03 msv/year 10 msv/year Incidents (at most once a year) 0.05 msv/occurrence 20 msv/occurrence Table 1 ESS general safety requirements. 3

4 4. BEAM LOSS MODES Calculations are performed for so- called normal operations and catastrophic accidents. - Catastrophic accident is defined as a full beam loss (average 5 MW at 2.5 GeV) on a thick steel target. Steel was chosen to represent a common accelerator component. Copper could ve been chosen as well resulting in similar beam- on dose rates. Note that we are talking about prompt dose rates only, which are same (within a factor of 2) for both steel and copper. The radionuclide production though is completely different for these two materials. - Normal operations are defined as maximum allowed beam loss on a similar thick target as described above. Allowed loss limit is set to be no more than 1 W/m for proton energies ~ 100 MeV and higher [2, 3]. This limit is derived from the residual dose rate limits for hands on maintenance for tunnel components [2, 3]. This means that the residual dose rates at 30 from the activated objects for 100 days of continuous irradiation and 4 hours of cool down time should be in the following range [4]: Unconstrained access - < 0.1 msv/hr Limited access time msv/hr Very limited access time msv/hr Remote handling required - > 100 msv/hr. At ESS similar limits are used to label blue, yellow and red zones [5]. Blue area unconstrained access < msv/hr Yellow area limited access msv/hr Red area very limited access > 1 msv/hr. We should assume that at > 100 msv/hr access is prohibited and remote handling required. For low energies, up to ~ 100 MeV, roughly 10 times higher (< 10 W/m) beam losses can be tolerated [6]. Note that 10 W/m loss limit is an estimate and requires further analysis. A paper, describing the loss limits in detail, will be presented to the International Particle Accelerator Conference in Shanghai in May TOOLS USED MARS is a Monte Carlo code for inclusive and exclusive simulations for three- dimensional hadronic and electromagnetic cascades, muon, heavy- ion and low- energy neutron transport in accelerator, detector, spacecraft and shielding components in the energy range from a fraction of an electronvolt up to 100 TeV [7, 8, 9]. A version MARS 15 (2012) was used in all calculations. The default BNAB mode was used in the calculations. This mode has a limited number of built- in cross- section data. The built- in materials are He, Be, CH, C, Tissue, water, air, soil, ordinary concrete, Al, Si, Ar, Ti, Fe, Cu, W, Pb and U. Note that this database does not include capture of thermal neutrons which can be important in labyrinths, wave- guide penetrations, tunnels and shafts. BNAB gives reasonable results for the built- in materials, but can be quite misleading for others. For a specific composition of soil, concrete and iron shielding, that are quite different from those built- in can result in under/overestimation of the dose/flux of low- energy neutron component by a factor of several, therefore, for all these cases, MCNP mode is strongly recommended. In this report only built- in materials and basic BNAB mode was used. 4

5 6. MODELING BEAM LOSS MODES Both normal operations and full beam losses were modeled as a point beam loss. This gives reliable results, if assuming that a point loss of 10 W every 10 meters is equivalent to a distributed 1 W/m losses [10]. As mentioned above, steel was chosen as a target material (for earth berm shielding calculations). 20 in radius and 1 meter long cylinder was modeled. Target dimensions are chosen based on estimated length/width of material lost protons can travel through. 7. LINAC EARTH BERM THICKNESS ANALYSIS 7.1 Geometry model A simplified geometry model used in the calculations is shown in Figure 2. A 20 meter long tunnel section with 3.3 m x 5.4 m cross- section dimensions was modeled. Thickness of concrete on top of the tunnel is 70, and on the sides it is 50. Maximum prompt dose rates are calculated on top of the berm for a longitudinal full beam loss (1.25 E16 p/sec) on 1 m long cylindrical (20 radius) stainless steel 304 target. x x y x:y = 1:1.091e+00 z x:z = 1:6.667e-01 Figure 2 Cross section view (left) and side view (right) of the linac tunnel, target and earth berm shielding. x is the vertical axis and z is the beam direction. Brown earth, grey concrete, blue steel, white air. 7.2 Materials description The following MARS built- in materials were used in the calculations: Earth berm - SOIL 1.9 g/ 3 H 17O 27Al 2Si 9 (actual ESS soil density is g/ 3 ). Target - STST g/ 3 stainless Steel 304. Tunnel air - AIR g/ 3 air at 18 0 C and 58 % humidity. Tunnel walls/ceiling - CONC g/ 3 concrete with steel reinforcing O 2SiCaNaFeAl. 5

6 7.3 Results Maximum prompt dose rate on top of the earth berm was calculated as a function of its thickness. Figures 3 and 4 represent normal operations (< 1 W/m beam loss for ~ 100 MeV 2.5 GeV and ~ 10 W/m loss for < 100 MeV) and catastrophic accidents (full beam loss) respectively. Plots are shown for 80 MeV (end of the DTL), 200 MeV (end of the SPK), 628 MeV (end of the MB), 1.5 GeV (middle of the HB) and 2.5 GeV (end of the HB). During normal operations, 2000 hours of occupancy time is allowed to radiation workers. The 10 msv/year limit then gives ~ 5 usv/hr. A full year, 365 days of occupancy time is assumed for general public. Then the 0.03 msv/year limit translates into 3.4 nsv/hr. Note that natural background around the southern Sweden is ~ 3mSv/year (0.3 usv/hr) [11]. During catastrophic beam loss (full beam loss), it is assumed that the beam will be off in no more than 1 sec. Current MPS system is designed to handle the beam abort within a pulse. But even if the MPS fails, the beam will destroy the accelerator components and it will become impossible to continue operation within few pulses (1 sec at most). With this assumption, the 0.05 msv/accident (for general public) and 20 msv/accident (for radiation worker) limits translate to 180 msv/hr for GP and 7E4 msv/hr for RW. Based on the data and allowed dose rate limits, recommendations are made for the minimum thickness of the berm not to exceed the radiation levels for radiation workers or general public. The results are summarized in Tables 2 and 3. A safety factor of 2 is included in the calculations. Figure 3 Prompt dose rates as a function of earth berm thickness during normal operation. 6

7 Figure 4 Prompt dose rates as a function of earth berm thickness during catastrophic accident. NORMAL OPERATIONS Linac section Proton energy (MeV) Recommended minimum berm thickness (m) (Restricted access to RW) Recommended minimum berm thickness (m) (Access to public allowed) End of the DTL End of the SPK End of the MB Middle of the HB End of the HB Table 2 Recommended minimum earth berm thicknesses for normal operations. FULL BEAM LOSS Linac section Proton energy (MeV) Recommended minimum berm thickness (m) (Restricted access to RW) Recommended minimum berm thickness (m) (Access to public allowed) End of the DTL End of the SPK End of the MB Middle of the HB End of the HB Table 3 Recommended minimum earth berm thicknesses for catastrophic accidents. As we see, the berm thickness is driven by the normal operations and not the catastrophic beam losses. This is always the case since 10 watts of beam loss for a year (or even for 2000 hours) is larger than 5 mega- watts of beam loss for a second (2000 x 3600 sec x 10 W > 1 sec x 5E6 W). Therefore accumulated dose on top of the berm is always larger due to normal operations 7

8 compared to accidental full beam loss. Thus, recommendations based on normal operations must be followed. Proton energy (MeV) Figure 5 Earth berm thickness (minimum required) as a function of a location along the linac. Three cases are shown with 10 msv (ESS general safety requirement), 1 msv, 0.25 msv and 0.1 msv annual dose limits. As said in section 3, 10 msv/year is the general safety requirement for ESS radiation workers. However, a plot is given in Figure 5 and the data is summarized in Table 4 for 10 msv/year, 1 msv/year, 0.25 msv/year and 0.1 msv/year limits. Distance from MEBT (m) Recommended min berm thickness (m) 10 msv/y (2000 h) 5 usv/hr Recommended min berm thickness (m) 1 msv/y (2000 h) 0.5 usv/hr Recommended min berm thickness (m) 0.25 msv/y (2000 h) usv/hr Recommended min berm thickness (m) 0.1 msv/y (2000 h) 0.05 usv/hr Table 4 Recommended minimum earth berm thicknesses during normal operations for 10 msv, 1 msv, 0.25 msv and 0.1 msv annual dose limits. 8. Fences around the linac Due to a limited CPU time, the exact representation of the radiation propagation in air, outside the earth berm shielding could not be modeled at this time. Rather, a conservative example was considered when there is one meter of earth around the tunnel. For a point beam loss on a target in the middle of the tunnel, at 2.5 GeV, dose rate was calculated in air, over 100 meters from the source of beam loss. The following was obtained: 8

9 x z 2.7e x:z = 1:2.062e-01 Figure 6 Prompt dose rates in air, as a function of distance (vertical axes) from the earth berm. From Figure 6 one can generate a curve, which shows a relative drop of the dose rates as a distance in air. This is shown in Figure 7 and is used as a reference for further analysis. Figure 7 A relative drop of prompt dose rate in air, as a function of distance in air. By placing fences at a certain distance from the edge of an earth berm, we are decreasing the dose rates to publically accessible areas to an allowed 3.4 nsv/hr. Placement of fences should be done based on how thick an earth berm is. If one decides to choose a minimum required earth berm amount to achieve a 5 usv/hr dose rate for radiation workers outside the berm, then one would need to place fences rather far from the berm. However, if one adds some extra soil to the berm, then the distance to fences will be reduced. In Figure 8 several plots are given to show how the 9

10 distance to fences (from the edge of an earth berm) change as a function of an earth berm thickness. Plots are shown for different locations along the accelerator. Figure 8 Relationship between an earth berm thickness and distance to fences. Fences are supposed to limit an access to general public. Note that this approach is very conservative, because, in the real geometry, the radiation travels not only through the air (as in the example above), but also partially in the ground, which helps the dose rate reduction. 9. Linac tunnel air activation 9.1 Geometry model The same 3.5 m x 5.4 m cross- section of the tunnel was used in the studies. 10 meters of the tunnel was modeled at the end of the linac, at 2.5 GeV energy. Normal operation (~ 1W/m) was modeled as a point longitudinal proton beam hitting a target in the middle of the tunnel. Beam intensity was estimated to 2.5E10 protons/sec (a point loss of 10 W is equivalent to uniform 1 W/m [10]). Since the produced radioisotopes in air depend on the target material, few targets of the most common accelerator component materials were studied. 9.2 Materials The most common materials for accelerator structure components were studied and average was derived for produced air activity. These materials are stainless steel (STST 7.92 g/ 3 ), copper (CU), carbon steel (STCA 7.82 g/ 3 ), aluminum (AL) and niobium (NB). When interacting with STST, CU, or STCA, target was modeled as a thick cylinder with 20 radius and 1 m length. When hitting NB or AL, a thin target, cylinder with 1 radius and 1 m long was considered. The amount of air activation was estimated as an average of the contributions from the five different targets. 10

11 9.3 Results The production rates of different radioisotopes are summarized in Table 5. Radionuclide Half- life Production rate (Bq/3/hour) N min 0.37 N sec O sec 8.07E- 4 C min 4.04E- 4 H year 5.58E- 6 C E3 year 1E- 8 Be day 1.96E- 4 C sec 4.04E- 4 Cl min 2.01E- 4 Be E6 year 7.61E Table 5 Radionuclide production rates in linac air at the high- energy end of the linac (2.5 GeV). The production rates need to be translated into the absorbed doses. The conversion coefficient depends on the release rate, location of release (height of stack), age group of a person who potentially inhales radioactive air, wind conditions on the ESS site and distances to closest populated areas. Additional work has to be done separately for further analysis. 10. ASSESMENT of RADIATION LEVELS AROUND STUBS 10.1 Geometry model A simplified geometry model was used to assess the prompt radiation levels within the klystron building around the stubs area during normal operation. Figure 9 shows a cross section of the linac tunnel with a stub to the right of it, merging into a klystron building. Model is based on the drawing shown in Figure 10. In Figure 11, a schematic picture is shown for waveguides, in the right wall of the tunnel and a floor of the klystron building respectively. Waveguide openings are modeled as rectangular opening of 20 x 30. Stubs are 3.5 m high and 2.7 m wide. A 20 x 20 x 1 m long stainless steel target is simulated in the middle of the 10 m section of the tunnel. Target is located across the beam direction and a pencil proton beam with 2.5 GeV hits it in the center. Normal operation, with 1 W/m loss limit is simulated. 11

12 x z x:z = 1:1.000e+00 Figure 9 A cross section of a linac tunnel, with a stub in the right, merging to the klystron building. Figure 10 A drawing of a cross section, showing a stub to the right of the linac tunnel, merging into a klystron building. 12

13 Side view Top view Tunnel wall Beam Direction Waveguide openings Waveguide openings Stub Beam Direction Tunnel x y x:y = 1:1.925e+00 y z y:z = 1:2.333e+00 Figure 11 Waveguide openings in the right wall of the tunnel (left) and a floor of the klystron building (right) Results Two cases were considered: one - with air inside the stub and another - with a concrete block inside the stub, in the space between the waveguides. For 2.5 GeV, the radiation map shows that the maximum prompt dose rate in the klystron building is ~ 10 msv/hr (air in the stub) vs ~ 1 msv/hr (concrete block in the stub). High prompt dose rates require remodeling the stubs geometry and placing shielding blocks at different locations. x z 5.0e x z 4.8e x:z = 1:3.333e+00 x:z = 1:3.333e+00 Figure 12 Radiation dose rate map along the floor of the klystron building with an empty stub (left) and with a space in between the waveguides filled with concrete (right). Dose rates (maximum value) drop from ~10 msv/hr to ~1 msv/hr. 13

14 Supplement A A guide to radiation and radioactivity levels near high- energy particle accelerator, by A. H. Sullivan was used to estimate the required soil thickness for 2.5 GeV only. The following notations are used in succeeding text:! Shield thickness, in our case! = 0.7 +!!"#$ (m)! Mean free path, 0.43 m for concrete (2.35 g/ 3 ) and 0.53 m for earth (1.9 g/ 3 ) [12] see Table 6. R Distance from a beam spill location to a point of interest, in our case !!"#$ (air in tunnel + concrete + soil) (m)!! A source term, an effective equilibrium dose equivalent, normalized to 1 m from an interaction.! Dose equivalent outside a shield. Table 6 Mean free path for several common accelerator component materials. Two cases were assessed by Sullivan s handbook: a uniform beam loss along a line and a point beam loss. 1. A uniform beam loss along a line: source term was taken from Figure 13 (Figure 2.8 in Sullivan [12]) and attenuation was calculated according to formula 1 (Formula 2.9 in Sullivan [12]):! =!!(!"#$) exp!! 0.94! (1)!! (!"#$) is a source term and for 1 W/m and 2.5 GeV equals to approximately 400 Sv/h (see Figure 13). 14

15 Figure 13 Source term for 1 W/m loss level. This leads to the following minimum amount of required soil to achieve different annual dose limits below. Recommended minimum berm thickness (m) 10 msv/y (2000 h) 5 usv/hr 1 msv/y (2000 h) 0.5 usv/hr 0.25 msv/y (2000 h) usv/hr 0.1 msv/y (2000 h) 0.05 usv/hr Table 7 Recommended minimum amount of soil to meet required annual dose limits. By Sullivan, for 2.5 GeV, 1 W/m uniform line spill model. An additional safety factor of two is included. 2. A point beam loss: three different approaches were taken: a) A source term was taken from Figure 14 (Figure 2.4 in Sullivan [12]) and attenuation was calculated according to formula 2 (Formula 2.5 in Sullivan [12]):!! =!!!!! exp!!"#$!!! (2)! is in Sv per proton. Only! = 90 0 was studied. 15

16 Figure 14 Source term for protons/sec. Then the source term for 10 W point loss (the case studied) is approximately 2.5 Sv/h (10 W at 2.5 GeV is 2.5x10 10 protons/sec) and results for soil thicknesses are given in Table 8. b) A source term was calculated from formula 3 (Formula 2.4 in Sullivan [12]) and attenuation was calculated from formula 4 (Formula 2.1 in Sullivan [12]).!! is in Sv m 2 per proton and!! is in GeV.!! 90 = !!"!!!.! (3)! =!!!! exp (!! ) (4) Then!! is approximately 3.18 Sv/h and the results are summarized in the Table 8. c) In principle radiation dose rate is maximum not at 90 0 from the beam axis, but a different angle. To take this angular dependence into account, Formula 5 (Formula 2.3 in Sullivan) was used as a source term!!! = !!"!!!.!" (! + 35/!! )! (5)!! is in Sv m 2 per proton,! is in degrees and!! is in GeV. Combining formulas 4 and 5 together with simple geometry judgment and known parameters of the shield we obtain: 16

17 !!!,! = !!"!.!"!!!"#! (!) 0.7 exp (! + 35/!! )! (2.35 +!)! 0.43sin! exp! 0.53sin! (6) As an example!!!,! is plotted as a function of! for! = 6 m and 2.5 GeV. Figure 15 Dose rate on surface of the shield as a function of an angle from the beam axis. Plot is given for a point 10 W beam loss at 2.5 GeV and 6 m of earth shield. Maximum occurs at ~ 86 degrees. The results for recommended berm thickness from a), b) and c) are given in Table 8. Recommended minimum berm thickness (m) Method 10 msv/y (2000 h) 1 msv/y (2000 h) 0.25 msv/y (2000 h) 0.1 msv/y (2000 h) 5 usv/hr 0.5 usv/hr usv/hr 0.05 usv/hr a b c Table 8 Recommended minimum amount of soil to meet required annual dose limits. By Sullivan s point spill model, for 2.5 GeV and 10 W spill. An additional safety factor of two is included. Supplement B In addition to previous MARS simulations, more calculations were performed for a radially symmetric geometry. The radially symmetric model was studied to make comparison with FLUKA simulation and analytical calculations. In this case the linac tunnel was modeled as 3.3 m diameter tube, with 0.7 m concrete around it and soil around the concrete (Figure 16). 17

18 x y x:y = 1:1.000e+00 x z x:z = 1:6.667e-01 Figure 16 Cross section view (left) and side view (right) of the linac tunnel, target and earth berm shielding. x is the vertical axis and z is the beam direction. Brown earth, grey concrete, blue steel, white air. Figure 17 shows the prompt dose rate as a function of earth berm thickness obtained from MARS, FLUKA and analytical calculations [12]. The plot is shown for 2.5 GeV only. Note that difference in the results come for different target assumptions, thus one should be careful to choose a proper target dimensions/material. Figure 17 Comparison between MARS, FLUKA and Sullivan s point loss models. MARS target 20 radius, 1 m long steel, FLUKA target 20 radius, 1 m long copper. FLUKA simulations were done by Michal Jarosz. 18

19 In Figure 18 one can see a beam- on dose rate as a function of soil thickness for 2.5 GeV, 1.5 GeV, 628 MeV and 200 MeV. These data are calculated for 20 radius and 1 m long steel target. An exponential fit is performed to data in between 100 and 300 soil and the curves are extrapolated for larger soil thicknesses. Figure 18 MARS data is shown for zero to 400 range. Fit is in between 100 and 300 and extrapolation is drawn for > 300. As already mentioned above, the dose rates somewhat depend on the dimensions/material of the target. This is shown Figures 19 and 20, for 628 MeV and 200 MeV, respectively. 19

20 Figure 19 MARS data is shown for zero to 400 range. Fit is in between 100 and 300 and extrapolation is drawn for > 300. Figure 20 MARS data is shown for zero to 400 range. Fit is in between 100 and 300 and extrapolation is drawn for > 300. Further, another prompt dose rate plot is given for 80 MeV and a thin 2 mm radius 1 m long steel target. 20

21 Figure 20 MARS data is shown for zero to 400 range. Fit is in between 100 and 300 and extrapolation is drawn for > 300. Based on the curves above a set of recommended minimum berm thicknesses is derived and summarized in Table 10. Two sets of numbers are given for 200 and 628 MeV, for thin and thick targets, but generally a more conservative approach should be taken. Recommended minimum berm thickness (m) 10 msv/y (2000 h) 1 msv/y (2000 h) 0.25 msv/y (2000 h) 5 usv/hr 0.5 usv/hr usv/hr / / / / / / / / Proton energy (MeV) 0.1 msv/y (2000 h) 0.05 usv/hr Table 10 Recommended minimum earth berm thicknesses during normal operations for 10 msv, 1 msv, 0.25 msv and 0.1 msv annual dose limits. For radially symmetric tunnel geometry. An additional safety factor of 2 is included. All the results are summarized on the first page of the report, in the SUMMARY section. 21

22 References [1] P. Jacobsson, General Safety Objectives for ESS, EDMS (2011) [2] R.A. Hardekoph, Beam loss and activation at LANSCE and SNS, The 7 th ICFA mini- workshop on high intensity high brightness hadron beams (1999) [3] A.I. Drozhdin, O.E. Krivosheev and N.V. Mokhov, Beam loss, collimation and shielding at the Fermilab proton driver, FERMILAB- FN- 639 (2000) [4] S. Halfon, D. Berkovits, Y.Eisen, A. Pernick and J. Rodnizki, Hands- on and beam loss criterion at SARAF SC linac (2009) [5] T. Hansson, Compilation of the Swedish Radiation Legislation (2011) [6] L.Tchelidze and J.Stovall, Estimation of Residual Dose Rates and Beam Loss Limits in the ESS Linac, ESS AD Technical Note ESS/AD/0039 (2012) [7] N.V. Mokhov, The Mars Code System User s Guide, Fermilab- FN- 628 (1995) [8] N.V. Mokhov, S.I. Striganov, AIP Conf. Proc. 896, pp (2007) [9] ap.fnal.gov/mars/ [10] E. Mauro, Radiation Protection Studies for CERN LINAC4/SPL Accelerator Complex [11] 13.html [12] A.H. Sullivan, Guide to radiation and radioactivity levels near high energy particle accelerators. 22