HELIOS Injector Shielding in the Free Electron Laser Equipment Room

Transcription

1 HELIOS Injector Shielding in the Free Electron Laser Equipment Room Scott O. Schwahn, Radiation Control Group Introduction The HELIOS injector (a.k.a., FEL Injector Test Area) project is intended to take place in the equipment room in the back room of the Free Electron Laser (FEL) facility, building 18, at Jefferson Lab. In order to run the injector, an area has to be built that has adequate shielding to prevent excessive personnel exposure. This Tech Note describes the calculations that were done to ensure that the estimated shielding is adequate for that purpose. HELIOS Injector Parameters and Source Term Assumptions The HELIOS injector is assumed to have the following parameters, which are based on the literature and on conversations with Ron Lauzé of Jefferson Lab and s Lorraine Marceau-Day of Louisiana State University s Center for Advanced Microstructures & Devices (CAMD). Energy: Current: Duty Factor: Ring fill time: Injection: 100 MeV 10 ma peak 2x10-7 (200 ns, 1Hz) ~6 minutes, including ramping time 3 times per day Shielding Calculations The first step in the process was to make some rough estimates of shielding needs, based on both photon production and on neutron production. This step was principally done using NCRP 51. It is fairly routine to make these calculations, and the final results were not based on these calculations, rather on Monte Carlo simulation, so the steps will not be detailed here. Suffice it to say that the estimates based on NCRP 51 matched reasonably closely to the Monte Carlo simulation (which is considered to be more accurate), so as to verify the Monte Carlo calculation. The latter will be presented in this tech note, along with a summary of the NCRP 51 calculations. The initial design idea of the HELIOS injector (an idea other than that of the author s) was to dig out the dirt between the FEL and the equipment room and to use that area as the injector test cave. However, initial Monte Carlo calculations demonstrated that the amount of shielding needed to protect workers in the HELIOS injector area from radiation from the FEL (as the dirt was a part of the integral FEL shielding), as well as that necessary to protect the FEL workers from HELIOS injector radiation, was not reasonable. Essentially, it required a very large amount of concrete or steel to be mounted on the ceiling of the FEL along the south wall. In addition to the shielding needs, there were concerns regarding the mounting of the shield and other issues such as ventilation, personnel access, cost for digging, etc. It was decided to look at other alternatives. Page 1 of 24

2 The final design involves building a cave in the existing equipment room. The wall thickness was estimated in the original NCRP-51 calculations to be 2 thick. This figure was estimated from the parameters above and also from the following personnel exposure limitations: Jefferson Lab Design Goal: Radiation Area Posting: 250 mrem per year 5 mrem per hour While it was relatively easy to determine the electron and photon source term, it was more difficult to determine the neutron component. The neutrons are generated mostly from the so-called Giant Photonuclear Resonance interaction of high-energy photons with machine components and shielding. The low energy threshold of the production in iron is about 10 MeV, and in aluminum about 13 MeV. The neutron spectrum was estimated using MCNPX, a code under development at Los Alamos National Laboratory. The beta version of MCNPX used was version ld=thu Mar 23 15:30:22 MST For conservatism, a thick copper target was assumed. The output neutron spectrum was then used as source term input to MCNP version 4b. It had already been predetermined that shielding for the neutrons would provide adequate photon shielding, though also attached are the photon results. After iterative design modifications that allowed for adequate ventilation, lighting, egress, radiation shielding etc., the final design was presented and was modeled in MCNP. It was assumed that 5% of the beam would be lost along the length of the injector with the last 95% in the dumplet itself. The results follow on page 5. Dose points were determined at all apparent worse-case spots. The results are in Table 1 below. The calculation uses 30 cm of concrete as dump shielding, which is very effective at removing neutrons of the energies involved. The dose rates are worst-case scenarios when the injector is being filled with electrons. The dose rates drop off dramatically (essentially to zero) when the machine is simply storing the electrons. TABLE 1 Neutron Dose Equivalent Rates 0 degree dump 24" Wall concrete shield 12" tally position Dose rate (mrem/h) Error (1 sigma) 5 outer door - FEL labyrinth % 15 outside bldg - main labyrinth (south) % 25 outside bldg - HELIOS labyrinth (south) % 35 FEL Door % 55 outside HELIOS labyrinth door % 65 equip. room - outside thin shielding % 75 below proposed duct % 85 above 0 degree dumplet % 95 dose inside vault (@ injector) % 105 ceiling, thin part % 115 in proposed duct % 125 Pen # % 135 Pen # % 145 Pen # % 155 Pen # % 165 Pen # % Page 2 of 24

3 As can be seen, the only accessible areas that become Radiation Areas (greater than 5 mrem/hr) during the injection are directly above the dumplet, and above Penetration #5, which is the one closest to the dumplet. These locations may need to be locally shielded if posting proves to be impracticable. The equipment room itself is best exemplified by tallies 55 and 65, which show dose rates of about 0.17 mrem/hr. This dose rate is consistent with a Radiologically Controlled Area, which is the posting originally assumed. The photon dose equivalent rates are relatively inconsequential, with a maximum of about 10% contribution at the hottest accessible spot (tally 85). The results of the calculation are in Table 2. Note that the errors cited are so large due to the very small photon flux at the positions. TABLE 2 Photon Dose Equivalent Rates 0 degree dump 24" Wall concrete shield 12" photon tally position mrem/h Error 5 outer door - FEL labyrinth % 15 outside bldg - main labyrinth (south) % 25 outside bldg - HELIOS labyrinth (south) % 35 FEL Door % 55 outside HELIOS labyrinth door % 65 equip. room - outside thin shielding % 75 below proposed duct % 85 above 0 degree dumplet % 95 dose inside vault (@ injector) % 105 ceiling, thin part % 115 in proposed duct % 125 Pen # % 135 Pen # % 145 Pen # % 155 Pen # % 165 Pen # % Outside of the building, dose rates approach 0.4 mrem/hr on the south side of the building (Tally 25) and even up to almost 0.8 mrem/hr outside the existing FEL door on the southwest side of the building. Clearly, these dose equivalent rates are undesirably high. If one uses the in one hour posting criterion, and assumes that during the injection, the higher dose rates occur for 10% of the time (as were the initial assumptions), the area approaches 80 µrem/h, which exceeds the Radiologically Controlled Area posting limit. Some localized shielding may have to occur to prevent this condition. Originally, there were plans to put some sort of baffle in the air intake to the HELIOS injector, or some shadow shielding in the injector itself, as this is the area where the radiation escapes. That action will likely still be necessary. The 30 cm of concrete around the dump was a rough cut of the amount of local shielding that may be necessary to achieve the design goals. If some more shielding, perhaps even a combination of concrete and steel were used, some of the problems identified above may be solved. However, even with the above proposed shielding, the dose rate objections are not insurmountable. Page 3 of 24

4 Another scenario to be considered is that of a temporary dump that is near the temporary south wall of the HELIOS injector facility, essentially a last stop before it is transported to the HELIOS storage ring. Dose rates from neutrons at the points of interest are in Table 3. TABLE 3 Neutron Dose Equivalent Rates 90 degree dump 24" Wall Tally Position Dose rate (mrem/h) 5 outer door - FEL labyrinth outside bldg - main labyrinth (south) outside bldg - HELIOS labyrinth (south) FEL Door outside HELIOS labyrinth door equip. room - outside thin shielding below proposed duct above 0 degree dumplet dose inside vault (@ injector) ceiling, thin part in proposed duct Pen # Pen # Pen # Pen # Pen # Essentially, dose rates are fairly similar, with the main difference being directly outside the south wall. This situation may be acceptable if there is a building there at the time and the area can be adequately controlled. Conclusions The HELIOS injector project, as currently designed, should result in personnel dose equivalents within the Jefferson Lab design goal of 250 mrem/yr. Area postings will not need to be greater than the Radiologically Controlled Area posting if a bit of localized shielding or direct access control is employed during injection. Areas that need to be considered for local shielding are the ceiling above the dump and the ventilation duct between the injector area and the FEL stairwell on the west side. The dump itself needs to be shielded, and 30 cm of concrete was proposed as a first cut. Further specification of the dump shielding was not made due to uncertainty in the space requirements. Page 4 of 24

5 1mcnpx version ld=thu Mar 23 15:30:22 MST /19/** 16:26:33 *************************************************************************************** probid = 01/19/** 16:26:33 i=helios2 ***************************************************** * * * Copyright Notice for MCNPX * * * * This program was prepared by the Regents of the * * University of California at Los Alamos National * * Laboratory (the University) under contract number * * W-7405-ENG-36 with the U.S. Department of Energy * * (DOE). The University has certain rights in the * * program pursuant to the contract and the program * * should not be copied or distributed outside your * * organization. All rights in the program are * * reserved by the DOE and the University. Neither * * the U.S. Government nor the University makes any * * warranty, express or implied, or assumes any * * liability or responsibility for the use of this * * software. * * * ***************************************************** 1- HELIOS Estimate - including neutron production 2- c $ concrete wall 3- c #4 $ air spaces (void) 4- c 3 0-6:-12:-10:11:13:14 $ outside interest 5- c $ tungsten target $ copper target 7- c $ Tantalum target 8- c $ ceiling 9- c $ wall detector region 10- c $ ceiling detector region #4 $ 1-meter sphere $outside px px pz pz cz py py py pz pz px px py sz warning. this surface has been replaced by a surface of type so c sdef erg=45 x=0 y=0 z= vec=0 0 1 dir=1 par=3 $electron beam 31- c material 1 - ordinary concrete 32- c m c c c c material 2 - tungsten 37- c m $tungsten 38- c mpn c **************************************************************************** 40- c TALLIES HERE. FORMAT: FAB5, where A is distance (m), B is type (0 = flux, 41- c 1 = dose, 2 = detector response). Distance 0.8 -> A= c **************************************************************************** 43- m $copper 44- mpn c m u 1 $tantalum Page 5 of 24

6 warning. m 3:photonuclear event sees ZA= in place of ZA= f2:n e2 2.50E E E E E E E E E f102:n fc102 Dose (Sieverts) 52- c de E E E E E E E E df E E E E E E E E mode n p e 62- IMP:e IMP:n IMP:p phys:p phys:n phys:e c LCA 69- cut:e 1.e cut:n 1.e cut:p 1.e nps 1e5 73- mplot tally 102 tfc e freq print Page 6 of 24

7 INPUT DECK FINAL CALCULATION HELIOS injector neutron analysis ( ) $dirt between HELIOS and FEL ( #13):( #23) $west wall $east wall $FEL south wall $NORTH HELIOS WALL ( ):( ) $south equip room wall ( ) $ceiling left of plane ( ): ( #15 #35 #39 #40 #41 #42 #43) $ceiling right of plane ( ):( ): ( ):( ) $floor left of plane #5 #3 #6 $floor right of plane ( ) $big east wall :-56:-47:53:-60:(10 #24) $void E $vent hole ( ):( ) $MAIN HELIOS shield wall E-3 ( #37):( ): ( #37 #38) $HELIOS RANGE $EAST HELIOS SHIELD WALL ( ):( )$labyrinth wall for HELIOS E-3 ( ):( ): ( #38): ( #38) $HELIOS LABYRINTH E-3 ( ):( ): ( ):( ) $labyrinth E-3 ( ) $alcove (occupiable) ( ):( #23): ( ) $walls in labyrinth E-3 ( ):( ): ( #36 #35) $equipment room ( ):( ) $dirt areas E $outside dose zone E #34 $FEL vault $concrete above FEL $dirt above FEL $user lab floor E $user lab air space #2 #3 #4 #5 #6 #11 #21 #23 $subfloor helios, etc $concrete floor of FEL $concrete HELIOS injector floor (NOT) $dirt HELIOS injector floor c $copper target E $AIR target c $concrete shield c $iron shield c $water shield c E $air shield (none) c $5% borated poly c ( ) $duct through wall - B-poly e-3 ( ) $duct through wall - air ( ):( ): ( ):( ) $"DUCT LABYRINTH" $main dump shield ( ):( ) $8-inch concrete external wall e $penetration # e $penetration # e $penetration # e $penetration # e $penetration #5 1 pz 0 2 pz pz pz px px px px 0 Page 7 of 24

8 9 px px 580 $FAR SOUTH EXTREME 100 px 730 $south dose zone 11 py 272 $ceiling 110 py 287 $thinner ceiling 111 py 234 $top of east shield wall 1111 py 277 $borated poly zone/air zone 112 py $bottom of duct 113 py $bottom shielding of duct 12 py 310 $floor upstairs 13 py 0 $floor 14 py $subfloor 15 pz pz 2027 $FAR EAST EXTREME 17 pz pz px pz pz px px 198 $VENT HOLE LIMIT 24 px 381 $VENT HOLE LIMIT 25 py 10 $VENT HOLE LIMIT 26 py $VENT HOLE LIMIT 27 c/z pz p p p p py pz pz pz px px px pz $FAR WEST EXTREME 48 pz px pz px pz py 640 $overall ceiling 54 py $FEL ceiling 55 py -213 $FEL floor 56 px $FEL north wall 57 py 183 $top of concrete above FEL 58 py 295 $top of dirt above FEL 60 py -305 $bottom of FEL floor 61 py c 101 c/z c/z pz pz py px -365 c 300 s px px pz pz pz pz px pz pz px pz pz pz 1209 Page 8 of 24

9 213 px s s px c/y c/y c/y c/y c/y c sdef pos=d2 erg=d1 $straight ahead config si E E E E E E E E E E+01 sp e E E E E E E E E E-10 si2 L $discrete values of position sp c material 1 - ordinary concrete m c material 2 - air m $air m $iron m $water m $5% borated polyethylene c material 7 - pseudo-dirt m e e-012 m $copper fc5 Dose at Outer Door (Sieverts) f5:n de5 2.50E E E E E E E E-02 df5 2.32E E E E E E E E-11 fc15 Dose outside labyrinth (south) (Sieverts) f15:n de E E E E E E E E-02 df E E E E E E E E-11 fc25 Dose outside HELIOS inj. labyrinth (south) (Sieverts) f25:n de E E E E E E E E-02 df E E E E E E E E-11 fc35 Dose just inside FEL door (Sieverts) Page 9 of 24

10 f35:n de E E E E E E E E-02 df E E E E E E E E-11 c fc45 Dose behind FEL labyrinth wall c f45:n c c de E E E E E E E E-02 c c c c df E E E E E E E E-11 c c fc55 Dose outside HELIOS labyrinth f55:n de E E E E E E E E-02 df E E E E E E E E-11 fc65 Dose in equipment room, general area f65:n de E E E E E E E E-02 df E E E E E E E E-11 fc75 dose in equipment room, below duct f75:n de E E E E E E E E-02 df E E E E E E E E-11 fc85 dose above injector(sieverts) f85:n de E E E E E E E E-02 df E E E E E E E E-11 fc95 dose at injector itself f95:n de E E E E E E E E-02 df E E E E E E E E-11 fc105 dose on ceiling, thin part f105:n Page 10 of 24

11 de E E E E E E E E-02 df E E E E E E E E-11 fc115 Dose in duct (Sieverts) f115:n de E E E E E E E E-02 df E E E E E E E E-11 fc125 Dose in PEN#1 (Sieverts) f125:n de E E E E E E E E-02 df E E E E E E E E-11 fc135 Dose in PEN#2 (Sieverts) f135:n de E E E E E E E E-02 df E E E E E E E E-11 fc145 Dose in PEN#3 (Sieverts) f145:n de E E E E E E E E-02 df E E E E E E E E-11 fc155 Dose in PEN#4 (Sieverts) f155:n de E E E E E E E E-02 df E E E E E E E E-11 fc165 Dose in PEN#5 (Sieverts) f165:n de E E E E E E E E-02 df E E E E E E E E-11 imp:n c Page 11 of 24

12 c c wwg:n c wwp:n c wwe:n E+02 c wwn1:n E E E E E+01 c E E E E E-01 c.0000e e e e E+09 c E E E E E+00 c.0000e e E E E-03 c.0000e e e E+31 mplot tally 65 tfc e freq prdmp 5e5 j j 4 j nps 1e8 Page 12 of 24

13 tally 5 tally 15 tally 25 nps mean error vov slope fom mean error vov slope fom mean error vov slope fom E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-01 tally 35 tally 55 tally 65 nps mean error vov slope fom mean error vov slope fom mean error vov slope fom E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-02 tally 75 tally 85 tally 95 nps mean error vov slope fom mean error vov slope fom mean error vov slope fom E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E Page 13 of 24

14 tally 105 tally 115 tally 125 nps mean error vov slope fom mean error vov slope fom mean error vov slope fom E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-01 tally 135 tally 145 tally 155 nps mean error vov slope fom mean error vov slope fom mean error vov slope fom E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-01 tally 165 nps mean error vov slope fom E E E E E E E E E E E E E E E E E E E E E E-01 Page 14 of 24

15 Figure 1: Overhead view including surface numbers 10/11/** 16:07:24 HELIOS injector neutron analysis probid = 10/11/** 16:04:40 basis: ( , , ) ( , , ) origin: (.00, 30.00,.00) extent = ( , ) JLAB-TN Page 15 of 24

16 Figure 2: Cross section including ventilation duct 10/11/** 16:09:07 HELIOS injector neutron analysis probid = 10/11/** 16:04:40 basis: ( , , ) ( , , ) origin: ( ,.00,.00) extent = ( , ). JLAB-TN Page 16 of 24

17 Figure 3: Close up view of exit air vent /11/** 16:09:48 HELIOS injector neutron analysis probid = 10/11/** 16:04:40 basis: ( , , ) ( , , ) origin: ( , , ) extent = ( , ) Page 17 of 24

18 Figure 4: Close up view of exit air vent with surface numbers /11/** 16:10:00 HELIOS injector neutron analysis probid = 10/11/** 16:04:40 basis: ( , , ) ( , , ) origin: ( , , ) extent = ( , ) Page 18 of 24

19 Figure 5: Close up view of exit air vent with volume numbers /11/** 16:10:20 HELIOS injector neutron analysis probid = 10/11/** 16:04:40 basis: ( , , ) ( , , ) origin: ( , , ) extent = ( , ) cell labels are cell names Page 19 of 24

20 Figure 6: Cross sectional view with dump shielding and volume numbers 10/11/** 16:11:52 HELIOS injector neutron analysis probid = 10/11/** 16:04:40 basis: ( , , ) ( , , ) origin: ( 91.50,.00,.00) extent = ( , ) cell labels are cell names JLAB-TN Page 20 of 24

21 Figure 7: Cross sectional view with dump shielding 10/11/** 16:12:01 HELIOS injector neutron analysis probid = 10/11/** 16:04:40 basis: ( , , ) ( , , ) origin: ( 91.50,.00,.00) extent = ( , ). JLAB-TN Page 21 of 24

22 Figure 8: Overhead view including dump shielding and ventilation duct 10/11/** 16:13:28 HELIOS injector neutron analysis probid = 10/11/** 16:04:40 basis: ( , , ) ( , , ) origin: (.00, ,.00) extent = ( , ). JLAB-TN Page 22 of 24

23 Figure 9: Overhead view including dump shielding and ventilation duct and surface numbers 10/11/** 16:13:41 HELIOS injector neutron analysis probid = 10/11/** 16:04:40 basis: ( , , ) ( , , ) origin: (.00, ,.00) extent = ( , ) JLAB-TN Page 23 of 24

24 Figure 10: Overhead view including dump shielding and ventilation duct and volume numbers 10/11/** 16:13:54 HELIOS injector neutron analysis probid = 10/11/** 16:04:40 basis: ( , , ) ( , , ) origin: (.00, ,.00) extent = ( , ) cell labels are cell names Page 24 of 24