Borras, C.

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PFC/JA-94-17Photon Activation of the Alcator C-ModLimiter and RF AntennaM. C. Borris, C. L. Fiore, K. W. Wenzel, Y. TakaseJuly 1994Accepted for publication in: Rev. Sci. Instrum.Reproduction, translation, publication, use and disposal, in wholeor part, by or for the United States government is permitted.Plasma Fusion CenterMassachusetts Institute of TechnologyCambridge, MA 02139This work was supported by a fellowship from Diputaci6nGeneral de Arag6n (Spain) No. BCB1491, LLNL Subcontract B218775,and U.S. DOE Grant No. DE-FG02-93ER54235Photon activation of the Alcator C-MOD limiterand RF antennaM. C. Borris, C. L. Fiore, K. W. Wenzel, Y. TakaseMIT Plasma Fusion Center, 175 Albany St., Cambridge MA 02139AbstractThis paper summarizes the results from activation measurements of the limiterand the RF antenna in Alcator C-MOD. The measurements were taken after therun period from May to October of 1993, during which hydrogen and deuteriumdischarges were studied. We conclude that Bremsstrahlung photons generated byrunaway electrons striking these materials induced most of the high threshold acti-vation reactions. This means that electrons with energies higher than 10 MeV musthave been generated in runaway discharges.1INTRODUCTIONThe Alcator C-MOD tokamak has recently completed its second experimentalcampaign from May to October of 1993. The tokamak operated for a period of 6months at plasma currents in the range from 100 kA to 1 MA, peak electron densities2 1020 m-3 and ion temperatures ~ 1 keV.Following these experiments, a vacuum break allowed access to internal compo-nents of the machine. The RF antenna and a limiter to protect it were removed andplaced in a cage of restricted access. The activation measurements started one monthafter the last day of operation.Fig. 1 shows the limiter and the antenna. The limiter tiles at the front surfaceare made of TZM (99% Mo, 0.5% Ti) and are fixed with stainless steel pins andscrews to the tile mounting plate which is Inconel (80% Ni, 18.4% Cr). Some of thecentral tiles on the midplane of the torus were melted as a consequence of excessiveenergy deposition by the runaway electrons. A dose rate of 5 mRem/h at contactwith the melted spot was measured after a cool-down period of one month followingthe last day of operation. Only the melted region showed activation levels exceedingbackground.The antenna and its Faraday shield are made of Inconel. There are TZM protec-tion tiles on the sides, top and bottom of the structure, located 5 mm in front of theFaraday shield and placed typically 5 to 10 mm behind the shadow of the limiter. Nophysical damage was observed on the shielding. The measured dose rate was a factorof 10 smaller than at the limiter.In order to identify the source of radiation from the limiter and the antenna,gamma-ray spectra were obtained with a high-purity germanium (HPGe) detector.2We believe that most of the radiation from these two components has been causedby Bremsstrahlung photons produced by runaway electrons with energies ~ 10 MeV.Understanding the runaway phenomenon is of great interest in the study of plasmastability ' and also because these high energy electrons can seriously damage themachine components. 2,3DESCRIPTION OF RADIATION MONITORINGThe neutron production is measured with fission chambers and BF 3 counters,while bubble dosimeters and TLD's provide neutron dose rates. Several monitors(TLD's, self reading dosimeters, film badges) have been used to report x- and -y-raydoses. A detailed description of the Alcator C-MOD radiation detection system hasbeen previously reported.4Fig. 2 shows a top view of the machine where the two neutron stations (containingfission chambers and BF 3 detectors) are inside moderators 1 and 2. The limiter isplaced in the tokamak through E-port to protect the RF antenna situated in D-port.Radiation dosimeters are distributed over different locations in the experimental cell.During the first four months of operation (mainly with hydrogen plasmas), themeasured photoneutron peak-rates were of the order of 2x10" s-1 and even higherwhen the discharges were dominated by runaway electrons. We should, though, takeinto account that the neutron detectors were calibrated with a Cf25 2 source that has anaverage neutron energy of 2.14 MeV, which is close to the energy of thermal neutronsbut significantly greater than the photoneutrons energy. Thus, the measurements areonly a lower limit on the actual photoneutron flux.3As a result of the deuterium operation during the month of September 1993, thetotal neutron emission was 1013, half of it coming from photoneutrons. When thedischarges were dominated by runaway electrons, neutron and -y detection was muchhigher at E-port, where the limiter is located, than at KT-port. Fig. 3 shows neutronand hard x-ray measurements from a runaway dominated hydrogen discharge.ACTIVATION MEASUREMENTSUpon completion of the experimental campaign in the fall of 1993, the antenna andthe limiter were removed from Alcator C-MOD. Radiation levels above backgroundwere found at the melted region of the limiter and the left central part of the antenna.One month after the end of operation their dose rates at contact were 5 mRem/h and0.5 mRem/h respectively. No measurable activation was found on any other internalcomponent of the tokamak.The limiter is made of TZM tiles (99% Mo, 0.5% Ti, ...) that are fixed withstainless steel pins and screws to an Inconel mounting plate (80% Ni, 18.4% Cr, ... ).The antenna is made of silver plated Inconel and is surrounded by TZM protectiontiles coated with TiC/TiN. The antenna is behind an Inconel Faraday shield.A series of -y-ray measurements were carried out using the HPGe detector toidentify the remaining y peaks from activation. The half-life of the radioisotopes waschecked, when it was not too long, taking consecutive measurements over 10 days.The detector was placed 3.5 inches away from the melted spot at the center ofthe limiter. We also took a spectrum of one of the limiter sides, where there is someInconel, by turning the detector.For the antenna, the detector was placed 3.5 inches away from its Inconel Faradayshield. However, it was still viewing part of the TZM lateral tiles.4Several measurements of different duration (from 300 s to 10 h) were taken, eachwith its corresponding background.RESULTSRepresentative spectra obtained from the limiter and the antenna are shown inFigs. 4 and 5 respectively. Unfortunately, these measurements were taken 1 monthafter the last operational day, so the short-lived radioisotopes had completely decayed.Some possible photonuclear processes to explain the peaks of these figures are listedin Tables I and II. Alternative neutron-induced reactions that would give rise to someof the peaks are given in table III. The cross sections for these neutron reactions are ofthe order of a few mb around 2.5 MeV5'6 , except for the Ni5 8 (n,p)Co58 (120 mb at 2.5MeV) in which case part of the activation could have been caused by thermonuclearD-D neutrons. However, no other activated Inconel was found in the experiment.Doses above background were measured only at the limiter and at the antenna. Theradiation was mainly localized at the melted region of the limiter. This is strongevidence that neutron induced reactions have not contributed significantly to theactivation.Knowing that most of the activation processes were induced by Bremsstrahlungphotons generated by high energy electrons, we tried to obtain some informationabout the energy and intensity of the incident runaway electrons by relating themwith the intensity of each peak:C, = Ai x jE, N(Ey,E..) x o-(E,) x dE,5where C, is the number of counts under the activation peak labeled "i", Ai includesthe decay constant and the detector efficiency, o- and Eth are the cross section andthreshold energy of the photonuclear process and N(E,,Emax) is the Bremsstrahlungspectrum generated by electrons of energy Emax. The fact that the activation couldhave occurred any time during the run period and the lack of data for most of thecross sections were the main reasons why we could not get this information. Wecan say, however, that some of these electrons have energies higher than 10 MeVby looking at the thresholds of the reactions that they can induce. Therefore, thistechnique has allowed us to obtain some information concerning the energy of therunaway electrons in the absence of other diagnostics. In previous studies on AlcatorC.7 , the Mo reactions were also found, but the activation of Ni, Ti and Cr was notobserved. The presence of Co5 6 provides evidence of electron energies as high as 20MeV.We could see Ni and Cr activation at the limiter because there is a thick pieceof Inconel supporting the molybdenum tiles by stainless steel bolts. At the meltedregion, these elements are close enough to the surface for the runaway electrons tocause their activation. In a different spectrum for which we turned the detector tosee the side of the limiter, the intensity of the two Co"7 peaks from the Ni 8 (-yp)reaction was much higher than in cases where the detector was viewing the front ofthe limiter.The activation of the antenna was much smaller, but enough to see the Co andCr radioisotopes as well as the strongest Nb peaks.CONCLUSIONS AND FUTURE WORKPhotons from runaway electrons with energies around 10 MeV and up to 20 MeV6are believed to cause most of the activation of the materials inside the Alcator C-MODvacuum vessel.The induced activity was sharply localized at the melted region of the limiter anddoses above background were found only at the limiter and at the antenna. Fur-thermore, some of the identified radionuclides could not have their origin in reactionsinduced by neutrons of reasonable energy (below 3 MeV). These two facts point to thepresence of photoreactions induced by high energy Bremsstrahlung generated insidethe limiter and the antenna by runaway electrons.For future runaway dominated discharges a calibrated detector should be used todetermine the energy spectrum of the hard x-rays. As soon as any internal compo-nent of the machine is removed, its activation should be studied by taking consecutivespectra to try to infer some information about the intensity and energy of the run-away electrons.ACKNOWLEDGMENTSWe wish to thank Dr. R. D. Petrasso for lending us all the necessary equipmentfor the activation measurements. This work was supported by the fellowship NoBCB1491 from Diputaci6n General de Arag6n, LLNL Subcontract B218775, and U.S. Department of Energy (DOE) Grant No. DE-FG02-93ER-54235.7REFERENCES1 H. Knoepfel and D. A. Spong, Nucl. Fus. 19, No 6, 785 (1979).2 J. D. Strachan, E. B. Meservey, W. Stodiek, R. A. Naumann, F. Girshick, Nucl.Fus. 17, No 1, 140 (1979).3 T. F. R. Group, Physics Letters 60 A, No 3, 219 (1977).C. L. Fiore, T. P. Fuller, R. J. Boivin, R. S. Granetz, PFC/JA-93-30, (1993).5I. Langner, J. J. Schmidt, D. Woll, KFK 750 (EANDC-E-88 U), 1968.6 D. I. Garber and R. R. Kinsey, Neutron Cross Sections Vol. II, BNL 325 (PhysicsNuclear - TID-4500), 1976.' D. S. Pappas, R. Furnstahl, G. P. Kochanski, PFC/RR-81-22 (1981).8FIGURE CAPTIONSTable I. Possible photonuclear reactions responsible for the observed activation atthe limiter.Table II. Possible photonuclear reactions responsible for the observed activationat the antenna.Table III. Neutron induced reactions that could generate some of the radioisotopesfound on the activation of the limiter and the antenna.Fig. 1. Antenna (front and side views) and Limiter (side view) of Alcator C-Modin 1993.Fig. 2. Top view of Alcator C-Mod showing the location of the neutron detectors.Fig. 3. Neutron and hard x-ray measurements from a runaway dominated hy-drogen discharge with a plasma current Ip=0.485 MA. The neutron detectors werecalibrated for D-D neutrons with a Cf25 2 source which has an average energy of 2.14MeV.Fig. 4. -y spectrum from the limiter activation obtained with a HPGe detectorduring 300 s.Fig. 5. -y spectrum from the antenna activation obtained with a HpGe detectorduring 300 s.9Table I.ElementNbamZros/Y88Nb 92 mNb9Sm /Nb95N10 99/Tc 9 9mCr 51Sc 46Co 58Co 56ReactionNlo92(yp)MO92 (',a)Mo 94 (y,np)Mo 96 (y,p)Mo' 00 (yn)Cr52 (-y,n)Ni4(yp)NiG0(-y,np)NiaS(-y,np)Threshold (MeV)7.5517.39.38.312102019.6Half-life62d85d/ 105d10 d90h/35d67h/6h27.8d84d71d77dE,(MeV)0.103, 1.2050.392/0.899, 1.8360.9350.234/0.7660.181, 0.740/0.1390.3200.889,1.1200.8110.847, 1.238, 1.771Table II.ReactionMo 2 (-yp)Nb 93 (y,n)/Mo 9 4 (-y,np)Mo 96 (yp)Cr5 2 (y,n)Ni 8( y,np)Ni 58(y,np)Threshold (MeV)7.58.8/17.39.312208.219.6Half-life62d10d35d27.8d71d27077dE,(MeV)1.2050.9350.7660.3200.8110.121, 0.1360.847, 1.238. 1.771Table III.ElementNb 92mNb 95m/Nb 95Mo99 /Tc 99mCr5 5Co 58ReactionMo12(n,p)Mo 95(np)Mo 98 (ny)Fe14 (n,a)Ni58(n*p)9AElementNb91MNb 9 2mNb 95Cr 5 5Co 58Co 57Co 56I-.xo,,'~ -b NC:Gxp0NS- -I- - -+ -0£0jo~)< .~r--4*) .4-)4),.jCdCL. 5E-Port DosimetryNeutron Moderator 1RF LimiterVacuum vesselKT-Port DosimetryNeutron Moderator 2Fig. 211neutron rate5.0x104.0x10c 3.0x10 1 -0s 2.0x101.0x10 101 n a an -k0.0 0.2 0.4 0.6 0.8time (s)hard x-ray rate0.20-0.15-0.10-00.05-0.00 -0.0 0.2 0.4 0.6 0.8time (s)Fig. 3Counts0tII I I600000I C I C I tell0000, , . , a tealI0-0 _04 0- q ~. I-43c01co(0 (0N' z0) 11~C o cf cozc- 0o -A ,)3Cf)~T100-z-nN)001zc-000N,0 - 1 1 1 1 1 1 144 00 0O-. I I I II . i I I I a 1 1111 - 1 I I I 1 I I I I. . . i , , , . A a A- I I I I I - ---Counts0I I I I lilt!00I I 3 3 31311 - I-I _______________________________________________________________________________________0000-0-00(Az0.. 0C.)U.'z(0wco(nU.Lr-4-t-a.-az0 UM (I 111111.1 I 11111g.. . I-.-.-I I I I Iill-0-nm0zImz-z-0-zrCD0(0.I.,, ... I. . .. :

Photon activation of the Alcator C-MOD limiter and RF antenna

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Photon activation of the Alcator C-MOD limiter and RF antenna

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