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Fusion Engineering and Design
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eutronics analysis of the IVVS/GDC plug in ITER
. Leichtle, U. Fischer ∗, A. Serikovssociation KIT-Euratom, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany
r t i c l e i n f o
rticle history:vailable online xxx
eywords:TERiagnosticseutronicsonte Carlo
a b s t r a c t
A neutronics analysis has been performed to provide the input required for the design strategy for the In-Vessel Viewing System (IVVS) and the Glow Discharge Cleaning (GDC) plug units in the ITER tokamak. Thefocus of the analysis has been on operational loads to the GDC electrode head in the shielding position andon the activation and the decay photon radiation absorbed in the structural components of the entiresystem. To estimate the conditions for maintenance scenarios, the occupational dose rate around theisolated IVVS/GDC head has been calculated assuming the ITER SA2 irradiation scenario. The Rigorous 2Step (R2S) method, developed previously at KIT, has been employed for the calculation of the shutdown
ccupational dose rates dose rates. The GDC head, which is subjected to the highest neutron loads, gets heavily activated anddominates the decay gamma activity of the entire plug. Accordingly, the shutdown dose rate around theIVVS/GDC plug is dominated by the GDC electrode head. It is therefore recommended to separate theGDC head from the system prior to further operations inside the Hot Cell. All components, except the Beprotective layer of the GDC probe, were shown to be classifiable as low level radwaste according to theFrench regulations.
The In-Vessel Viewing System (IVVS) and the Glow Dischargeleaning (GDC) units have been designed to share common portlugs which are mounted in penetrations at the lower level of the
TER Vacuum Vessel (VV). The IVVS is a fundamental tool to performn-vessel inspections between plasma pulses or during a shut-down1]. The Glow Discharge Cleaning (GDC) system utilizes an elec-rode to produce glow discharges in the vacuum vessel (VV) for theleaning and wall conditioning during intermediate maintenanceeriods. ITER employs in total six identical IVVS/GDC plug unitsounted in six lower ports.Neutronics analyses are required to provide the input needed
or decisions on the design strategy of the IVVS/GDC plug unit.his works reports on analyses to assess the operational loadsn the GDC electrode head in the shielding position and on thectivation and the decay photon radiation absorbed in the struc-ural components of the entire system. To estimate the conditionsor maintenance scenarios, the occupational dose rate around thesolated IVVS/GDC head is calculated employing the Rigorous 2tep (R2S) method [2] for the calculation of the shutdown doseates.
Please cite this article in press as: D. Leichtle, et al., Neutronics andoi:10.1016/j.fusengdes.2012.02.068
uhe Institute of Technology. Published by Elsevier B.V. All rights reserved.
2. The IVVS/GDC plug unit
A IVVS/GDC plug unit, as assumed for this work, consists ofthe following components (Fig. 1): the IVVS probe, capable of per-forming the laser-based in-vessel viewing and metrology; the GDCelectrode, capable of producing glow discharge in the vacuumvessel; the IVVS deployment system, capable of moving the IVVSalong the tube from the parking position up to the various work-ing positions; the GDC deployment system, able to move the GDCin three positions, i.e. parked position (rear), shielding positionduring plasma operation, and working position (front) above thedivertor cassette dome; the housing structure, which provides thesupport/guidance to the deployment systems (rails, racks, stops,etc.); and the VV port tube which is also provided with an end flangeand is equipped with feed-troughs for the various services given tothe deployment systems and to IWS and GDC. The IVVS/GDC plugextends over a length of about 11 m from the GDC tip to the rearend at the bioshield level.
3. Computational approach
The methodological approach for the neutronics analysis com-prises three computational steps: neutron transport, activationand decay photon transport calculations. Transport calculations are
alysis of the IVVS/GDC plug in ITER, Fusion Eng. Des. (2012),
performed with the Monte Carlo code MCNP5 [3] using FENDL-2.1[4] cross-section data. The inventory code FISPACT [5] is used for theactivation calculations with EAF-2007 [6] activation cross-sectionsto provide the decay gamma sources in the activated material cells.
2 D. Leichtle et al. / Fusion Engineering and Design xxx (2012) xxx– xxx
Fig. 1. Schematic view of the IVVS/GDC plug unit assumed for this work (blue:housing structure, purple: deployment systems, green: IVVS probe/GDC head) [1].(For interpretation of the references to color in this figure legend, the reader isr
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by the R2S interface software to provide decay photon sources for
eferred to the web version of the article.)
hutdown dose rate calculations are performed utilizing the Rig-rous 2-Step (R2S) approach developed previously at KIT [2]. The2S code system couples neutron transport, activation and decayhoton transport through automated interfaces enabling thus theallying of neutron flux spectra and the sampling of decay pho-on sources on a user defined set of geometry cells of the MCNP
odel. Neutron and decay photon transport calculations are thuserformed with the same detailed geometry model providing even-ually the shut-down dose rate distributions. Nuclear responsesre provided both in the IVVS/GDC cells and on a fine mesh griduperimposed to the IVVS/GDC plug and its surrounding.
The geometrical model used for the neutronics calculations isased on the standard ITER Alite 3D-model, version 4.1. A pre-
iminary MCNP model of the IVVS/GDC plug has been provided byusion for Energy, Barcelona [7]. The model has been corrected atIT and integrated into the Alite 4.1 ITER model (Figs. 2 and 3).he main material of the plug is austenitic steel SS316L(N)-IG usedor the structure. The GDC electrode includes a thin Be protectiveayer and a CuCrZr heat sink. Furthermore, cooling water and somel2O3 isolation layers are incorporated in the model. Note the exact
Please cite this article in press as: D. Leichtle, et al., Neutronics andoi:10.1016/j.fusengdes.2012.02.068
eometrical arrangement of the GDC head in the shielding positionhere the VV opening, assumed for the IVV/DGC system, is plugged
Fig. 2. Vertical (left) and horizontal (right) cuts of the GDC pro
Fig. 3. Vertical cut of the IVVS/GDC along its major axis.
by the GDC head, see Fig. 2, left side. According to the final MCNPmodel, the GDC tip is recessed by 105 cm from the inner vessel shell.
The coupled R2S calculation is based on the neutron flux dis-tributions obtained in the IVVS/GDC plug and its vicinity. Theactivation and decay photon source calculations are performed forthe full set of material cells according to the SA2 irradiation sce-nario prescribed for ITER safety analyses [8]. The SA2 operationscenario is based on the most recent understanding of plant avail-ability, maximum pulse rates etc., but retains a reasonable degree ofconservatism so as to provide the maximum activation in the short,medium and long terms after shutdown. The SA2 scenario refersto an irradiation extending over 20 (calendar) years according tothe scheduled ITER operation. All FISPACT calculations are initiated
alysis of the IVVS/GDC plug in ITER, Fusion Eng. Des. (2012),
the final shutdown decay photon transport calculations. A specialMCNP source routine is required to sample those photons from the
be head. Cuts are adjusted to the GDC probe orientation.
D. Leichtle et al. / Fusion Engineering and Design xxx (2012) xxx– xxx 3
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Table 1Decay photon (SA2 20 years irradiation scenario) and operational nuclear heatingrates (in Gy/s) at the GDC tip.
Material Operationaldose [Gy/s]
Absorbed decay photonradiation dose [Gy/s]
0 s 12 days aftershutdown
CuCrZr heat sink 67.3 0.47 3.9 × 10−3
Be layer 286 1.4 4.1 × 10−3
ing operation. After 12 days of cooling time the decay photon heat
ig. 4. Vertical cut of the neutron flux density distribution [cm−2 s−1] along theVVS/GDC plug calculated with MCNP’s mesh tally.
ctivated material cells. As the IVVS/GDC plug remains in the toka-ak after shutdown, the same MCNP geometry model as for the
eutron transport run can be used.The Monte Carlo transport calculations were mainly performed
n the HPC-FF/JUROPA high-performance supercomputer at FZülich (FZJ) running MCNP in the parallel mode under the MPIommunication technique. MCNP’s mesh-based weight windowenerator technique has been employed to ensure statistically reli-ble results in all regions of interests across the IVVS/GDC port andt’s vicinity up to the cryostat. Typically, one billion source neu-ron histories were tracked in a transport calculation utilizing aew hundred processors on the HPC-FF computer cluster.
. Analyses for operational and shutdown periods
.1. Neutron flux and operational nuclear heating
Total neutron fluxes and neutron energy spectra have been cal-ulated in the VITAMIN-J group structure (175 groups) in the MCNPells of the IVVS/GDC plug and its close surroundings. The resultsere normalized to the nominal fusion power of ITER, 500 MW.
× 109 source neutron histories were tracked in a calculation con-uming about 6200 CPUh on 560 CPU of the HPC-FF Computer at FZJ.he statistical errors achieved for the flux tallies in the geometryells of interest are in general below 5%.
The neutron flux distribution has been also calculated on apatial mesh which extends over a length of about 11 m fromhe plasma chamber up to the rear part of the IVVS/GDC at theevel of the bioshield. The weight window mesh generator wasmployed as variance reduction technique with a point detectorally located in the back of the IVVS/GDC plug for optimization ofhe weight window map. The resulting neutron flux density dis-ribution is shown in Fig. 4 for illustration purposes. The highesteutron flux values are observed at the level of the first wall (ca.014 cm−2 s−1). At the GDC electrode tip, the flux is already attenu-ted to a level of 1013 cm−2 s−1 and decreases rapidly further alonghe IVVS/GDC system due to the appreciable shielding performancef the steel/water mixture in the electrode head which compen-ates effectively the blanket module cut out. Over the length of the
Please cite this article in press as: D. Leichtle, et al., Neutronics andoi:10.1016/j.fusengdes.2012.02.068
ousing the flux gradient is rather small, and the flux level is atten-ated by 3 orders of magnitude from the VV exit to the bioshieldaround 105 cm−2 s−1).
SS316IG core rod 24.1 0.08 1.2 × 10−3
SS316IG shaft 4.5 0.03 6.4 × 10−4
The operational nuclear heating was calculated for all materialscells of the IVVS/GDC system. The photon contribution was shownto be dominant for the heating deposition in structural materialssuch as steel and copper. For the light mass element beryllium, theneutron heating is the main contributor. The operational nuclearheating rates are at very moderate levels. The maximum of about0.6 W/cm3 is observed for the copper cap (CuCrZr-IG alloy) of theGDC tip. The total nuclear heat in the GDC electrode (Be, CuCrZr,and steel parts) amounts to 3.2 kW.
4.2. Activation inventory and waste classification
The activation levels of the IVVS/GDC components have beenassessed to enable their classification according to French radwasteregulations, adopted by ITER [9]. Accordingly, radioactive waste canbe classified depending on specific nuclides’ activity, half-life andradio-toxicity. A so-called LMA limit (maximum level of activity)discriminates low active A-type waste from medium active B-typewaste; only those types are relevant for ITER tokamak components.
To have a conservative estimate regarding the waste treatmentand the dismantling strategy, the full 20 years SA2 operation sce-nario with 12 days cooling time after shutdown was assumed. Allcomponents, except the Be protective layer of the GDC probe, wereshown to be classifiable as A-type waste. The Be cover will be B-type due to tritium whose specific activity is 3.85 × 108 Bq/g (LMAlimit: 2 × 105 Bq/g). Thus only the Be layer of the GDC head has to betreated separately from the other activated parts of the IVVS/GDCplug.
4.3. Shutdown nuclear heating and absorbed decay photonradiation dose
Shutdown decay photon heating calculations have been per-formed both for the MCNP5 cells of the IVVS/GDC unit and ona mesh superimposed to the IVVS/GDC plug and its close sur-rounding. The decay photon source distributions were obtained byFISPACT calculations for each specified material cell and each con-sidered decay time making use of the neutron flux spectra providedby the preceding MCNP calculation for the material cells. Thoseoutput files can be read by a specifically designed MCNP source rou-tine. MCNP5 has been modified with this R2S photon decay sourceroutine to simulate the decay photon transport in the activatedcomponents.
The maximum decay photon heating at shutdown is only about4 mW/cm3 in the Cu heat sink of the GDC probe. This is less than 1%of the respective maximum operational heating. The decay photonheating decreases rapidly to values of the order of 10−8 W/cm3 atthe entrance to the bioshield, see Fig. 5. The heating due to decayphotons is thus insignificant compared to the nuclear heating dur-
alysis of the IVVS/GDC plug in ITER, Fusion Eng. Des. (2012),
is further reduced by two orders of magnitude in the GDC head.Table 1 compares the absorbed photon decay radiation dose ratesto the operational doses in units of Gray/s.
4 D. Leichtle et al. / Fusion Engineering and Design xxx (2012) xxx– xxx
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ig. 5. Vertical cut of decay photon heating distribution at shutdown [W/cm3] after0 years SA2 operation of ITER.
The decay-photon heating was shown to dominate the shut-own nuclear heating (afterheat) since there are no stronglpha-emitters in the materials of the IVVS/GDC and the beta radi-tion is 1–2 orders of magnitude lower than the decay photoneating. Based on the SA2 irradiation scenario, the total accumu-
ated absorbed dose during the 20 years ITER operation wouldmount to 1100 MGy in CuCrZr and 400 MGy in the Steel core rodf the GDC probe. After shutdown, up to 12 days, the absorbedecay photon radiation sums up to 0.01 MGy in the heat sink and.004 MGy in the Steel core rod.
.4. Occupational shutdown dose rate assessment
The IVV/GDC plug need to be maintained during shutdown peri-ds or occasionally be extracted from the torus and transportedy a Transfer Cask System to the ITER Hot Cell for repair and/oreplacement of components. The shutdown dose rate thus needso be assessed to enable the safe handling of the activated IVV/DGClug. To this end, the equivalent dose rate distribution around the
VV/DGC port plug has been calculated 12 days after ITER shutdownssuming the full SA2 irradiation scenario. Only the decay photonources in the materials of the IVVS/GDC system have to be consid-red in this case assuming the plug is extracted from the torus. Thiss accomplished in the decay photon transport calculation without
odifying the ITER model just by removing the activated mate-ials from the ITER components surrounding the IVV/GDC systemhile not touching the materials inside the IVV/GDC. Thus the dose
ate for the isolated IVV/GDC can be obtained without extracting itctually from the ITER model.
MCNP’s mesh tally feature was utilized to provide the shut-own dose rate distribution on a fine spatial grid in and aroundhe IVV/GDC plug. The mesh size amounted to 6 m height and 3 midth covering the full length of the IVVS/GDC plug and extend-
ng also into the plasma chamber. Making use of the decay photonource distribution inside the IVVS/GDC plug and the specific R2Source sampling routine. Decay photon transport simulations were
9
Please cite this article in press as: D. Leichtle, et al., Neutronics andoi:10.1016/j.fusengdes.2012.02.068
erformed for about 4 × 10 source histories. The ICRP-74 flux-to-ose conversion factors [10] have been used to convert the decayhoton fluxes to effective biological radiation dose rates in units ofv/h.
Fig. 6. Vertical cut of shutdown dose rate distribution [Sv/h] around the IVV/GDCplug (12 days after shutdown, 20 years SA2 operation scenario).
Fig. 6 shows the resulting map of the equivalent shutdown doserate around the IVVS/GDC in a vertical cut. Note that the chosencolor scale allows to differentiate the dose rate levels with two dis-tinctive colors per decade. As expected, the dose rate around theIVVS/GDC plug is dominated by the heavily activated GDC electrodehead with peaking values around 5 Sv/h in the center. Already about1.5 m behind the GDC head the dose rate level is two orders of mag-nitude lower (<50 mSv/h) than at the tip of the electrode. Furtherdownstream, the dose rate falls below 5 mSv/h. To facilitate themaintenance of the IVVS/GDC system it is therefore recommendedto separate the GDC head from the other parts of the system thusreducing substantially the radiation loads for further operationswith the system inside the Hot Cell.
5. Conclusion
A dedicated neutronics analysis has been performed to provideinput to the design process of a common IVVS/GDC plug unit. Thefocus has been on operational loads to the GDC electrode headin the so-called shielding position and on the activation and thedecay-photon radiation absorbed in the structural components ofthe entire system. To estimate the conditions for maintenance sce-narios the occupational dose rate around the isolated IVVS/GDChead has been calculated assuming the ITER SA2 irradiation sce-nario. The GDC head, which is subjected to the highest neutronloads, gets heavily activated and dominates the decay gamma activ-ity of the entire plug. Accordingly the shutdown dose rate aroundthe IVVS/GDC plug is dominated by the GDC electrode head. It istherefore recommended to separate the GDC head from the systemprior to any further operations inside the Hot Cell. The shutdownnuclear heating in the materials of the IVV/GDC is mainly due toabsorbed decay photon radiation and is significantly less than theoperational nuclear heating. All components, except the Be protec-tive layer of the GDC probe, were shown to be classifiable as lowlevel radwaste (“A-type” according to the French regulations).
alysis of the IVVS/GDC plug in ITER, Fusion Eng. Des. (2012),
Acknowledgments
The work leading to this publication has been funded partiallyby the European Joint Undertaking for ITER and the Development of
usion Energy (Fusion for Energy) under contract no. F4E-OPE-144.he views and opinions expressed herein reflect only the author’siews. Fusion for Energy is not liable for any use that may be madef the information contained therein.
The analyses made use of an adaptation of the Alite MCNP modelhich was developed as a collaborative effort between the FDS
eam of ASIPP China, ENEA Frascati, JAEA Naka, and the ITER Orga-ization.
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