406 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 38, NO. 3, MARCH 2010

Neutronic Analysis of FASTRosaria Villari, Antonio Cucchiaro, Basilio Esposito, Daniele Marocco,

Fabio Moro, Luigino Petrizzi, Aldo Pizzuto, and Giorgio Brolatti

Abstract—As part of the Fusion Advanced Studies Torus(FAST) project, a neutronic analysis has been performed, aimedto design optimization and radiological safety assessment. Theneutron emissivity source foreseen for various FAST scenarios hasbeen calculated and used as input for Monte Carlo calculations.The shielding analysis and nuclear heating calculations have beencarried out with MCNP5 using a detailed 3-D model of the ma-chine. The energy and spatial distributions of neutron fluxes havebeen used to perform activation analysis by means of the FISPACTcode for safety assessment. The implications of the results on thedesign of the machine and on safety issues are presented anddiscussed.

Index Terms—Activation, Fusion Advanced Studies Torus(FAST), neutronics.

I. INTRODUCTION

FUSION ADVANCED Studies Torus (FAST) is the projectrecently proposed by the Italian Association on fusion for

a European satellite tokamak in support of the ITER programand researches toward DEMO [1] in the frame of the EuropeanAccompanying Program [2]. It will work with deuterium plas-mas in a dimensionless parameter range close to that of ITER.FAST main aims are the following:

1) to assess technical solutions for the first wall/divertordirectly relevant for ITER and DEMO at significant wallpower load (P/R = 22 MW/m) and ELMs (1–2 MJ/m2

at the same ITER plasma density and collisionality);2) to exploit advanced tokamak (AT) regimes with long

pulse duration with respect to the current diffusion time(up to 170 s);

3) to investigate nonlinear dynamics relevant to understandα-particle behavior in burning plasmas by using fast ions(3He) accelerated by heating and current drive systems(Ptot = 40 MW).

Table I summarizes the main parameters of the machine; therange refers to the main plasma scenarios (from H-mode refer-ence to a full noninductive current drive scenario) described indetail elsewhere [1].

Although FAST is a normal conducting machine operatingwithout tritium, the assessment of the radiation fluxes, loads,

Manuscript received June 29, 2009; revised October 5, 2009 andNovember 5, 2009. First published February 2, 2010; current version publishedMarch 10, 2010.

The authors are with Fusion Technology Unit, Associazione EURATOM–ENEA sulla Fusione, Centro Ricerche Frascati, Frascati, Italy (e-mail:rosaria.villari@enea.it; cucchiaro@frascati.enea.it; basilio.esposito@enea.it;daniele.marocco@enea.it; fabio.moro@enea.it; luigino.petrizzi@enea.it; aldo.pizzuto@enea.it; giorgio.brolatti@enea.it).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPS.2009.2038915

TABLE IMAIN PARAMETERS OF FAST

and radiation damage is crucial in the design of the machineas a significant DD neutron yield is expected. Concerningsafety, the neutron-induced radioactivity, although not critical,needs careful consideration for licensing, maintenance, anddecommissioning, as well as for waste management. This paperdescribes the neutronic and activation analyses performed onthe basis of the latest design specification aimed to designoptimization and radiological safety assessment.

II. FAST NEUTRON SOURCE

The neutron source (neutron emissivity [n/(m3∗s)] along apoloidal section of the plasma) foreseen for different FASTscenarios (reference H-mode, extreme H-mode, and AT [1])has been calculated using a code specifically developed inENEA for ITER (Measurement Simulation Software Tool(MSST) [3]). The source is evaluated by MSST using theequilibrium configuration for the scenario, the ion temperatureprofile, the ion density profile, and a parameterization of theneutron reactivity in terms of the ion temperature [4]; constantneutron emissivity on surfaces of constant poloidal magneticflux is assumed. Once the neutron source is given, the totalneutron rate (n/s) can easily be evaluated by integration: Testsperformed on Frascati Tokamak Upgrade (FTU) have showngood agreement (within 10%) between the output of the codeand the measurements of the FTU neutron flux monitor system.

0093-3813/$26.00 © 2010 IEEE

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Fig. 1. Calculated neutron source for the FAST H-mode reference scenario.

TABLE IINEUTRON RATES AND YIELDS FOR DIFFERENT FAST SCENARIOS

A graphical representation of the FAST neutron source forthe reference H-mode scenario is shown in Fig. 1. The neutronrates and yields foreseen for the mentioned FAST scenarios arereported in Table II.

III. MONTE CARLO CALCULATIONS

The neutron emissivity source obtained with MSST has beenadapted to match the source routine used by MCNP to generatesource particles. The energy spectrum is that produced by a DD(2.45-MeV neutrons) plasma with an ion temperature of 10 keV.The shielding analysis and nuclear heating calculations havebeen carried out with MCNP5 [5] with FENDL 2.1 nuclear datalibrary using a detailed 3-D model of the machine.

The geometrical configuration of FAST has been modeledfor MCNP calculations on the basis of the last design specifi-cations. The 3-D MCNP model is shown in Fig. 2. It representsa 20◦ toroidal sector of the machine. Reflecting boundarycondition has been imposed to approximate the full extent ofthe machine. The material compositions and the dimensions ofthe FAST components are reported in Table III, including theacronyms used in the text for their identification. The model in-cludes the water-cooled first wall made of copper and stainlesssteel armored with tungsten, the vacuum vessel made of inconel625, and the full tungsten divertor. Ferromagnetic insert, copperstabilizer located in the outboard zone, mechanical structures,and cryostat are also described. The magnet system of thesector consisting of six poloidal, one toroidal, and six centralcoils is described with different mixtures of copper oxygen-free (CuOFH) and reinforced epoxy. In order to calculate the

Fig. 2. Three-dimensional MCNP model of FAST.

TABLE IIICOMPONENTS AND MATERIALS OF FAST

absorbed dose, the insulation in the magnets is homogenizedexcept in the inner ground toroidal coil insulation. In thepresent analysis, one equatorial, two vertical, and two obliqueports are assumed to be empty. The chemical composition ofstainless steel (SS316LN), Eurofer, and tungsten (Plansee AG)

408 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 38, NO. 3, MARCH 2010

Fig. 3. Poloidal sectors.

Fig. 4. NWL in MW/m2 for 3.4 × 1017 n/s.

has been taken from the ITER Materials Assessment Report [6],inconel 625 from [7].

In order to make a conservative estimate, the presentedresults refer to the maximum expected neutron rate, i.e., 3.4 ×1017 n/s, corresponding to the H-mode extreme scenario.

The energy and spatial distributions of neutron fluxes havebeen used to perform activation analysis by means of theFISPACT code (EASY 2007) [8] for safety assessment. Thecalculation of the shutdown dose rate has been performedusing the direct one-step (D1S) method [9] in various ex-vesselpositions, at different cooling times.

A. NWL

The neutron wall loading (NWL) has been calculated on thepoloidal sectors shown in Fig. 3. Sectors U to N correspond todivertor regions.

The nuclear power generated during H-mode extreme shotsis 0.13 MW. The NWL on the first wall varies between1.14 × 10−3 and 2.7 × 10−3 MW/m2, and the average valueis 2.5 × 10−3 MW/m2 (Fig. 4). Comparing the peak values onthe midplane, the NWL in sector K is about 35% higher than in

TABLE IVNEUTRON AND GAMMA FLUXES

sector C because the plasma is closer to the outboard, as shownin Fig. 3.

The peak value on the divertor region is about 1.4 ×10−3 MW/m2.

B. Neutron and Gamma Fluxes and Spectra

The neutrons emitted by the plasma interact with surroundingstructures and generate gamma radiation. Neutrons and gammafluxes have been calculated in all machine components, andthe values on the equatorial midplane are reported in Table IV.E1 and E2 refer to detectors located inside the empty equatorialport at 120 and 250 cm from the center of the plasma, respec-tively. The maximum neutron flux is obtained in the outboardplasma-facing component.

The fast neutron flux (E > 0.1 MeV) contribution is ∼80%of the total in the components faced to the plasma and drops to∼10% in the cryostat. The ratio between gamma and neutronfluxes varies between 0.15 and 0.3 depending on the position.The neutron spectra (175 energy groups) have been calculatedin the most irradiated zones of all the FAST components

VILLARI et al.: NEUTRONIC ANALYSIS OF FAST 409

Fig. 5. Neutron spectra (a) inboard and (b) outboard for 3.4 × 1017 n/s.

and used as input for activation analysis. Some representativespectra in the inboard and outboard components are shown inFig. 5.

C. Nuclear Heating

The calculated nuclear heating densities due to neutron andgamma at the equatorial midplane are shown in Fig. 6. Themaximum nuclear heating on the plasma-facing component is∼180 mW/cm3 on the outboard zone mainly due to gammaenergy deposition on tungsten (98%). On the first layer of theinboard TF coil, it is ∼32 mW/cm3 and 13 mW/cm3 in its frontinsulator, where the gamma contribution is 32%.

The total nuclear heating of FAST is summarized in Table V.The overall power is 440 kW, with 15% deposited on the FW,∼32% on the TF coil, ∼7% on the divertor, less than 10% onthe VV, and the remainder on the other components.

D. FAST Site

The site of FAST has not yet been decided, and the choicewill depend on both strategic and cost issues. One of thepossible options is Frascati. The existing nuclear building ofFTU is undersized for FAST, and if Frascati will be the selectedsite, the construction of a new hall adjacent to the presentone is foreseen. The present FTU torus hall walls are 220 cmthick. Simulations of an enlarged bunker (17 000 m3) with the

same wall dimensions have been carried out to estimate if theshielding capabilities of the FTU hall walls are adequate forFAST. The effective dose due to neutrons and prompt gammascalculated just outside the shielding walls is attenuated by afactor 4.12 × 106 (see Table VI) with respect to the values ob-tained inside. This preliminary estimate shows that the existingFTU wall could be acceptable for a FAST building. The postir-radiation dose has been disregarded in the present calculationbecause it is negligible with respect to the prompt one.

IV. ACTIVATION CALCULATIONS

The neutron fluxes calculated with MCNP5 have been usedto estimate, by means of the FISPACT code, the radioactivityinduced in the FAST components after ten years of operations.

A. Operational Scenario

The operational scenario considered for activation and radia-tion damage assessment is reported in Table VII.

It is assumed that FAST will operate at the maximum per-formances after three years. A neutron production of about1.6 × 1021 n/year in the last six years of operations correspondsto ∼1200 high-performance shots/year in 150 days of oper-ation. Continuous irradiation per year has been assumed forthe first nine years of operations. In the last year, 143 days ofcontinuous irradiation are followed by pulsed irradiation duringthe last week (8 high-performance shots/day) in order to avoidunderestimation of short-term activation.

B. Activation

Activity and contact dose rate versus time after irradiation forthe main FAST components in the most irradiated positions areshown in Fig. 7. The higher short-term activation is obtainedin the plasma-facing components and the divertor due to tung-sten. At longer cooling times, higher induced radioactivity isobserved in VV mainly because of nickel, cobalt, and tantalumactivation. The expected radioactivity level may require thepreparation of an ad hoc temporary repository to store someof the dismounted activated components. Anyway, long-termactivation does not seem to be troublesome: Within 50 yearsfrom the shutdown, the contact dose of all components is belowthe hands-on limit (10 μSv/h), and the level of activity shouldnot cause waste management problems.

C. Shutdown Dose Rate

The shutdown dose rate has been calculated with the D1Smethod in the positions shown in Fig. 8. The resulting effectivedose rates versus the time after irradiation are shown in Fig. 9.

This analysis shows that, after one week, the dose level out-side the cryostat is below 10 μSv/h, and it is below 100 μSv/hinside the equatorial port. In such conditions, the radiationworkers can access to the torus hall for the normal workingprocedures on the device and on the diagnostic system withoutspecial planning or permission (supervised area).

Since a limited number of radionuclides have been includedin the present D1S calculation, the results are affected by large

410 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 38, NO. 3, MARCH 2010

Fig. 6. Neutron heating density for 3.4 × 1017 n/s: (n) Neutron, (g) gamma, and total.

TABLE VTOTAL NUCLEAR HEATING

TABLE VIEFFECTIVE DOSE INSIDE AND OUTSIDE THE BUNKER

uncertainties, and a more accurate analysis is required to con-firm the present results. Detailed calculation of the shutdowndose rate at shorter cooling times and inside the vessel will be

TABLE VIIOPERATIONAL SCENARIO

performed in the future. Anyway, due to the high activationof the most irradiated components at short cooling times, itis expected that the dose rate inside the vessel will be veryhigh, implying that maintenance and inspection of in-vesselcomponents at short cooling times must be remote.

V. RADIATION DAMAGE

The absorbed dose in the TF insulator has been evalu-ated along the poloidal profile. The maximum value is 2.6 ×10−23 MGy/n on the equatorial midplane. Assuming a designlimit of 10 MGy [10], the operations should be limited to a totalproduction of 3.85 × 1023 n. This figure is a factor 30 higherthan in the expected operations; hence, the maximum expecteddose to the insulator at the end of operation will be well belowthe allowed limit to avoid the replacement.

Displacement damage and problems for reweldability are notforeseen: The maximum calculated values after one year ofoperation (for an annual neutron production of 1.6 × 1021 n)are 2.30 × 10−5 He appm and 4.66 × 10−5 dpa obtained in thefirst wall.

VI. IMPACT OF TRITON BURNUP

The nuclear responses presented in the previous sectionshave been calculated for DD neutrons only. In a pure deuterium

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Fig. 7. (a) Activity and (b) contact dose rate versus the time after irradiation.

plasma, 1.01-MeV tritons are produced by the reaction D(d,p)Twith nearly equal probability as 2.45-MeV neutrons. A fractionof the generated tritons react to produce 14.1-MeV neutronsthrough DT reactions (triton burnup). The burnup fractiondepends on the ion confinement, slowing down time, deuteriumdensity, and DT cross section. The typical order of magnitudein an existing device is about 0.01 (1% 14-MeV/2.5-MeVneutrons) but varies depending on the operation conditions [11].Previous studies have shown that it increases with the decreaseof the plasma size and with the increase of the plasma current[12]. An accurate estimate of FAST triton burnup is not yetavailable. However, due to the compactness and high currentcapabilities of the machine, the DT component could exceedthe 1% value.

MCNP and FISPACT calculations have been performed toevaluate the effect of 14-MeV neutrons on the nuclear responsesof FAST. Assuming a triton burnup fraction of 1% (i.e., DTneutron rate: 3.4 × 1015 n/s), an ∼2% increase on the globalnuclear heating and a 6% increase on the doses to the insulatorare obtained. The effect on the insulator is higher because theneutron contribution to its dose is dominant, while the gammacontribution dominates for almost all the other components (seeTable V).

Concerning the induced radioactivity, the effect on somecomponents can be higher since the neutron energy increases,and further radionuclides may be generated. The impact onthe safety-related issues depends on the material and varieswith the decay time. The DT-to-DD activity ratio is reported

412 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 38, NO. 3, MARCH 2010

Fig. 8. Scoring positions for the shutdown dose rate calculation.

Fig. 9. Effective dose rate versus time after irradiation.

TABLE VIIIIMPACT OF DT ON FAST ACTIVATION

in Table VIII using the DD activity presented in the previoussection and assuming a 1% triton burnup fraction (i.e., DTneutron yield: 1.31 × 1020 n). The effect is within 20%: Thehigher increases are observed on the copper stabilizer due to60Co and on steel components due to 55Fe decay.

Referring to the shutdown dose rate calculation (seeSection IV-C), an increase up to 4% is obtained when includingthe DT contribution. However, since a limited number of nu-clides have been assumed, a higher impact could be expected.

VII. CONCLUSION

Detailed 3-D neutronic and activation analyses have beenperformed on the present design of FAST using the mostvalidated tools and data. All nuclear loads have been quantifiedand will be used for design optimization, safety assessment, andlicensing.

Radiation damage on the main components is negligible.Long-term activation is not troublesome, but high short-/

medium-term activation is expected. Hence, remote handlingis necessary, and dismounted activated components should bestored in a temporary repository.

After one week, the dose level outside the cryostat is ex-pected to be < 10 μSv/h; the access to the bunker at shortercooling times requires the adoption of special precautions.

Concerning the bunker, the shielding capabilities of the ex-isting FTU wall seem adequate for the FAST building.

More accurate analyses for shutdown dose rate assessmentwill be performed, as well as for the triton burnup contribution.

Shielding could be further optimized, if necessary, toprotect diagnostic systems and electronics and to reduceactivation.

REFERENCES

[1] A. Pizzuto, F. Gnesotto, M. Lontano, R. Albanese, G. Ambrosino,M. L. Apicella, A. Bruschi, G. Calabro’, A. Cardinali, R. Cesario,F. Crisanti, V. Cocilovo, A. Coletti, R. Coletti, P. Costa, S. Briguglio,P. Frosi, F. Crescenzi, V. Coccorese, A. Cucchiaro, C. Di Troia,B. Esposito, G. Fogaccia, G. Granucci, G. Maddaluno, R. Maggiora,M. Marinucci, D. Marocco, P. Martin, G. Mazzitelli, F. Mirizzi,S. Nowak, L. Panaccione, G. L. Ravera, F. Orsitto,V. Pericoli Ridolfini, G. Ramogida, C. Rita, A. A. Tuccillo,R. Zagórski, M. Valisa, R. Villari, G. Vlad, and F. Zonca, “TheFusion Advanced Studies Torus (FAST): A proposal for an ITER satellitefacility in support of the development of fusion energy,” presented at the22nd IAEA Fusion Energy Conf., Geneva, Switzerland, 2008, submittedto Nuclear Fusion.

[2] T. Hartkopf, “R&D needs and required facilities for the development offusion as an energy source. Report of the fusion facilities review panel,”European Commission Report, Oct. 2008.

[3] A. Peres, “Fusion cross sections and thermonuclear reaction rates,”J. Appl. Phys., vol. 50, no. 9, pp. 5569–5571, Sep. 1979.

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[6] ITER Document, G 74 MA10 W 0 Materials Assessment Report,Appendix B—Chemical Composition of Materials, Jul. 2004.

[7] C. Sublet, “Activation considerations relevant to the decommissioning offusion reactors,” Ph.D. dissertation, Imperial College London, London,U.K., 1989.

[8] R. Forrest, FISPACT-2007 User Manual EASY 2007 DocumentationSeries UKAEA FUS 534. Oxfordshire, U.K.: EURATOM/UKAEAFusion Assoc., Culham Sci. Centre, 1996, pp. 53–58.

[9] D. Valenza, H. Iida, R. Plenteda, and R. T. Santoro, “Proposal of shutdowndose estimation method by Monte Carlo code,” Fus. Eng. Des., vol. 55,no. 4, pp. 411–418, Sep. 2001.

[10] H. Iida, V. Khripunov, L. Petrizzi, and G. Federici, “Nuclear analysisreport (NAR),” Nuclear Analysis Group, ITER Naka&Garching JointWork Sites, ITER Report, G 73 DDD 2 W 0.2, Jul. 2004.

[11] S. Conroy, O. N. Jarvis, G. Sadler, and G. B. Huxtable, “Time resolvedmeasurements of triton burnup in JET plasmas,” Nucl. Fus., vol. 28,no. 12, pp. 2127–2134, 1988.

[12] M. Hoek, T. Nishitani, M. Carlsson, and T. Carlsson, “Triton burnupmeasurements by neutron activation at JT-60U,” Nucl. Instrum. MethodsPhys. Res. A, Accel. Spectrom. Detect. Assoc. Equip., vol. 368, no. 3,pp. 804–814, Jan. 1996.

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Rosaria (Sara) Villari received the M.Sc. degree in physics from the Univer-sity of Rome “La Sapienza,” Rome, Italy, in 2000 and she post-graduated inhealth physics in 2004.

She is currently a Research Scientist with Fusion Technology Unit, Associ-azione EURATOM–ENEA sulla Fusione, Centro Ricerche Frascati, Frascati,Italy. Her current research interests include neutronic and activation analysesand experiments in nuclear fusion field. She is involved in several internationalprojects in the frame of fusion technology development and analyses.

Antonio Cucchiaro received the M.Sc. degree in nuclear engineering andthe M.Sc. degree in mechanical engineering from University of Rome “LaSapienza,” Rome, Italy, in 1978 and 1982, respectively.

Since 1988, he has been a Researcher with Fusion Technology Unit, As-sociazione EURATOM–ENEA sulla Fusione, Centro Ricerche Frascati,Frascati, Italy. He is an expert on mechanical engineering design, integration,and commissioning. He participates in international groups for collabora-tion, assessment, analysis, and reviewing of several fusion projects. He is theProject Manager and Responsible Officer of several projects for the technolog-ical development of fusion devices and component testing.

Basilio Esposito received the M.Sc. degree in physics from the University ofRome “La Sapienza,” Rome, Italy, in 1987.

He is currently a Senior Scientist with Fusion Technology Unit, AssociazioneEURATOM–ENEA sulla Fusione, Centro Ricerche Frascati, Frascati, Italy. Heis a Member of the FTU plasma operation group, and his current researchinterests include disruptions in tokamak plasmas and development of neutrondiagnostics for ITER.

Daniele Marocco received the M.Sc. degree in physics from the University ofRome “La Sapienza,” Rome, Italy, in 2000.

He is currently a Researcher with Fusion Technology Unit, AssociazioneEURATOM–ENEA sulla Fusione, Centro Ricerche Frascati, Frascati, Italy. Heis a Member of the FTU plasma operation group, and his research activity ismainly devoted to neutron diagnostics design and data analysis.

Fabio Moro received the M.Sc. degree in physics from “Roma Tre” University,Rome, Italy, in 2002.

He is currently a Research Fellow with Fusion Technology Unit, Associ-azione EURATOM–ENEA sulla Fusione, Centro Ricerche Frascati, Frascati,Italy. His current research interests include computational neutronics, nucleardesign of components and diagnostics, and activation analyses in controllerthermonuclear fusion.

Luigino Petrizzi received the M.Sc. degree in nuclear physics and the Ph.D.degree in nuclear engineering from University of Rome “La Sapienza,” Rome,Italy.

Since 1986, he has been with Fusion Technology Unit, AssociazioneEURATOM–ENEA sulla Fusione, Centro Ricerche Frascati, Frascati, Italy,where he is dealing with neutron and photon transport problems for fusiontechnology. He has several publications in related topics. He has done twostages at ITER, the international project during the engineering design activitybetween 1996 and 2001 in support of the neutronic group. He has severalcollaborations with European laboratories in the frame of neutronic activities,in design and experiments.

Aldo Pizzuto received the M.Sc. degree in nuclear engineering from Politec-nico di Torino, Torino, Italy, in 1976.

He is currently the Head of the Research Unit at Associazione EURATOM–ENEA, and he is the Head of the Fusion Technology Unit, ENEA, Frascati.

Giorgio Brolatti is currently an Assistant Engineer with Fusion Technol-ogy Unit, Associazione EURATOM–ENEA sulla Fusione, Centro RicercheFrascati, Frascati, Italy. He has been working with ENEA since 1981 mainly onthe design and construction of experimental systems for magnetically confinedfusion.