Journal of Nuclear Materials 417 (2011) 1280–1283

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Journal of Nuclear Materials

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Neutron-induced damage evolution under Beam Raster Scanner conditions for IFMIF

Fernando Mota ⇑, Christophe J. Ortiz, Angel Ibarra, Rafael VilaLaboratorio Nacional de Fusión por Confinamiento Magnético – CIEMAT, 28040 Madrid, Spain

a r t i c l e i n f o

Article history:Available online 1 January 2011

0022-3115/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.jnucmat.2010.12.271

⇑ Corresponding author. Address: Avda/ComplutenSpain. Tel.: +34 91 346 6578; fax: +34 91 346 6068.

E-mail addresses: fernando.mota@ciemat.es (Fciemat.es (C.J. Ortiz),Angel.ibarra@ciemat.es (A. Ibarra),

a b s t r a c t

The formation and evolution of defects in materials irradiated with a homogeneous neutron source andwith the Beam Raster Scanner (BRS) solution was investigated. The intensity neutron source fluctuationsinherent to the BRS system were determined using the neutron transport McDeLicious code. Defects gen-erated during irradiation were calculated using the binary collision approximation MARLOWE code, usingthe primary knock-on atom (PKA) energy spectrum resulting from neutron interactions with the material.In order to predict the evolution of defects during irradiation, a Rate Theory model based on ab initioparameters was developed. Our model accounts for the migration of mobile defects, the formation ofclusters and their recombination. As an example, we investigated defect evolution in Fe irradiated atroom temperature in both beam configurations. Simulation results clearly indicate that the defectevolution expected in the BRS configuration is nearly the same as the one expected in a homogeneousirradiation system.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

The goal of the International Fusion Material Irradiation Facility(IFMIF) [1] is to provide an accelerator based deuterium–lithium(d�6,7Li) intense source to produce neutrons with a suitable energyspectrum at sufficient intensity and irradiation volume to test can-didate fusion materials up to the full lifetime of their anticipateduse in a future fusion power reactor (DEMO). Therefore, IFMIF willconstitute an essential tool for the international strategy towardsthe achievement of future fusion reactors. The McDeLicious MonteCarlo code [2] was developed to simulate the neutron generationon the basis of the d+6,7Li cross section, which was previously eval-uated in Refs. [3,4].

The IFMIF facility requires two 125 mA deuteron beams of40 MeV energy to combined onto a flowing liquid lithium targetto produce a neutron flux of approximately 1018 n/m2 s [1]. Inthe IFMIF reference design, the required beam distribution at thetarget is a rectangular flat top of dimensions 50 mm vertical by200 mm horizontal and uniform to within ±5% [1]. The proposalin the IFMIF Comprehensive Design Report (CDR) report [1] is touse high order optical component to tailor the beam distributionto meet the target requirement, that is, in those report was pro-posed a deuteron beam profile of dimensions 200 mm � 50 mmto meet the requirements of the neutron source. However, theproduction and monitoring of such a neutron distribution is

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se 22, CP 28040, Ed.2.P0.28k,

. Mota), christophe.ortiz@Rafael.vila@ciemat.es (R. Vila).

technologically not trivial. As an alternative option, using a beamraster scanning device to define a time averaged distribution hasbeen suggested. This would eliminate the need for the high orderoptics and use the Gaussian distribution of the two D+ beams,scanned across the target surface, to produce the required profile.The feasibility of this approach to produce the necessary unifor-mity and the requirements for the scanning system were the sub-ject of the TW5-TTMI-001 EFDA report [5].

It is therefore crucial, before any technological solution is cho-sen, to determine whether both irradiation solutions will producethe same number of defects and if these latter will follow the sameevolution. The aim of this work is to simulate the formation andevolution of defects in materials that will be irradiated in the highflux test module (HFTM) rigs subject to the d+6,7Li neutron source[2] under the two configurations of deuteron beam irradiation sys-tem considered, that is, the homogeneous irradiation with a rect-angular beam profile [6] – reference design – and the beamraster scanning system (BRS). The high flux test module (HFTM)[7] is located down-stream behind the lithium target in the regionshowing the highest neutron radiation level. The HFTM is a steelcontainers housing a number of irradiation rigs that contain encap-sulated irradiated specimens made of the reduced activation fer-ritic-martensitic (RAFM) steel (for example EUROFER ferriticsteel). Since Fe is one of the main elements composing EUROFER,in this work we focussed on the evolution of defects in Fe.

In Section 2 a description of the Beam Raster Scanner system isgiven. In Section 3 we explain the methodology used to calculatethe damage created during irradiation and its subsequent evolu-tion. The results obtained on the evolution of defects in Fe irradi-ated with the homogeneous irradiation solution and with the

F. Mota et al. / Journal of Nuclear Materials 417 (2011) 1280–1283 1281

BRS system are reported in Section 4. Finally, the conclusions of ourwork are given in Section 5.

2. Description of the Beam Raster Scanner

The deuteron beam parameters were selected from those calcu-lated in the EFDA report [5] to ensure optimal irradiation of thelithium target. The raster scanner was deemed to achieve theequivalent deposition on the target than in the reference designand following three criteria: (a) the time averaged uniformitywithin the 200 mm � 50 mm target envelope is within the ±5%specified for the reference design; (b) the fraction of the beam fall-ing outside the 200 mm � 50 mm target envelope must not be lar-ger than the one for the reference design; (c) for the conditions thatboth beams travel the same distance in one complete scan, thecontribution to the summed distribution is shared equally. Theseconditions can be fullfilled by a raster scanning system [5] in whichthe footprint is scanned by two deuteron beams orthogonal withroot-mean-square beam radii r � 3 mm. In order to scan the entirefootprint surface, the vertical beam must perform 60 passes andthe horizontal beam 15 passes (see Fig. 1). The velocity of the beamacross the target surface defines the frequency of oscillation of thescanning system power supplies. The frequency of the horizontalbeams is 12140 Hz and the one of the vertical beam is 3035 Hz.

3. Methodology to calculate defect creation and evolutionduring irradiation

In order to test the capability of the BRS to produce the samedamage than in the homogeneous irradiation solution, we simu-lated the evolution of the defects created by the BRS at two distantirradiated positions of the HFTM, namely, the center and one cor-ner, as indicated by the circles in Fig. 1. To do so, we first calculatedthe intensity neutron source fluctuations inherent to the BRS as afunction of time at these two positions using McDeLicious code.Since the d-Li IFMIF neutron source generates a neutron spectrumextending up to 55 MeV, transport, activation, transmutation anddamage calculations require cross sections exceeding the tradi-tional 20 MeV, which were used up to now in the field of fission.Therefore, the LANL 150 MeV nuclear data libraries (ENDF/B.VI.6)[8] and INPE–FZK [3] were used for one of the most importantnucleides, including neutron reaction cross sections up to

Fig. 1. HFTM-Rigs geometrical model vertical cut, plane XY, for the McDeLiciouscalculation. The circles specify the positions of the samples studied. The squareshows the deuteron beam footprint. The dashed line indicates a part of the scannedpath of each deuteron beam.

150 MeV. For other nucleides the ENDF/B.VII [9] and JEFF-3.1[10] nuclear data libraries were used. More specifically, the li-braries chosen for each nucleus are the following: LANL 150 MeVdata file: 1H, 2H, 9Be, natC, 14N, 16O, 27Al, 28Si, 29Si, 30Si, 31P,natCa, 50Cr, 52Cr, 53Cr, 54Cr, 54Fe, 56Fe, 57Fe, 58Ni, 60Ni, 61Ni,62Ni, 64Ni, 63Cu, 65Cu, 93Nb, 182W, 183W, 184W, 186W; JEFF-3.1: 22-Ti-46, Ti-47, Ti-48, Ti-49, Ti-50; ENDF/B-VII library: 32S,23Na, 39K, 58Fe; INPE/FZK library: 6Li, 7Li. The neutron flux fluc-tuations calculated at the center and at the corner of first row ofthe HFTM rigs are reported in Figs. 2 and 3, respectively. For bothpositions we can see that there are two types of fluctuations con-tributing to the neutron flux. The short-period fluctuation, of about0.2 ms, corresponds to the scan of each deuteron beam and is thusthe same for both positions. The fluctuation with the longer periodcorresponds to a complete scan in a given direction. Periods of 3 msand 4.8 ms were obtained for the center and the corner positions,respectively. For the position at the center a minimum flux of2.24 � 1014 n/cm2 s and a maximum flux of 8.91 � 1015 n/cm2 swere found. In the case of the position at the corner, minimumand maximum fluxes are 7.27 � 1013 n/cm2 s and 6.63 � 1015 n/cm2 s, respectively. These values are to be compared with the neu-tron flux expected in the homogeneous irradiation solution, that is,1.02 � 1015 n/cm2 s.

To calculate the defects resulting from the neutron interac-tions in the material of the first row of the HFTM rigs, the pri-mary knock-on atom (PKA) energy spectrum was taken fromthe work carried out by Simakov et al. [11]. Then, the defectsgenerated in Fe by such PKA spectrum, i.e. self-interstitials (I)and vacancies (V), were simulated by means of the Monte Carlocode MARLOWE [12], which is based on the binary collisionapproximation. In our calculations, possible recombinations be-tween I and V were taken into account. Frenkel pairs with a sep-aration less than a recombination distance of 3.3 latticeparameters, as it was previously determined for Fe in Ref. [13],were considered to recombine spontaneously. Thus, simulationsshow that the PKA spectrum considered here generates in aver-age 88 stable Frenkel pairs in Fe per neutron. In addition, simu-lations predict that in average, 115860 I–V pairs/neutron/cm areexpected to form in Fe irradiated with such PKA spectrum. Thisdata, multiplied by the neutron flux, gives the generation rateof I and V in cm�3 s�1, which can be used as input of our kineticmodel to predict the evolution of defects during irradiation. Todo so, the atomistic processes that govern the evolution ofdefects in Fe, which are presented in Ref. [14], were described

Fig. 2. The neutron flux fluctuations calculated at the center of first row of theHFTM rigs. The dashed line corresponds to the fit used in the damage calculation.

Fig. 3. The neutron flux fluctuations calculated at the corner of first row of theHFTM rigs. The dashed line correspond to the fit used in the damage calculation.

Fig. 4. Temporal evolution of the mean radius and the density of interstitial clustersfor a homogeneous neutron irradiation solution (open squares); a neutronirradiation with BRS at center position (full line) and a neutron irradiation withBRS at corner position (dotted line).

Fig. 5. Temporal evolution of the mean radius and the density of vacancy clustersfor a homogeneous neutron irradiation solution (open squares); a neutronirradiation with BRS at center position (full line) and a neutron irradiation withBRS at corner position (dotted line).

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following a rate equation formalism. In this formalism, it is as-sumed that reaction rates follow the kinetic law of mass actionderived by Bronsted [15]. Then, according to the mass conserva-tion, the temporal evolution of the concentration of mobile de-fects and immobile clusters is governed by a set of couplednon-linear partial differential equations and ordinary differentialequations. In practice, this leads for mobile defects to a set ofspatial diffusion-reaction equations that are obtained by takinginto account their diffusion and generation–recombination termscorresponding to all possible reactions in which they are in-volved, as done in a previous work [14]. On the other hand, whendefects are immobile, such as interstitial or vacancy clusters, theyfollow a markovian chain process and their kinetics can then bedescribed by a master equation, as done in Ref. [14]. The reactionrate constants corresponding to the atomistic processes consid-ered here were calculated following the methodology used inRef. [14] and using the binding and migration energies of defectsobtained by ab initio calculations [16,17]. The rate equations cor-responding to the atomistic processes considered in this workwere solved using the partial differential equation solver PROMIS1.5 [18].

4. Results

In order to compare the damage obtained in the case of the ref-erence design with the one obtained with the BRS solution, wesimulated the defect evolution in Fe under irradiation for a temper-ature of 300 K. In the case of the continuous irradiation (referencedesign), the evolution of defects was simulated for a constant neu-tron flux of 1.02 � 1015 n/cm2 s. In the case of the BRS, the defectevolution was simulated in the center and in the corner (seeFig. 1) of the HFTM taking into account the neutron flux fluctua-tions shown in Figs. 2 and 3. For simplicity, in our kinetic modelthe neutron intensity fluctuations are described using the analyti-cal functions reported in Figs. 2 and 3 (dashed lines). The simula-tion results of the evolution of interstitial and vacancy defectsare reported in Figs. 4 and 5, respectively. Fig. 4 clearly evidencesthat the evolution of the mean radius of interstitial clusters ob-tained at the center and the corner of the HFTM (full and dottedline) is very close to the one expected in the homogeneous irradi-ation solution (open squares). We can remark that the cluster den-sities only differ at short times (t � 1 ms), i.e. for times in the orderof magnitude of the fluctuation period. This is expected since atshort times, the constant flux of the homogeneous system signifi-cantly differs from the fluxes expected at the center and at the

corner of the BRS system. At longer times, i.e. when fluctuationsare averaged and become negligible, the density of interstitial clus-ters predicted at the two positions of the HFTM is in very goodagreement with the one calculated for the homogeneous irradia-tion system. In Fig. 5 we reported the results obtained for va-cancy-type defects. The same conclusions can be drawn, that is,the evolution of vacancy defects obtained with the BRS system isexpected to be very close to the one expected in a homogeneousirradiation system.

5. Conclusions

In this paper we have studied the influence of the neutronsource fluctuation, expected in a Beam Raster Scanner system in IF-MIF, on the formation and evolution of defects in materials thatwill be neutron-irradiated. These results were compared to thoseobtained on a homogeneous irradiation solution. In particular, wecompared the evolution of the mean radius and the density ofinterstitial and vacancy clusters in both systems. Results shownto be nearly identical, meaning that a Beam Raster Scanner systemwill generate the same damage as a homogeneous irradiation sys-tem. Hence, if beam raster technology is established technically,magnetic components to make a rectangular beam profile of20 � 5 cm is not needed in the high energy beam transport (HEBT)section of the accelerator.

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