Chlorine diffusion in uranium dioxide under heavy ion irradiation

NIM B Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 257 (2007) 527–531 www.elsevier.com/locate/nimb

Chlorine diffusion in uranium dioxide under heavy ion irradiation Y. Pipon

a,e,*

, N. Be´rerd

a,e

, N. Moncoffre a, C. Peaucelle a, N. Toulhoat L. Raimbault b, P. Sainsot c, G. Carlot d

a,f

, H. Jaffre´zic a,

a

e

Universite´ Claude Bernard Lyon-1/Institut de Physique Nucle´aire de Lyon (IPNL), 4, rue Enrico Fermi, 69622 Villeurbanne cedex, France b Ecole des Mines de Paris, Centre de Ge´osciences, 35 rue Saint Honore´, F-77305 Fontainebleau cedex, France c Institut National des Sciences Applique´es de Lyon (INSA), UMR 5514, F-69621 Villeurbanne cedex, France d Commissariat a` l’Energie Atomique (CEA), Centre de Cadarache, DEN/DEC/SESC/LLCC, 13108 Saint-Paul lez Durance, France Universite´ Claude Bernard Lyon-1, Institut Universitaire de Technologie (IUT A), 94, boulevard Niels Bohr, 69622 Villeurbanne cedex, France f Commissariat a` l’Energie Atomique (CEA), DEN/Saclay, 91191 Gif sur Yvette cedex, France Available online 19 January 2007

Abstract The radiation enhanced diffusion of chlorine in UO2 during heavy ion irradiation is studied. In order to simulate the behaviour of Cl, present as an impurity in UO2, 37Cl has been implanted into the samples (projected range 200 nm). The samples were then irradiated with 63.5 MeV 127I at two fluxes and two temperatures and the chlorine distribution was analyzed by SIMS. The results show that, during irradiation, the diffusion of the implanted chlorine is enhanced and slightly athermal with respect to pure thermal diffusion. A chlorine gain of 10% accumulating near the surface has been observed at 510 K. This corresponds to the displacement of pristine chlorine from a region of maximum defect concentration. This behaviour and the mean value of the apparent diffusion coefficient found for the implanted chlorine, around 2.5 · 1014 cm2 s1, reflect the high mobility of chlorine in UO2 during irradiation with fission products.  2007 Elsevier B.V. All rights reserved. 36

PACS: 29.27.a; 66.30.h; 81.05.Je; 82.80.Ms Keywords: Diffusion; Irradiation; Chlorine; UO2

1. Introduction Among fission or activation products (such as 129I, Cs, 99Tc, 14C and 36Cl), 129I and 36Cl dominate annual dose rates at the outlet in most reference and degraded scenarios of spent fuel disposal [1]. Due to its solubility and poor retention in the near field, 36Cl is known to significantly contribute to the instant release fraction. Moreover, due to its volatile behaviour, it is likely to be released to the atmosphere in case of a reactor incident. It is therefore

135

*

Corresponding author. Address: Universite´ Claude Bernard Lyon-1/ Institut de Physique Nucle´aire de Lyon (IPNL), 4, rue Enrico Fermi, 69622 Villeurbanne cedex, France. Tel.: +33 4 72431057; fax: +33 4 72448004. E-mail address: [email protected] (Y. Pipon). 0168-583X/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.105

important to assess its behaviour during in reactor processes. Pristine chlorine (35Cl: 75.77% and 37Cl: 24.23%) is present as an impurity in the nuclear fuel (<0.5 ppm). During reactor operation, 36Cl is produced by neutron capture according to the 35Cl(n, c)36Cl reaction which has a rather large cross-section for thermal neutrons (around 33 barns) [2]. During in reactor processes, part of the 36Cl atoms is displaced from its original position due to recoil or to collisions with the fission products. It is of primary importance to elucidate the involved diffusion mechanisms and both irradiation and thermal effects must be assessed. In order to simulate the behaviour of the displaced 36Cl atoms, 37Cl was implanted into the UO2 pellets and its behaviour during irradiation was followed by SIMS analysis. The thermal diffusion of chlorine in depleted UO2 in the 1273–1473 K temperature range has been presented in a

528

Y. Pipon et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 527–531

previous paper [3]. It was shown that Cl thermal diffusion starts at relatively low temperatures (1273 K). The apparent diffusion coefficient at this temperature is around 1.2 · 1016 cm2 s1. A mean activation energy of 4.3 eV has been calculated. This value is rather low, compared to those found from the literature for other radionuclides [4,5]. These results point out a great ability of chlorine to migrate in UO2 at relatively low temperatures. Moreover, the thermal migration process appears to be rather complex, involving mechanisms such as atomic or grain boundary diffusion, directed diffusion along preferential paths as well as trapping into sinks before successive effusion. These results indicate the importance to study the role of the irradiation defects. 2. Experimental procedure Sintered depleted UO2 pellets (0.3% 235U) of 8 mm diameter and 200 lm thickness were used for these experiments. The mean size of the UO2 grains is around 10 lm. The samples were first polished and successively annealed at 1673 K for 4 h in a reducing atmosphere (Ar + H2 5%) in order to remove most of surface defects produced by polishing and keep the samples stoichiometry [6]. The samples were implanted with 37Cl+ at the Nuclear Physics Institute of Lyon (implantation energy of 360 keV) with an ion fluence of 1013 ions cm2. This relatively low fluence was chosen in order to minimize probable effects due to release and/or trapping usually observed at higher ion fluences (above 1014 ions cm2) for volatile fission products (as Xe and I). The 37Cl projected range (Rp) calculated with SRIM [7] is 200 nm and the maximum of the 37Cl concentration is around 5 ppm at. The implanted samples were then irradiated at the TANDEM facility of Orsay University (Paris XI) using a

Fig. 1.

127

I beam of 0.5 MeV/u (corresponding to a total energy of 63.5 MeV). Such a beam is considered to be representative for the fission product mean mass and energy distribution. Fig. 1 displays the 37Cl distribution and the defect profile created by the 127I irradiation. It shows that in the whole Cl implanted zone, the mean dpa concentration value is 0.2 and that the projected range of the vacancy distribution due to the 127I irradiation is 5 lm. A dedicated irradiation cell was used to perform the 127I irradiations. It allows to control precisely pressure and temperature parameters [8]. The irradiation device is shown in Fig. 2(a). The target is made of a stainless steel plate with two slots. The 200 lm thick UO2 pellet is placed in the first slot. A chromium doped alumina target is placed in the second slot to visualize the beam. These targets are positioned on a silver plate. A tantalum resistance allows the target heating by Joule effect up to 973 K. The whole set is fixed on a stainless steel support. To maintain the electric isolation, alumina and MACOR plates are inserted respectively between the silver support and the tantalum resistance and between the tantalum resistance and the stainless steel support. These materials are excellent electric insulators and alumina has a good thermal conductivity. A variable power supply is used to control the current passing through the resistance and to monitor the target temperature. A K thermocouple is fixed on a stainless steel plate. This whole device is placed in the irradiation cell. The residual vacuum in the cell is 104 Pa. H2 can be introduced through a microleak (Fig. 2(b)) at a pressure of 5 · 104 Pa in order to avoid any oxidation of the UO2 sample [9]. A 1.5 lm thick aluminum window is placed between the irradiation cell and the beam line in order to prevent the beamline from pressure degradation and uranium contamination. The initial charge distribution of the ion beam is centred on 9+. It is shifted to 21+ after passing through the Al foil.

Cl implantation profile (E = 360 keV, U = 1013 ions cm2) together with the defect profile created by the

37

127

I irradiation.

Y. Pipon et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 527–531

529

Fig. 2. Irradiation set up allowing the pressure and temperature control. Detail of the irradiation cell (a) and detail of the target holder (b).

The irradiation fluence was fixed to 5 · 1014 ions cm2 and the flux and temperature conditions were varied. Two irradiation fluxes (1.5 · 1010 and 4.5 · 1010 ions cm2 s1) and two temperatures (510 K and 300 K, without external heating) were used. The 37Cl depth profiles on the as-implanted and irradiated samples were analyzed with a CAMECA IMS 6f SIMS facility at the ‘‘Ecole des Mines de Paris’’ of Fontainebleau. The focused primary beam (around 10 nA) was scanned over an area of 150 lm by 150 lm on the sample surface. Secondary ions were collected from a smaller region (62 lm in size) located in the center of the sputtered area to minimize crater-edge effects. The analysis were made with a Cs+ primary ion beam and the negative secondary ions 37Cl (mainly implanted ions), and 254(UO) ions were collected. As the 254(UO) signal does not evolve with the annealing time, it was used as a reference to normalize the 37Cl signal. The depth scale for each sputter profile was determined by measuring the crater depth by optical interferometry at the INSA (Institut National des Sciences Applique´es) of Lyon, using a FOGAL device [10]. Differences in the sputtering rates depending on the different UO2 grains orientations result in a significant roughness at the bottom of the eroded craters. Therefore, an average depth value was determined by recording around 80 scans across the central region of each crater. 3. Results and modeling Fig. 3 displays the evolution of the implanted 37Cl profile with temperature for fluxes of 1.5 · 1010 ions cm2 s1 (Fig. 3(a)) and 4.5 · 1010 ions cm2 s1 (Fig. 3(b)). For both fluxes, a similar behaviour is observed which is enhanced with the temperature: it consists in a diffusion phenomenon coupled with a chlorine accumulation at the near surface that we will call the surface peak. This type of accumulation near the surface has already been observed in [3] for the thermal diffusion of chlorine. At 300 K, the total chlorine amount remains constant whereas at 510 K, a slight increase of this amount is observed (Table 1).

In order to model the evolution of the experimental spectra and to quantify the diffusion as a function of time, we used the Fick’s second law: oC o2 C ¼ D 2 ot ox

ð1Þ

where C = C(x, t) is the Cl concentration (at cm3) in UO2 at a depth x and a time t, D* is the apparent Cl radiation enhanced diffusion coefficient (cm2 s1). In order to numerically solve this equation, the following boundary and initial conditions were set: C(0, t) = K, where K is a constant value given by the experimental profiles. It was considered as equal to the maximum value recorded at the surface peak. Cð1; tÞ ¼ 0 C(x, 0) is the as-implanted distribution profile. The radiation induced diffusion (RID) coefficient for in reactor fission has been shown to be proportional to the fission rate in UO2 and independent of the temperature [11]. We have estimated these diffusion coefficients D* according to the formula (2) for chlorine. D ¼ A  F

ð2Þ

where A is a constant value with units to the fifth power and F is the flux divided by the mean projected range (around 5.5 lm for 127I at 63.5 MeV in UO2). Taking the upper limit of the A value calculated by Hocking et al. [12] (2 · 1029 cm5), we have calculated the values for the 1.5 · 1010 ions cm2 s1 and 4.5 · 1010 ions cm2 s1 flux. Table 2 resumes the experimental D* values deduced from the fits as well as the calculated D* [11]. It is worth to note that: (i) for a given temperature, the experimental D* is independent on the flux; (ii) at 300 K, the calculated values are lower by a factor around ten compared to the experimental D* whereas at 510 K they are lower by a factor around fifty. 4. Discussion We have extrapolated, from a previous study [3], the thermal diffusion coefficient of chlorine in UO2 at 510 K

530

Y. Pipon et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 527–531

Fig. 3. SIMS spectra of the evolution of the implanted chlorine profiles with temperature for a flux of 1.5 · 1010 ions cm2 s1 (a) and 4.5 · 1010 ions cm2 s1(b).

Table 1 Evolution of the chlorine concentration (%) as a function of the irradiation conditions in comparison with the as-implanted sample profile Irradiation conditions

Chlorine amount compared to the as-implanted quantity (%)

1.5 · 1010 cm2 s1 1.5 · 1010 cm2 s1 4.5 · 1010 cm2 s1 4.5 · 1010 cm2 s1

101 112 98 113

300 K 510 K 300 K 510 K

and found a value of 1043 cm2 s1. In fact, D* can be written as D* = Dirr + Dth, Dirr and Dth being respectively the diffusion under irradiation and the pure thermal diffusion contributions [13]. It is clear that Dth is negligible with

regard to Dirr, and that D* is very close to Dirr. It evidences that chlorine diffusion is mainly enhanced by the defects created by the heavy ion irradiation and that it is thermally activated. This is in agreement with the theory of diffusion under irradiation [13–15], which points out a mixed regime at low temperature called the radiation enhanced diffusion (RED) as it was observed by Van Sambeek et al. [16]. We have compared the thermal and radiation enhanced diffusion coefficients of chlorine in UO2 with those of uranium [11]. This is illustrated in Fig. 4 on the Arhenius plot. This figure points out a greater mobility of chlorine compared to that of uranium. For this last element, the diffusion process is athermal.

Table 2 Experimental and calculated diffusion coefficients as a function of the irradiation conditions Apparent diffusion coefficient

Experimental D* (cm2 s1) Calculated D* (cm2 s1)

Irradiation conditions 1.5 · 1010 ions cm2 s1

1.5 · 1010 ions cm2 s1

4.5 · 1010 ions cm2 s1

4.5 · 1010 ions cm2 s1

300 K

510 K

300 K

510 K

(5.0 ± 0.4) · 1015 6 · 1016

(2.1 ± 0.4) · 1014 6 · 1016

(5.5 ± 0.5) · 1015 2 · 1015

(2.8 ± 0.4) · 1014 2 · 1015

The calculated D* have been deduced from the relation D* = A Æ F [11].

Y. Pipon et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 527–531

531

Fig. 4. Arrhenius plot of chlorine diffusion in UO2 for thermal [3] and under irradiations experiments comparing to uranium diffusion in UO2 [11].

Moreover, a chlorine gain of 10% can be observed when the irradiation temperature is increased. On the experimental SIMS profiles, it corresponds to the peak near the surface in Fig. 3. It can be explained by a rapid diffusion of the pristine chlorine from the bulk. As shown in Fig. 1, iodine irradiation leads to a high concentration of defects which is maximum at a depth of 5 lm. We have estimated that the accumulation of pristine Cl, coming from the first 5 lm into a 80 nm thick surface peak is consistent with such an hypothesis. In order to interpret the formation of the surface peak, we can assume the existence of (pristine) chlorine atom/vacancy complexes [17] that could migrate faster than the implanted chlorine towards the surface during irradiation. Finally, the (pristine) chlorine atoms would be trapped at the very near surface. The vacancy distribution has been studied by positron annihilation spectroscopy. The results (not presented here) show that chlorine can be trapped in near surface vacancy defects remaining from the polishing. The diffusion coefficient of the vacancies induced by iodine irradiation and estimated from [18] is equal to 1012 cm2 s1. This high value is coherent with our hypothesis. 5. Conclusion In this paper, we have studied the radiation enhanced diffusion of chlorine in UO2 with the aim of a better understanding of the evolution of this activation product in the nuclear spent fuel. Chlorine diffusion appears to be dependent on temperature and irradiation in the studied temperature range (a factor 4 is observed between 300 and 510 K). However, it is independent on the irradiation flux. Further analysis of the chlorine environment is now in progress to explain such a behaviour and complementary experiments at the Orsay Tandem are necessary.

Acknowledgement The authors are very grateful to the staff of Orsay Tandem for his help during heavy ion irradiation. References [1] L. Johnson, C. Poinssot, C. Ferry, P. Lovera, Estimates of the instant release fraction for UO2 and MOX fuel at T = 0, A Report of the Spent Fuel Stability [SFS] Project of the 5th Euratom Framework Program, Nagra Technical Report NTB 04-08, March, 2005, Wettingen. [2] G.L. Mo´lnar, Zs. Revay, T. Belgya, Nucl. Instr. and Meth. B 213 (2004) 32. [3] Y. Pipon, N. Toulhoat, N. Moncoffre, H. Jaffre´zic, S. Gavarini, P. Martin, L. Raimbault, A.M. Scheidegger, Radiochim. Acta 94 (2006) 705. [4] S.G. Prussin, D.R. Olander, W.K. Lau, L. Hansson, J. Nucl. Mater. 154 (1988) 25. [5] F. Schmitz, R. Lindner, J. Nucl. Mater. 17 (1965) 259. [6] Hj. Matzke, A. Turos, J. Nucl. Mater. (1983) 349. [7] J.F. Ziegler, J.P. Biersaack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. [8] N. Be´rerd, A. Chevarier, N. Moncoffre, H. Jaffre´zic, E. Balanzat, H. Catalette, J. Appl. Phys. 97 (2005) 083528. [9] PRECCI report CEA-R-5958 (E), vol. 1, 2001, p. 168. [10] S. Petitgrand, R. Yahiaoui, K. Danaie, A. Bosseboeuf, J.P. Gilles, Opt. Lasers Eng. 36 (2001) 77. [11] Hj. Matzke, Radiat. Effects 75 (1983) 317. [12] W.H. Hocking, R.A. Verrall, I.J. Muir, J. Nucl. Mater. 294 (2001) 45. [13] S.M. Myers, D.E. Amos, D.K. Brice, J. Appl. Phys. 47 (1976) 1812. [14] G.J. Dienes, A. Damask, J. Appl. Phys. 29 (12) (1958) 1713. [15] A. Barbu, G. Martin, Solid State Phenom. 30–31 (1993) 179. [16] A.I. Van Sambeek, R. S Averback, CP. Flynn, M. H Yang, W. Ja¨ger, J. Appl. Phys. 83 (1998) 7576. [17] D. Mathiot, S. Martin, J. Appl. Phys. 70 (1991) 3071. [18] H. Stehle, H. Assmann, F. Wunderlich, Nucl. Eng. Des. 33 (1975) 230.