Burnup of some refractory metals in a fusion neutron spectrum

ELSEVIER

Journal of Nuclear Materials 212-215 (1994) 640-643

Burnup of some refractory metals in a fusion neutron spectrum C.B.A. Forty a, G.J. Butterworth

b, J.-Ch. Sublet a

aAEA Technology, Fusion, C&ham, Euratom / UKAEA Fusion Association, Abingdon, Oxfordshire OX14 3DB, United Kingdom b Butterworth and Associates, West Il.&y, Newbury, Berkshire RG16 OAL, United Kingdom

Abstract

For some materials, neutron-induced transmutations are predicted to lead to substantial changes in elemental composition over neutron exposure times comparable to expected component service lives. These changes involve the partial burnup of elements initially present and the accumulation of transmutation products, to a point at which materials properties may be adversely affected. Tantalum, tungsten and the alloy W-26Re are studied as examples and allowance is made in inventory predictions for ‘self-shielding’ effects that are found to be important in these materials. The evolution of elemental compositions is examined as a function of exposure time and the implications for materials properties are briefly discussed.

1. Introduction

The transmutation of elements in materials exposed to fusion neutrons may result in the accumulation of transmutation products detrimental to engineering properties. Moreover, the rapid burnup of particular constituents of the host material may also alter its composition and further promote degradation. The most vulnerable components are those nearest the plasma, namely the first wall, divertor and limiter, where both the flux and the proportion of high energy neutrons are greatest. For a number of pure elements, substantial compositional changes are predicted to occur [l] over timescales comparable to the expected service life of the component of which they form a constituent. Certain alloys will also be susceptible to this kind of degradation. In some cases, the transformations may be expected to degrade the performance of the material more than the changes arising from radiation damage. To illustrate the potential magnitude of transmutational changes, three technologically-important materials, namely the elements tantalum and tungsten and the alloy W-26Re (compositions are given in weight percent) are selected for study. The evolution of elemental compositions and generation of specific nuelides in these host materials is firstly predicted as a function of neutron exposure time using the standard

‘infinite dilution’ cross-section data. These results are then compared with predictions from a second set of calculations which take account of ‘self-shielding’ effects due to resonant neutron capture in the epitherma1 region of the neutron spectrum. Self-shielding is known to be important in tungsten and is also found to be appreciable in tantalum and rhenium. The two sets of calculations place upper and lower bounds on the reaction rates and thus indicate the limits on the time-dependent variations in material composition. The implications of these compositional changes with respect to the properties of these materials are briefly discussed.

2. Activation calculations The neutron activation calculations were performed using the inventory code FISPACT3 [2] and the associated neutron cross-section library EAF3 [3] generated in the GAM-II lOO-group structure. A continuous exposure to the first wall flux of the EEF fusion power plant [4], having a wall loading of 5 MWm-‘, was assumed. The multi-group approximation used in the collapsing process is necessarily cOarse in relation to the resonance structure of many materials. As a result, the effect on the neutron flux of the sharp resonance

0022-3115/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDZ 0022-3115(93)E0310-6

641

C.B.A. Forty et al. /Journal of Nuclear Materials212-215 (1994) 640-64.3

peaks that occur in the epithermal energy region with some nuclides is poorly represented. At low concentrations, the presence of nuclides exhibiting strong capture resonances does not greatly perturb the smoothly-varying flux distribution. In the limit of zero concentration, this case may be referred to as one of “infinite dilution”. The cross-section data of EAF3 are implicitly conditional upon this assumption and so are referred to here as the infinite dilution cross-section data. In a material containing an appreciable density of resonant absorber nuclei, e.g. tungsten, the transmitted flux will be strongly attenuated at the resonance energies and the material exposed downstream to the modified flux distribution will experience reduced reaction rates at these energies. The material exhibiting resonance behaviour is said to be ‘self-shielded”. The magnitude of the self-shielding effect is a complex function of geometry, temperature and isotopic composition of the system. For the example of tungsten, four of the five natural isotopes have giant resonances in the epithermal region that will cause large dips in the associated neutron flux. Since the activation code uses constant infinite dilution cross-section data over the entire exposure history, inventories are liable to be overpredicted. An approximate method may be used to correct the reaction rates calculated in the infinite dilution case. The multi-group cross sections are adjusted to provide an effective cross section by applying a simplified firstiteration variation of the Background Cross-Section method [5]. For each element, a “background factor” is applied to the resonance peaks in any of its constituent isotopes to reduce their magnitude to background levels. This adjustment is employed for all resonant isotopes present at the beginning of the neutron exposure but takes no account of the production

of transmutation products which may themselves exhibit resonance. Moreover, it is fully appreciated that this method is unable to correctly take account of the interactive self-shielding of many competing resonant isotopes. A full treatment of the problem would necessitate the use of a 3D pointwise Monte Carlo transport calculation or a detailed case-dependent resonance treatment at the time when the cross-section library is generated and, as such, is beyond the scope of the present work. Nevertheless, the approximate method adopted provides a reasonable indication of the reaction rate lower limit and the resulting transmutation parameters. Self-shielded capture cross sections have been calculated for the natural isotopes of tungsten, rhenium and tantalum. The calculations assume exposure of a homogeneous mixture of the isotopes in the pure element and thus yield the effective cross sections to be used in place of the infinite dilution cross sections.

3. Compositional changes during neutron exposure The compositional changes in the elements tantalum and tungsten and in the alloy W-26Re were calculated for neutron exposure times of 113 days, 1 year and 2.5 years. The results are presented in Table 1 for two sets of transmutation calculations; firstly, employing the infinite dilution cross sections (IDXS) and, secondly, using the self-shielded cross sections (SSXS). The SSXS results are shown in parenthesis. 3.1. Tantalum Tantalum undergoes teration during neutron

substantial compositional alexposure and is transmuted

Table 1

Compositional changes with exposure time, calculated using infinite dilution cross sections and, in parenthesis, self-shielded cross sections Storting material

Element

Tantalum

Hf Ta W all others

Qc 1% 80% 19% K 1%

(< 1%) (95%) (5%) (g: 1%)

< 1% 42% 57% =z 1%

(1%) (81%) (18%) (Q 1%)

Tungsten

W Re OS all others

94% (99%) 5% (1%) < 1% (& 1%)
84% 12% 4% < 1%

(95%) (4%) (1%) (< 1%)

73% (88%) 12% (6%) 15% (6%) -=+=l%(acl%)

W-26Re

W Re OS all others

70% (73%) 24% (26%) 6% (< 1%) *l%(a:l%)

63% 21% 16% Q: 1%

(71%) (27%) (2%) (< 1%)

55% 14% 31% +C 1%

Exposure time 113 d

2.5 y

lY

1% 11% 87% < 1%

(2%) (59%) (39%) (* 1%)

(66%) (28%) (6%) (a: 1%)

C.B.A. Forty et al. /Journal

642

of Nuclear Materials 212-215

(1994) 640-643

mainly to tungsten together with a smaller proportion of hafnium. After 113d, which might roughly represent the expected life of a divertor component, calculations using the IDXS data predict that the composition will have changed to Ta-19W. After 2.5 y, the notional lifetime of a first wall/blanket component, only 11% of the initial Ta is predicted to remain. With the SSXS data, reduction of the one-group average cross sections for the reactions i8’Ta(n, y)“‘Ta and 181Ta(n, y)‘82Ta by factors of 0.6 and 0.16, respectively, results in a lower tantalum burnup. The transmutation rates are still substantial, however, leading to Ta-SW after 113 d and Ta-39W-2Hf after 2.5 y. Several other transmutation elements are also generated in concentrations above lappm and their buildup over neutron exposure times up to 25 y is illustrated in Fig. 1. exposure

3.2. Tungsten For tungsten, calculations based on the IDXS data indicate that the composition will have changed to W-5Re after 113 d and to W-12Re-150s after 2.5 y. The transmutation rate is significantly decreased when, using the SSXS data, the one-group average cross-sections for the (n, y) reactions lBoW dlsl W, 182W +183 W 182W -Psm W, 183~ +184 W and law +187 W are reduced by the respective factors 0.42, 0.14, 0.15, 0.38 and 0.19. The compositions predicted using the SSXS data are W-1Re at 113 d and W-6Re-60s at 2.5 y.

10'

’

’

““,‘I

“““1

7

“--‘I *

time

(d)

Fig. 2. Change in isotopic concentrations exposure of W-26Re alloy.

during neutron

3.3. W-26Re alloy The transmutation characteristics of the binary alloy W-26Re involve a complex interplay between the two constituent elements. The transmutations after 113 d predicted from the IDXS data yield a composition W-24Re-60s. The osmium content comes partly from the tungsten but mostly from the rhenium. It is notable that the rhenium is consumed faster than it is generated from the more abundant tungsten. For a 2.5 y exposure time the composition is W-310s-14Re. The SSXS modifications for the tungsten reactions mentioned above and for the reactions 18’Re(n, y)18’jRe, ‘85Re(n, y)‘86Re and ‘87Re(n, y)188Re, by factors 0.1, 0.19 and 0.1 respectively, result in a slight reduction in the transmutation rates for tungsten and a marked reduction in the rates for rhenium. The SSXS data yield compositions W-26Re-10s at 113d and W28Re-60s at 2.5 y. Fig. 2 shows the isotopic variation together with the production of the osmium isotopes 1860s, 1870s and 1880s. While the isotopes 18’W, 182W, is6W and ‘85Re are consumed, le3W hardly changes with exposure whilst l”W and 187Re actually increase in concentration.

4. Discussion

exposure

time

(d)

Fig. 1. Evolution of elemental composition in tantalum during neutron exposure.

Whilst calculations of the transmutation rates of materials containing resonance capture nuclides should, strictly speaking, be performed on a specific-case basis, the present results are believed to represent approximate upper and lower limits. Even with the lower of

CBA

Forty et al. /Journal of Nuclear Materials212-215 (1994) 640-643

the predicted results, however, the metallurgica con=quences of the compositional changes may be a cause for concern with some of the materials. Tantalum has been proposed as a coating material for divertors [61 and as a minor constituent in some low activation steels 171.Tantalum and tungsten, the main transmutation product, form a continuous range of solid solutions and modest additions of tungsten to tantalum, as in the commercial ahoy Ta-lOW, confer a number of property advantages. The hafnium generated at lower concentrations in tantalum also forms a solid solution and its beneficial effects are exploited in commercial alloys such as Ta-8W-2Hf. Thus exposures up to the anticipated fhrence Iimits would not be expected to cause any drastic property changes in tantalum at the lower transmutation rate predictedusing the SSXS data. However, in circumstances where self-shielding is not fully effective, e.g. in plasma-faking surface layers, the burnup rate in tantalum is very high and the resulting metallurgical changes are likely to be significant. In tungsten the main tr~smutation products are rhenium and osmium. Rhenium can beneficially be added to tungsten up to about 25% but high-rhenium tungsten alloys are metastable and liable to form brittle intermetallic compounds such as the x-phase formed above 20% Re 181. Osmium is a more potent alloying element than rhenium and is limited to concentrations in the region !i-10% before the onset of u-phase formation 181.The calculations indicate that detrimental phases could be formed during exposure to a neutron fluence of about 4 MWym-’ in the absence of self-shielding and around 10 Mwymm2 when selfshielding is included. The consequences of predicted ~m~sitional changes in the alloy W-26Re are difficult to assess but it may be noted that the initial alloy is metastable and that any increase in rhenium content as well as the introduction of osmium as a transmutation product are liable to promote the precipitation of brittle phases. 5, conclusions The usual inventory codes predict very high transmutation rates for certain elements such as tantalum, tungsten and rhenium in a fusion neutron flux. These elements contain nuclei exhibiting large resonance absorption peaks in the epithet-ma1 neutron energy region that would, in reality, lead to “self-shielding” characterised by a reduction in flux at the resonance energies and corresponding reductions in the tr~smutation rates at these energies. The magnitude of the selfshielding effect depends on several case-specific variables and is not easily quantified, though upper and lower bounds to the transmutation rates have been estimated using the normal “‘infinite dilution” cross

643

sections and the modified “self-shielded” cross sections. Although the transmutation rates calcuiated with allowance for self-shielding are generally lower by a significant factor than those predicted by the standard code calculations, they are still appreciable in terms of likely consequent property changes. More detailed studies are evidently needed to quantify more exactly the transmutation effects in these materials and to take account of the time-dependent changes in cross-section data. A particularly high transmutation rate is predicted for tantalum, though the main products, tungsten and hafnium, form solid solutions with tantalum and are not expected to lead to drastic property changes in bulk samples, though they may cause significant alterations in near-surface layers. In the case of tungsten, the concentrations of the major products rhenium and osmium predicted for exposure to anticipated lifetime fhtences may approach the levels at which deleterious phase changes could occur. In high rhenium alloys such as W-26Re, the propensity to intermetallic phase formation will be intensified with the introduction of the transmutation osmium and the maintenance of phase stability may demand the adoption of a lower initial rhenium content.

Acknowledgement This work was jointly funded by the UK Department of Trade and Industry and Euratom.

References 111 C.B.A. Forty, R.A. Forrest, D.J. Compton and C. Rayner, Handbook of Fusion Activation Data, Part 1: Elements Hydrogen to Zirconium, AEA Technology Report AEA FUS 180 (1992) and Part 2: Elements Niobium to Bismuth, AEA Technology Report AEA FUS 232 (1993). [2] R.A. Forrest and J.-Ch. Sublet, FISPACT3 User Manual, AEA Technology Report AEA FUS 227 (1993). {3] J. Kopecky and D. Nierop, Contents of EAF-3, Netherlands Energy Research Foundation, ECN Report ECN-I92-023 (1992). [4] M.G. Sowerby and R.A. Forrest, eds., A Study of the Environmental Impact of Fusion, AEA Hat-well Report AERE R 13708 (1990). [5] R.E. MacFarlane, R.B. Kidman and R.J. LaBauve, The Background Cross Section Method as a General Tool for Reactor Analysis, Proc. ANS Topical Meeting, CONF78040, Gatlinberg, Tennessee, April 1978, ed. E.G. Silver (1978). [6] R.F. Mattas et al., J. Nucl. Mater. 103&104 (1981) 217. [7] K.W. Tupholme, D. Dulieu and G.J. Butterworth, J. Nucl. Mater. 179-181 (1991) 684. [8] T.B. Massalski, ed., Binary Alloy Phase Diagrams (ASM Intemational, 1990%