Fusion activation of ferrous alloys - dependence on flux, irradiation time and fluence

Fusion Engineering and Design 22 (1993) 279-321 North-Holland

279

Fusion activation of ferrous alloys - D e p e n d e n c e on flux, irradiation time and fluence J,-Ch. S u b l e t a n d G.J. B u t t e r w o r t h

AEA Fusion (Euratom / UKAEA Fusion Association), Culham Laboratory, Abingdon, Oxfordshire, OXI4 3DB, United Kingdom Submitted 17 August 1992, accepted 23 November 1992 Handling Editor: P. Komarek

The induced activity, contact dose-rate and decay heat of a number of ferrous alloys have been calculated using the European Activation SYstem EASY for the first wall position in a number of different conceptual reactor designs, namely CCTR, EEF, DEMO, STARFIRE, ITER and NET. In the first instance all neutron fluxes were normalised to 1 MWm -2 to permit direct investigation of the influence of the different spectra on the predicted activation. The activation levels of ferrous alloys predicted for the various reactor designs at a standardised flux are found to vary by orders of magnitude at cooling times relevant to reactor decommissioning. The ways in which the predicted activation properties of a material scale with the duration of the irradiation, the first wall neutron loading and the corresponding fluence are systematically examined. The activation behaviour of candidate structural materials is governed by a relatively limited number of specific radionuclides and can best be understood by following the production pathways for these nuclides and by examining their dependence on the flux and irradiation time. These pathways can be strongly modified by changes in the irradiation conditions and the amount of a particular radionuclide generated cannot be simply scaled if multiple step reactions are involved in its production. The cases of selected ferrous alloys are considered in some detail; the production pathways for dominant radionuclides are identified and the way in which the pathways evolve as irradiation proceeds is described. The results demonstrate that an appreciation of the main generation routes for radionuclides produced by multiple-stage reaction chains is essential to a proper application of the flux-time scaling relationships in safety and environmental studies.

1. Introduction The main objective of calculations to predict the activation characteristics of fusion reactor components is to provide data for safety and environmental assessments. Safety analyses of normal operations, potential accident events and finally decommissioning need detailed data on the amount and types of radioactive products present. Assessments of this kind are needed to predict the safety and environmental implications of the various strategies which could be considered, such as the selection of materials and design options so as to minimise operational risks and environmental impact. The consequences of the radioactive inventory, taking into account the timescale of the relevant event, need to be evaluated as input to decisions to be taken on safety and environmental issues. It has become clear in recent years that these issues have to be addressed with the same degree of planning and attention as other more immediate aspects of reactor deE l s e v i e r S c i e n c e P u b l i s h e r s B.V.

sign. The potential advantages of any new major energy generation system must be weighed, at a very early stage of its development, against its likely safety and environmental impact. Even at this early stage, therefore, conceptual fusion reactor design needs to take account of the potential risks associated with accidental release events during reactor operation and with disposal of the radioactive wastes generated. The development and validation of the computational techniques and nuclear databases needed for prediction of activation characteristics clearly forms an essential part of the overall fusion development programme. It is also important to gain some appreciation of the degree of confidence that can be attached to the theoretical predictions, given the uncertainties in the large amount of nuclear data required and in the design, materials choice and operational conditions applying in future reactors. An examination of the results from several existing studies on fusion reactor activation reveals significant

280

Z-Ch. Sublet, G.J. Butterworth / Fusion activation o f ferrous alloys

disparities between the values of the activation parameters predicted by different workers. These differences are mainly attributable to variations in the reactor design, choice of materials, irradiation conditions or nuclear data used in such conceptual studies. It is instructive, however, to focus on a particular subject area and to investigate the causes of such differences. The aim of this study is to quantify the variations in predicted properties and relate them to their causes, for the particular case of several alternative ferrous alloys in the first wall region of six different fusion reactor designs. It is clear from earlier work that the assumed irradiation conditions, which must be related to the operational scenarios for a power-producing reactor, have a substantial impact on the inventory predictions and this dependence will be considered in some detail. Inventory calculations allow the identification of the isotopes that will be important for operational, decommissioning or waste disposal issues. This study examines the influence of the neutron spectrum, total flux and irradiation time on the activation behaviour of two standard high performance stainless steels 316 and FV448 and two experimental low activation counterparts, OPTSTAB [1] and LA12TaLC [2]. The ways in which the predicted activation properties of each material vary with the duration of irradiation, the total flux and their product, the fluence, were systematically examined. At a given cooling time, the activation behaviour of the candidate material is governed by a limited number of specific radionuclides, namely those which contribute more than 1% to the predicted parameter. This set of isotopes evolves with the cooling time and, for convenience, three distinct periods can be considered: O p e r a t i o n a l issues - timescale: shutdown to 25 years D e c o m m i s s i o n i n g issues - timescale: 25-150 years W a s t e disposal issues - timescale: 150 years to millennia. Once the principal radionuclides have been identified, the activation properties of even the most complex materials can be understood by focusing attention on the few specific isotopes important for the parameter considered, e.g. activation, dose-rate and heat output at the relevant cooling time. The routes for production of these particular isotopes can be delineated and the ways in which the quantities of isotopes produced vary with the irradiation conditions readily understood. An analytical solution of the equations governing mixed decay and reaction channels is only possible if certain assumptions are made, such as a constant parent nuclide concentration. Such assumptions are not

acceptable in a real case involving many nuclides produced through multistep chains. The dominant radionuclides formed via multistep chains of reactions and decays are of importance in the present context, where the final concentrations of the product nuclides are not directly proportional to the product of the flux and the irradiation time. A similar problem occurs in the case of fission reactors when it is required to calculate the spent fuel inventory of each pin or sub-assembly taking into account its radial and axial power distribution history. Thus it is necessary to know the main generation routes for radionuclides produced by multiple-stage reaction chains in order to understand their evolution under the time-varying irradiation conditions that are bound to be encountered in the operation of a fusion reactor.

2. Variations in reactor spectra

Six different conceptual and experimental reactor designs have been considered in the present study; CCTR [3], EEF [4], DEMO [5], STARFIRE [6], ITER [7] and NET [8]. The methods used to predict the first wall neutron fluxes reported for these designs encompassed SN and Monte-Carlo calculations with a very broad range of nuclear data libraries. The neutron spectra are usually normalised to a given power loading on the mid-plane of the reactor, which corresponds to a given set of plasma parameters. The only plasma parameter relevant to the neutronic calculation is the source shape and strength, ie. the D - T reaction rate spatial distribution. The averaged neutron power loading on the first wall is used extensively as a starting point for conceptual design studies and specifies the power that impinges on the first wall, not what is actually absorbed in it. The power carried by the neutron flux incident on the first wall is related to the 14 MeV neutron current and so is directly proportional to the source strength through a factor dependent on design-specific three-dimensional geometrical effects. For present purposes, the neutron fluxes for all the reactor designs have been normalised to a neutron power loading of 1 MW m-z. This normalisation allows the activation properties for the different designs to be uniformly compared on the basis of a fixed neutron loading while retaining the backscattered neutron spectrum specific to each design. Figure 1 shows the first wall neutron flux versus energy profile for each reactor in a GAM-II scheme. The GAM-II energy group structure is composed of 100 groups defined by lethargy intervals of 0.1 for the

281

J.-Ch. Sublet, G.Z Butterworth / Fusion activation of ferrous alloys

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groups 1-49 and 0.25 for the groups 50-99. The upper energy of the first group is 14.918 MeV and, because of the normalisation, this group contains 6.97 × 1013 neutrons for every spectrum, This group flux corresponds to a current of 4.44×1013 n c m - 2 s -1, with the as-

sumption that the f l u x / c u r r e n t ratio is constant for each reactor and neutronic calculation, given 1 M W m - 2 of 14 MeV n e u t r o n power load. The 14 MeV fusion peak may be distinguished from the typical broader peak centred at around 1 MeV. The structure

Table 1 Total integrated neutron flux values Reactor Total flux [n cm-2 s- 1]

EEF 4.27 x 10 TM

CCTR 3.26 X 10TM

DEMO 3.59 X 1014

STARFIRE 4.77 × 1014

ITER 4.82 X 1014

NET 3.78 X 10TM

282

J.-Ch. Sublet, G.Z Butterworth / Fusion activation of ferrous alloys

of each spectrum is characteristic of each design and dependent on the reactor materials. Apart from thc C C T R spectrum, the unmistakable pattern of the spectrum profile of a fusion reactor first wall is easily recognizable. The fusion tail, below 0.1 MeV, is representative of the rate and type of neutron interaction occurring behind the first wall and especially in the blanket. Table 1 demonstrates that the total energy-integrated flux can vary by as much as 20% from the mean value of 4.08 × 1014 n c m -2 s I for the six reactors and this variation has a direct influence on the inventory predictions for a fixed spectrum. The deviation could be as high as 40% if experiments such as J E T and more exotic versions of D E M O and N E T are included in the sampling. Thus disparities in activations predictions could be due to differences in the integrated flux, even when a normalisation process such as the one described above has been applied. In order to compare the activation predictions for the various reactors on a common basis, the total energyintegrated flux values given in table 1 were employed in the calculations, corresponding to a normalised 14 MeV wall loading of 1 M W m 2. It is important to be aware of the limits and approximations which have to be implemented in the calculational procedure to allow the computation to be performed economically. Such limits and approximations could lead to erroneous results when attention is focused, without knowledge of their effect, on particular aspects of the calculation. In order to understand how the predictions separately depend on the collapsed cross-section values and on the integrated flux one has to be aware of the method employed in the inventory code itself to solve the set of first order differential equations. The cross-sections are flux-averaged over the spectrum (a process known as collapsing) to obtain the one-group averaged cross-section, for each isotope and reaction. Secondly, this set of cross-sections is used to solve the several hundred differential equations, for each isotope. This process is, of course, performed iteratively because of the changes in the number of atoms as the calculation proceeds. It would be perfectly legitimate to solve the set of differential equations in each neutron energy group and subsequently balance the number of atoms. This method would, however, necessitate a substantial amount of inner iteration in the solution of the differential equations in order to reach convergence. This second method may, however, be more efficient in cases where the neutron energy spectrum is variable in space and if allowance has to be made in the neutronic calculation for the

time d e p e n d e n c e of the spectrum. This could occur if the materials sustained significant elemental modification between the beginning of life and end of life in the reactor. This kind of behaviour will certainly occur in the blanket region of a fusion reactor and will have to be taken into account in calculations involving tritium breeding materials, for example. With the assumption that the neutron spectrum profile in the first wall will not change during the life of the reactor, at least during the lifetime of the first wall, the E A F - 2 library was collapsed with the spectrum appropriate to each reactor design.

3. Activation behaviour of first wall materials Four different materials have been considered in this study, the standard austenitic stainless steel type 316 and a low activation experimental analogue O P T S T A B [1], the conventional Firth-Vickers martensitic steel FV448 and a low activation variant L A 1 2 T a L C [2]. Table 2 indicates the essential compositions of these materials, omitting for present purposes any potential tramp elements in order to avoid unnecessary complication. Calculations were performed using the E u r o p e a n Activation SYstem EASY, based on the inventory code F I S P A C T [9] linked with the data library E A F - 2 [10]. The E u r o p e a n Activation File, version EAF-2, contains data for neutron-induced reactions on stable and unstable nuclides with half-lives exceeding 0.5 day. The library covers 669 target nuclides with 11,852 reactions kinematically allowed below 20 MeV. In addition to the cross-section data for the target nuclides, the inventory code also requires radioactive decay data for the target and product nuclides formed either by reac-

Table 2 Elemental compositions of the steels in mass percent

Fe Ni Cr Si Mn Mo C W Ta V Nb

316

OPTSTAB

FV448

LA12TaLC

Bal. 12.0 17.0 1.0 2.0 2.5 0.8

Bal. 15.4 0.27 11.6 0.072 2.05 0.5 -

Bal. 0.75 10.5 0.35 0.75 0.65 0.15 0.25 0.3

Bal. 8.9 0.03 1.0 0.09 0.76 0.09 0.39 -

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J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

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286

J.-Ch. Sublet, G.J. Butterworth

/ Fusion activation of ferrous alloys years of continuous irradiation at a n e u t r o n power loading of 1 M W m - 2 , for all six reactor spectra. Specific activity, gamma dose rate and decay heat are plotted in figs. 2 - 4 for each material and reactor as a function of cooling time. From fig. 2 a variation in the specific activity by up to a factor 10 is evident on comparing the predictions for the different reactor designs for a given material. The differences are more noticeable at cooling times below 1 month or greater than 100 years and hence are relevant only to issues important on those time scales. The close agreement of the predicted activities for the

tion or decay. For all the 1377 nuclides up to Po219, the chemical symbol, mass number, isomeric states, isotopic abundance, half-life, n u m b e r of decay modes, average gamma, beta and alpha energies, and branching ratio to each daughter are required, although for the 266 stable isotopes only identifying information is needed. These data are contained in the data library DECAY-1.

3.1. Reference irradiation conditions In the first instance, calculations were performed with the reference irradiation conditions, namely 2.5

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Z-Ch. Sublet, G.Z Butterworth / Fusion activation of ferrous alloys

tion values. The trend for the decay heat, shown in fig. 4, is similar to that for the specific activity, the dominant isotopes being to a large extent identical. Since the variations in the integrated neutron flux values for the different reactors cannot solely account for the wide range in predicted activation parameters, the source must be sought in the variations of the cross-section values due to changes in the neutron energy spectrum. Using EASY, it is possible to compare selected cross-section values and the code allows the user to print the one-group averaged cross-section

EEF and STARFIRE reactor concepts and the tendency for the experimental devices, NET and ITER to exhibit higher levels of activation may be noted. The contact y dose-rates (fig. 3) arising from the irradiated steels show a much wider range of disparity, by up to 2 orders of magnitude, particularly for the low activation steels and cooling times greater than 100 years. However, the largest disparity occurs systematically for the predictions for the CCTR design, which is characterised by a flux profile with few neutrons below 1 keV and is thus associated with low (n,y) reaction cross-see-

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J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys I

and affecting particularly the reactions important in the region below 1 keV. In figs. 5-10 the activation parameters specific activity, gamma dose-rate, decay heat output are predicted as a function of cooling time together with an identification of the principal radionuclides responsible under conditions of an EEF first wall flux of 1 MW m 2 for 2.5 y. It is notable that only a few isotopes dictate the profile of a given activation parameter as a func-

value with its reaction path from parent to daughter. For single step paths this facility makes it possible to assess the influence of the neutron spectrum on the cross-section value and hence on the concentration of the nuclide produced. On re-examining the results shown in figs. 2-4 in the light of the variations in neutron spectra given in fig. 1, it is evident that wide variations can arise in the collapsing of the cross-section values, especially in the tail region of the spectrum

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J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

tion of the cooling time. These principal radionuclidcs can be identified in the graphs at the time that they are predominant, by plotting points corresponding to the value of the activation parameter at shutdown, at the point on the cooling time axis corresponding to their half-life. Any radionuclide will have decayed to 1/128 of its original amount after a cooling time equal to 7 times its half-life. On logarithmic plots this behaviour appears as a rapid fall in the activity value on that timescale. The only exception to such a pattern arises when a predominant nuclide is produced, after irradia-

289

tion, by decay or isomeric transition from a less dominant radionuclide. Such a channel of production influences the timescale at which the product isotope is predominant, either by delaying its appearance by a time corresponding to the half-life of the parent isotope or by adding what can be called a retarded channel. As an example, the dose-rate of LA12TaLC after 10 years cooling includes a major contribution from Re186 (half-life 90.7h) and Ir192 (half-life 74d) produced respectively from Re186m (half-life 2001o9) and Ir192n (half-life 240y)by isomeric transition alone

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J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

during cooling, even though they have been produced by other channels during irradiation. Whilst a relatively large number of isotopes contribute significantly to the activation parameters at shorter post-irradiation times, only a few are important for longer cooling times up to millennia. The isotopes important with respect to specific activity and decay heat are generally identical and only a few additional isotopes need to be introduced when considering the gamma dose-rate. The calculational system EASY in-

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corporates a pathway analysis routine that permits a ready identification of the reaction and decay routes leading to the production of particular nuclides, normally those making the major contributions to the overall activation. Tables 3-6 demonstrate the pathway facility by indicating the "daughter", the "parent" and the production routes between them, with information on the percentage contributions made by the individual pathways to the generation of the predominant isotopes pertinent to each steel. It may be noted that it is

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J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

terms of materials optimisation, it is of interest to compare the activation characteristics of the four steels at different cooling times. Figs. 11-13 demonstrate clearly the benefits gained from elemental substitution in the low activation variants, particularly in terms of the 7 dose-rate after several years of cooling. It is evident, however, that such low activation steels do not exhibit any advantages at short cooling times or during the life of the reactor and that the slim advantage in terms of the specific activity and heat output could easily be lost through a slight shift in the neutron

sufficient to follow the production of fewer than 10 radionuclides for each material in order to adequately describe its activation behaviour for cooling times greater than 1 year; over the whole range of cooling times no more than 20 isotopes per steel need be included. Such detailed study of the activation behaviour down to the individual isotope level, should allow materials scientists or engineers to optimise materials with respect to safety and environmental issues or indeed any issues which can be linked to the isotopic inventory. In

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J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

292

3.2. Dependence on wall loading

energy distribution. The compositional design of such steels has essentially focused on reduction of the contact dose-rates after 100 years cooling rather than improvements at short times, which are in any case limited by the generation of a substantially greater number of dominant isotopes through multi-channel pathways and by the fact that the major contributors, Mn56 and Fe55 arise mainly from Fe56 in the iron base.

IRRADIATION t '

106

Over the first wall surface of a tokamak reactor the unscattered neutron current (i.e the 14 MeV neutron wall loading) exhibits considerable deviation both poioidally and toroidally from its value at the midplane, upon which the normalisation has been based. These deviations are usually taken into account by averaging the neutron load over the surface and refer-

OF FV448 I l

EEF FW 1.0 MW/M2 I '

M n 56

10 4

V 52

~

Mn 54

k

I0 °

\

94

10-2

104

!

din

10~ 10~

Hour

L 10-4

Day

,

Mth

Year

t L i L 10-2 100 Time after irradiation (years)

lo 2

I 104

Fig. 8. Graphs of contact 3' dose-rate versus cooling time for the two martensitic steels irradiated for 2.5y at 1 MWrn -2 with the EEF spectrum.

293

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

hot spots due to a sensitivity to plasma heat distribution and the fuelling process [12]. The result could be enhanced neutron loads rising to a few tens of MW m - 2 on the first wall surfaces in the direct radiation paths of these "plasma islets". In order to investigate the dependence of the predicted activation parameters on the wall loading, further calculations were performed with the EEF neutron spectrum, changing only the wall loading. Values up to 50 MWm -2 were used, largely for illustration purposes since such high loadings are unlikely to be

ring the predicted activation to the averaged neutron load value. Non-trivial three dimensional geometrical effects in a real life design exacerbate such deviations and need to be taken into account if a true representation of the irradiation conditions is required. Three-dimensional neutronics calculations [11] show in fact that the neutron wall loading can vary by up to an order of magnitude in the poioidal direction. For a full-scale fusion reactor, nominally between 1 and 10 MWm -2 could be expected. Moreover, the volumetric D - T reaction rate in the plasma could be subject to localised

lO 6

'

IRRADIATION [ '

OF LA12 I '

EEF I

FW

'

1.0 MW/M2 I '

104

V 52

WT a 1 8 3 ~ 1 8 7

Cr51

lO ~

/

1o o

8 10-2

104

Hour

10-6IMin

10"6

l 10 4

\

Day

Mth

} lO-2

,

Hf178n ~ . _

Year

I

I

lO 0

Time after irradiation (years) Fig. 8 (continued).

I lo 2

I 10 4

294

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

experienced in a reactor. The nominal condition of 1 MW m - 2 has been taken as a base and the calculations repeated for values of 10, 20 and 50 M W m -2. The cooling time steps used in the calculation are identical to those employed earlier, so it is possible to plot the ratio of the activation parameters corresponding to the increased wall loading and the nominal 1 M W m -2 loading, against cooling time. Thus, with this normalised parameter plot, deviations from simple linear scaling with the wall loading become immediately ap-

i

IRRADIATION I ~

I

parent as departures from straight lines parallel to the time axis. Figures 14-16 demonstrate the pattern of variation of the three main activation parameters with respect to their dependency on the wall loading for the four steels. Examination of the figures reveals departures from linear scaling that vary with the material, the activation parameter in question and the cooling time. In some cases, for example the dose rate of OPTSTAB, deviations by orders of magnitude are apparent. As a gen-

OF 316 EEF FW ' f

1.0 MW/M2 ' I

'

102

"--'-'"--"-~-~Mn56

V 52 Al 28

~

~

'

~

n 5 4 854

100 Ni 57 Mo99 Cr 51 P 32

60 Fe55

10-2

Y

10-4

Nb 94 N ;9

10-6 Min

Hour

i

[

Day I

Mth

Year

I

[

1

I

,

I

102 104 i0-2 10o Time after irradiation (years) Fig. 9. Graphs of decay heat versus cooling time for the two austenitic steels irradiated for 2.5y at 1 MWm -2 with the EEF neutron spectrum. 10-6

104

295

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

ergy fusion neutrons of long-lived radionuclides into less active species. It is a feature observed in nearly all the steels and represents a strong advantage for management of fusion reactor waste. As a generic feature, however, an increase in wall loading not only leads to nearly direct scaling of the value of the activation parameters, but also supralinear scaling for particular isotopes that dominate the activation response function between 10 and 100 years cooling time. The explanation for this highly non-linear

eral feature, the radionuclides that predominate between 10 and 100 years seem to be most strongly influenced by the magnitude of the wall loading. A trend may be discerned in fig. 14 when the balance between burnup and build-up rate of long-lived radionuclides turns in favour of the former with increases in wall loading and the activities converge toward a common value at long cooling times, corresponding to the prediction for a wall loading of 10 M W m -2. This behaviour results from the transmutation by high en-

IRRADIATION OF OPTS EEF FW 1.0 M W / M 2 I _ ---'~.------~n

~

f

'

I

'

I

'

I

56

102 Mn54 V 52

W 187Ta183 T a ~ ' k Re188

lOo

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10.2

8 =

10-4 H 3 ~,,~

C14

10-6 Min 10"6

Hour I

10-4

Day I

i

Mth

Year

I

I

,

i

10-2 10o 102 Time after irradiation (years) Fig. 9 (continued).

,

f 104

296

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys as a function of wall loading, as a consequence of the multichannel multistep routes for its production.

behaviour must be sought in the way in which the production pathways of the dominant isotopes vary with the wall loading. It emerges from the analysis presented in section 4 that the scaling behaviour depends on the sometimes delicate balance between production and removal of key radionuclides that form links in the multiple stage reaction chains. As an extreme example the departure from a straight line in the dose-rate curves predicted for 316 steel can be traced to the high gradient of the build-up rate of Co60

3.3. Dependence on irradiation time The notional operational life of about 25 years for a fusion reactor represents the maximum irradiation time to which any component will be subjected and so can be taken as the upper limit. Whilst the "permanent" structures such as coils, shield and other components

I R R A D I A T I O N O F FV448 EEF F W 1.0 M W / M 2 I

'

I

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Mn 54 100

A128

C o ~ Cr51 Mo99 Fe59 Nb 92rn

Fe I

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Nb91 C 14 10-6 Min

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100

J

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102

~

L

104

Time after irradiation (years) Fig. 10. Graphs of decay heat versus cooling time for the two martensitic steels irradiated for 2.5y at 1 MWm -2 with the EEF neutron spectrum.

297

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys far from the plasma, may last for the whole reactor lifetime, most of the components in the first wall and blanket region will have much shorter service lives of a few years. In order to examine the dependence of activation on the irradiation time, the neutron wall loading has been fixed at 1 M W m -2 and the irradiation time varied in steps from a baseline value of 2.5 years up to 25 years, assuming the E E F spectrum. As before, the results are presented in terms of the ratio between the activation parameter calculated for the chosen irradiation time and that calculated for 2.5 years irradiation,

as a function of the cooling time. For the four steels, figs. 17-19 demonstrate the pattern of evolution of the three main activation parameters with respect to their dependence upon the irradiation time. As was observed for the wall loading, the activation exhibits a pronounced non-linearity with irradiation time, to a degree varying with the material and the parameter considered. A similar trend with cooling time is apparent, too, with higher ratios occurring around 100 years. The curves again highlight the difference in scaling behaviour between the radionuclides dominant at short cooling time and those important at

I R R A D I A T I O N O F L A 1 2 EEF F W 1.0 M W / M 2 I

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Year

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10-2 100 Time after irradiation (years) Fig. 10 (continued).

102

104

298

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

times of order 100y. It is remarkable that an increase in irradiation time by a factor 10 hardly changes any of the activation parameters examined, for cooling times up to a year. It is also notable that the activation parameters predicted for both of the conventional steels scale less than linearly with the irradiation time, whereas for the low activation steels the scaling is supralinear. This difference in behaviour has interesting implications for the design of reduced activation compositions. As in the case of the wall loading dependence, the irradiation time dependence of the activation response function for a material is dependent upon the evolu-

tion of the pathways of production of the predominant isotopes. The routes for production of the short-lived isotopes are insensitive to the irradiation times assumed, whereas those leading to the production of the dominant long-lived isotopes follow a scaling relation that is at least linear and frequently of a higher order. The latter behaviour is observed for the routes for production of Ir192 and Co60 in LA12TaLC, which account for the sharp increase in the dose-rate curve for that steel. The relative importance of the various isotopes changes with the cooling time. Since the build-up of each isotope obeys a different law of dependence on

Table 3 Pathway analysis for 316 Radionuclides

Reaction chain

Leads to contribution total for radionuclides

Fe55

Fe54(n,g)Fe55 Fe56(n,2n)Fe55 Ni58(n,a)Fe55

4.5% 91.5% 3.9% of the total amount of Fe55

Mn54

Co60

Ni63

Mn55(n,2n)Mn54 Fe54(n,p)Mn54 Fe54(n,d)Mn53(n,g)Mn54 Ni60(n,p)Co60 Ni60(n,d)Co59(n,g)Co60 Ni60(n,p)Co60m(IT)Co60 Ni60(n,d)Co59(n,g)Co60m(IT)Co60 Ni62(n,g)Ni63 Ni64(n,2n)Ni63 Ni6l(n,g)Ni62(n,g)Ni63

45.7% 50.9% 2.3% 98.9%

of the total amount of Mn54

49.8% 0.8% 46.1% 0.9% 97.6%

of the total amount of Co60

78.7% 10.9% 10.1% of the total amount of Ni63

Nb91

Mo92(n,d)Nb91 Mo92(n,d)Nb91m(IT)Nb91 Mo92(n,2n)Mo91(b + )Nb91 Mo92(n,2n)Mo91m(b+ )Nb91

79.2% 3.2% 16.1% 0.6% of the total amount of Nb91

Nb94

Mo94(n,p)Nb94 Mo95(n,d)Nb94 Mo94(n,p)Nb94m(IT)Nb94 Mo95(n,d)Nb94m(IT)Nb94

38.5% 11.9% 4.6% 44.0% of the total amount of Nb94

Mo93

Mo92(n,g)Mo93 Mo94(n,2n)Mo93

31.8% 67.1% of the total amount of Mo93

J.-Ch. Sublet, G.£ Butterworth / Fusion activation of ferrous alloys irradiation time, each of the activation parameters for a steel exhibits a different d e p e n d e n c e on irradiation time. The detailed structure seen in plots such as Figs. 17-19 results from the combination of these effects.

3.4. Fluence dependence The neutron fluence is defined as the product of the total energy-integrated neutron flux and the irradiation time. Problems arise when this product is employed in a too simplistic m a n n e r to represent uniquely the irradiation conditions experienced by a component. An integrated fluence of, for example, 10 M W y m -2 could be modelled in different ways, such as 1 M W m -a over 10 years, 10 M W m -2 over 1 year or 4 M W m -2

over 2.5 years. Whilst activation values are, for convenience, often referred to a particular fluence, it would be expected from the foregoing results that the use of fiuence alone as a parameter to specify irradiation conditions uniquely could lead to difficulties when the flux and irradiation time are both variable. If, for instance, a fusion reactor or experiment operates in a pulsed mode, it may not be possible to model, in complete detail, the irradiation conditions. In an experiment such as J E T the predicted full D - T Phase total neutron fluence, in reality given by a few thousand pulses lasting around 10 seconds and scattered over 2 years, has been time-averaged over those two years to allow a realistic calculation to be made [13]. It has been proven that such modelling does not affect

Table 4 Pathway analysis for OPTSTAB Radionuclides

Reaction chain

Leads to contribution total for radionuclides

Fe55

Fe54(n,g)Fe55 Fe56(n,2n)Fe55

4.6% 95.1% 9-WYgo

of the total amount of Fe55

81.7% 17.1% 9-g-:ggo

of the total amount of Mn54

63.7% 33.4% 0.7% . 1.4,% 9.9~2-go

of the total amount of Ta182

Mn54

Ta182

Re 186

Mn55(n,2n)Mn54 Fe54(n,p)Mn54 Ta181(n,g)Ta182 Ta181(n,g)Ta182m(IT)Ta182 W182(n,2n)W181(b + )Ta181(n,g)Ta182 Ta181(n,g)Ta182n(IT)Ta182m(IT)Ta182 W 184(n,g)W 185(b - )Re 185(n,g)Re 186 W186(n,2n)W185(b - )Re185(n,g)Re186 W186(n,g)W187(b-)Re187(n,2n)Re186 W183(n,g)W184(n,g)W185(b - )Re185(n,g)Re186 W184(n,g)W185m(IT)W185(b - )Re185(n,g)Re186 W186(n,2n)W185m(IT)W185(b - )Re185(n,g)Re186

45.5 % 20.2% 18.5% 2.5% 3.3% 7.8% of the total amount of Re186

Co60

Fe58(n,g)Fe59(b - )Co59(n,g)Co60

44.8% of the total amount of Co60

C14 Hf178n

N14(n,p)C14 W182(n,na)Hf178n W182(n,a)Hf179(n,2n)Hf178n Ta181(n,na)Lu177(b - )Hf177(n,g)Hf178n W182(n,a)Hf179m(IT)Hf179(n,2n)Hf178n Ta181(n,3n)Ta179(b + )Hf179(n,2n)Hf178n Ta181(n,g)Ta182(b - )W182(n,na)Hf178n

299

99.9% 9-gg-go

of the total amount of C14

22.1% 6.3% 1.0% 0.6% 11.6% 1.6% of the total amount of Hf178n

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

300

the predicted long-term activation [14], though it will have an influence on the short-term activation response. Thus, if pulsed modes are encountered in fusion reactor operation it will be necessary to revise the calculational scheme so as to be able to predict more accurately the isotopic inventory relevant during the lifetime of the reactor and shortly thereafter, a feature particularly important with regard to accidental or thermal related issues.

In order to explore the sensitivity of the predicted activation to variations in the individual factors of the wall load and irradiation time at a fixed value of fluence, calculations were performed for 316 steel under conditions of an E E F first wall flux of 1 M W m : for 2.5y and a wall load of 10 M W m -2 for 0.25y. The influence of these factors is shown by fig. 20, from which it is evident that the long-term activation, beyond about 10y cooling, is insensitive to the individual

Table 5 Pathway analysis for FV448 Radionuclides

Reaction chain

Leads to contribution total for radionuclides

Fe55

Fe54(n,g)Fe55 Fe56(n,2n)Fe55

4.6% 95.1% 0077~

Mn54

Mn55(n,2n)Mn54 Fe54(n,p)Mn54 Fe54(n,d)Mn53(.n,g)Mn54 Fe56(n,d)Mn55(n,2n)Mn54

of the total amount of Fe55

19.0% 76.1% 3.4% 0.6% of the total amount of Mn54

Co60

Nb93m

Ni60(n,p)Co60 Ni60(n,d)Co59(n,g)Co60 Ni60(n,p)Co60m(IT)Co60 Fe58(n,g)Fe59(b - )Co59(n,g)Co60 Ni60(n,d)Co59(n,g)Co60m(IT)Co60 Nb93(n,n')Nb93m Mo94(n,d)Nb93m Nb93(n,g)Nb94(n,2n)Nb93m

46.1% 0.7% 41.9% 3.7% 0.9% 93.4%

of the total amount of Co60

96.9% 0.6% 0.6% of the total amount of Nb93m

Nb94

Ni63

Nb93(n,g)Nb94 Nb93(n,g)Nb94m(IT)Nb94 Mo95(n,d)Nb94m(IT)Nb94 Ni62(n,g)Ni63 Ni64(n,2n)Ni63 Ni61(n,g)Ni62(n,g)Ni63

30.5% 67.8% 0.6% 0g-........9~o

of the total amount of Nb94

78.7% 10.9% 10.0% of the total amount of Ni63

Nb91

C14 Mo93

Mo92(n,d)Nb91 Mo92(n,d)Nb91m(IT)Nb91 Mo92(n,2n)Mo91(b + )Nb91 N14(n,p)C14 Mo92(n,g)Mo93 Mo94(n,2n)Mo93

78.8% 3.3% 15.9% 96.0~

of the total amount of Nb9l

99.9% 00-........9~o

of the total amount of C14

31.8% 67.1% of the total amount of Mo93

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys Table 6 Pathway analysis for LA12TaLC Radionuclides

Reaction chain

Leads to contribution total for radionuclides

Fe55

Fe54(n,g)Fe55 Fe56(n,2n)Fe55

4.6% 95.3% 99.-ffff~o

Mn54

Mn55(n,2n)Mn54 Fe54(n,p)Mn54 Fe54(n,d)Mn53(n,g)Mn54 Fe56(n,d)Mn55(n,2n)Mn54

of the total amount of Fe55

23.5% 71.9% 3.2% 0.6% of the total amount of Mn54

Ta182

Re186

Ta181(n,g)Ta182 Ta181(n,g)Ta182m(IT)Ta182 W182(n,2n)W181(b+)Ta181(n,g)Ta182 Ta181(n,g)Ta182n(IT)Ta182m(IT)Ta182

W184(n,g)W185(b - )Re185(n,g)Re186 W 186(n ,2n)W 185(b - )Re 185(n,g)Re 186 W186(n,g)W187(b-)Re187(n,2n)Re186 W183(n,g)W184(n,g)W185(b-)Re185(n,g)Re186 W184(n,g)W185m(IT)W185(b - )Re185(n,g)Re186 W186(n,2n)W185m(IT)W185(b - )Re185(n,g)Re186

62.7% 32.9% 1.5% 1.4% 9-g-.-.-.-.-.-.-.-~o

of the total amount of Ta182

45.5% 20.2% 18.5% 2.5% 3.3% 7.8% of the total amount of Re186

Co60

Fe58(n,g)Fe59(b - )Co59(n,g)Co60

44.8% of the total amount of Co60

C14

Hf178n

Ir192

N14(n,p)C14

W182(n,na)Hf178n W182(n,a)Hf179(n,2n)Hf178n W182(n,2n)W181(n,a)Hf178n Ta181(n,na)Lu177(b-)Hf177(n,g)Hf178n W182(n,a)Hf179m(IT)Hf179(n,2n)Hf178n Ta181(n,3n)Ta179(b + )Hf179(n,2n)Hf178n Ta181(n,g)Ta182(b - )W182(n,na)Hf178n

W186(n,g)W187(b - )Re187(n,g)Re188(b - ) Osl88(n,g)Osl89(n,g)Osl90(n,g)Osl91(b - ) Ir191(n,g)Ir192 W186(n,g)W187(b - )Re 187(n,g)Re 188m(IT) Re188(b - )Os188(n,g)Os189(n,g)Os 190(n,g) Os191(b - )Ir191(n,g)Ir192 W186(n,g)W187(b- )Re187(n,g)Re188(b - ) Osl88(n,g)Osl89m(IT)Osl89(n,g)Osl90(n,g) Osl91(b - )Ir191(n,g)Ir192 W186(n,g)W187(b - )Re187(n,g)Re188(b - ) Os 188(n,g)Os 189(n,g)Os190(n,g)Os 19 lm(IT) Osl91(b-)Irl91(n,g)Irl92 W186(n,g)W187(b - )Re187(n,g)Re188(b - ) Os 188(n,g)Os 189(n,g)Os190(n,g)Os 191(b - ) Ir191(n,g)Ir192m(IT)Ir192

99.9% 9q-.......9~o

of the total amount of C14

30.2% 8.6% 0.7% 0.7% 0.8% 7.7% 1.1% 4~-.....K~o

of the total amount of Hf178n

4.8%

2.2%

7.2%

8.7%

4.4%

301

302

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

Table 6 (continued) Radionuclides

Reaction chain

Leads to contribution total for radionuclides

W186(n,g)W187(b - )Re 187(n,g)Re188m(IT) Re188(b - )Os188(n,g)Os189m(IT)Os 189(n,g) Osl90(n,g)Osl91(b - )Ir191(n,g)Ir192 3.2% W186(n,g)W187(b - )Re187(n,g)Re 188m(IT) Re188(b- )Os188(n,g)Os189(n,g)Os190(n,g) Oslglm(IT)Osl91(b -)lr191(n,g)Ir192 3.9% W 186(n,g)W 187(b - )Re 187(n ,g)R e 188(b - ) Os188(n,g)Os189m(IT)Os 189(n,g)Os 190(n,g) Osl91m(IT)Osl91(b-)Irl91(n,g)Irl92 13.1% W186(n,g)W187(b - )Re 187(n,g)Re188m(IT) Re188(b - )Os188(n,g)Os189(n,g)Os190(n,g) Osl91(b - )Ir191(n,g)Ir192m(IT)Ir192 2.0% W186(n,g)W187(b - )Re 187(n,g)Re 188(b - ) Os 188(n,g)Os 189m(IT)Os 189(n,g)Os 190(n,g) Os191(b- )Ir 191(n,g)Ir192m(IT)Ir 192 6.7% W186(n,g)W187(b - )Re187(n,g)Re188(b - ) Osl88(n,g)Osl89(n,g)Osl90(n,g)Osl91m(IT) Oslgl(b - )lr 191(n,g)Ir192m(IT)lr 192 8.2% W186(n,g)W187(b - )Re 187(n,g)Re188m(IT) Re188(b-)Os188(n,g)Os189m(IT)Os189(n,g) Os190(n,g)Os 19 lm(IT)Os 191(b - )Ir191(n,g)Ir192 5.9% W 186(n,g)W 187(b - ) Re 187(n,g)Re 188m(IT) Re188(b-)Os188(n,g)Os189m(IT)Os189(n,g) Os190(n,g)Os191(b - )lr 191(n,g)Ir 192m(IT)lr 192 3.0% W186(n,g)W187(b - )Re187(n,g)Re188m(IT)Re188(b - ) Os188(n,g)Os 189(n,g)Os 190(n,g)Os 191 m(IT) Osl91(b- )Ir 19 l(n,g)Ir192m(IT)Ir192 3.7% W 186(n,g)W 187(b - )Re 187(n,g)R e 188(b - )Os 188(n,g ) Os189m(IT)Os189(n,g)Os190(n,g)Os191 m(IT) Osl91(b - )lr 191(n,g)Ir192m(IT)Ir192 12.3% of the total amount of Ir192

factors making up the fluence. At short cooling time, however, the combination of higher flux with shorter irradiation time leads to activation values that are higher by up to an order of magnitude, ie. the activation scales roughly as the neutron wall load. This pattern of behaviour is expected to be similar for the other steels considered. The isotopes dominant during the operational lifetime of the first wall and up to a few years of subsequent cooling, as well as their routes of production, are clearly sensitive not only to the integrated power load but also to the way in which it is applied. Although the fluences and the reaction rate are equal for the two cases, the differences in the predicted amounts of short-lived isotopes demonstrate the sensitivity to the way the irradiation conditions are modelled. Such behaviour could be explained, in a broad sense, if it is noted that for these two cases there is an equal

fluence but that it has been distributed over two different periods of time, 2.5 years and 0.25 years. The set of differential equations involved are identical and the response function is directly influenced by the only p a r a m e t e r varied, namely the irradiation time. If the irradiation time is reduced, the pre-equilibrium concentrations of the short-lived isotopes will be higher, at equal neutron load, thus the term representing the loss by decay of the isotopes will be smaller. It may be noted that the saturation activity of a radionuclide produced through a single reaction path is reached when the irradiation time is equal to 7 times the half-life of that radionuclide. However, this result is based on a simplified equation which does not take account of any loss by reactions of the product nuclide nor production by decay from other radionuclides. Nevertheless, it demonstrates that the concentration of short-lived isotopes scales directly with the flux. The

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

303

wall loading will break down when the neutron reaction rate is so high that depletion of the parent nuclide becomes significant within the irradiation time. In view of this behaviour it is clear that when concerned with operational issues, namely issues involving short-term activation properties, it is important to be able to model with some degree of exactness the power loading history sustained by first wall materials, with particular attention to the actual duty cycle.

numerical predictions of the inventory code FISPACT, which takes account of all production and depletion channels, illustrate this typical behaviour and confirm this pattern for the dominant short-lived isotopes. In this context the meaning of the description "shortlived" must be considered in relation to the irradiation time and it appears from the results that the class of short-lived isotopes would include those having half lives up to a few months. This direct scaling with the

I R R A D I A T I O N O F STEELS EEF FIN 1 M W / m 2 I

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I lo 4

Time afier irradiation (Years)

Fig. 11. Graphs of activity versus cooling time for the four steels irradiated for 2.5y at 1 MWm -2 with the EEF neutron spectrum.

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

304

4. P a t h w a y a n a l y s i s o f g e n e r a t i o n

isotope, a cutoff value has been applied. In the present paper only those pathways that contribute more than 0.5% to the total amount of the final radionuclide are included. Although other pathways may exist, the amount of the isotope generated via them is at least 200 times less. For a given material the way in which the activation behaviour depends on the irradiation conditions can be understood by focusing on the production pathways for the major isotopes and the manner in which these depend on the irradiation parame-

routes

For each steel the principal isotopes governing the activation response function at various cooling times, as well as their production pathways, are identified in tables 3-6. These tables correspond to the reference irradiation condition, namely a wall load of 1 MW m - 2 for 2.5 y, and will be modified if either the wall load or irradiation time is changed. It is important to note that in presenting the production pathways for a given

I R R A D I A T I O N O F STEELS EEF F W 1 M W / m 2 I I ' ' I '

t0 +

104

-

-

SS316

~

1o2

OPTSTAB

J=

I0 °

,: \

!; \

FV448

\

LA12TaLC

\

,+

10-2

\

10 4

'\'. ,\ •. ,\ "-- . . . . . . . . . . . . . . . . . 10 -6 ~lin

10-6

I

Houri

10-4

I Day

I

Mth[

year[

]

10.2 100 102 Time after irradiation (Years)

~ - l~- " . . . . . . . . f

104

Fig. 12. Graphs of ~/ dose-rate dependence versus cooling time for the four steels irradiated for 2.5y at 1 MWm -2 with EEF neutron spectrum.

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys ters. For the steel 316, for example, it is found that only 6 isotopes, namely Fe55, Co60, Ni63, Nb91, Nb94 and Mo93, are important for issues relevant after 1 year cooling time. Their production pathways, shown in table 3, indicate the complexity of the routes by which they are produced and the delicate balance which exists between competing reaction paths. In attempting to understand the effect of the changes in irradiation conditions, it is helpful to plot the amounts of the dominant radioisotopes produced as a function of the varying wall loading or irradiation time. Figure 21 shows the dependence of the rate of IRRADIATION I '

lo 2

D

I

305

growth of the isotopes predominant in the activation response function of 316 steel. These values are normalised by dividing by the amount predicted for the selected isotopes under the reference irradiation conditions of 1 M W m -2 and 2.5 years. The normalised build-up value represents the direct scaling factor that needs to be applied to the value predicted for the reference conditions so as to take account of the changes in either wall loading or irradiation time. It is seen that most of these curves are not linear and indicate build-up rates that are less than proportional to the wall loading or irradiation time. The routes for

OF STEELS EEF ~ ' I '

1MW/rn2 T '

I

.,.,

lo 0

........OPTSTAB 10-2

\(''~ \'ii~

'!/

FV448

LAI2TaLC 104 i"

" " \

\

..

10~ 'N

', \.

Hour

Mtn

,

10~

I 104

Day

,

I 10.2

Mth

Year

L

J 10o

', \

,

I

I

I

",

Time afterirradiation(Years)

Fig. 13. Graphs of decay heat versus cooling time for the four steels irradiated for 2.5y at 1 MWm -2 with the EEF spectrum.

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

306

W A L L L O A D I N G D E P E N D E N C E O F SS316 100

I

I

I

I

I

o

~" 10 50MW/m2

.~ x

........

20 M W / m 2

10 MW/m2

¢lin

1 10 .6

Hour

~

I 10 .4

Day

J

Mth

Year

I ~ I , i 10"2 10 0 102 Time after irradiation (Years)

;

I 10 4

h 106

Fig. 14. Graphs of activity dependence versus cooling time for the four steels.

production of Fe55, Ni63, Mo93, Nb91 and Nb94 are single step reactions or single step reactions followed

multiplication factor applied to the flux. In fact, it is not quite equal but always less, thus the term in the

by decay, h e n c e t h e r a t e o f a c c u m u l a t i o n o f t h e s e i s o t o p e s w i t h t h e wall l o a d i n g is, at m o s t , e q u a l to t h e

d i f f e r e n t i a l e q u a t i o n s r e p r e s e n t i n g t h e loss by r e a c tions clearly b e c o m e s n o n - n e g l i g i b l e .

WALL LOADING DEPENDENCE OF OPTSTAB

100

I

I

0

I

.---

.

.

T

.

.

.

.

I

.

,

10 <

50 MW/m2 ........

20 M - W / m 2 10 M W / m 2

~n

1 10 .6

Hour

i

10-4

Mth

Day

,

I

~

Year

I

~

I

10-2 100 102 Time after irradiation (Years) Fig. 14 (continued).

J

I

10 4

J

106

307

J.-Ch. Sublet, G.Z Butterworth / Fusion activation of ferrous alloys W A L L L O A D I N G D E P E N D E N C E O F FV448

100

I

!

I

I

I

O

10 50 M W / r n 2

-.

20 M W / m 2

........

lOiviW/r~

din

1

I~y

Hour

,

I

10"6

,

Mth I

104

J

Yelr I

,

I

,

I 10 4

10-2 lo0 102 Time after irradiation (Years) Fig. 14 (continued).

The differential equation representing the changes in the number density of a nuclide with time involves the following four terms: B is the loss from reactions, B ' is the loss from decays,

1@

P is the production by reactions, P ' is the production by decays. Although an analytical solution of the differential equations necessitates the neglect of the terms B and P', this approximation is avoided in the numerical

WALL L O A D I N G D E P E N D E N C E O F L A 1 2 T a L C

1000

I

I

I

I

I

50 MW/m2 ........

20 MVq/m2

A

100 0

L~

< 10

~n

1

104

,

Hour I

lO4

Day A

I

kith t

yc~r I

~

I

lo-2 lo 0 lo 2 Time after irradiation (Years)

Fig. 14 (continued).

~

I

lO4

10 6

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

308

WALL LOADING

1000

I

I

DEPENDENCE I

OF SS316 l

l

50 MW/m2 .......

20 M W / m 2

I@3 0

I0

~irt 1

Hour I

~

10-6

Day ~

Mth ,

I

Year I

,

I

t

1

,

10-4

10.2 10° 102 104 Time after irradiation (Years) Fig. 15. Graphs of 3' dose-rate dependence versus cooling time for the four steels. solution provided by F I S P A C T . In this case the buildup of an isotope is governed by the balance existing between these four terms, which is often a delicate one. If some of them are directly proportional to the flux their combined effect is certainly not. WALL LOADING

10000

The routes for production of Co60 involve both multiple and single step reactions and the rate of growth of such an isotope with respect to the wall loading is much higher than might have been expected. This behaviour can be understood if one takes the DEPENDENCE

50 M W / m 2 "

"

10 6

OF OPTSTAB

!:'

20 M W / m 2

1000 10 M W / m 2

o~

100 ,: l

-

.

............

~

-

~'

'

I0

1

~n

10-6

,

H~ur

10-4

,

Day

}

Mth ,

y~ar

,

I

10.2 10° 102 Time after irradiation (Years) Fig. 15 (continued).

~

[

104

i

106

309

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

WALL LOADING DEPENDENCE OF FV448 1000

I

I

1

I

I

50 M W / m 2

I00 O

/

\ !

10

1 10-6

Hour i

Day ,

Mth I

10-4

~

Year I

~

I

,

10.2 10° 102 Time after irradiation (Years)

I

,

104

10"

Fig. 15 (continued). example of the second paths of table 3: Ni60(n,d)Co59(n,7)Co60 gives 2.05 x 10 TM atoms of Co60 at 1 MW m - 2 Ni60(n,d)Co59(n,7)Co60 gives 1.54 × 1020 atoms of Co60 at 10 M W m -2

Ni60(n,d)Co59(n,7)Co60 gives 1.39 × 1021 atoms of Co60 at 50 MW m - 2 Relative to the reference conditions, the numbers of atoms increase by factors of 75 and 678, respectively, for increases in wall loading of 10 and 50 times. Thus,

WALL LOADING DEPENDENCE OF LA12TaLC

10000

I

I

I

I

I

50 MW/m2 ........

S/,""

20 MW/m2

",:":.~

1000

100

10

1

,tin

10"6

Hfmr

104

Day

Mat

10.2 100 102 Time after irradiation (Years) Fig. 15 (continued).

104

10'

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

310

W A L L L O A D I N G D E P E N D E N C E O F SS316

1000

I

I

I

I

I

50 MW/m2

o

100

//

\

-..

lO

4in

1 10 -6

Hour

,

I 10 -4

,

l)ay

1 10 "2

Mth

~

Y~r

I 10 °

,

t 102

,

I. 10 4

, 106

Time after irradiation (Years) Fig. 16. Graphs of decay heat dependence versus cooling time for the four steels.

scaled with such large factors that they become predominant. This behaviour can be understood qualitatively in terms of the steps involved since, if the flux is in-

at the higher wall loadings, the paths of table 3 no longer represent the major routes for production of Co60 because other multi-step reactions, less important under the reference irradiation conditions, have

WALL LOADING DEPENDENCE OF OPTSTAB 1000 !

]

I

I

I

l

50 MW/m2 .....

o

20 M W / m 2

100

©

10

V[in

1

10-6

H~ur

104

l Day

I

Mth

Year

,

I

,

10 .2 10 0 10 2 Time after irradiation (Years) Fig. 16 ( c o n t i n u e d ) .

I

10 4

10 6

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

311

W A L L L O A D I N G D E P E N D E N C E O F FV448

1000

I

I

I

I

I

50 M W / m 2

20MW/m2

. . . . . . . .

,~ /

0

---

100

10MW/m2

......................................................

8

\

/

/

° .'"'/" " , ///\\

:;

10

1

Hour

~n

10"6

l

,

Day

104

I

Mth ,

Year I

I

,

~

I

10.2 100 102 Time after irradiation (Years)

104

106

Fig. 16 (continued).

creased by a factor 10 the number of Co59 nuclei produced by the first (n,d) reaction is almost, but not exactly, multiplied by 10 and if the second (n,7) reaction behaves in the same way then the number of Co60

WALL LOADING DEPENDENCE OF LA12TaLC

lOOO

o

nuclei produced by that particular route would be increased by nearly, but never quite, as much as 10 × 10. If such a path is termed a two-step reaction route, it is clear that a more rapid scaling in the production of

I

l

-

50 ~'n~/m2

........

20 M W / m 2

I

' A

I

/,

/

I

.

lOO

~ ................

.:/,'"

,.

. ..,."°"

/

N

""-,.

lO

~n

1 10"6

Hour I

10-4

,

I~y

I

M~ ,

Year I

,

I

10.2 100 102 Time after irradiation (Years) Fig. 16 (continued).

,

I

104

106

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

312

T I M E DEPENDENCE OF S S 3 1 6 10

I Hour

..Min

. -

I Year

Mth

I

I

/

5 Years 7.5Years lOYears

-

-. .....

o

I Day

\

[

'

~N\

I:."-.-., Ill .. fl

15 Y e a r s 20 Y e a r s 25 Y e a r s

", - , \,

/!! r - - -

l/ tl

//i' ,'l; '//

"~--

\ ~.

........

-

',

.......

f!

~:'/.

1 10"6

,

I 10 -4

,

I ~ I , I 10 -2 10 ° 102 T i m e a f t e r i r r a d i a t i o n (Years)

~

I 10 4

,

lOs

Fig. 17. G r a p h s of activity d e p e n d e n c e versus cooling t i m e for the four steels.

the number of steps in the reaction chain, though the exact scaling law must be determined by calculation. These scalings explain, for the specific nuclei involved,

Co60 is due to the presence of some paths with 3 or 4 reaction steps. The amounts of radionuclides generated via such paths can scale with a power as high as

TIME DEPENDENCE OF OPTSTAB 100.0

2Vlin

Hlour

Day

I

Mt'h

V~Jar

I

5 Years 7.5 Y e a r s 10 Y e a r s 15 Y e a r s 20 Y e a r s

10.0 o

/.---'~-~

_'~- .

/l'f . . . . . . . . . . . . . . i ' /. . . . . . . . . ,,., .......................

25 Y e a r s

-?

,.....~...~ "/."

< 1.0

---

0.1 10"6

I

10-4

,

I

L

I

,

I

10 -2 10 ° 102 T i m e a f t e r i r r a d i a t i o n (Years) Fig. 17 ( c o n t i n u e d ) .

L

I

104

106

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys lO

313

T I M E D E P E N D E N C E O F FV448 I Hour

.Mln

I

l~y

Mlh

I Yeir

i

I

lr ....... ........ ..... .....

5 Years

"li"

7.5 Years

! .- . . . .

10 Years

~r

\ " ' " "'-

\ "",,\

--_..

..

".,

15 Years 20 Years 25 Years

~! .. - _ _ i

" -

~ t .- . . . . l; ............. I,:

< ,~

1 10"6

I

I

,

10.4

,

I

,

. . . . ""

;

I

~

102 10o 102 T i m e after irradiation (Years)

I

104

lO 6

Fig. 17 (continued).

the non-linearity observed in the figs. 14-19, and illustrate the dominant effect of Co60 on the activation parameters of 316 steel.

100.0

In the case of the LA12TaLC steel the set of important isotopes comprises C14, Mn54, Fe55, Co60, Re186 and Ir192, with routes of production depicted in

TIME DEPENDENCE OF LA12TaLC ~lln

H~r

........ 10.0

<

..... .....

Day

I

Mth

Y~r

5 Years 7.5 Years 10 Years 15 Years 20 Years 25 Years

I

I

/ \ / / /./~-. / / .-

=.-~-.__ .-~___..._~=÷..~...~..._..-~.~:. . ~ T ~ ---~'-~-.-'~'-~" - " '~'-"""-'~" ~

\ ~ - - - - .~. "

=-~"-Y

1.0

0.1 10-6

104

10 .2 100 102 T i m e after irradiation (Years) Fig. 17 (continued).

104

106

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

314

TIME DEPENDENCE

10

I

I

Hour

o

;

Day

I

I

Year

1

5 Years 7.5 Years 10 Years 15 Years 20 Years 25 Years

.....

OF SS316

I

Mth

/f. / __ _." / /

//,,, O

1"1' t ., " / / I,." /,, / / I/ // // J ...

""

]

///,///

/..'//..'/

106 10-2 100 102 104 Time after irradiation (Years) Fig. 18. Graphs of 3' dose-rate dependence versus cooling time for the four steels. 10-6

10-4

table 6. The lengthy and complex paths for production of Ir192 from W186 are readily apparent. Such paths are bound to be very sensitive to changes in irradiation

conditions and this fact is borne out by fig. 22, in which the isotopic build-up is plotted against irradiation time and wall loading. The quantities of Ir192 and Co60

TIME DEPENDENCE

10000.0

-

I

~Vlin

1000.0

0

•~

I

Hour

Day

~

Mth

OF OPTSTAB

I

I

5 Years

100.0

]/

~. . . . "\\ ~'!l ~'~".~

15 Years 20 Years

~/ \\.\ ~r ~ ~,', , ~'\!,

I-.,'i'I /,-'",1:

10.0

1.0

~.~

7.5 Years 10 Years

25 Years

o r~

I

Year

frillI,,,- -. ",'

......

.

~ ,(,\ \ .\~

~ ' ~

",. "..\'~---~: \V~_ --

~

_

-

0.1 10-6

1

10-4

I

~

I

J

I

10"2 100 102 Time after irradiation (Years) Fig. 18 (continued).

,

I

104

106

J.-Ch. Sublet, G.J. Butterworth / Fusion activation o f ferrous alloys

10

315

TIME DEPENDENCE OF F V 4 4 8 I Hour

.Min

I

I Year

MtE

Day

I

J

/\

I!,\

.....

5 Years 7.5 Years 10Years 15 Years

.....

20 Years

........

I[ "'\ k. . . . . . . l:J .., " .

/if- - .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

................

Iii:f /I:, /I/, r " . .........

25 Years

Iii/ .................................. I/i~! /." d j I .~ "/1"'" . . . . . . . . . .

1

,

10"6

.~-~

..o

I

_

,

10 -4

,'l I: .')1' I" ~ 1 ,

I" --"

........

-

_

_

.

I

~

/

I . : 1 .

.

_ /

/

I

,

I

,

I

10-2 10o 102 T i m e after i r r a d i a t i o n (Years)

10 6

10 4

Fig. 18 (continued).

increase strongly with the wall loading, whilst the concentration of only the first of these behaves in the same m a n n e r with respect to t h e irradiation time. The

1oooo.0 ~,~

particular d e p e n d e n c e of the build up of Re186 with wall loading should also be noted and can be explained by the presence of a decay reaction in all channels of

TIME DEPENDENCE OF LA12TaLC I Hour

I Day

-lCalt

I

I

Year

5 Years 7.5 Years 10 Years 15 Years 20 Years 2,5 Years

1000.0

100.0

[

i

\

.\\

li[ " ~ '~\ / ..;i.. ...... ... ~ ,~\

I."_ J."

/.I /

10.0

'.. ,'A

."

".. ~'~

;z."/-----~

/,'i:,--/

"..v~

..s:3:-.-

\

/i~.../

1.0

0.1 lO ~

I

I

104

1o 2

,

I

,

I

lOo lO2 Time after irradiation (Years)

Fig. 18 (continued).

J

I

lO*

i

,

106

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

316

TIME D E P E N D E N C E 10

I

.Min

I

Hour

O F SS316

1

Day

Mlh

I

Year

/

f

~,

//--. \\

0

-

-

-

.....

5 Years 7.5 Y e a r s 10 Y e a r s 15 Y e a r s 20 Years 25 Years

/: / //":.

'\.

\

\" \ •

"

\/"

/

/

-

i/ ,,' /://:.... .."

8

,

,

-

"----...

/

......

" I ,.

2:

/:'/I ,;

ii./i."

,'I I " S ///1" I ," /<,'/i..' i / ,"

-~,//'

1

10-6

== = :-:="= ....... 10 -4

~

10 .2

I

,

Time

Fig. 19. G r a p h s o f d e c a y h e a t d e p e n d e n c e v e r s u s c o o l i n g

production. That reaction forms a critical path and limits the impact of an increase in wall loading. The dependence on irradiation time of the concen-

Hlour

I0.0

I

Mtla

.... .....

g~

L

10 6

time

for the

f o u r steels.

OF OPTSTAB

Y~ar

I

5 Years 7.5 Y e a r s 10 Y e a r s 15 Y e a r s 20 Y e a r s 25 Y e a r s

.... 0

Diy

I

10 4

tration of Ir192 shows that even with such extended routes of production the amount of nuclide goes on increasing within the timescale studied. However, those

TIME D E P E N D E N C E 100.0 2vlin

J

10 ° 10 2 after irradiation (Years)

/ , : ,. ~ . ~ _ _ / / / , . \ -• . . . . . . . . . . . ~\ jl /

//.i'. \

0

//.."'.

.....................

~

I

~,' 1.0

0.1 10 -6

t

10 .4

L

I

,

10 -2 Time

I

10 ° after

irradiation

Fig. 19 ( c o n t i n u e d ) .

102 (Years)

,

I

10 4

10 6

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys 10

317

TIME DEPENDENCE OF FV448 H~

,MJn

•

I

Day

I

Mt5

I

Year

/ 5 Years 7.5 Years

........

-,..

,,f

10 Years 15 Years 20 Years 25 Years

..... .....

0

I

~ ............

, i. . . . . . . . . . . .

//

- _

Ii/

li'://1".-" . . . . . . . . . . . . . . . . . . . . . . . . . . /."l I ;'"

/.I.i ~.. ~':l I.: ...... "~" . . J l' I.: I"~

~

.~--- . . . . .

".'/'/':/

/

.-....-.-...-_ .T. - - . ~

1

10.4

i0 "6

104

10"2 10° 102 Time after irradiation (Years)

106

Fig. 19 (continued).

types of long multistep path should be considered as exceptional and treated with caution in numerical predictions.

100.0

TIME DEPENDENCE OF LA12TaLC .~o.

2~n

........ O

Given such complex scaling behaviour it is clear that the only way of reliably assessing the activation properties of fusion materials under variable irradia-

10.0

o,T

I

~

yj.

~

5 Years 7.5 Years

/\ \

/,

10 Years 15 Years

..... .....

\

//'~.

\

/I I/ / x X,~. ,, "",- ~ __/;/ ". ~ ~ " /.1'1 :" ". - - . . . . .

20 Years

\

"

25 Years

I I : / X "" . . . . . . //,.."

8 ;=

i

/.'/.: /

--

. . . . . . . . . . . . . ._.

\

1.0

0.1 10"6

J

I

10 -4

,

I

~

I

~

I

102 Time after irradiation (Years)

10-2

100

Fig. 19 (continued).

~

I

104

106

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

318

l0 Ih

•

•

FLUENCE I

I

DEPENDENCE

O F SS316

]

'

I

FLUENCE

DEPENDENCE

O F SS316

--

'

T

I

I i

I

1 M W / m 2 and 2.5 years

1 M W / r a 2 and 2.5 years

102

10 M W / m 2 and 0.25 year

10 M W / m 2 and 0.25 year

~.~ 1012 < 100

10 Itl

FI

I 10.2

~

, 10-6

I 104

L

L

,

I

,

I

L

10.4 M~. i 0 "6

I

I02 10.2 lift Time after irradiation (Years)

104

FLUENCE --T

I

DEPENDENCE I

'

t~T

7 10.4

,

~t~

Y.r

1 10-2 10° 102 Time after irradiation (Years)

O F SS316 I

I 1 M W / m 2 and 2,5 years

10o

"~

\ 10 M W / m 2 and 0,25

L~ 10-2 ,v

I

lO,I .o~r

10~ ~ - J 10~

f 104

oty

,

Mr,

i 1o-2

,

",.

Y.,

1 1oo

,

1 102

,

_I__ 104

Time after irradiation (Years)

Fig. 20. Graphs

of fluence

dependence

versus

cooling

time for 316 steel for the three

activation

parameters.

,__1

J 104

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

100

319

ISOTOPIC B U I L D - U P R A T E O F SS316 '

' ' ' ' ' ' ' 1 ' '

'

'

'

'

'

'

'

' ' '

' . " ' ' ' '

..

I ' ' ' '

'

'

'

'

'

I

. . . . . . . .

:

:"

Fe55 Co60 Ni63 Mo93 Nb91 Nb94

- ......

:'

80 :

'

..... .....

60

a~

40 o

Z 20

10

20 30 40 Wall loading (MW/m2) Fig. 21. Graphs of isotopic build-up rate dependence for 316 steel.

tion conditions, is by analysing the routes for production of all dominant isotopes and deducing from each channel the scaling relationship responsible for the

50

overall behaviour of the material. This approach requires a detailed knowledge of all channels of production identified during the computation of the general

ISOTOPIC B U I L D - U P R A T E O F SS316 10

'

'

'

'

I

'

'

'

'

I

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'

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'

I

'

'

'

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Fe55 ........

., "

Co60

,"

Ni63 .....

Mo93

.....

Nb91

/ .. "

Nb94 ,.0

."

/ -"

_

..->"

.. " .~/"w""~ .,¢t. ./'"

..f

~o -~:"

6 /

d-

/

/

4 /

~

,

°

.~.

,

I

,

10

,

15 Irradiation time (Years) Fig. 21. (continued).

20

25

320

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys

100|'

ISOTOPIC ,r ........

I.......

BUILD-UP , .........

RATE

OF LA12TaLC ,

. . . . . . . . .

. . . . . . . . .

I

i

I J ]_I 80}-I

I ,'

- Fe55 ~.-- Mn54 --Re186----Co60 -- C14 --Ir192

/ i

~-I

~[-I

/ /'

I

/

, ...-"

/'

40 Z

.--'""

/ t!

. .....

/'

,I

~

/

~

."

i

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~

•.

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-

--

.

.

.

"........ .

-

.

~.._.

.

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~ .... -"~

.

.

.

.

.

.

.

.

-

.

..~-

_

.

10

.

_--

.

.

.

20 30 Wall loading ( M W / m 2 )

40

50

Fig. 22. Graphs of isotopic build-up rate dependence for LA12TaLC.

ISOTOPIC

BUILD-UP

RATE

OF LA12TaLC

10000 Fe55 .....

/- ~

Mn54

1...--/-

Re186 ..... .....

1000

Ir192

/

/

Co60 C 14

i-

/ J

/

./

/

/ /

d~

/

100

/ / /

0

/

Z / 10

._.

./" j./ ,¢ / ,i.

/

.-~

.......

- ................

=

....

_.I" ..f"

~.

_ ~

5

_ .

_ _ ~ ....................................

10

.

.

.

.

T..'T. 7.?T..~.~_--..T-.7:,~----:

15 Irradiation time (Years)

Fig. 22. (continued).

20

..............

25

J.-Ch. Sublet, G.J. Butterworth / Fusion activation of ferrous alloys solution. Such analysis explains the profound effect of variations in the irradiation conditions on the pathways of production and hence on the amounts of isotopes produced through different channels.

5. Conclusion The activation properties of a first wall material can be strongly dependent on the reactor design through its influence on the neutron flux and spectrum. The associated variations in activation properties may have important implications for safety and environmental issues and should be taken into account iteratively in the design of reactors and the optimisation of materials to achieve safety and environmental goals. The activation properties of the steels considered scale in a non-linear manner with the duration of the irradiation, the total energy-integrated flux and the corresponding fluence. In each case the activation properties at a particular cooling time are governed by a limited number of dominant radionuclides. In general, the isotopes responsible for the residual activity at long cooling times arise as products of multiple stage reactions and decay chains. The concentrations of radionuclides generated by such multiple step reactions scale supralinearly with the neutron flux and irradiation time and are thus sensitive to the irradiation conditions. The non-linearity of the production is greater the number of reaction or decay steps present in the chain and needs to be analyzed on an individual radionuclide basis in order to correctly predict the appropriate scaling relationship. Since the activation properties of the materials considered here can vary strongly and in a non-linear manner with the irradiation conditions represented by the neutron wall loading and the duration of the irradiation, it is clearly desirable in predicting the activation properties to model the anticipated irradiation conditions as closely as possible and also to examine the sensitivity of the predicted results to variations in the assumed conditions. A further point should be mentioned concerning the uncertainties. The calculational procedure employed to predict the activation behaviour relies heavily on the available cross-section database covering a very large number of nuclides. The uncertainties inherent in many of these cross-section values must appear as uncertainties in the predictions and the accumulated uncertainty may be substantial in the cases of nuclides

321

arising as the end-products of long reaction and decay sequences or as products of generation pathways which involve delicate balances between the production and removal of intermediate radionuclides by competing pathways. The uncertainties arising from the preprocessing of the cross-sections should also be included. A more accurate description of activation behaviour must therefore include a consideration of the effects of uncertainties in the nuclear data employed.

References [1] A.Ho Bott, F.B. Pickering and G.J. Butterworth, Development of high manganese high nitrogen low activation austenitic stainless steels, J. Nucl. Mater. 141-143 (1986) 1088. [2] K.W. Tupholme, D. Dulieu and G.J. Butterworth, Grain-refined low activation martensitic steels--The effects of composition and tempering treatments and longterm ageing on properties and structures, Culham Laboratory Report, AEA-FUS109 (1991). [3] J.A.S. Guthrie and N.H. Harding, Culham Conceptual Tokamak Reactor Mark III, Culham Laboratory Report, CLM-R-215 (1981). [4] M.G. Sowerby and R.A. Forrest, eds., A study of the environmental impact of fusion, Harwell Laboratory Report, AERE-R-13708 (1990). [5] P.I.H. Cooke, P. Reynolds et al., A DEMO tokamak reactor, Culham Laboratory Report, CLM-R-254 (1985). [6] C.C. Baker, M.A. Abdou et al., STARFIRE-a commercial tokamak fusion power plant study, Argonne National Laboratory Report, ANL/FPP-80-1/2 (1980). [7] K. Shibanuma et al., Japanese Contributions for ITER, JAERI Naka Report, JAERI-M-91-080 (1991). [8] L.J. Baker et al., Study of the reactor relevance of the NET design concept, Culham Laboratory Report, CLMR-278 (1987). [9] R.A. Forrest and J. Kopecky, The European Activation SYstem (EASY), IAEA Advisory Group meetings on FENDL-3, Vienna, November 1991. [10] J. Kopecky and D. Nierop, Contents of EAF-2, ECN Petten Report, ECN-1-91-053 (1991). [11] K.A. Verschuur, Poloidal variation of the NET blanket nuclear response functions, The NET Team, Garching, EUR/FU/XII-80/88/82 (1988). [12] M. Hugon, private communication [13] J.-Ch. Sublet, Activation considerations relevant to the decommissioning of fusion reactors, Ph.D Thesis, Imperial College, London (1989). [14] C. Ponti, Calculation of radioactive decay chains produced by neutron irradiation, JRC Ispra, EUR 9389 EN (1984).