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The use of present fission reactors tosimulate radiation damage in fusion reactorfirst wallsH. I. Avci a & G. L. Kulcinski ba Battelle's Columbus Laboratories , 505 King Ave., Columbus, Ohio, 43201b Nuclear Engineering Department , University of Wisconsin , Madison,Wisconsin, 53706Published online: 19 Aug 2006.

To cite this article: H. I. Avci & G. L. Kulcinski (1979) The use of present fission reactors to simulate radiationdamage in fusion reactor first walls, Radiation Effects, 45:1-2, 61-68, DOI: 10.1080/00337577908208411

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Radiation Effects, 1979, Vol. 45, pp. 61-68 00330033-7579/79/4501-0061$04.50/0

@ 1979 Gordon and Breach Science Publishers, Inc. Printed In United States of America

THE USE OF PRESENT FISSION REACTORS TO SIMULATE RADIATION DAMAGE IN FUSION REACTOR FIRST WALLS

H. I. AVCI

Battelle's Columbus Laboratories, 505 King Ave., Columbus, Ohio 43201

and

G. L. KULCINSKI

Nuclear Engineering Department, University of Wisconsin, Madison, Wisconsin 53706

(Received May 25, 1979)

The placement of passive shields (ISSECs) between the first wall and the source of neutrons has been considered as a mechanism for extending first wall lifetimes in fusion reactors. Solid carbon, Mo, Nb, W, and V were considered as ISSECs for tokamaks and liquid metals Li, Ph, and a Pb-Li eutectic alloy Pb,Li for laser fusion reactors. In addition to their radiation damage reducing capabilities in the first walls, it is shown that ISSECs also soften the Primary Knock-on Atom energy spectrum in the first wall to close to that found in fast or thermal fission test reactors. Such damage modi- fications would allow more confidence in applying data from current fission test facilities to components in future fusion devices. The appm He/dpa ratio has also been shown to be about equal in both the first wall and in fission reactors when 40&50 cm of liquid Pb or Pb,Li is used (except for Ni containing alloys in HFIR). At present, the required thicknesses for the other ISSEC materials to give the same reduction in damage effects are considered to he too large for practical reasons (ix. > 1 meter). An overall conclusion of this study is that aside from the time dependence of the radiation damage in inertial confinement fusion reactors, the structural materials to be used in such reactors employing liquid Pb or Ph,Li ISSECs at thickness of 45-60 cm can be tested in fission test facilities already present today.

INTRODUCTION

It has been shown that a passive shield placed between the source of neutrons and the first structural wall in fusion reactors can modify and reduce the various first wall response functions such as displacement and internal gas generation rates, radioactivity, Primary Knock-on-Atom (PKA) spectra, etc.1-6 In previous studies, the passive shield was given the name ISSEC, short for Internal Spectral Shifter and Energy Converter. For example, it has been calculated that ISSECs made of graphite, molybdenum, niobium, vana- dium and tungsten can decrease the displacement, and helium and hydrogen gas production rates in structural first walls of magnetic confinement fusion reactors such as Tokamaks by factors of 2 to 50 and 3 to 275, respectively. 1*3,6 In inertial confinement fusion reactors, higher reduction factors can be achieved by the employment of liquid metal ISSECs

such as liquid Li, Pb, or a Pb-Li alloy4-'j with a composition of 4 lead atoms to each Li atom. (This alloy is represented as Pb4Li in this study, even though it is not a stoichiometric compound). It is believed that the benefits of such lower displace- ment and gas production rates will be higher first structural wall lifetimes, which, in turn, have a great effect on the economics of fusion power.7

One other advantage of the ISSEC concept and the softened neutron spectra is anticipated to be softer PKA spectra in the first structural walls of fusion reactors. Obviously, if the PKA spectra behind the ISSECs are similar to that already available to the materials scientists, projections of the first wall behaviour can be made with much more confidence. The object of this paper is to calculate the PKA spectra in various first wall materials behind different ISSECs and compare them to the PKA spectra that would be obtained in an unprotected first wall and in current fission reactor testing facilities.

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62 H . I. AVCI AND G. L. KULCINSKI

I1 CALCULATION PROCEDURES

The neutron flux spectra used in calculating the PKA spectra in the first structural wall were obtained by solving the discrete ordinates form of the neutron transport equation using the ANISN8 program with a S,-P, approximation. The nuclear data set used was generated with the AMPX modular code system' from nuclear data in ENDF-B/IV." The model fusion blanket used in these calculations was a one-dimensional, homogeneous, cylindrical geometry blanket. A variable thickness ISSEC zone was placed between the neutron source and the first structural wall. The first wall material used in all cases was 316 SS with 0.5 cm thickness. The first wall was followed by a 60 cm thick breeding zone composed of 95 % natural lithium and 5% 316 SS for structural material. Behind the breeding zone was a 30 cm thick reflector zone with a composition of 95 % C and 5% 316 SS. Albedos of 0.5-0.25 were used to simulate the final shield for neutrons of energy from 14.9 MeV down to 9 MeV. An albedo of 0.3 was used for neutrons of lower energy. A uniform cylindrical neutron source was used consistently for both solid and liquid ISSECs. For the inertial confinement fusion reactors, which are more likely candidates to utilize liquid ISSECs than are the magnetic confinement reactors, these assump- tions regarding the geometry of the blanket and the neutron source geometry are different than the typical point source in a spherical cavity." How- ever, some previous studies performed by the author^^,^ show that for liquid ISSEC thicknesses in excess of 40 cm, the response of the first wall is independent of the blanket geometry and neutron source distribution used.

The PKA spectra were calculated using the DISCSM code which is a modified version of the DISCS code originally written by G. R. Odette and D. R. Doiron", later simplified by L. R. Greenwood' by directly accessing ENDF-B" files for input, and modified to include (n, n'x) and thermal neutron (n, y) recoil reactiom6

I11 RESULTS AND ANALYSIS

The primary effect of an ISSEC is to soften the neutron flux spectrum in the first structural wall of a fusion reactor. The magnitudes of this modification can be seen in Figure 1 where we have plotted the normalized neutron flux versus neutron

\ lo- ' \

\

Herd Fusion

10-1 I 10 102 10' 104 log 10' lo' ENERGY - E V

FIGURE 1 ISSECs.

Normalized first wall neutron flux behind various

energy in the first structural wall for three cases ; when there is no ISSEC protection, when there is 25 cm of carbon, and when there is 91 cm of liquid Pb4Li in front of the first structural wall. The 25 cm of carbon represents the maximum thickness due to heat transfer limitations. As we see in Figure 1, the 14 MeV component of the flux is reduced by about one order of magnitude by 25 cm of carbon, and by about 4 orders of magnitude by 91 cm of the alloy Pb,Li. Due to low parasitic neutron capture cross-sections, and good modera- tion in carbon, the low energy part of the flux spectrum is increased when carbon is used as an ISSEC.

The softening of the neutron spectrum can also be regarded as a reduction in the average energy of the neutrons undergoing scattering in the first structural wall. This is illustrated in Figure 2 where we have plotted the average neutron energy in the first wall as a function of C and Pb4Li ISSEC thicknesses. On the same Figure, the average neutron energies are also shown for the HFIR, EBR-11, and LAMPF-(Be target) experimental test facilities. 14,1s

Of more concern in radiation damage studies is the energy of the PKAs rather than the energy of the neutrons. We have calculated the average Ni PKA energy in a 316 SS first wall behind various ISSEC thicknesses and plotted it in Figure 3 as a function of increasing ISSEC thickness. On the same figure also shown are the average Ni PKA energies in HFIR and EBR-I1 reactors' neutron test spectra and the average Ni PKA energy from 5 MeV Copper ion irradiations.I6 It is apparent from Figures 2 and 3 that about 15 cm of Pb4Li

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DAMAGE IN FUSION REACTOR 63

ISSEC Thickness cm

FIGURE 2 Average neutron energy in the 316 SS first wall behind carbon and PbaLi ISSECs.

HFlll-

10 M Y) 40 s w 70 n o ISSEC THICKNES¶. em

FIGURE 3 Pb,Li ISSECs.

Average nickel PKA energy behind carbon and

will give the same average neutron and Ni PKA energies in a fusion reactor first wall as in a fast fission reactor spectrum like EBR-11. (For HFIR, the required Pb4Li ISSEC thickness is about 25 cm). Thicknesses much larger than 25 cm is needed for carbon to get the same effect.

Figure 4 shows the differential Ni PKA energy

I I I

16' 1 6 ~ I# 161 I d 1 '

I

NI P K A E N E R G Y , T-MeV

FIGURE 4 Differential Ni PKA distributions in various neutron spectra.

probability distributions in 5 different neutron spectra; in an unprotected fusion reactor first wall, in the fast neutron test reactor EBR-11, a thermal neutron test reactor HFIR, and in fusion reactor first wall with two kinds of solid ISSEC protection; one with 25 cm of carbon and the other with 25 cm of molybdenum. Figure 5 shows similar curves for cases when the first wall is protected by 115 cm of Li and 91 cm of Pb4Li liquid ISSECs and the three reference cases, hard fusion, EBR-11, and HFIR of Figure 4. The results shown in Figures 4 and 5 are sometimes easier to understand when the data is integrated and normalized to unity (Figure 6). This latter figure shows the fraction of PKAs with energy greater than a given value T. Figures 7-9 are similar to Figure 6 except they are for first walls of aluminum, titanium, and niobium, res- pectively.

Since the object of the article was to see how close one could make the PKA spectra in a fusion reactor first wall to that characteristic in already available neutron test facilities, let us consider the EBR-I1 and HFIR spectra separately.

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Id -

I .o

.9

.8

. I

.6

.5

.4

. 3

.2

. 1

.o 10-4 10-3 lo-* 10-1 1

I I I , I 167 I 1

NI P K A ENERGYVT - HEV

FIGURE 6 Fractional Ni PKA distributions in various neutron spectra.

I- A * (3

W 2 W

I

a

t 3 v,

Y a a LL 0

z I- V

0

a a LL

AL P K A ENERGY, T - M E V

FIGURE 7 various neutron spectra

Fractional A1 PKA probability distributions in

_/ I15 Crn LI

HARD FUSION

to- 3 10-2 10-1 I

Ti PKA ENERGY, T-MEV

FIGURE 8 various neutron spectra.

Fractional Ti PKA probability distributions in

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DAMAGE IN FUSION REACTOR 65

NB P K A E N E R G Y ~ T - nEv FIGURE 9 Fractional Nb PKA distributions in various neutron spectra.

3a EBR-11 Spectra

It can be seen from Figures 6-9 that the PKA spectrum in a fast neutron facility such as EBR-I1 is more biased to low energies than in HFIR or in an unprotected fusion first wall. Above energies of 10 KeV, there are more PKAs in the unprotected fusion reactor first wall than in EBR-I1 and this, of course, is the major concern of materials scientists faced with simulation of such damage. The insertion of an ISSEC structure between the neutron source and the first wall does a great deal to soften the high energy PKA spectra in a fusion reactor. This can be seen in Figures 4 and 5 . However, of the four ISSEC materials shown in Figures 4 and 5 , Pb,Li is the only one that can reduce the high energy part of the Ni PKA spectrum in a fusion reactor first wall close to that found in EBR-11. In fact, our calculations536 indicate that about 50 to 65 cm of either liquid Pb or liquid Pb4Li ISSEC would give enough pro- tection to the first wall so that the fraction of high energy (> 0.1 MeV) Ni, or Fe or Cr PKAs in a fusion reactor first wall is actually less than those

found in EBR-11. However, the softening of the PKA spectrum by the ISSECs also occurs at the low energy end and increases the discrepancy below - 50 KeV. For example, in Ni (Figure 6), the probability of producing PKAs with T > 1 KeV is almost twice as large in EBR-I1 as behind a 25 cm thick Mo or C ISSEC. The use of thicker liquid ISSECs makes the situation even worse on the low energy side while at the same time softening the PKA spectra even further at the high energy side. The probability of producing a nickel PKA of T > 1 KeV is 3.5 times as great in EBR-I1 than behind 91 cm thick Pb4Li liquid ISSEC whereas the probability of producing a PKA of T > 100 KeV is 10 times greater in EBR-11.

It is clear that the use of ISSECs to protect the first wall not only reduces the absolute damage rate, but it softens the neutron spectra in all the materials considered to the point that the fast neutron facilities produce even harder PKA spectra than found in potential fusion reactor first walls. At this stage of our understanding of the radiation damage process, this is the same as saying that, on a dpa basis, the displacement damage per neutron will be higher in EBR-I1 than in an ISSEC protected fusion reactor, not less as is the case today.

3b HFIR PKA Spectra

Unlike the EBR-I1 case, there is a large number of thermal neutrons present which can cause (n, y ) reactions. The recoils from these reactions can produce large numbers of low energy PKAs and we see from Figure 6, for example, that the HFIR PKA spectrum is softer than that for the unprotected fusion reactor first wall over the entire energy range. Without any modification to the fusion neutron spectrum, this can have serious implications for present simulation studies.

However, when solid or liquid ISSECs are used, the softened PKA spectra are closely simulated by the HFIR spectra over a wide range of energies. For example, it is seen from the fractional PKA distributions plots for Ni, Al, and Nb that the HFIR spectra closely simulates the PKA spectrum below several hundred KeV behind a 25 cm of carbon ISSEC.

While the ISSECs cannot reduce the maximum recoil energy, they can reduce the number of those recoils to insignificant levels. Again, it can be seen that 91 cm of Pb,Li in front of a Ni first wall can reduce the number of PKAs above 100 KeV to less

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66 H. I . AVCI AND G. L. KULCINSKI

0.01

than l/lOth of those produced in HFIR (Figure 6). The similar comparison shows that the HFIR spectrum produces a factor of 10 less PKAs above 100 KeV than for the hard fusion case (i.e., the 91 cm of Pb4Li reduces the number of PKAs above 100 KeV by a factor of 100 over the hard fusion spectrum case).

The conclusion that one arrives at after these comparisons is that the displacement damage behind the various ISSECs considered can best be simulated by a thermal reactor spectrum, and if anything, the damage per incident neutron should be less in the modified fusion reactor environment than in the fission neutron test reactor. Fast neutron facilities can simulate the ISSEC modified high energy PKA spectra but they do not do a very good job of simulating the low energy spectra. If there are processes that are sensitive to large numbers of low energy collisions (such as stimu- lated diffusion or annealing of point defects) then the fast neutron spectra may again not be suitable for simulation.

'\ 'iW4 1 Pb \ -

\ * l l l l l l l i l l

IV DISCUSSION

In order for the simulation to be accurate in any fusion reactor materials testing facility, one other important factor has to be taken into considera- tion. It has to do with the synergistic effects between the gas atoms that are generated inside the material from such nuclear reactions as (n, a) and (n, p ) and the displacement damage. In other words, the ratio of appm He/dpa should be the same in the two systems for a true simulation. Table I lists the appm He/dpa ratios for different materials in a hard fusion neutron spectrum, and in EBR-I1 and HFIR. It is found that this ratio

TABLE I

Calculated ratio of appm He production to displaced atom density in potential CTR first structural wall materials (3, 17)

appm He/dpa Material Fusion EBR-I1 HFIR

AL 37.5 0.31 0.30 V 6.1 0.03 0.014

Nb 3.9 0.09 0.11 Mo 7.7 0.10 0.087

316 SS 24.8 0.63 60-70"

After 1 year of irradiation

' O 0 U

varies from 1 to 40 for unprotected walls. Appro- priate values for a fast reactor (EBR-11) and a thermal reactor (HFIR) test facility are generally much lower ranging from 0.014 to 0.63 for most elements. The one exception to this statement has to do with Ni containing alloys in a thermal neutron spectrum. The value in 316 SS is 60-70 in HFIR because of the Ni-58 to Ni-59 conversion and subsequent high (n, a) cross section of Ni-59 at thermal energies.

Figure 10 shows the appm He/dpa ratio in a 316 SS first structural wall of a CTR divided by the same ratio in the 316 SS in EBR-I1 for varying thicknesses of all the ISEEC materials considered in this study. We see in this figure that about 47 cm of liquid Pb,Li or 42 cm of liquid Pb ISSEC exactly reproduces the appm He/dpa ratio from EBR-I1 in the 316 SS first structural wall of a CTR. One needs more than 2 meters of liquid Li ISSEC, or - 1.5 m C, 80 cm V, 75 cm W, 70 cm Mo, or - 50 cm of Nb ISSEC to get the same effect. These thicknesses are really not realistic for solid ISSECs because of the excessive tem eratures (even with conductively cooled shields). s

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DAMAGE IN FUSION REACTOR 61

The significance of these results is that 40-50 cm of liquid Pb or 45-55 cm of liquid Pb4Li ISSEC reduces the appm He/dpa ratio in a 316 SS first structural wall of an inertial confinement fusion reactor to that value one would get if the 316 SS sample was irradiated in EBR-IT. As discussed previously, the high energy part of the PKA spectrum will also be similar in the two cases. Therefore, the data obtained from the already present fast fission test facilities could be used with much more confidence in the design of fusion reactors if the liquid Pb or Pb4Li ISSECs were to be used at thicknesses given above. As far as the other solid ISSEC materials (and liquid Li) considered in this study are concerned, the gas to dpa ratio behind any practical thickness of them is still so much higher than for EBR-I1 that any process that depends on the interaction of gas atoms and point defects will not be correctly simulated in fission reactors.

The effect of neutron moderation is even more dramatic for the non-nickel containing first wall materials (Fe, Cr, Nb, and V) in the HFIR spec- trum. Even though calculations were not done for these elements with liquid ISSECs, the Pb and Pb4Li ISSECs would also reduce the He/dpa ratio down to levels characteristic of the HFIR with -40 cm of flowing metal. The simulation of the appropriate fusion PKA spectra is even better in HFIR than EBR-11. This means that contrary to current thinking, the thermal fission neutron facilities would be extremely valuable as a simula- tion facility for non-Ni containing materials if such materials were to be used behind 40-50 cm of Pb or Pb4Li material in fusion reactors.

One final point is worth noting about the time structure of irradiation in fusion and fission devices. It is well-known that tokamaks are quasi-steady-state machines and from the point of view of the response of materials to the time of irradiation, such reactors could be considered steady-state. On the other hand, liquid ISSECs are originally conceived to be used in laser fusion reactors which are inherently pulsed systems. If one could assume that the time structure of the irradiation in a pulsed device such as a laser fusion reactor does not have any drastic effect on the final damage state of the material, the simu- lation in fission reactors would give accurate results. However, it is expected that the inherent damage incurring in the irradiated material will be different in pulsed and steady-state systems. Even though in the process of slowing down the

neutrons, the ISSECs help spread out the time interval during which neutrons arrive at the first structural wall, it will still be far from a steady- state irradiation. Therefore, before one could use the fission reactor data in the design of laser fusion reactors employing liquid ISSECs reliably, one has to answer the question of what effect pulsed irradiation has on the material as opposed to steady-state irradiation.

V CONCLUSIONS

The major conclusions that can be gathered from this study are as follows:

1) The probability of producing a PKA with energy greater than about 0.1 MeV is 3 4 orders of magnitude higher in an unsoftened fusion first wall neutron spectrum than it is in either EBR-I1 or HFIR fission reactor neutron spectrum.

2) The only practical ISSEC materials that can reduce the high energy part of the PKA distri- bution in a fusion reactor first wall neutron spec- trum to EBR-I1 or HFIR levels are Pb and Pb4Li with thicknesses around 50-65 cm. With the other ISSEC materials considered in this study either the required thickness becomes too large ( > 2 m) or heat transfer limitations prohibit the ISSECs from operating at the required thicknesses.

3) On the overall energy scale, the PKA spec- trum obtained in a fusion reactor first wall pro- tected by ISSECs is better represented by spectra obtained in a thermal fission reactor like HFIR than in a fast fission reactor such as EBR-11.

4) The calculated appm He/dpa ratio in A., V, Nb and Mo first walls is much higher in a hard fusion reactor neutron spectrum than it is in either EBR-I1 or HFIR spectrum. Because of the Ni5' (n, y) Ni59 (n, a) reaction sequence, 316 SS has a higher appm He/dpa ratio in HFIR than in an unmodified fusion spectrum.

5) Either 42 cm of liquid Pb or 47 cm of liquid Pb4Li ISSEC can reduce the appm He/dpa ratio in the 316 SS structural first wall of a CTR to same value one would have obtained in 3 16 SS in EBR-11. Unreasonable thicknesses of the other ISSEC materials would be required to accomplish the same thing. For example, to reach fission reactor gas atom to displacement ratios, one would need more than 2 meters of Li, about 1.5 meters of C, 70-80 cm of V, W or Mo and - 50 cm of Nb.

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68 H. 1. AVCI AND G. L. KULCINSKI

6 ) An overall conclusion of this study is that if the time dependence of the radiation damage in a laser fusion reactor is not overriding (i.e., at low operating temperatures), the structural materials to be used in laser fusion reactors employing liquid Pb and Pb4Li ISSECs at thicknesses of 45-60 cm can be tested in fission test facilities already present today. This would also allow the vast amount of data that have already been obtained in such facilities to be used in the design of these fusion reactors.

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R. W. Conn, G. L. Kulcinski, H. I. Avci, and M. EI- Maghrabi, Nucl. Tech., 26, 125 (1975). H. I. Avci and G. L. Kulcinski, The response of ISSEC protected first walls to DT and DD plasma neutrons, Proc. Int. Confer, on Rad. Eff. and Tritium Tech. for Fusion Reactors at Gatlinburg, Tenn., October 1-3, 1975. H. I. Avci, et al., Nucl. Engr. and Des., 45, 285 (1978). H. I. Avci and G. L. Kulcinski, Trans. Am. Nucl. Soc., 28, 40 (1978). H. I. Avci and G. L. Kulcinski, The Effect of Liquid Metal Protection Schemes in Inertial Confinement Fusion Re- actors, Nucl. Tech., 44, 333 (1979). H. I. Avci, Protection of CTR first structural walls by neutron spectrum shifting, Ph.D. thesis, Univ. of Wisc. (1978). G. L. Kulcinski and J. R. Young, The influence of first wall lifetime on the cost of electricity in UWMAK type fusion reactors, Proc. Int. Conf. on Rad. Eff. and Tritium Tech. for Fusion Reactors, Gatlinburg, Tenn., October 1- 3,1975.

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17. G. L. Kulcinski, “Fusion Reactors: Their Challenge to Materials Scientists” to be published in Contemporary Physics. Also report UWFDM-277, Nucl. Engr. Dept., Univ. of Wisc., Madison, Wisc. (October 1978).

18. N. M. Ghoniem and G. L. Kulcinski, Nucl. Engr. and Des., 52, 111, (1979). See also N. M. Ghoniem and G. L. Kul- cinski, “Swelling of metals under pulsed irradiation,” Report UWFDM-179, Nuclear Engr. Dept., Univ. of Wisc., Madison, Wisc. (October 1976).

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