- Email: [email protected]
Hydrogen recycling properties of stainless steels
453
~~~~~~~
ir r~~,nt~~ and releas_e c~~~~~~t~~~ of st~~sss $eeb that co~t~b~te to ze recychng of a rn~et~~~~~ confined plasma are renewed- Detaxls are presented cm labora~~ rne~ur~m@nts of hydrogen resections decor tion, trapping and refease. CriticrEtldata needs are shown to include reflection below 100 E!tp I de~rFtxon cross sections at realistic energies, hydrogen surface coverage, and molecular recombination r&es for ~h~a~teri~ed fir& wall surfaces.
e
of isotope changeover in DITE [I]$ TFR f2lt XSX-B f3], and PDX [4].
0.1
2
fn the next ge~@ratio~ of rn~neti~~~ confined pIasma devices, such as TFTR and JE I , 8, major coneern is wall tritium inventor,zy, Since the ftrst wall of TFTR has 10R cm2 of surfme area, and a total volume of IO8 cm3 [5Jt its trapping characteristics can piay a significant role in the inventory. An addiis the tritium ertional concern over fuel recycli meation throu h the first walrY of advanced IL T devices such a~% ED, INTER, or ~~~. Excessive tritium permeation would require the impfementation of permeation barriers or additional tritium recovery equipment. Knowledge of the processes of hydrogen re~y~~in~is therefore important not only for the M~der~tand~n~ of the operation of present day devices but for the optimization of future reti@ designer aa v&l. This phper reviews recent development in three main me~ha~~rns for hvdroraen rttcvcfina from Srst wa#s: (1) reflection; ~2~-pho~~, el&tro< and ion desorp tion; and (3) hydrogen trapping, d~ff~~o~ and molecular reeomb~nat~on~ Au~~~~tjc stabinless steels were chosen &Bthe first wall materials of this review since the are found in the majority of megueticalfy con I ned plasma devices operatin today, end are the baseline materials in many of tIf,e advanced reactor designs uEder ~~~truc~~o~ or de~lopme~~. For further lnf~rmation on recycling properties of
4
8
10
$2
~~g~~ 1, The fractions amonnt of hyd~e~ fdeuterium) in the discharge as a function of the number of discharges after ~h~~~ver to d~uterium (hydragen) fill gas. The data are based on optical and residual gas rne~urern~~~~ [l-4]. first wall materiel the reader is referred to several other detailed review articles [6-141. Reviews of in&S recycling rn~as~rern@~t~can also be found in the Ii~rat~e fI5-20f.
The three main recyclirq rne~a~s to be: considered in this paper are &&rated in Figure 2. A fraction of the incident h dro en ffux on a solid surface is immed~tely re Hecte % as neutral atoms that retain a sign&ant &a&ion of their incideqt ener 7. A second mechanism for prompt reieme IS the duect de& tion of ada4rbed surface hydrogen by the incident Ttux of photons, electrons, iona and
454
K. L. Wiiron / fi>ldrogcn recwling properties of’stuinless steels
ENERGY (keV)
(O-Fe) 0. I I
1 .C
+=‘A00
/ .C
I C.0
01 . O8
5
% *P
8
a@ i
0.1
:
35
D---w
I H.D--SS
.
THOMAS
.
ECKSTEIN SiDENlVS
H-SS
.
H--C”
0
d
H--Fe
11
#!
D--Fe
A
o---Fe
0
i
-
W--Fe
I241 12 h .2 21
and LENSKJAERIZ
OEN and ROBlNSoN HAGGMAFx
IPA
and “ERBEEK
t? 6,2
and SlERSmx
ROWSON et al /2 0 JBCKSON 13 !i
*8
3i
. ,301
sj
@Cl! 0.0 1
m i
r,
i 0. I
t .o
REOUCEOENERGY
Figure 3. A comparison of theory and experiment for hydrogen reflection [21-301.
Figure 2. An ~lu~~ation of the various ion-solid interaction processes responsible for hydrogen recycling. charge-exchange neutral atoms. The deeorbed h drogen typically has a few electron volts of energ F”ln~l~,~dr~en atoms that come to rest in the so1:id beginto diffuB+, and ma undergo trappin and detra ing at various defec Ps (e.g. a vacancy) l!efore reac“g.mg the surface and recombining with other hydrogen atoms to desorb as molecules.
3. ~FLECTION Hydrogen reflection from stainless steel surfaces has been studied in detail by several investigators, including Eckstein ctnl[21,22], Sideniue and Lenekjaer [23], and Thomas and coworkers [24,251. Recent binary collision computer calculations have been carried out by Oen and Robinson [26,27], Robinson et Jackson [29], and Haggmark and Biersack implantation energies above 100 eV, the agreement between theory and experiment has been remarkably good, a~ shown in Figure 3. Calculations and experiments are also in reasonable agreement for the energy and angular distribution of reflected hydrogen 122,251. Typic~ly~the rellected particles come off with a continuous B ectrum of energies extending down from the inci*Bent energy [21,25]. However, below 160 eV little data axiet, and.calcu+ tions unng the binary collision approxrmutron with zero surface binding energy begrn to break down. Since most charge exchange neutrals in present day machines have energies tiO0 eV, the reflection coefffcient at low energies is of crucml importance for modelling of recycling [19,20]. Several new theoreti-
cal approaches are currently being tested. Jackson 311 has added a surface attractive field to simuIate the binding of hydrogen to a metal surface. A second theoretical approach is the use of molecuiar dynamics calculations [32], where the multi-body collisions present at low energies can be more realistically treated. 4. DESORPTION Hydrogen can be directly desorbed from surfaces by the incident flux of hotons, electrons, or h drogen ions and charge exe Kange neutrals created Buring a plasma discharge. The rate of release of hydrogen (JH) is given by: JH = J,N,+
(1)
where Ji is the incident flux {photon, electron, or ion), NH is the hydrogen surface coverage, and u is the cross section for desorption. The efficiency (11) is defined as: 1)= -Ju = NHO (2). Jt While the cross sections (g) is a fuRdament~ property of the desorption process, the efficiency (1))depends additionally on the particular hydrogen surface coverage at the time of the me~urement [33,343. Discussion of thermal desorption can be found in seetion 5.2. 4.1 Pho~e~rption Lichtman and co-workers [35-371 have found that the efficiency for photodesorption of residual gases from chromrum oxide surfaces of stainless steel increase8 from lo+ molecules/photon at 4 eV photon energy ta 5 x 1O-3 molecules/photon at 6.7 eV. (Sign~~cantly lower efficiencies were observed for
K.L. Wilson/ Hydrogen recycling properties of stainlesssteels
clean (i.e. non-oxidiaed) metal surfaces,) Recent experiments at energies in the vacuum ultraviolet and soft X-ray region (5 - 100 eV) have shown that the desorption cross sections rise dramatically when the photon energy equals the energy of a core electron [38]. This effect has been explained by a mechanism known as core-hole Auger decay 1391.Finally at X-ray photon energies (i.e. 210 keV) Brumbach and Kaminsky [40-421 did not observe any significant hydro en desorption from stainless steel. Hence, photodesor %ed hydrogen is likely to be produced primarily from the line and recombinaton radiation flux (at 5 - 500 eV photon energy], although the cross sections are generally not known. 4.2 Electron Stimulated Desorpt~on Early me~urements of electron stimulated desorption (ESD) of hydrogen found high efficiencies of (9 N 1 molecule/electron) for unbaked stainless steel surfaces bombarded with N 500 eV electrons 133, 43,441. While CO was the nrincipal was desorbed. hydrogen release was also detected. F&owing bake: out or long bombardment, the ef&iencv drooned to TZ?10-8~molecules/elec~on. A similar de&&se in hydrogen desorption efficiency with baking has recently been reported by Achard ebai [45] for 316 LN stainless steel, and by Leiegard and Schram 6461 for 304 L stainless steel Lelegard and Schram 1471 have also me~ured the desorption efficiency ss a function of electron energy for 316 L stainless steel. They observed a decrease in desorption efficiency from N 2 x to- 3 Hz/electron at 100 eV, to N 1 x lO-4 Hz/electron at 12 eV. They also concluded that the surface hydr en was constantly being replenished by diffusiono! rom the bulk. The ESD cross section for hydrogen desorption has been measured as a function of electron energy by Drinkwine and Lichtman [48]. As shown in Figure 4, the cross section is quite large, reaching a value of
0.0
0.5
455
l.0
15
20
incident energy k#IH>
Figure 5, IID cross section for deuterium desorption from stainless steel by hydrogen. Also shown are TRIM calculations for two values of the surface binding energies (E,) [52]. 5 x IO-'~cm2 at 200 eV. These data are in good Agreement with earlier cross section meas~ements of McCracken et txl133], who reported cross sections N tO-*ecm* for 600 eV electrons . 4.3 Ion Impact Desorption The ion impact desorption (BD) of hydrogen from stainless steel has been studied as a function of sample temperature by McCracken [49], and Erents and McCracken [50]. For liquid helium cooled surfaces, an efficiency of lo2 - lo3 atoms/incident ion was observed for 5 keV hydrogen, and attributed to a thermal spike desorption of a condensed hydrogen layer [50]. At 77 K, similar large Hz efficiencies were reported, resulting from the dissociation of water condensed on the surface 1491. At 300 K unbaked targets had cross sect’ons ,?Z&Yi6cm2 for adsorbed residual gases. with e h. ciencies of 1 - 10 atoms/ion. FollowinK bakeout at 343 K a cross section of 2 x lo-l7 cm was observed for the desor.ption of hydrogen by 5.6 keV D+ bombardment 1491.Preliminary BD me~urements have a&o been reported by Lelegard and Schram 1471. The most detailed IID measurements of hydrogen desorption from stainless steel are those of Bastasa [51,52]. Figure 5 shows his measurements of the deeorption of adsorbed deuterium by hydrogen in the energy range of 0.3 to 1.5 keV/hydrogen atom 1521.The cross section is quite far e over the relevant energy range for the charge exe f ange neutral ffux, indicating that IID can be a significant source of recycling. 4.4 Comparison of Desorption Processes
Figure 4. Comparison of total cross-section (a~) data for ESD of h dro en with cross-section data for ionisation of H! to&$, as a function of bombarding electron energy [48].
The relative importance of hoton, electron, and ion desorption of stainless steel is incident mean summ~ised in Table 1, energies are 10 eV for photons and electrons [54], and 200 eV for charge exchange neutral hydrogen [533. The three desorption processes may be comparrable, given the uncertainties in the photon flux,
456
K. L. Wilson / Hydrogen recycling properties
of stainlesssteels
+ 1.0 B
'r: 0.8 d 2 0.6 3
Table 1. Desorbed hydrogen flux per unit hydrogen cover age. and the photodesorption and ESD cross sections at the a propriate energies. Since the absolute desorption 1 uxes depend critically on the hydrogen wall coverage (NH), the typical surface hydrogen coverage durin a discharge must be determined before the overal“I contribution of desorption recesses to recycling can be quantitatively assesse B 5. HYDROGEN TRAPPING, DIFFUSION, AND MOLECULAR RECOMBINATION At 1 keV, only 30 % of the incident flux is directly reflected from stainless steel (see Figure 3). The remaining flux comes to‘ rest in the solid. As illustrated in Figure 2, the rate of release of hydrogen from a solid is governed by: (1) diffusion; (2) trapping at defects or impurities; (3) molecular recombination at the surface. Since these recesses are thermally activated, the hydrogen re Pease characteristics are a stron function of temperature. In addition, athermal e fiects such as ion beam enhanced detrappmg and diffusion must also be considered. 5.1 Thermal Diffusion and Trapping Many of the early studies of hydrogen trapping and release for stainless steels were carried out using the ion beam reemitrsion technique, where the partial ressure of the implanted gas in the target cham!Jer is measured during ion bombardment. With this techni ue the partial pressure than es can be related to tB e rate of release of the impf anted gas from the sample. This rate of release of gas is normaliaed to the incident ion flux, and termed the reemi’ssion rate (R). Early reemission measurements showed that implanted hydrogen was readily released from stainless steel during bombardment at room temperature [55-601. However, a large temperature dependence in the reemission rate was also observed. Figure 6, by Freeman et al [58], shows the trapping coefficient (defined as 1-R) as a function of 50 keV D+ fluence at 170, 300, and 390 K. For the 170 K implant a high trapping rate (i.e. virtually no reemiasion of implanted deuteriurn) was observed for fluences up to 1018 D/cm2, while for the 390 K implant no additional retention was found for fluences 22 x 10” D/cm2. Based on these considerations Freeman concluded that the diffusivity plays an important role in the deuterium release characteristics of stainless steel. More recent reemission measurements [24,61-641,have confirmed these general features for hydrogen release from stainless steels.
J 1016
1Ol7 Dose
1018
1Ol9
(atoms/anq
Figure 6. The trapping coefficient for stainless steel bombarded with 60 keV D+ at (a) 390 K (b) 300 K (c) 170 K [58].
Not all of the hydro en implanted into stainless steel is immediately ref eased during bombardment. Techniques such as thermal desorption spectroscopy (TDS), secondary ion mass spectroscopy (SIMS) and D(3 He, a)P and D(3 He, P)a nuclear reaction analysis (NRA) have been used to determine the post-impllt retention characteristics of stainless steels. ure 7 presents TDS data by Clausing et CJ [65] for 304 stainless steel ex osed to a 300 eV glow discharge for an hour. TYle desorption spectrum after bombardment corresponds to 2 x 1OleH/cm2 retention, while after 17 hours at room temperature the retention has decreased significantly. TDS measurements by Wilson and co-workers [66-691, and Thomas (241have shown that part of this retention is due to hydrogen that had diffused into the sample bulk during implantation. Farrell and Donnelly [70] have also observed trapping attributed to Ti atoms present in 321 stainless steel. Ah-Khan et 01have also postulated the trapping of hydrogen as molecular gas in pre-existing voids (71-731.
Figure 7. Hydr en desorption from stainless steel samples bombar8 ed with 4 milliamperes per square centimeter of 300 eV hydrogen for one hour [65].
K. L. Wilson/ Hydrogen recycling properties of stainlesssteels
agreement with this picture. Detailed transmission electron microsco y studies of the microstruc.P ess steels have been reported ture of implanted stain by Thomas and Wilson [78], and Jager and Roth [79]. Hydrogen implantation results in extensive interstitial loop radiation damage, but no bubbles were observed, to a 1 nm diameter detection limit. A significant enhancement of trap ing, with perhaps an 1: 0.1 - 0.2 eV increase in indm energy, ‘& stee“r followhas also been observed for stainless ing He+ predamsge [67,80,81]. Myers [Sll has sugested that the increased binding energy may result from deuterium chemisorption on internal surfaces of helium bubbles. Although no measurements have been made on hydrogen trapping in neutron-damaged stainless steels, the deuterium trapping measurements on Ni by Besenbacher et al [82] have shown no significant increase in the trap binding energy of deuterium in Ni preirradiated with Ni+ fa neutron damage simulation) compared to D+ damage alone.
304 LN 55 6 keV D3+ 5x1014 atomsicm2s 3x1018 atoms/cm*
A
295K IMPLANT
0
335K IMPLANT
457
-CALCULATION
5.2 Molecular Recombination
D oo,
,&J-F& _I
0.0
0.1
0.2 DEPTH
0.3 l&m
0.4
0I.5
J
Figure 8. Calculations and experimental data for D(“He, a)H nuclear reaction profiling of 304 LN samples impl~ted at 295 or 335 K, and rapidly quenched to 77 K afterwards [SQ]. Much of our information on hydrogen trapping at radiation damage sites (e.g. vacancies, interstitial loops, etc.) in stainless steels has come from profiling measurements. SIMS data by Roth et 0![74] and NRA data bv Altstetter et at 1751showed that hvdro en tra p&g in the radiation damage generatid bv t‘h.e ion Eearn can be a sixnificant source of retent&n. Figure 8, from the recent NRA profiling of Rohdansky et of [69] shows deuterium retention from tra ping as well as from bulk diffusion. Immediate Py after implantation at 295 K with 6 keV Dgz deuterium is found: (a) trapped in the near surface radiation damage (to a depth of N 0.1 pm, which equals the maximum range of the ions) at a concentration of a.03 D/metal atom; (b) diffusing into the bulk beyond the 0.1 pm ion range, at a concentration SO.01 D/metal atom. In a similar implant at 335 K, the deuterium concentrations in ~~~n~~~~~~~t~N~~~~~~~~~r~~~~ [sQ],indicate that there are two dominant radiation damage traps with binding energies (measured with respect to the interstitial hydrogen energy) of z 0.1 and 0.3 eV. ProfIling measurements by Wampler et at [76] and Picraux et d 1771are also in general
At higher temperatures (i.e. 2400 K) the radiation damage traps in stainless steel are not significantly populated owing to the relatively small trap binding energies ( SO.5 ev). Hydrogen release then becomes controlled b molecular recombination at the surface. Possib *Ke rate limiting steps include associative recombination of chemisorbed atoms on the surface [52,84,85] and direct recombination from bulk subsurface sites without equilibration at chemisorption sites [84,86]. In general, the rate of release of hydrogen from the solid (JH) can be expressed as: JH = krc2
(3)
where k, is the overall recombination rate constant and c is the h drogen concentration just beneath the surface, as d”etermined by the solution to the diffusion equation [84]. Measurements of k, are often done with the “plasma simulator” experiment [87921, where a glow discharge or plasma is initiated, and the chamber pressure is monitored. As the ions bombard and are retained in the walls, the chamber pressure decreases. Alternatively, when the dischar e is terminated the pressure rises as hydrogen is re&eased from the wall. Measurements of these pressure changes combined with numerical calculations are able to yield values of k,. An example of these measurements by Waelbroeck et al [87] is presented in Figure 9. Also shown are theoretical fits to the data usmg the PER1 computer code fQ3,94], which is a numerical solution to the diffusion equation with a recombination limited kinetics boundary condition. Figure 10 summsriaes plasma simulator, permeation, and ion beam measurements of k, for stainless steels [81,87-89,95-981. Also shown are theoretical c~c~ations for k, by Ali-Khan et d [71] and Baskes [84] that assume direct recombination as the rate limiting step. There is an enormous variation in the me~ureme~ts and c~cuiations, with over four
1, 1
68
: 3OLOC
PC, : 3*10
Torr
Iqr,. 0557A
1
5_
1 initiai
on the effects of oxygen on the release of hydrogen from the stainless steel walls of a plasma simulation experiment. Oxygen exposure clearly affects the recombination rate. In addition, Baskes [98] has shown that calculations of k, a.re sensitive to the assumed energy (i.e. depth distribution) of the impinging ions. Thus any measurement of k,. must consider the surface condition and the ion energy distribution in the analysis.
-
slope
0
5
10
15
20
25 tisl
Figure 9. Experimental data (line) and calculations (symbols) for the pressure change upon initiation of glow discharge bombardment of a stainless steel.
I
I
,
,
!
I
I
I
,
STAINLESS STEEL
\
THEORY ---
19dl,a=o.5
-*-
[841 .a= 5x I O-5
The release of hydrogen from stainless steels is therefore dependent on three thermally-activated rate limiting steps: (a) diffusion; (b) defect trapping; and (c) molecular recombination. The relative importance of these processes is illustrated in the recent ion beam reemission experiments of Wilson and Baskes [97]. Samples of 304 LN stainless steel were prebombarded with 10 keV D$ to fluences 25 x 1018 D/cm* at a chamber pressure of lo-’ Pa, in order to produce an oxide free surface [99]. The samples were then subjected to a cyclic implantation scenario at various temperatures. Figure 12 shows the reemission rates as a function of time after many cycles at 273, 323, or 373 K Also shown are DIFFUSE numerical calculations [loo] of hydrogen diffusion in the presence of point defect traps. Values for diffusivity, trapping, and recombination are consistent with previous measurements and calculations [69,84]. The data and calculations show a small reemission rate at 273 K, because of the low diffusivity at this temperature. At
80
-
:?oco ‘: QSOI
,
-29 ‘O
I.0
, 2.0
,
,
,
,
, 3.0
,
,
,
lOOO/T(K) Figure 10. Summary of recombination rate constant (k,) theory and experiment [71,81,84,87-89,9598]_ The factor, a, in the Baskes theory 1841 is the molecuIar sticking coefficient. orders of magnitude difference at room temperature, because the recombination rate constant is dependent not only on the material but also on the specific surface conditions (i.e. surface contamination, oxide composition, etc.) As an example, Figure 11 shows measurements by Clausing et aI [91]
Figure 11. The effect of wall conditioning on hydrogen recycling. The pre-conditioning is as follows (a) several hours exposure to the plasma during previous runs. (b,c,d) Exposed to 1 atm oxygen for l/2 h and,then exposed to bombardment with 100 eV hydrogen ions at 6~10’~ ions/cm% for the time indicated. Oxygen is removed during the plasma exposure so that the l/4 h, l/2h and 1 h exposure have decreasing amounts of oxygen [91].
459
K. L. Wilson / Hydrogen recycling properties of stainless steels
323
K the reemission rate is limited primarily by presence of 0.1 eV binding energy radiation damage traps. These traps fill during the beginning of each implantation period, but empty between bombardments. Finally at elevated tern eratures such as 373 K, the traps are not significant Py populated, and the recombination rate begins to limit hydrogen release.
IO keV D$ --+304
’
LN SS
2.7x
- ID 14’
D/W?8
120 ~
!
CALC.
0
1
2
3
4 FLIJENCE d
I
IO
1
t
20
30
I
40
i
50
6 cm*
7
8
Y
10
1
Figure 13. Deuterium saturation and isoto e exchange.in 316 stainless steel at 153 K, for di ! erent incident energies. Implantation was switched from D+ toH+ at the inflection points (e.g. 3.5 x 10’8/cm2 fluence at 14 keV). The symbols are data by Blewer et al [loll, and the solid lines are predictions of the local mixing model of Doyle et d [103].
273
d0
5
I
60
I
TIME (s)
Figure 12. Experimental data and calculations of reemission for 10 keV D3+ implantation of 304 LN stainless steel at various temperatures. Calculation arameters include trap concentration: 0.07 atom Praction; trap binding energy: 0.1 eV; diffusivity: 0.1 exp (-0.6/kT) cm*/s; k,: (4.5 x 10-15/ X/T) exp(0.52/kT) cm*/s; reflection: 0.15 [Q?]. 5.3 Athermal Processes In addition to thermally activated processest h droen can migrate in stainless steel via atherm J. , ion !!Iearn induced mechanisms. These effects are most easily observed in isotope exchange measurements conducted at low temperatures where thermal diffusion is nenliaible. Figure 13 shows NRA data for deuterium %&tion in-316 stainless steel at 153 K by Blewer et crf[loll. During D+ implantation all non-reflected deuterium is retained up to a critical fluence (e.g. N 1 x lo’* / cm* at 14 keV). Further implantation leads to blistering and to saturation in the deuterium retention. When the implanting ion is switched to protium (e.g. after 3.5 x lOI* /cm2 for 14 keV) the deuterium retention is observed to decrease. Since deuterium is immobile at 153 K in stainless steel, athermal mechanisms must be responsible for the observed decrease. Other similar measurements of isotope exchange in stainless steel at low temperature include those of Braganaa et al [63] and Farrell et al 1701. Models of isotope exchange generally employ radiation enhanced diffusion or trap filling mechanisms [63,101-1031. As an
example, Fi ure 13 also contains a calculation of the isotope c!l angeover made with the local mixing model of Doyle et al [103]. The model assumes that after ~turation at a s ecific depth, additional implanted hydrogen will 1 e released, with the specific isotope (i.e. H or D) determined by the local H/D concentration. The model provides an excellent fit to the data for stainless steel at 153 K, and also for other materials at temperatures where hydrogen thermal mobility is negligible. At higher temperatures (e.g. 2300 K) Bragansa et 0l[63] and Clausing et al [90] have demonstrated that recycling in stainless steel increases markedly due to the co-operation of thermally activated and ion induced processes. Hence the relative importance of ion induced effects on the release of hydrogen changes with increasing wall temperature.
6. APPLICATIONS 6.1 Fuel Recycling An important application of the measurements and theory of hydrogen tra ping and release from stainless steel is the model f!ing of fuel recycling in resent day tokamaks. A recent plasma-wall recyc Ying calculation by Howe [20] shows the success, as well aa the limitations en~untered because of the lack of critical data. Howe has cou led a plasma transport model with a first wall an a limiter model that ineludes repection, thermal diffusion and trapping, and ion induced detrapping. Figure 14 compares experiment and calculations for the line average plasma density (ii,) during ohmic and neutral beam injection dischar es in ISX-B. The observed decrease in ii, upon neutr 9 beam injection is explained in his calculations by a decrease in recycling (R,) that
460
K. L. Wilson / Hydroge?~ rec)ding
0
I
I
I
I
//OH
--_ - - - CALCULATED
o~gmo-11025
properties of stainless steels
combination rate on stainless steels near room temperature vary by four orders of magnitude. This ran e of values can have an enormous effect on pre I!!!. acted recycling and tritium inventory. As an example, Figure 15 shows a calculation by Baskes [104] of the bulk tritium inventory in TFTR o erated for 25 full power D-T shots per day for t Kree months with a wall temperature of 373 K. The three values of the recombination rate (k,) are within the reported variation shown in Figure 10. For k, = 2 x 1O-23 cm4/s, the tritium inventory is acceptably low. However, for k, = 2 x 1O-27 cm*/s the recombination is extremely slow, causing the bulk tritium inventory to exceed 5 x lo5 Ci within two months. Bakeout at 525 K for one week reduces this inventory by only a factor of three since the low recombination rate continues to inhibit release. Permeation through the first wall also increases to significant levels during this bakeout. Future experiments must determine the correct recombination characteristics for realistic first wall surfaces. New in-situ diagnostics to monitor surface properties of reactor first wall surfaces must also be developed.
t(msl
Figure 14. Top: Experimental and calculated line average density (n,) for ISX-B with and without neutral beam injection. Bottom: Calculated average pumping fraction (p) and recycle coefficient (R,)
results from the ion temperature rise upon injection. Hotter charge exchange neutral atoms have a lower reflection coefficient, and the non-reflected portion of this flux is deposited further into the wall, requiring longer to diffuse back to the SUTface. Hence there is a transient period after neutral beam injection where recycling is lowered (i.e. the wall “pumps” more hydrogen), and the plasma density decreases. While the model gives good qualitareement with measurements, Howe discusses tive severaa? deficiencies in the wall model that need to be resolved. Most of the charge exchange neutrals are below 200 eV, and since reflection accounts for 8090% of the recycling in this case, the lack of data and theor for reflection below 100 eV is a critical problem. 6 esorption rates of hydrogen by photons, electrons and ions must be determined before these processes can be included in the model. A third problem area is the proper recombination rate for a tokamak first wall. 6.2 Tritium
Inventory
in TFTR
Radiation damage trapping can influence hydrogen retention in devices operated near room ternperature but it will become less important in St steel at the elevated first-wall temperatures oiY!ki!R because of the lo9v hydrogen-defeet binding ener ‘es. The amount of hydrogen that is retained in the &t wall is then critically dependent on the recombination rate at the surface. Unfortuneately, present calculations and measurements for the hydrogen re-
ld4,
10
20
30
40
50
TIME
60
( days
70
80
90
)
Figure 15. Calculated bulk tritium retention 304 LN stainless steel wall of TFTR [104].
6.3. Tritium Reactors
Inventory
and Permeation
in the
in Advanced
The retention of hydrogen isotopes in the stainless steel first wall is of concern for both the tritium inventory and the potential for hydrogen embrittlement. Calculations of the hydrogen inventory in EPR and NUMAK have been carried out by Look and Baekes [105], and by Wienhold et 01 [94] for INTOR. In the temperature range of 600-900 K both calculations indicated that the average hydrcen bulk concentration will be &O a m. At these evels, hydrogen embrittlement is un fl! I ely, and the P; tritium inventory is acceptably low. Wienhold [94] calculates a tritium inventory in the first wall of only 3g tritium for INTOR operated at 675 K.
XL.
Wilson / Hydrogen recycling properties of stainless steels
However, the tritium permeation throu h the stainless steel first wall of a fusion reactor drst wall can be quite large. Wienhold’s calculations [94] for tritium permeation flux (OT) through the INTOR first wall into the coolant are given in Figure 16 for various incident wall fluxes and operating temperatures. The steady state permeation rate of 10 x 1021tritium atoms mm2 d-l corresponds to a loss of 16g tritium per day(l.6 x lo5 Ci/day). Techniques for reduction of these high permeation rates include permeation barriers at the first wall-coolant interface [53], and the use of less permeable materials in regions where the first wall or limiter is exposed to an energetic tritium charge exchange neutral flux [106].
$ I
2.
3
4
5
6
t[hlFi ure 16. Permeating tritium flux density @r through a BS wall of thickness 0.5 cm as function of time during 200 discharges of 100s and 10s interruption (solid curve: incident flux = 1V7 atoms cm-2 s-l; dashed curve: incident flux = 1Ol6atoms cm-2 s-l) 1941.
7. SAY Recent advances in the stud of ion-solid interactions have led to a detaile d picture of hydrogen recycling between the lasma and stainless steel wall of a magnetically con %ned fusion reactor. Processes that contribute to recycling include reflection, desorption, diffusion, tra ping, and molecular recombination. However, bePore accurate assessments of h drogen recycling in a fusion reactor can be m d, progress must be made in several areas. Critical data needs include low energy ( SlOO eV) reflection; desorption cross sections at appropriate energies; first wall hydrogen surface coversges; and molecu~areecombmation rates for realmtic first wall sur-
ACKNOWLEDGEMENTS I wish to acknowled e the many fruitful discussions with the etafI of spandia National Laboratories. Special thanks also go to J. Roberto (ORNL), J. Winter (KFA Jiilich), S. Myers, R. A. Kerst, and
461
M. I. Baskes (SNL) for use of their data prior to ublication. This work wss supported by the U. S. b epartment of Energy.
REFERENCES
Ill
G. M. McCracken, S. J. Fielding, S. K. Erents, A. Pospiesacsyk, P. E.Stott, Nucl. Fusion 18 (1978) 35. T&R Group, J. Nucl. Mater. 93 85 94 (1980)
J. Roberto, S. Kasai, L. E. Murray, J. E. Simpkins, S. P. Withrow, R. A. Zuhr, this conference. H. F. Dylla, private communication. J. L. Cecchi, J. Nucl. Mater. 93 & 94 (1980) 28. S. K. Erents, in B. Navinsek (ed) Physics of Ioniaed Gases 1976, J. Stefan Institute, Ljubljana, Yugoslavia, (1976) 299. [71 G. M. McCracken in Proc. Int. Symp. on Plasma Wall Interaction, Jiilich, Pergamon Press, (1976) 339. [81 B.M.U. Scheraer, J. Vat. Sci. Technol. 13 (1976) 420: r91 R. Behrisch, J. de Physique 38 (1977) C3. S. T. Picraux, Physics Today 30 (1977) 42. I;!$ W. Bauer, J. Nucl. Mater. 76 & 77 (1978) 3. 112: K. L. Wilson, IEEE Transactions on Nuclear Science NS-26 (1979) 1296. P31G. M. McCracken, P. E. Stott, Nuclear Fusion 19 (1979) 889. WI R. Behrisch in Physics of Plasmas Close to Thermonuclear Conditions, Pergamon Press (1980). WI E. S. Hotston and G. M. McCracken, J. Nucl. Mater. 63 (1976) 177. WI I$ S. Marmar, J. Nucl. Mater. 76 & 77 (1978)
[171S. ‘J. Fielding,
G. M. McCracken, P. E. Stott, J. Nucl. Mater. 76 & 77 (1978) 273. P81G. M. McCracken, J. Nucl. Mater. 85 & 86 (1979) 943. WI D. E. Voss, S. A. Cohen, J. Nucl. Mater. 93 & 94 (1980) 405. PO1y7 C. Howe, J. Nucl. Mater. 85 t 86 (1980)
Pll W: Eckstein,
F.E.P. Matschke, and H. Verbeek, J. Nucl. Mater. 63 (1976) 199. P21W. Eckstein and H. Verbeek, J. Nucl. Mater. 93 & 94 (1980) 518. (231 G. Sidenius, T. Lenskjaer, Nucl. Instr. Methods 132 (1976) 673. E. W. Thomas, J. Appl. Phys. 51 (1980) 1176. [:;I E. W.Thomas, R. Young, J. E. Harris, J. Nucl. Mater. 93 & 94 (1980) 524, [26] 0. S. Oen aad M. T. Robinson, Nucl. Instr. Methods 132 (1976) 647. [27] OS. Oen, M. T. Robinson, J. Nucl. Mater. 76 & 77 (1978) 370.
462
J. E. Robinson, K. K. Kwok, D, A. Thompson, Nucl. In&r. Methods 132 (1976) 667. PQIFb7P, Jackson, J. Nucl. Mater. 93 & 94 (1980) [=I
WI L. d.
Haggmark, J. P. Biersack, J. Nucl. Mater. 85 t 86 (1979) 1031. PI D.P. Jackson, Rad. Effects 49 (1980) 233. in Proc. Plasma Edge Exper1321M. T. Robins& iments and Modeling Workshop, UCLA; June 4-5. 1980. PPG510. D12. k. S. Barton, W. Dillon, [331G. ‘M. MiCrackenj Nuovo Cimento Suppl. 5 (1967) 146. Y. Shapira, CRC Crit. Rev. 1341D. Lichtman, Solid State & Mater. Sci. 8 (1978) 93. Surf. I351G. W. Fabel, S. M. Cox, D. Lichtman, Sci. 40 (1973) 571. [361 D. Lichtman, Y. Shapira, J. Nucl. Mater. 63 (1976) 184. Y. Shapira, D. Lichtman, 1371 M. J. Drinkwine, Adv. Chem. Ser. 158 (1976) 171. [381 M. L. Knotek, V. 0. Jones, V. Rehn, Phys. Rev. Lett. 43 (1979) 300. [3Ql M. L. Knotek and P. J. Feibelman, Phys. Rev. Lett. 40 (1978) 964. [401 S. Brumbach and M. Kaminsky, J.Nucl. Mater. 63 (1976) 188. 1411 S. Brumbach, M. Kaminsky, in Radiation Effects on Solid Surfaces, Adv. Chem. Ser. No. 158 (1976) 183. [421 J. Brumbach, M. Kaminsky, J. Appl. Phys. 47 (1976) 2844. [431 R. E. Clausing, 11th Natl. Vat. Symp., AVS Chicago, 1964. 1441 U. Cummings, N. Dean, F. Johnson, J. Jurow, J. Voss, J. *ai. Sci. Technol. 8 (1971) 348. M. H. Achard, R. Calder, A. Mathewson, Vac1451 uum 29 (1978) 53. I461 J. Lelegard, A. Schram, J. Nucl. Mater. 76 & 77 (1978) 637. 1471J. Lelegard and A. Schram, in Proc. Int. Symp. on Plasma Wall Interaction, Pergamon Press (1977) 293. I481M. J. Drinkwine and D. Lichtman, J. Vat. Sci. Technol. 15 (1978) 74. 49 G. M. McCracken, Vacuum 24 (1974) 463. t50 1 S. K. Erents and G. M. McCracken, J. Appl. Phys. 44 (1973) 3139. 1511 R. J. Bastass, private communication. [52] R. J. Bastasa and L. G. Haggmark, this conference. [531 W. M. Stacey, M. A. Abdou, J. A. Schmidt, T. E. Shannon, J. K. Griffith and the U. S. INTOR Group, U.S INTOR $he U.S. Contribution to the International To&nak Reactor Phase-l Workshop, Conceptual Design, INTOR/81-1 (1981). [541 Fusion Reactor Materials Program Plan, Section III, U.S. D.O.E./ET-0032/3 (1978), Appendix. [551 V. A. Simonev, G. F. Kleimonov, A. G. Mileshkin and K. A. Kochnev, Nucl. Fusion Suppl. 1,
(1962) 325. E. S. Borovik, 1561
N. P. Katrich and G, T. Nikolaev, Atomn Energyia 18, (1965) 91. [571 E. S. Borovik, N. P. Katrich and G. T. Nikolaev, Atmn Energyia 21, (1966) 339. 1581 N. J. F’reeman, I. D. Latimer and N. R. Daly, Nature 212, (1966) 1346. 1591 G. M. McCracken and J. H. C. Maple, Brit. J. Appl. Phys. 18, (1967) 919. A. if301 L. Hunt, C. C. Damm, and R. K. Goodman, Lawrence Livermore Laboratory, UCID-16408 (1964) Draft; 1973 MS). WI W. Bauer and G. J. Thomas, J. Nucl. Mater. 53, (1974) 127. PI K. L. Wilson, G. J. Thomas, and W. Bauer, Nucl. Technol. 29, (1976) 322. PI C. M. Braganaa, S. K. Erents, E. S. Hotston, and G. M. McCracken, J. Nucl. Mater. 76 & 77, (1978) 298. PI B. Navinsek, M. Peternel, A. Zabkar, S. K. Erents, J. Nucl. Mater. 93 & 94 (1980) 739. 1651R. E. Clausing, L. C. Emerson, L. Heatherly, and R. J. Colchin, in Proc. Intl. Conf. on Plasma Wall Interaction, Jiilich (1976) 573. P61K. L. Wilson and M. I. Baskes, J. Nucl. Mater. 74 (1978) 179. PI K. L. Wilson and M. I. Baskes, J. Nucl. Mater. 76 & 77 (1978) 291. WI K. L. Wilson and A. E. Pontau, J. Nucl. Mater. 85 & 86 (1979) 989. PQIJ. Bohdansky, K. L. Wilson, A. E. Pontau, L. G. Haggmark, M. I. Baskes, J. Nucl. Mater. 93 & 94 (1980) 594. G. Farrell and S. E. Donnelly, 6. Nucl. Mater. 1701 76 &s 77 322, (1978) 132. VI I. Ah-Khan, K. J. Dieta, F. Waelbroeck, P. Wienhold, J. Nucl. Mater. 74 (1978) 132. P211. Ah-Khan, K. J. Dieta, F. Waelbroeck, P. Wienhold, J. Nucl. Mater 76 & 77 (1978) 263. F31I. Ali-Khan, K. J. Diets, F. G. Waelbroeck, P. toehold, J. Nucl. Mater. 85 & 86 (1979)
P4
J. Rbth, J. Bohdansky and W. 0. Hofer, in Proc. Intl. Symp. on Plasma Wall Interaction, Jiilich, Pergamon Press, (1976) 309. [75] C. J. Altstetter, R. Behrisch, J. Bottiger, F. Pohl, B.M.U. Scheraer, Nucl. Instr. Methods 149 (1978) 5s. 1761W. R. Wam pfer, B. L.Doyle, D. K. Brice and S. T. Picraux, SAND80-li84 (1980). S. T. Picraux and W. R. Wampler, J. Nucl. 1771 Mater. 93 & 94 (1980) 853. i781 G. J. Thomas and K. L. Wilson, J. Nucl. Mater. 76 & 77 (1978) 332. L7QlW.J&ger and J. Roth, J. Nucf. Mater. 93 & 94 (1980) 756. 1801 K. L. Wilson, A. E, Pontau, L. G. Haggmarjc, EieLcpkes, J. Roth, J. Bohdansky, this con1811 S. Mye&, to be published
in the Proceedings
of
K. L. Wilson / Hydrogen recycling properties of stainless steels
the Environmental Degradation of Engineering Materials, V,P.I., Blacksburg, VA, September 21-23, 1981. F. Besenbacher, J. Bottiger, T. Laursen, W. If321 Moller, J. Nucl. Mater. 93 & 94 (1980) 617. I. Ali-Khan, K. J. Dietz, F. G. Waelbroeck, and 1831 P7Wienhold, J. Nucl. Mater. 76 & 77 (1978) M. i. Baskes, J. Nucl. Mater. 92 (1980) 318. R. J. Madix, G. Ertl, K. Christman, Chem. Phys. Lett. 62 (1979) 38. G. Comsa, R. David, B. J. Schumacher, Surf. Sci. 95 (1980) L210. F. Waelbroeck, J. Winter, P. Wienhold, this conference. R. A. Kerst, this conference. P. Wienhold, R. E. Clausing, F. Waelbroeck, J. Nucl. Mater. 93 & 94 (1980) 540. 1901R. E. Clausing, L. C. Emerson, L. Heatherly, J. Nucl. Mater. 85 & 86 (1979) 542. ]911 R. E. Clausing, L. C. Emerson and L. Heatherly, J. Nucl. 76 & 77 (1978) 267. F. [921 Waelbroeck, K. J. Diets, P. Wienhold, J. Winter, I. Ah-Khan, H. Merkens, E. Rota, J. Nucl; Mater. 93 & 94 (1980) 839. 1931P. Wienhold, I Ah-Khan, K. J. Dietz, M. Profant, ytioyaelbroeck, J. Nucl. Mater. 85 & 86 (1979) 1941P. Wienhold, M. Profant, F. Waelbroeck, J. Winter, J. Nucl. Mater. 93 & 94 (1980) 866. 1951M. Braun, B. Emmoth, F. Waelbroeck, P. Wienhold, J. Nucl. Mater. 93 & 94 (1980) 861. 96 I. Ah-Khan, referenced in [95]. I97I K. L. Wilson and M. I. Baskes, unpublished data. 1981M. I. Baskes, accepted for publication in J. Nucl. Mater. 1991R. J. Bastasz and G. J. Thomas, J. Nucl. Mater. 77 (1978)183. 100 M. I. Baskes, SAND80-8201 (1980). 101 R. S. Blewer, R. Behrisch, B.M.U. Scherzer, R. Schulz, J. Nucl. Mater 76 & 77 (1978) 305. [102]E. S. Hotston, J. Nucl. Mater. 88 (1980) 279.
II
[103]B. L. Doyle, W. R. Wampler, D. K. Brice, S. &Picraux, J. Nucl. Mater. 93 & 94 (1980) [104]M. I. Baskes, private communication. [105]G. W. Look and M. I. Baskes, J. Nucl. Mater. 85 & 86 (1979) 995. [106]C. A. Flanagan, D. Steiner, G. E. Smith, Fusion En ‘neering .Design Center Staff, Initial Trade an f Design Studies for the Fusion Engineering Device, ORNL/TM-7777 (1981).
463
~~~~~~~
ir r~~,nt~~ and releas_e c~~~~~~t~~~ of st~~sss $eeb that co~t~b~te to ze recychng of a rn~et~~~~~ confined plasma are renewed- Detaxls are presented cm labora~~ rne~ur~m@nts of hydrogen resections decor tion, trapping and refease. CriticrEtldata needs are shown to include reflection below 100 E!tp I de~rFtxon cross sections at realistic energies, hydrogen surface coverage, and molecular recombination r&es for ~h~a~teri~ed fir& wall surfaces.
e
of isotope changeover in DITE [I]$ TFR f2lt XSX-B f3], and PDX [4].
0.1
2
fn the next ge~@ratio~ of rn~neti~~~ confined pIasma devices, such as TFTR and JE I , 8, major coneern is wall tritium inventor,zy, Since the ftrst wall of TFTR has 10R cm2 of surfme area, and a total volume of IO8 cm3 [5Jt its trapping characteristics can piay a significant role in the inventory. An addiis the tritium ertional concern over fuel recycli meation throu h the first walrY of advanced IL T devices such a~% ED, INTER, or ~~~. Excessive tritium permeation would require the impfementation of permeation barriers or additional tritium recovery equipment. Knowledge of the processes of hydrogen re~y~~in~is therefore important not only for the M~der~tand~n~ of the operation of present day devices but for the optimization of future reti@ designer aa v&l. This phper reviews recent development in three main me~ha~~rns for hvdroraen rttcvcfina from Srst wa#s: (1) reflection; ~2~-pho~~, el&tro< and ion desorp tion; and (3) hydrogen trapping, d~ff~~o~ and molecular reeomb~nat~on~ Au~~~~tjc stabinless steels were chosen &Bthe first wall materials of this review since the are found in the majority of megueticalfy con I ned plasma devices operatin today, end are the baseline materials in many of tIf,e advanced reactor designs uEder ~~~truc~~o~ or de~lopme~~. For further lnf~rmation on recycling properties of
4
8
10
$2
~~g~~ 1, The fractions amonnt of hyd~e~ fdeuterium) in the discharge as a function of the number of discharges after ~h~~~ver to d~uterium (hydragen) fill gas. The data are based on optical and residual gas rne~urern~~~~ [l-4]. first wall materiel the reader is referred to several other detailed review articles [6-141. Reviews of in&S recycling rn~as~rern@~t~can also be found in the Ii~rat~e fI5-20f.
The three main recyclirq rne~a~s to be: considered in this paper are &&rated in Figure 2. A fraction of the incident h dro en ffux on a solid surface is immed~tely re Hecte % as neutral atoms that retain a sign&ant &a&ion of their incideqt ener 7. A second mechanism for prompt reieme IS the duect de& tion of ada4rbed surface hydrogen by the incident Ttux of photons, electrons, iona and
454
K. L. Wiiron / fi>ldrogcn recwling properties of’stuinless steels
ENERGY (keV)
(O-Fe) 0. I I
1 .C
+=‘A00
/ .C
I C.0
01 . O8
5
% *P
8
a@ i
0.1
:
35
D---w
I H.D--SS
.
THOMAS
.
ECKSTEIN SiDENlVS
H-SS
.
H--C”
0
d
H--Fe
11
#!
D--Fe
A
o---Fe
0
i
-
W--Fe
I241 12 h .2 21
and LENSKJAERIZ
OEN and ROBlNSoN HAGGMAFx
IPA
and “ERBEEK
t? 6,2
and SlERSmx
ROWSON et al /2 0 JBCKSON 13 !i
*8
3i
. ,301
sj
@Cl! 0.0 1
m i
r,
i 0. I
t .o
REOUCEOENERGY
Figure 3. A comparison of theory and experiment for hydrogen reflection [21-301.
Figure 2. An ~lu~~ation of the various ion-solid interaction processes responsible for hydrogen recycling. charge-exchange neutral atoms. The deeorbed h drogen typically has a few electron volts of energ F”ln~l~,~dr~en atoms that come to rest in the so1:id beginto diffuB+, and ma undergo trappin and detra ing at various defec Ps (e.g. a vacancy) l!efore reac“g.mg the surface and recombining with other hydrogen atoms to desorb as molecules.
3. ~FLECTION Hydrogen reflection from stainless steel surfaces has been studied in detail by several investigators, including Eckstein ctnl[21,22], Sideniue and Lenekjaer [23], and Thomas and coworkers [24,251. Recent binary collision computer calculations have been carried out by Oen and Robinson [26,27], Robinson et Jackson [29], and Haggmark and Biersack implantation energies above 100 eV, the agreement between theory and experiment has been remarkably good, a~ shown in Figure 3. Calculations and experiments are also in reasonable agreement for the energy and angular distribution of reflected hydrogen 122,251. Typic~ly~the rellected particles come off with a continuous B ectrum of energies extending down from the inci*Bent energy [21,25]. However, below 160 eV little data axiet, and.calcu+ tions unng the binary collision approxrmutron with zero surface binding energy begrn to break down. Since most charge exchange neutrals in present day machines have energies tiO0 eV, the reflection coefffcient at low energies is of crucml importance for modelling of recycling [19,20]. Several new theoreti-
cal approaches are currently being tested. Jackson 311 has added a surface attractive field to simuIate the binding of hydrogen to a metal surface. A second theoretical approach is the use of molecuiar dynamics calculations [32], where the multi-body collisions present at low energies can be more realistically treated. 4. DESORPTION Hydrogen can be directly desorbed from surfaces by the incident flux of hotons, electrons, or h drogen ions and charge exe Kange neutrals created Buring a plasma discharge. The rate of release of hydrogen (JH) is given by: JH = J,N,+
(1)
where Ji is the incident flux {photon, electron, or ion), NH is the hydrogen surface coverage, and u is the cross section for desorption. The efficiency (11) is defined as: 1)= -Ju = NHO (2). Jt While the cross sections (g) is a fuRdament~ property of the desorption process, the efficiency (1))depends additionally on the particular hydrogen surface coverage at the time of the me~urement [33,343. Discussion of thermal desorption can be found in seetion 5.2. 4.1 Pho~e~rption Lichtman and co-workers [35-371 have found that the efficiency for photodesorption of residual gases from chromrum oxide surfaces of stainless steel increase8 from lo+ molecules/photon at 4 eV photon energy ta 5 x 1O-3 molecules/photon at 6.7 eV. (Sign~~cantly lower efficiencies were observed for
K.L. Wilson/ Hydrogen recycling properties of stainlesssteels
clean (i.e. non-oxidiaed) metal surfaces,) Recent experiments at energies in the vacuum ultraviolet and soft X-ray region (5 - 100 eV) have shown that the desorption cross sections rise dramatically when the photon energy equals the energy of a core electron [38]. This effect has been explained by a mechanism known as core-hole Auger decay 1391.Finally at X-ray photon energies (i.e. 210 keV) Brumbach and Kaminsky [40-421 did not observe any significant hydro en desorption from stainless steel. Hence, photodesor %ed hydrogen is likely to be produced primarily from the line and recombinaton radiation flux (at 5 - 500 eV photon energy], although the cross sections are generally not known. 4.2 Electron Stimulated Desorpt~on Early me~urements of electron stimulated desorption (ESD) of hydrogen found high efficiencies of (9 N 1 molecule/electron) for unbaked stainless steel surfaces bombarded with N 500 eV electrons 133, 43,441. While CO was the nrincipal was desorbed. hydrogen release was also detected. F&owing bake: out or long bombardment, the ef&iencv drooned to TZ?10-8~molecules/elec~on. A similar de&&se in hydrogen desorption efficiency with baking has recently been reported by Achard ebai [45] for 316 LN stainless steel, and by Leiegard and Schram 6461 for 304 L stainless steel Lelegard and Schram 1471 have also me~ured the desorption efficiency ss a function of electron energy for 316 L stainless steel. They observed a decrease in desorption efficiency from N 2 x to- 3 Hz/electron at 100 eV, to N 1 x lO-4 Hz/electron at 12 eV. They also concluded that the surface hydr en was constantly being replenished by diffusiono! rom the bulk. The ESD cross section for hydrogen desorption has been measured as a function of electron energy by Drinkwine and Lichtman [48]. As shown in Figure 4, the cross section is quite large, reaching a value of
0.0
0.5
455
l.0
15
20
incident energy k#IH>
Figure 5, IID cross section for deuterium desorption from stainless steel by hydrogen. Also shown are TRIM calculations for two values of the surface binding energies (E,) [52]. 5 x IO-'~cm2 at 200 eV. These data are in good Agreement with earlier cross section meas~ements of McCracken et txl133], who reported cross sections N tO-*ecm* for 600 eV electrons . 4.3 Ion Impact Desorption The ion impact desorption (BD) of hydrogen from stainless steel has been studied as a function of sample temperature by McCracken [49], and Erents and McCracken [50]. For liquid helium cooled surfaces, an efficiency of lo2 - lo3 atoms/incident ion was observed for 5 keV hydrogen, and attributed to a thermal spike desorption of a condensed hydrogen layer [50]. At 77 K, similar large Hz efficiencies were reported, resulting from the dissociation of water condensed on the surface 1491. At 300 K unbaked targets had cross sect’ons ,?Z&Yi6cm2 for adsorbed residual gases. with e h. ciencies of 1 - 10 atoms/ion. FollowinK bakeout at 343 K a cross section of 2 x lo-l7 cm was observed for the desor.ption of hydrogen by 5.6 keV D+ bombardment 1491.Preliminary BD me~urements have a&o been reported by Lelegard and Schram 1471. The most detailed IID measurements of hydrogen desorption from stainless steel are those of Bastasa [51,52]. Figure 5 shows his measurements of the deeorption of adsorbed deuterium by hydrogen in the energy range of 0.3 to 1.5 keV/hydrogen atom 1521.The cross section is quite far e over the relevant energy range for the charge exe f ange neutral ffux, indicating that IID can be a significant source of recycling. 4.4 Comparison of Desorption Processes
Figure 4. Comparison of total cross-section (a~) data for ESD of h dro en with cross-section data for ionisation of H! to&$, as a function of bombarding electron energy [48].
The relative importance of hoton, electron, and ion desorption of stainless steel is incident mean summ~ised in Table 1, energies are 10 eV for photons and electrons [54], and 200 eV for charge exchange neutral hydrogen [533. The three desorption processes may be comparrable, given the uncertainties in the photon flux,
456
K. L. Wilson / Hydrogen recycling properties
of stainlesssteels
+ 1.0 B
'r: 0.8 d 2 0.6 3
Table 1. Desorbed hydrogen flux per unit hydrogen cover age. and the photodesorption and ESD cross sections at the a propriate energies. Since the absolute desorption 1 uxes depend critically on the hydrogen wall coverage (NH), the typical surface hydrogen coverage durin a discharge must be determined before the overal“I contribution of desorption recesses to recycling can be quantitatively assesse B 5. HYDROGEN TRAPPING, DIFFUSION, AND MOLECULAR RECOMBINATION At 1 keV, only 30 % of the incident flux is directly reflected from stainless steel (see Figure 3). The remaining flux comes to‘ rest in the solid. As illustrated in Figure 2, the rate of release of hydrogen from a solid is governed by: (1) diffusion; (2) trapping at defects or impurities; (3) molecular recombination at the surface. Since these recesses are thermally activated, the hydrogen re Pease characteristics are a stron function of temperature. In addition, athermal e fiects such as ion beam enhanced detrappmg and diffusion must also be considered. 5.1 Thermal Diffusion and Trapping Many of the early studies of hydrogen trapping and release for stainless steels were carried out using the ion beam reemitrsion technique, where the partial ressure of the implanted gas in the target cham!Jer is measured during ion bombardment. With this techni ue the partial pressure than es can be related to tB e rate of release of the impf anted gas from the sample. This rate of release of gas is normaliaed to the incident ion flux, and termed the reemi’ssion rate (R). Early reemission measurements showed that implanted hydrogen was readily released from stainless steel during bombardment at room temperature [55-601. However, a large temperature dependence in the reemission rate was also observed. Figure 6, by Freeman et al [58], shows the trapping coefficient (defined as 1-R) as a function of 50 keV D+ fluence at 170, 300, and 390 K. For the 170 K implant a high trapping rate (i.e. virtually no reemiasion of implanted deuteriurn) was observed for fluences up to 1018 D/cm2, while for the 390 K implant no additional retention was found for fluences 22 x 10” D/cm2. Based on these considerations Freeman concluded that the diffusivity plays an important role in the deuterium release characteristics of stainless steel. More recent reemission measurements [24,61-641,have confirmed these general features for hydrogen release from stainless steels.
J 1016
1Ol7 Dose
1018
1Ol9
(atoms/anq
Figure 6. The trapping coefficient for stainless steel bombarded with 60 keV D+ at (a) 390 K (b) 300 K (c) 170 K [58].
Not all of the hydro en implanted into stainless steel is immediately ref eased during bombardment. Techniques such as thermal desorption spectroscopy (TDS), secondary ion mass spectroscopy (SIMS) and D(3 He, a)P and D(3 He, P)a nuclear reaction analysis (NRA) have been used to determine the post-impllt retention characteristics of stainless steels. ure 7 presents TDS data by Clausing et CJ [65] for 304 stainless steel ex osed to a 300 eV glow discharge for an hour. TYle desorption spectrum after bombardment corresponds to 2 x 1OleH/cm2 retention, while after 17 hours at room temperature the retention has decreased significantly. TDS measurements by Wilson and co-workers [66-691, and Thomas (241have shown that part of this retention is due to hydrogen that had diffused into the sample bulk during implantation. Farrell and Donnelly [70] have also observed trapping attributed to Ti atoms present in 321 stainless steel. Ah-Khan et 01have also postulated the trapping of hydrogen as molecular gas in pre-existing voids (71-731.
Figure 7. Hydr en desorption from stainless steel samples bombar8 ed with 4 milliamperes per square centimeter of 300 eV hydrogen for one hour [65].
K. L. Wilson/ Hydrogen recycling properties of stainlesssteels
agreement with this picture. Detailed transmission electron microsco y studies of the microstruc.P ess steels have been reported ture of implanted stain by Thomas and Wilson [78], and Jager and Roth [79]. Hydrogen implantation results in extensive interstitial loop radiation damage, but no bubbles were observed, to a 1 nm diameter detection limit. A significant enhancement of trap ing, with perhaps an 1: 0.1 - 0.2 eV increase in indm energy, ‘& stee“r followhas also been observed for stainless ing He+ predamsge [67,80,81]. Myers [Sll has sugested that the increased binding energy may result from deuterium chemisorption on internal surfaces of helium bubbles. Although no measurements have been made on hydrogen trapping in neutron-damaged stainless steels, the deuterium trapping measurements on Ni by Besenbacher et al [82] have shown no significant increase in the trap binding energy of deuterium in Ni preirradiated with Ni+ fa neutron damage simulation) compared to D+ damage alone.
304 LN 55 6 keV D3+ 5x1014 atomsicm2s 3x1018 atoms/cm*
A
295K IMPLANT
0
335K IMPLANT
457
-CALCULATION
5.2 Molecular Recombination
D oo,
,&J-F& _I
0.0
0.1
0.2 DEPTH
0.3 l&m
0.4
0I.5
J
Figure 8. Calculations and experimental data for D(“He, a)H nuclear reaction profiling of 304 LN samples impl~ted at 295 or 335 K, and rapidly quenched to 77 K afterwards [SQ]. Much of our information on hydrogen trapping at radiation damage sites (e.g. vacancies, interstitial loops, etc.) in stainless steels has come from profiling measurements. SIMS data by Roth et 0![74] and NRA data bv Altstetter et at 1751showed that hvdro en tra p&g in the radiation damage generatid bv t‘h.e ion Eearn can be a sixnificant source of retent&n. Figure 8, from the recent NRA profiling of Rohdansky et of [69] shows deuterium retention from tra ping as well as from bulk diffusion. Immediate Py after implantation at 295 K with 6 keV Dgz deuterium is found: (a) trapped in the near surface radiation damage (to a depth of N 0.1 pm, which equals the maximum range of the ions) at a concentration of a.03 D/metal atom; (b) diffusing into the bulk beyond the 0.1 pm ion range, at a concentration SO.01 D/metal atom. In a similar implant at 335 K, the deuterium concentrations in ~~~n~~~~~~~t~N~~~~~~~~~r~~~~ [sQ],indicate that there are two dominant radiation damage traps with binding energies (measured with respect to the interstitial hydrogen energy) of z 0.1 and 0.3 eV. ProfIling measurements by Wampler et at [76] and Picraux et d 1771are also in general
At higher temperatures (i.e. 2400 K) the radiation damage traps in stainless steel are not significantly populated owing to the relatively small trap binding energies ( SO.5 ev). Hydrogen release then becomes controlled b molecular recombination at the surface. Possib *Ke rate limiting steps include associative recombination of chemisorbed atoms on the surface [52,84,85] and direct recombination from bulk subsurface sites without equilibration at chemisorption sites [84,86]. In general, the rate of release of hydrogen from the solid (JH) can be expressed as: JH = krc2
(3)
where k, is the overall recombination rate constant and c is the h drogen concentration just beneath the surface, as d”etermined by the solution to the diffusion equation [84]. Measurements of k, are often done with the “plasma simulator” experiment [87921, where a glow discharge or plasma is initiated, and the chamber pressure is monitored. As the ions bombard and are retained in the walls, the chamber pressure decreases. Alternatively, when the dischar e is terminated the pressure rises as hydrogen is re&eased from the wall. Measurements of these pressure changes combined with numerical calculations are able to yield values of k,. An example of these measurements by Waelbroeck et al [87] is presented in Figure 9. Also shown are theoretical fits to the data usmg the PER1 computer code fQ3,94], which is a numerical solution to the diffusion equation with a recombination limited kinetics boundary condition. Figure 10 summsriaes plasma simulator, permeation, and ion beam measurements of k, for stainless steels [81,87-89,95-981. Also shown are theoretical c~c~ations for k, by Ali-Khan et d [71] and Baskes [84] that assume direct recombination as the rate limiting step. There is an enormous variation in the me~ureme~ts and c~cuiations, with over four
1, 1
68
: 3OLOC
PC, : 3*10
Torr
Iqr,. 0557A
1
5_
1 initiai
on the effects of oxygen on the release of hydrogen from the stainless steel walls of a plasma simulation experiment. Oxygen exposure clearly affects the recombination rate. In addition, Baskes [98] has shown that calculations of k, a.re sensitive to the assumed energy (i.e. depth distribution) of the impinging ions. Thus any measurement of k,. must consider the surface condition and the ion energy distribution in the analysis.
-
slope
0
5
10
15
20
25 tisl
Figure 9. Experimental data (line) and calculations (symbols) for the pressure change upon initiation of glow discharge bombardment of a stainless steel.
I
I
,
,
!
I
I
I
,
STAINLESS STEEL
\
THEORY ---
19dl,a=o.5
-*-
[841 .a= 5x I O-5
The release of hydrogen from stainless steels is therefore dependent on three thermally-activated rate limiting steps: (a) diffusion; (b) defect trapping; and (c) molecular recombination. The relative importance of these processes is illustrated in the recent ion beam reemission experiments of Wilson and Baskes [97]. Samples of 304 LN stainless steel were prebombarded with 10 keV D$ to fluences 25 x 1018 D/cm* at a chamber pressure of lo-’ Pa, in order to produce an oxide free surface [99]. The samples were then subjected to a cyclic implantation scenario at various temperatures. Figure 12 shows the reemission rates as a function of time after many cycles at 273, 323, or 373 K Also shown are DIFFUSE numerical calculations [loo] of hydrogen diffusion in the presence of point defect traps. Values for diffusivity, trapping, and recombination are consistent with previous measurements and calculations [69,84]. The data and calculations show a small reemission rate at 273 K, because of the low diffusivity at this temperature. At
80
-
:?oco ‘: QSOI
,
-29 ‘O
I.0
, 2.0
,
,
,
,
, 3.0
,
,
,
lOOO/T(K) Figure 10. Summary of recombination rate constant (k,) theory and experiment [71,81,84,87-89,9598]_ The factor, a, in the Baskes theory 1841 is the molecuIar sticking coefficient. orders of magnitude difference at room temperature, because the recombination rate constant is dependent not only on the material but also on the specific surface conditions (i.e. surface contamination, oxide composition, etc.) As an example, Figure 11 shows measurements by Clausing et aI [91]
Figure 11. The effect of wall conditioning on hydrogen recycling. The pre-conditioning is as follows (a) several hours exposure to the plasma during previous runs. (b,c,d) Exposed to 1 atm oxygen for l/2 h and,then exposed to bombardment with 100 eV hydrogen ions at 6~10’~ ions/cm% for the time indicated. Oxygen is removed during the plasma exposure so that the l/4 h, l/2h and 1 h exposure have decreasing amounts of oxygen [91].
459
K. L. Wilson / Hydrogen recycling properties of stainless steels
323
K the reemission rate is limited primarily by presence of 0.1 eV binding energy radiation damage traps. These traps fill during the beginning of each implantation period, but empty between bombardments. Finally at elevated tern eratures such as 373 K, the traps are not significant Py populated, and the recombination rate begins to limit hydrogen release.
IO keV D$ --+304
’
LN SS
2.7x
- ID 14’
D/W?8
120 ~
!
CALC.
0
1
2
3
4 FLIJENCE d
I
IO
1
t
20
30
I
40
i
50
6 cm*
7
8
Y
10
1
Figure 13. Deuterium saturation and isoto e exchange.in 316 stainless steel at 153 K, for di ! erent incident energies. Implantation was switched from D+ toH+ at the inflection points (e.g. 3.5 x 10’8/cm2 fluence at 14 keV). The symbols are data by Blewer et al [loll, and the solid lines are predictions of the local mixing model of Doyle et d [103].
273
d0
5
I
60
I
TIME (s)
Figure 12. Experimental data and calculations of reemission for 10 keV D3+ implantation of 304 LN stainless steel at various temperatures. Calculation arameters include trap concentration: 0.07 atom Praction; trap binding energy: 0.1 eV; diffusivity: 0.1 exp (-0.6/kT) cm*/s; k,: (4.5 x 10-15/ X/T) exp(0.52/kT) cm*/s; reflection: 0.15 [Q?]. 5.3 Athermal Processes In addition to thermally activated processest h droen can migrate in stainless steel via atherm J. , ion !!Iearn induced mechanisms. These effects are most easily observed in isotope exchange measurements conducted at low temperatures where thermal diffusion is nenliaible. Figure 13 shows NRA data for deuterium %&tion in-316 stainless steel at 153 K by Blewer et crf[loll. During D+ implantation all non-reflected deuterium is retained up to a critical fluence (e.g. N 1 x lo’* / cm* at 14 keV). Further implantation leads to blistering and to saturation in the deuterium retention. When the implanting ion is switched to protium (e.g. after 3.5 x lOI* /cm2 for 14 keV) the deuterium retention is observed to decrease. Since deuterium is immobile at 153 K in stainless steel, athermal mechanisms must be responsible for the observed decrease. Other similar measurements of isotope exchange in stainless steel at low temperature include those of Braganaa et al [63] and Farrell et al 1701. Models of isotope exchange generally employ radiation enhanced diffusion or trap filling mechanisms [63,101-1031. As an
example, Fi ure 13 also contains a calculation of the isotope c!l angeover made with the local mixing model of Doyle et al [103]. The model assumes that after ~turation at a s ecific depth, additional implanted hydrogen will 1 e released, with the specific isotope (i.e. H or D) determined by the local H/D concentration. The model provides an excellent fit to the data for stainless steel at 153 K, and also for other materials at temperatures where hydrogen thermal mobility is negligible. At higher temperatures (e.g. 2300 K) Bragansa et 0l[63] and Clausing et al [90] have demonstrated that recycling in stainless steel increases markedly due to the co-operation of thermally activated and ion induced processes. Hence the relative importance of ion induced effects on the release of hydrogen changes with increasing wall temperature.
6. APPLICATIONS 6.1 Fuel Recycling An important application of the measurements and theory of hydrogen tra ping and release from stainless steel is the model f!ing of fuel recycling in resent day tokamaks. A recent plasma-wall recyc Ying calculation by Howe [20] shows the success, as well aa the limitations en~untered because of the lack of critical data. Howe has cou led a plasma transport model with a first wall an a limiter model that ineludes repection, thermal diffusion and trapping, and ion induced detrapping. Figure 14 compares experiment and calculations for the line average plasma density (ii,) during ohmic and neutral beam injection dischar es in ISX-B. The observed decrease in ii, upon neutr 9 beam injection is explained in his calculations by a decrease in recycling (R,) that
460
K. L. Wilson / Hydroge?~ rec)ding
0
I
I
I
I
//OH
--_ - - - CALCULATED
o~gmo-11025
properties of stainless steels
combination rate on stainless steels near room temperature vary by four orders of magnitude. This ran e of values can have an enormous effect on pre I!!!. acted recycling and tritium inventory. As an example, Figure 15 shows a calculation by Baskes [104] of the bulk tritium inventory in TFTR o erated for 25 full power D-T shots per day for t Kree months with a wall temperature of 373 K. The three values of the recombination rate (k,) are within the reported variation shown in Figure 10. For k, = 2 x 1O-23 cm4/s, the tritium inventory is acceptably low. However, for k, = 2 x 1O-27 cm*/s the recombination is extremely slow, causing the bulk tritium inventory to exceed 5 x lo5 Ci within two months. Bakeout at 525 K for one week reduces this inventory by only a factor of three since the low recombination rate continues to inhibit release. Permeation through the first wall also increases to significant levels during this bakeout. Future experiments must determine the correct recombination characteristics for realistic first wall surfaces. New in-situ diagnostics to monitor surface properties of reactor first wall surfaces must also be developed.
t(msl
Figure 14. Top: Experimental and calculated line average density (n,) for ISX-B with and without neutral beam injection. Bottom: Calculated average pumping fraction (p) and recycle coefficient (R,)
results from the ion temperature rise upon injection. Hotter charge exchange neutral atoms have a lower reflection coefficient, and the non-reflected portion of this flux is deposited further into the wall, requiring longer to diffuse back to the SUTface. Hence there is a transient period after neutral beam injection where recycling is lowered (i.e. the wall “pumps” more hydrogen), and the plasma density decreases. While the model gives good qualitareement with measurements, Howe discusses tive severaa? deficiencies in the wall model that need to be resolved. Most of the charge exchange neutrals are below 200 eV, and since reflection accounts for 8090% of the recycling in this case, the lack of data and theor for reflection below 100 eV is a critical problem. 6 esorption rates of hydrogen by photons, electrons and ions must be determined before these processes can be included in the model. A third problem area is the proper recombination rate for a tokamak first wall. 6.2 Tritium
Inventory
in TFTR
Radiation damage trapping can influence hydrogen retention in devices operated near room ternperature but it will become less important in St steel at the elevated first-wall temperatures oiY!ki!R because of the lo9v hydrogen-defeet binding ener ‘es. The amount of hydrogen that is retained in the &t wall is then critically dependent on the recombination rate at the surface. Unfortuneately, present calculations and measurements for the hydrogen re-
ld4,
10
20
30
40
50
TIME
60
( days
70
80
90
)
Figure 15. Calculated bulk tritium retention 304 LN stainless steel wall of TFTR [104].
6.3. Tritium Reactors
Inventory
and Permeation
in the
in Advanced
The retention of hydrogen isotopes in the stainless steel first wall is of concern for both the tritium inventory and the potential for hydrogen embrittlement. Calculations of the hydrogen inventory in EPR and NUMAK have been carried out by Look and Baekes [105], and by Wienhold et 01 [94] for INTOR. In the temperature range of 600-900 K both calculations indicated that the average hydrcen bulk concentration will be &O a m. At these evels, hydrogen embrittlement is un fl! I ely, and the P; tritium inventory is acceptably low. Wienhold [94] calculates a tritium inventory in the first wall of only 3g tritium for INTOR operated at 675 K.
XL.
Wilson / Hydrogen recycling properties of stainless steels
However, the tritium permeation throu h the stainless steel first wall of a fusion reactor drst wall can be quite large. Wienhold’s calculations [94] for tritium permeation flux (OT) through the INTOR first wall into the coolant are given in Figure 16 for various incident wall fluxes and operating temperatures. The steady state permeation rate of 10 x 1021tritium atoms mm2 d-l corresponds to a loss of 16g tritium per day(l.6 x lo5 Ci/day). Techniques for reduction of these high permeation rates include permeation barriers at the first wall-coolant interface [53], and the use of less permeable materials in regions where the first wall or limiter is exposed to an energetic tritium charge exchange neutral flux [106].
$ I
2.
3
4
5
6
t[hlFi ure 16. Permeating tritium flux density @r through a BS wall of thickness 0.5 cm as function of time during 200 discharges of 100s and 10s interruption (solid curve: incident flux = 1V7 atoms cm-2 s-l; dashed curve: incident flux = 1Ol6atoms cm-2 s-l) 1941.
7. SAY Recent advances in the stud of ion-solid interactions have led to a detaile d picture of hydrogen recycling between the lasma and stainless steel wall of a magnetically con %ned fusion reactor. Processes that contribute to recycling include reflection, desorption, diffusion, tra ping, and molecular recombination. However, bePore accurate assessments of h drogen recycling in a fusion reactor can be m d, progress must be made in several areas. Critical data needs include low energy ( SlOO eV) reflection; desorption cross sections at appropriate energies; first wall hydrogen surface coversges; and molecu~areecombmation rates for realmtic first wall sur-
ACKNOWLEDGEMENTS I wish to acknowled e the many fruitful discussions with the etafI of spandia National Laboratories. Special thanks also go to J. Roberto (ORNL), J. Winter (KFA Jiilich), S. Myers, R. A. Kerst, and
461
M. I. Baskes (SNL) for use of their data prior to ublication. This work wss supported by the U. S. b epartment of Energy.
REFERENCES
Ill
G. M. McCracken, S. J. Fielding, S. K. Erents, A. Pospiesacsyk, P. E.Stott, Nucl. Fusion 18 (1978) 35. T&R Group, J. Nucl. Mater. 93 85 94 (1980)
J. Roberto, S. Kasai, L. E. Murray, J. E. Simpkins, S. P. Withrow, R. A. Zuhr, this conference. H. F. Dylla, private communication. J. L. Cecchi, J. Nucl. Mater. 93 & 94 (1980) 28. S. K. Erents, in B. Navinsek (ed) Physics of Ioniaed Gases 1976, J. Stefan Institute, Ljubljana, Yugoslavia, (1976) 299. [71 G. M. McCracken in Proc. Int. Symp. on Plasma Wall Interaction, Jiilich, Pergamon Press, (1976) 339. [81 B.M.U. Scheraer, J. Vat. Sci. Technol. 13 (1976) 420: r91 R. Behrisch, J. de Physique 38 (1977) C3. S. T. Picraux, Physics Today 30 (1977) 42. I;!$ W. Bauer, J. Nucl. Mater. 76 & 77 (1978) 3. 112: K. L. Wilson, IEEE Transactions on Nuclear Science NS-26 (1979) 1296. P31G. M. McCracken, P. E. Stott, Nuclear Fusion 19 (1979) 889. WI R. Behrisch in Physics of Plasmas Close to Thermonuclear Conditions, Pergamon Press (1980). WI E. S. Hotston and G. M. McCracken, J. Nucl. Mater. 63 (1976) 177. WI I$ S. Marmar, J. Nucl. Mater. 76 & 77 (1978)
[171S. ‘J. Fielding,
G. M. McCracken, P. E. Stott, J. Nucl. Mater. 76 & 77 (1978) 273. P81G. M. McCracken, J. Nucl. Mater. 85 & 86 (1979) 943. WI D. E. Voss, S. A. Cohen, J. Nucl. Mater. 93 & 94 (1980) 405. PO1y7 C. Howe, J. Nucl. Mater. 85 t 86 (1980)
Pll W: Eckstein,
F.E.P. Matschke, and H. Verbeek, J. Nucl. Mater. 63 (1976) 199. P21W. Eckstein and H. Verbeek, J. Nucl. Mater. 93 & 94 (1980) 518. (231 G. Sidenius, T. Lenskjaer, Nucl. Instr. Methods 132 (1976) 673. E. W. Thomas, J. Appl. Phys. 51 (1980) 1176. [:;I E. W.Thomas, R. Young, J. E. Harris, J. Nucl. Mater. 93 & 94 (1980) 524, [26] 0. S. Oen aad M. T. Robinson, Nucl. Instr. Methods 132 (1976) 647. [27] OS. Oen, M. T. Robinson, J. Nucl. Mater. 76 & 77 (1978) 370.
462
J. E. Robinson, K. K. Kwok, D, A. Thompson, Nucl. In&r. Methods 132 (1976) 667. PQIFb7P, Jackson, J. Nucl. Mater. 93 & 94 (1980) [=I
WI L. d.
Haggmark, J. P. Biersack, J. Nucl. Mater. 85 t 86 (1979) 1031. PI D.P. Jackson, Rad. Effects 49 (1980) 233. in Proc. Plasma Edge Exper1321M. T. Robins& iments and Modeling Workshop, UCLA; June 4-5. 1980. PPG510. D12. k. S. Barton, W. Dillon, [331G. ‘M. MiCrackenj Nuovo Cimento Suppl. 5 (1967) 146. Y. Shapira, CRC Crit. Rev. 1341D. Lichtman, Solid State & Mater. Sci. 8 (1978) 93. Surf. I351G. W. Fabel, S. M. Cox, D. Lichtman, Sci. 40 (1973) 571. [361 D. Lichtman, Y. Shapira, J. Nucl. Mater. 63 (1976) 184. Y. Shapira, D. Lichtman, 1371 M. J. Drinkwine, Adv. Chem. Ser. 158 (1976) 171. [381 M. L. Knotek, V. 0. Jones, V. Rehn, Phys. Rev. Lett. 43 (1979) 300. [3Ql M. L. Knotek and P. J. Feibelman, Phys. Rev. Lett. 40 (1978) 964. [401 S. Brumbach and M. Kaminsky, J.Nucl. Mater. 63 (1976) 188. 1411 S. Brumbach, M. Kaminsky, in Radiation Effects on Solid Surfaces, Adv. Chem. Ser. No. 158 (1976) 183. [421 J. Brumbach, M. Kaminsky, J. Appl. Phys. 47 (1976) 2844. [431 R. E. Clausing, 11th Natl. Vat. Symp., AVS Chicago, 1964. 1441 U. Cummings, N. Dean, F. Johnson, J. Jurow, J. Voss, J. *ai. Sci. Technol. 8 (1971) 348. M. H. Achard, R. Calder, A. Mathewson, Vac1451 uum 29 (1978) 53. I461 J. Lelegard, A. Schram, J. Nucl. Mater. 76 & 77 (1978) 637. 1471J. Lelegard and A. Schram, in Proc. Int. Symp. on Plasma Wall Interaction, Pergamon Press (1977) 293. I481M. J. Drinkwine and D. Lichtman, J. Vat. Sci. Technol. 15 (1978) 74. 49 G. M. McCracken, Vacuum 24 (1974) 463. t50 1 S. K. Erents and G. M. McCracken, J. Appl. Phys. 44 (1973) 3139. 1511 R. J. Bastass, private communication. [52] R. J. Bastasa and L. G. Haggmark, this conference. [531 W. M. Stacey, M. A. Abdou, J. A. Schmidt, T. E. Shannon, J. K. Griffith and the U. S. INTOR Group, U.S INTOR $he U.S. Contribution to the International To&nak Reactor Phase-l Workshop, Conceptual Design, INTOR/81-1 (1981). [541 Fusion Reactor Materials Program Plan, Section III, U.S. D.O.E./ET-0032/3 (1978), Appendix. [551 V. A. Simonev, G. F. Kleimonov, A. G. Mileshkin and K. A. Kochnev, Nucl. Fusion Suppl. 1,
(1962) 325. E. S. Borovik, 1561
N. P. Katrich and G, T. Nikolaev, Atomn Energyia 18, (1965) 91. [571 E. S. Borovik, N. P. Katrich and G. T. Nikolaev, Atmn Energyia 21, (1966) 339. 1581 N. J. F’reeman, I. D. Latimer and N. R. Daly, Nature 212, (1966) 1346. 1591 G. M. McCracken and J. H. C. Maple, Brit. J. Appl. Phys. 18, (1967) 919. A. if301 L. Hunt, C. C. Damm, and R. K. Goodman, Lawrence Livermore Laboratory, UCID-16408 (1964) Draft; 1973 MS). WI W. Bauer and G. J. Thomas, J. Nucl. Mater. 53, (1974) 127. PI K. L. Wilson, G. J. Thomas, and W. Bauer, Nucl. Technol. 29, (1976) 322. PI C. M. Braganaa, S. K. Erents, E. S. Hotston, and G. M. McCracken, J. Nucl. Mater. 76 & 77, (1978) 298. PI B. Navinsek, M. Peternel, A. Zabkar, S. K. Erents, J. Nucl. Mater. 93 & 94 (1980) 739. 1651R. E. Clausing, L. C. Emerson, L. Heatherly, and R. J. Colchin, in Proc. Intl. Conf. on Plasma Wall Interaction, Jiilich (1976) 573. P61K. L. Wilson and M. I. Baskes, J. Nucl. Mater. 74 (1978) 179. PI K. L. Wilson and M. I. Baskes, J. Nucl. Mater. 76 & 77 (1978) 291. WI K. L. Wilson and A. E. Pontau, J. Nucl. Mater. 85 & 86 (1979) 989. PQIJ. Bohdansky, K. L. Wilson, A. E. Pontau, L. G. Haggmark, M. I. Baskes, J. Nucl. Mater. 93 & 94 (1980) 594. G. Farrell and S. E. Donnelly, 6. Nucl. Mater. 1701 76 &s 77 322, (1978) 132. VI I. Ah-Khan, K. J. Dieta, F. Waelbroeck, P. Wienhold, J. Nucl. Mater. 74 (1978) 132. P211. Ah-Khan, K. J. Dieta, F. Waelbroeck, P. Wienhold, J. Nucl. Mater 76 & 77 (1978) 263. F31I. Ali-Khan, K. J. Diets, F. G. Waelbroeck, P. toehold, J. Nucl. Mater. 85 & 86 (1979)
P4
J. Rbth, J. Bohdansky and W. 0. Hofer, in Proc. Intl. Symp. on Plasma Wall Interaction, Jiilich, Pergamon Press, (1976) 309. [75] C. J. Altstetter, R. Behrisch, J. Bottiger, F. Pohl, B.M.U. Scheraer, Nucl. Instr. Methods 149 (1978) 5s. 1761W. R. Wam pfer, B. L.Doyle, D. K. Brice and S. T. Picraux, SAND80-li84 (1980). S. T. Picraux and W. R. Wampler, J. Nucl. 1771 Mater. 93 & 94 (1980) 853. i781 G. J. Thomas and K. L. Wilson, J. Nucl. Mater. 76 & 77 (1978) 332. L7QlW.J&ger and J. Roth, J. Nucf. Mater. 93 & 94 (1980) 756. 1801 K. L. Wilson, A. E, Pontau, L. G. Haggmarjc, EieLcpkes, J. Roth, J. Bohdansky, this con1811 S. Mye&, to be published
in the Proceedings
of
K. L. Wilson / Hydrogen recycling properties of stainless steels
the Environmental Degradation of Engineering Materials, V,P.I., Blacksburg, VA, September 21-23, 1981. F. Besenbacher, J. Bottiger, T. Laursen, W. If321 Moller, J. Nucl. Mater. 93 & 94 (1980) 617. I. Ali-Khan, K. J. Dietz, F. G. Waelbroeck, and 1831 P7Wienhold, J. Nucl. Mater. 76 & 77 (1978) M. i. Baskes, J. Nucl. Mater. 92 (1980) 318. R. J. Madix, G. Ertl, K. Christman, Chem. Phys. Lett. 62 (1979) 38. G. Comsa, R. David, B. J. Schumacher, Surf. Sci. 95 (1980) L210. F. Waelbroeck, J. Winter, P. Wienhold, this conference. R. A. Kerst, this conference. P. Wienhold, R. E. Clausing, F. Waelbroeck, J. Nucl. Mater. 93 & 94 (1980) 540. 1901R. E. Clausing, L. C. Emerson, L. Heatherly, J. Nucl. Mater. 85 & 86 (1979) 542. ]911 R. E. Clausing, L. C. Emerson and L. Heatherly, J. Nucl. 76 & 77 (1978) 267. F. [921 Waelbroeck, K. J. Diets, P. Wienhold, J. Winter, I. Ah-Khan, H. Merkens, E. Rota, J. Nucl; Mater. 93 & 94 (1980) 839. 1931P. Wienhold, I Ah-Khan, K. J. Dietz, M. Profant, ytioyaelbroeck, J. Nucl. Mater. 85 & 86 (1979) 1941P. Wienhold, M. Profant, F. Waelbroeck, J. Winter, J. Nucl. Mater. 93 & 94 (1980) 866. 1951M. Braun, B. Emmoth, F. Waelbroeck, P. Wienhold, J. Nucl. Mater. 93 & 94 (1980) 861. 96 I. Ah-Khan, referenced in [95]. I97I K. L. Wilson and M. I. Baskes, unpublished data. 1981M. I. Baskes, accepted for publication in J. Nucl. Mater. 1991R. J. Bastasz and G. J. Thomas, J. Nucl. Mater. 77 (1978)183. 100 M. I. Baskes, SAND80-8201 (1980). 101 R. S. Blewer, R. Behrisch, B.M.U. Scherzer, R. Schulz, J. Nucl. Mater 76 & 77 (1978) 305. [102]E. S. Hotston, J. Nucl. Mater. 88 (1980) 279.
II
[103]B. L. Doyle, W. R. Wampler, D. K. Brice, S. &Picraux, J. Nucl. Mater. 93 & 94 (1980) [104]M. I. Baskes, private communication. [105]G. W. Look and M. I. Baskes, J. Nucl. Mater. 85 & 86 (1979) 995. [106]C. A. Flanagan, D. Steiner, G. E. Smith, Fusion En ‘neering .Design Center Staff, Initial Trade an f Design Studies for the Fusion Engineering Device, ORNL/TM-7777 (1981).
463