Abstract

1. Introduction
Safe handling techniques of tritium will have to be established before D-T burning
M. Matsuyama, K. Watanabe, S. Morozumi,
experiments in large-scale devices such as ITER, DEMO and so on. Metallic uranium has been widely used as a reversible tritium getter in a variety of tritium handling systems including laboratory scale devices and is a potential candidate for the D-T burning devices. However, the uranium getter is not necessarily advantageous from the viewpoints such as its high dissociation pressure, complicated acitivation processes, flammability against air exposure and so on ¹³ . Accordingly, it is required to search for more feasible getter materials than uranium for the safe handling of tritium gas. We have reported 21 that an intermetallic compound, ZraNi 11 , in Zr-Ni alloys shows some advantageous features in comparison with uranium getter. In the present paper, we will describe the results of feasibility tests of a tritium container packed with ZrsNi 1 alloy. Figure 1 shows a cross-sectional view of the tritium continer fabricated in the present study. The container was mainly constructed of 316 stainless steel (SS316), following double containment concept to minimize tri¬ tium release to the environment. This stain- less stell, SS316, is one of austenitic stain¬ less steels showing low permeability to tri¬ tium ³³. An inner tube, which was connected to a vacuum system via two metal bellows valves, was 9.53 mm in outer diameter and 1.25 mm in thickness. The outer surface of the inner tube was electroplated with gold to prevent corrosion of the SS316 surface and permeation of tritium at elevated tem¬ peratures. The Zra Ni,, alloy used in the present study was purchased from Japan Metals and Chemicals Co. The alloy had been prepared with argon arc melting method and pulverized with exposing it to hydrogen of 10 atm at 200℃. Subsequently, the absorbed hydrogen had been evacuated at 800℃. The powder of ZrsNi,, alloy was examined with a conventional X-ray diffractometer. It was revealed that the alloy powder consisted of ZrNi, ZrsNi,,, Zr,Ni,, and Zr, On account of the small diameter of the inner tube, the alloy powder was screened Fig. 1. A cross-sectional view of a tritium con ­ tainer constructed of SS316,
by using mesh sieves. The size and weight of the powder used were over 60 mesh (0.25 mm) and 1.5g , respectively. A small handmade ohmic heater, which was insulated from the inner tube by using quartz glass tubes, was installed in the tritium container for activation of the powder and for temperature control during the measurements of dissociation pressure. The temperature was measured at two different points of the outer surface of the inner tube with two thermocouples spot-welded. Figure 2 shows a schematic dia- gram of the experimental apparatus used. This is essentially an ultra- high vacuum system equipped with three kinds of pumps : sputter ion 13 pump, turbomolecular pump and oil- 10 sealed rotary pump. The residual pressure of the system was usually below 3×10~ Torr (1 Torr=133 D Pa). Two precise capacitance mano- meters (MKS Baratron : model 390 HA with 1 or 1000 Torr head) were used to measure the dissociation pre- ssure of Zr , Ni , , deuteride at various temperatures and for other pressure G measurements. Feasibility tests of the tritium container were carried Fig. 2 . Block diagram of the vacuum system connected out by using deuterium gas as an with the tritium container : A, quadrupole mass alternative of tritium, because 20 spectrometer ; B, ionization gauge ; C, sputter ion Ci/day is the maximum amount of pump ; D, vacuum chamber ; E, turbomolecular pump ; F, rotary pump ; G, Ds cylinder ; H, capaci- tritium permitted to use in our fa- tance manometers (1 and 1000 Torr heads); I, tri- cilities. This amount is equivalent tium container. All of numbers in the figure des- cribe the metal bellows valve. to about 8 cm (STP)/day. With use of a quadrupole mass spectrometer, the purity of the deuterium gas was confirmed to be over 97% : main inpurities were hydrogen and carbon monoxide. Dissociation pressures of deuterides having various stoichiometric compositions were measured with a closed system. Namely, after a given amount of deuterium gas was absorbed at room temperature, the valve Na10 was closed. Subsequently, the tem- perature of the sample was raised stepwise from room temperature and the dissociation pressure was measured at each temperature. After a series of measurements, a given amount of deuterium gas was added at room temperature and the dissociation pressure was again measured through similar processes.
3. Results and discussion
3. 1. Heating ability
Table 1 shows the electric power needed to heat the inner tube up to a given tem- perature. In this test, the outer tube had been evacuated in advance. The table shows that even a little electric power was suffic- ient to heat the inner tube up to 500℃. In addition, the temperature of the outer tube went up to only about 100℃ even though the inner tube was heated up to 500℃. This suggests that no special cooling device is necessary for this tritium container.
3. 2. Activation processes
Figure 3 represents the variation of absorption rates of deuterium gas with the temperatures of vacuum heating of the load- ed alloy powder to examine activation. The vacuum heating was performed for 1 hour at temperatures of 300, 400 and 500℃. Those procedures will be denoted as "processing" beolw. After each vacuum heating, the alloy powder was cooled to room temperature. The alloy powder was exposed to deuterium gas of 9 Torr and the pressure decrease was measured with the capacitance manome- ter. In the case that the absorption rate obeys the first-order kinetics with respect to deuterium pressure (i.e., dP/dt = — kP, where P is the deuterium pressure at a time,
k the rate constant of absorption), the variation of the deuterim pressure should be described by a simple exponential function * ' . No linear relation, however, was observed at any processing temperature except for the initial stage of absorption. In addition, the absorption was accelerated with time and/or absorption. Such behavior is attributed to generation of heat owing to the ad/absorption of deuterium. Supposing the adsorption of deuterium molecules takes place initially at certain active sites, a given amount of heat is generated and liberated around the active sites. This causes local heating of the respective parts of crystallites because of the low thermal conductivity of the packed alloy powder, accelerating the absorption rate. This is an advantageous point for fast absorption. However, appropriate counterplans against the local heating would be important for the design of a tritium container since the residual pressure of tritium is basically determined by the temperature of those parts. Figure 3 also shows that the absorption rate increases with the processing temper- ature. On the basis of these observations, we adopted the evacuation at 400℃ for 1 hr as the standard activation process of the as-received ZraNi,, alloy. In general, the absorption rate reduces owing to the adsorption of contaminatns such as HzO, CO and COs. Therefore, it will be necessary to re-activate of the alloy powder to regain the absorption rate. In this case, the standard activation process could be employed for the re-activation. With respect to the processing at 400℃, the alloy powder released considerable amounts of gaseous products such as H : O and CO as well as H : , whereas the gas release decreased substantially above 400℃. This is very similar to the observations for ZraNi by Ichimura et al 5 ³ . According to Ichimura, in addition, zirconium is in oxidized states on the as-received surface of Zr:Ni, whereas it gradually changes to metallic state with heating above 400℃. It appears plausible to expect similar phenomena for ZraNi...
3. 3. Dissociation pressure
Figure 4 shows the temperature dependence of the dissociation pressure over ZrsNi,,Dx, where x is the value determined at room temperature. As seen in the figure, the logarithm of the dissociation pressure varied linearly with the reciprocal temperature in a relatively low temperature region (below about 150C) for all values of x examined. Above this temperature region, the dissociation pressure deviated downward from the respective straight lines : the higher the temperature was, the larger the deviation. It is considered that the deviation is due to the large variation of deuterium concentration in a series of measurments. In the case of Zr, Ni, , D3, , , however, the dissociation pressure reached about 500 Torr at 350℃. In this case, the initial absorption amount of deuterium in ZroNi, Ds. 4, was 3.03 x 10' Torr•cm (STP) at room temperature, which is equivalent to about 100 Ci of tritium gas. On the other hand, the released amount was 1.87×10* Torr•cm (STP) at 350℃. Namely, the ratio of them attained to 0.62. This indicates that most of the initially loaded tritium can be supplied to gas phase at 350℃. Table 2 is a summary of thermodynamic data evaluated from the linear part of the lines shown in Fig. 4 . The thermodynamic data were evaluated by applying the equa- tion, d(lnKp)/d(1/T) = A Ho/R, where Kp is the equilibrium constant, T the absolute
temperature, R the gas constant and AHo the standard enthalpy change. The standard enthalpy change was about —25 kcal/mol(D x ) irrespective to the stoichiometric composi­ tions of deuterium and agreed well with the results obtained from our previous work ²³. On the other hand, the standard entropy change( Δ S°) increased with the deu- terium concentration. The absolute entropy of deuterium gas is 34.6 cal/mol(D,) • K. The observed values were apparently larger than this. In our previous work on the heat of deuteride formation of Zr 9 Ni 1 , alloy 3 ' , the standard entropy change was close to the absolute entropy of deuterium gas. In this experiment, the as-received Zr ,Ni,, alloy used was in a form of plate and consisted mostly of ZraNi,, intermetallic compound, which was confirmed with the X-ray diffrac- tometer. The alloy was pulverized under milder conditions than the present sample, exposing it to deuterium below 1 atm at room temperature before measurements of the dissociation pressure. The primary difference between the previous and the present works lies in the fact that the sample consisted mostly of a metallic compound, ZrsNi.. in the former, whereas it was a mixture of metallic compounds in the latter. The dissociation pressure observed in the present study should be a function of the number and type of coexisting metallic compounds, and deuterium concentration in each compound and/or respective deuterides. The temperature dependence of the dissociation pressure should be related to those factors. Namely, it is likely that the coexistence of intermetallic compounds plays an important role for large apparent standard entropy changes observed in the present study.
3. 4. Absorption rate
Effects of stoichiometric deuterium con¬ centration in the alloy on an absorption rate were examined. Figure 5 shows the change in the absorption rate of deuterium gas at room temperature with the deuterium concentration in the alloy. The deuterium pressure was set low enough to keep the deuterium concentration almost constant during the absorption measurements. Namely, the amount of deuterium gas added to deu- teride at a given stoichiometric deuterium concentration was 63 Torr • cm in each run, which increases the deuterium composition only 0.004. It is seen in this figure that the pressure decreased exponentially with time, irrespective to the initial deuterium concen- tration. As mentioned above, this indicates that the absorption rates of deuterium obeys the first-order kinetics with respect to deuterium pressure. Therefore, slopes of the lines in the figure correspond to the rate constants of the absorption. It is clear from this figure that the absorption rate constant is influenced by the initial deuterium concen- tration : one can see about four times differnce in the rate constants in Fig. 5 . This appears due to variation of the number of adsorption sites or activation energy of adsorption of deuterium molecules with deuterium concentration. Anyhow, in the prac- tical use, such effect must be considered to determine the appropriate working range of the alloy for tritium handling. We also examined the absorption process under near practical conditions. Namely, after closing the valve Na10 and opening two valves of Na11 and 12, most of the deute- rium absorbed as Zr , Ni,, D , , , , , was first released to gas phase with heating it up to 350℃, where the gas pressure went up to 488 Torr. Subsequently, the container was allowed to cool with switching off the power supply. The pressure decrease is shown in Fig. 6 . The gas pressure initially ran parallel with temperature decrease. The absorption rate, however, gradually decreased in spite of the further decrease of temperatrure. Supposing the equilibrium is established quite readily at each temperature, the deuterium pressure is expected to decrease as a dashed line shown in this figure. It is seen that the observed pressure deviates further from the dashed line with time. Three mechanisms appear to be plausible for this phenomenon: ( 1 ) the temperature difference between the wall and the alloy powder, ( 2 ) the change in the material properties and ( 3 ) wall effects. As for the first mechanism, the measured temperature appears to be evidently lower than that of the alloy because heat is evolved in the inner tube owing to the absorp­ tion of deuterium. To examine this effect, the valve Na12 shown in Fig.2 was tempo- rarily closed for about 10 minutes during the measurements. This is shown as "CLOSE" along with arrow in Fig. 6 . A pressure rise was observed after closing the valve, indicating
the desorption of deuterium from the surface of stainless steel tubes. Opening the valve (indicated "OPEN" in the figure) caused sudden pressure decrease. The pressure, however, did not attain to a level expected as shown by the dashed line, although the temperature decreased further. This behavior was reproducible as seen in the figure. In addition, the pressure did not decrease further even after sufficient cooling time and was kept almost constant at about 4.5× 10-3 Torr. These observations indicate that the pressure differ- ence between the observed and expected is not due to the temperature difference bet- ween the outer surface of the inner tube and alloy. To examine the second mechanism, namely the possibility of changes in material characteristics affecting the dissociation pressure, a part of tubes was evacuated over- night rigorously by closing the valve Na12. Subsequently, the pressure was again meas- ured by opening the valve. It was observed that pressure was at the level expected from the dashed line. This indicates that substantial characteristics concerning with the dis- sociation pressure did not change in adsorption processes. Namely, it appears that the third mechanism would play an important role for the pressure difference. It is seen that the pressure difference is mostly due to the desorption of deuterium from the inner surface of the tubes. Similar phenomena would be also expected to occur in tritium handling systems. Namely, tritium storage materials can not fully display their ability under such condi- tions. Consequently, it is of importance to development materials inactive to tritium adsorption and/or absorption. Matsuyama et al. have reported 6 ' that the adsorption as well as desorption of tritium on/from SS304 can be reduced to below 1/100 by pre- paring surface passivation layers ; for example, ceramic coating with such as boron nitride and electrochemical passivation.
4 . Conclusions
A tritium container packed with powdered ZraNiu alloy was fabricated from 316 stainless steel. its feasibility was examined by using deuterium gas as an alternative of tritium. The conclusions obtained were as follows : ( 1 ) A compact heater incorporated into the container was sufficient to heat up the alloy powder to temperatures above 500℃. This was enough to activate the alloy powder and liberate deuterium gas at 1 atmospheric pressure. ( 2 ) The as-received Zr 9 Ni 1 alloy could be activated by the evacuation at 400℃ for 1 hr. These conditions would be also applicable to reactivation of ZraNi, r-tritide. ( 3 ) It was possible to supply deuterium gas near i atm at about 350℃ and to absorb most of it at room temperature, although the as-received alloy was a mixture of intermetallic compounds such as ZrNi, ZrsNi,, and Zr,Ni, ( 4 ) The apparent absorption rate was affected by the stoichiometric deuterium con- centration in the alloy and the desorption of deuterium from the inner surface of constructing materials. Especially, the latter effect will impaire functions of reversible tritium getters.