Ionization chamber system to eliminate the memory effect of tritium

Nuclear Instruments and Methods in Physics Research A278 (1989) 525-531 North-Holland, Amsterdam

525

IONIZATION CHAMBER SYSTEM TO ELIMINATE THE MEMORY EFFECT OF TRITIUM Masabumi NISHIKAWA, Toshiharu TAKEISHI, Yuzuru MATSUMOTO and Isao KUMABE Department of Nuclear Engineering, Kyushu University, Fukuoka 812, Japan Received 19 September 1988 and in revised form 15 December 1988

The memory effect due to the adsorption of tritium onto the electrodes of an ionization chamber sometimes reduces the accuracy of the results. Ionization chambers are generally used to monitor the tritium level in a gas stream because of their reliability, flexibility and wide range of measurement . In this paper we shown that the memory effect is mainly brought about by the transfer of tritium from the gas stream to surface water on the electrode wall by adsorption or isotope exchange reactions. A way to simulate the extent of the memory effect is proposed . An ionization chamber system which can eliminate the memory effect by applying an isotope exchange reaction is also proposed .

1. Introduction A monitoring system utilizing ionization chambers is generally used to measure the tritium level in a gas stream of various conditions due to its reliability, flexibility and wide range of measure. However, more study about the behavior of tritium on the surface of electrodes of an ionization chamber is required to understand the memory effect because it reduces the accuracy in the measurement of tritium, which itself is essential to watch the tritium balance in a fusion power plant. The memory effect observed for an ionization chamber with electrodes made of oxygen-free, high conductivity copper or 304 stainless steel in various conditions is discussed in this paper. A tritium monitoring system which can eliminate the memory effect is also proposed . ( 2. Theory The ionization current I generated by the ß decay of tritium in a gas stream in an ionization chamber is given by I= 3 .7 X l0 10EeTcg Ve/W [A], where E: average energy of ß-particles (eV), charge of electron (C), e: Tcg : concentration of tritium in the gas stream (Ci/m3), W: average energy expended in the gas per ion pair formed (eV), Ve effective volume of chamber (m3). 0168-9002/89/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)

The ionization current I' from tritium on the surface of the electrodes is given by (2) I'=3 .7X101 °Ee(Tes/2) Se / W [A], where Tcs: concentration of tritium on the surface of the electrodes of the chamber (Ci/m2 ), Se : total surface area of electrodes in the effective volume (m2 ). is Tes halved in the above evaluation because only those ß-particles emitted toward the gas stream can form ion pairs that can be measured . As I' is a measure of the memory effect, the relative error is obtained from eqs. (1) and (2) as

(3) (I /I)=(Se1V,)[(Tes/2)/Teg] . The minimum measurable concentration of tritium by an ionization chamber, Teg,rnin, is given by 4) Tcg, min = 2 .82 X 10 4WImm/ Ve [C'/M2 ] , where Imin is the minimum detectable current. The present authors postulate that surface water on the electrode material plays a dominant role in the memory effect . Then the mass balance of hydrogen isotopes on the surface is given by the following equations, considering (1) water adsorption, (2) the isotope exchange reaction between tritiated water in the gas stream and surface water, and (3) the isotope exchange reaction between gaseous tritium and surface water : a4Hz o/at - (KF,ad)(CHzo + CTZO - C*) X3 +(KF,exl)X+(KF .ex2)xz -0 , * . a 9T zo/al - (KF,ad) (CHZ0 + CT20 C )X4 - (KF,ext) X - (KF,ex2) X2 = 0,

526

M. Nishikawa et al. / System to eliminate the memory effect of tritium

where X1 =CT2- (CH/K+ CT2)[gT2o/(qH2o+gTZ0)], X2 - CT2 0 - (CH 2 0 + CT20) [ gT2 o/(gH 2 0 + qT,O )] > (8)

X3 - CH20/(CH20 + CT20 )

for (CH20 + CT20 - C * ) >_ 0, X3 - 9H 2 0/( gH20 + gT20)

analog to digital converter circuit

(9)

for (CH,O + CT20 - C* ) < 0,

interface

X4 - CT20/(CH20 + CTZ O ) for (CH20 + CT 2 0 - C * ) Z 0, X4 - gT2 0/(gH2 0 + gT20)

micro computer

(10)

for (CH20 + CH 2 0 - C * ) < 0, and where C: concentration in gas stream (mol/m3 ), C* : equilibrium concentration of water vapor in gas stream with adsorbed water (mol/m3), concentration of surface water (mol/m2 ), g: KF,ad : overall mass transfer coefficient of adsorption (m/s), KF, exl overall mass transfer coefficient of isotope exchange reaction between gaseous tritium and surface on electrode (m/s), KF, ex2 : overall mass transfer coefficient of isotope exchange reaction between tritiated water in gas stream and surface water (m/s), K: equilibrium constant. From previously published results we assume that the isotope effect in water adsorption and that in the isotope exchange reaction between tritiated water and surface water are assumed negligible [1,2]. In the case of the isotope exchange reaction between gaseous hydrogen isotopes and surface water, K is equal to 4 in the hydrogen-tritium system, and equal to 4/3 in the deuterium-tritium system [2] . 3. Experimental A vertical section of the ionization chamber used in this study is shown in fig. 1. The casing is made of brass and oxygen-free, high conductivity copper or 304 stainless steel is used for the electrode material . The high purity copper used in this study does not disturb the background level of the ionization chamber though copper is said to contain some radioactive impurities.

Fig. 1. Ionization chamber and its measuring circuit. The effective volume of the ionization chamber shown in fig. 1 is 48 cm 3 and its total surface area is 55 cm2. A 1 .5 times longer ionization chamber than that shown in fig. 1 was also constructed. A potential difference of 90 V is applied between the two electrodes using a cell . At this potential difference, the ion pair recombination losses are calculated to be negligible by using equations of Price [3] . Calculation using the method of Colmenares [4] also indicates that the case of a potential difference of 20 V is already in the plateau region . A schematic diagram of the measuring circuit is also shown in fig. 1. The minimum measurable tritium concentration using the ionization chamber shown in fig. 1 is about 100 lr Ci/m3 because the detectable limit of the current meter used in this study is 5 x 10 -15 A. The gas stream conditions used in this study are listed in table 1 . Though the average energy per ion pair formed (W) varies a little for different gases, a constant value of 35 eV can be used with negligible error over the range of this experiment . The flow diagram of the experimental apparatus is shown in fig. 2. An MS5A bed packed with 1 mm MS5A spherical particles is used to remove water vapor from the process gas. Table 1 Experimental conditions Carrier gas Hydrogen concentration Water vapor concentration Tritium level Hg /T g ratio Flow rate

N2 30-7600 ppm 0-25800 ppm 0.24-360 mCi/m3 0.033-668 mCi/mol 0.6 1/min

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M. Nishikawa et al. / System to eliminate the memory effect of tritium

When oxygen is included in the gas stream, oxidation of gaseous tritium is performed using a platinumalumina catalyst bed though it uptakes tritium through adsorption or isotope exchange reaction between structure water of the catalyst substrate and gaseous tritium or tritiated water in the gas stream as reported previously [1,2,6] . The effects which the following parameters have on the sorption and desorption behavior of tritium on the electrodes of ionization chambers are discussed after measuring changes of the background value. (1) The tritium level in the gas stream. (2) The chemical form of tritium in the gas stream (gaseous tritium or tritiated water) . (3) The concentration of hydrogen isotopes in the gas stream . (4) The kinds of gas components flowing with tritium and their concentration. (5) The temperature of the gas stream . (6) The humidity in the gas stream.

4. Results and discussion Tritium handling facilities usually have tritium cleanup systems consisting of a catalyst bed and an adsorption bed to minimize the environmental release of tritium. The tritium trapped in a catalyst bed or an adsorption bed is recovered by drying or isotope exchange reaction . Tritium in the chemical form of water is released to the purge gas when drying is applied. Fig. 3 shows a typical change in the output of an ionization chamber when it is used to measure tritiated water released from a catalyst bed with a hydrophilic substrate or from an adsorption bed to the hot dry purge gas in case of recovery of tritium trapped in a tritium cleanup system .

Fig. 2. Flow diagram of experimental apparatus. It is dried at 300 ° C in NZ gas flow for several hours before each experiment . A spongy CuO bed is attached to convert gaseous tritium to tritiated water when no oxygen is included in the gas stream and the CuO bed temperature is adjusted to get the desired conversion ratio using results reported elsewhere [5].

158

152 155

bed temp( °C)

146 134 116 150 142 128

62

92

27

carrier gas N2 Q=0.6 [/min

\

background 160

120

100

0.51

mCilcm 80

initial background

3

60

40

20

0

time(min)

Fig. 3. Output of an ionization chamber measuring tritium released from catalyst bed.

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M. Nishikawa et al. / System to eliminate the memory effect of tritium

3

ionization chamber 50cm H2=50ppm H

HT=0,28ppm in N2

ag / T ay = 357

Q=0.6 11 min

50

40

30

20

10

0 time(mln)

Fig. 4. Output of an ionization chamber measuring continuously flowing tritium . From this figure it appears that 0.51 MCi/m3 of tritium is still contained in the purge gas 3 h from the beginning of tritium recovery even though it should have been exhausted within an hour or so . This indicates that the memory effect due to the transfer of tritium from tritiated water in the gas stream proceeds rapidly and that tritium on the surface of the copper electrodes cannot be returned to the dry gas. This can be deduced from the duration of the memory effect seen in fig. 3 although no tritium is detected in the gas stream an hour after initiation . The change of output of the ionization chamber with time when a gas flow containing somes tritiated water is continuously introduced into the ionization chamber is shown in fig. 4. In this experiment the gas flow containing gaseous tritium is switched to pass through a hot spongy CuO bed at time zero . The initial decrease of the reading mostly corresponds to a decrease of tritium concentration in the gas stream caused by adsorption of tritium onto the internal surface of the gas pipes or to the body of the ionization chamber. This is because the tritium exchange rate from tritiated water in the gas stream to the surface water is much faster than the rate from gaseous tritium as discussed later. It is also seen from fig. 4 that the isotope exchange reaction between gaseous tritium and surface water is much slower because almost no increase of the reading is observed while gaseous tritium is passed through the ionization chamber before time zero . The apparent tritium concentration in the gas stream, however, increases with time to more than twice of the inlet tritium concentration . It is also confirmed in this study that the elevated background reading can be rapidly returned to the

initial background value when the gas flow with some water vapor is introduced to the ionization chamber, though only a slow return is obtained when hydrogen gas is added to the gas flow. From the above observations, and from a knowledge of the quantities involved as stated below, it is concluded that tritium is transferred from the gas stream to the wall surface mainly through the isotope exchange reaction and that the amount of surface water on the copper electrodes is constant . Accordingly, tritium transfer through adsorption can be ignored when the tritium balance is considered using eq . (5) or (6). The observed quantity of tritium adsorbed on the copper electrodes facing a gas stream containing tritiated water is plotted in fig. 5 against the atomic ratio of tritium in the gas stream. The following equation is obtained notwithstanding the wide variation in the tritium level or humidity in the gas: TC S=14 .0(Tag/Hag ) [Ci/ M2 ] ,

(I1)

where Hag means the total atomic concentration of hydrogen isotopes in the gas. The number of tritium atoms per unit surface area of copper is derived from the above equation as 20 2 (12) Tag =2 .9x10 (Tag /Hag ) [ atom/m ] .

Eq. (11) or (12) shows that a constant concentration of hydrogen atoms, 2.9 x 10 20 atoms/m2 , exists on the copper surface in the range of vapor pressure, 3-2500 Pa, and temperature, 10-35 o C.

10

2

3

10

104 10 5 106 10~ Hog /T ag ratio in ionization chamber (H atom/ T atom)

Fig. 5 . Amount of tritium adsorbed and copper electrode.

Hag /Tg

ratio for a

M. Nishikawa et al. / System to eliminate the memory effect of tritium

The chemical form of these hydrogen atoms is supposed to be that of water in this study because the isotope exchange reaction proceeds quickly in such a case and only slowly when gaseous hydrogen isotopes are in the gas stream . The overall mass transfer coefficient of the isotope exchange reaction between tritiated water in the gas stream and the surface water is correlated as follows: (13) KF , ex2 = 5 .4 X 10 -5 exp(-1000/RT) [m/s], where the activation energy, 1000 cal/mol, is almost the same as that observed for precious metal catalysts with hydrophilic substrates . A tritium adsorption rate of about 8 X 10 13 T atoms/(cm2 Torr T20 s) is expected for the copper surface at room temperature from the above equation . The overall mass transfer coefficient of the isotope exchange reaction between gaseous tritium and the surface water was observe to be less than (1/400)th of the exchange rate between tritiated water and the surface water, though both rates show similar values in case of a precious metal catalyst [4]:

_ (14) KF, ex1/KF, ex2 2 .5 X 10 -3 . The experimental values by Hirabayashi and Saeki [7] for adsorption rates of the gaseous tritium onto the 316 stainless steel surface give 10 8-109 T atoms/(cm2 Torr T2 s) at 25 ° C. However, a higher tritium adsorption rate could be expected on the surface because they evacuated samples at 10 -3 Pa for 20 min at room temperature before the measurements . This treatment could drive off physically adsorbed water as reported by Okamoto and Tuji [8]. Using the experimental results, changes in the reading of the ionization chamber can be simulated as shown in fig. 6. For an ionization chamber with a larger electrode surface area, a longer time delay is expected before the memory effect reaches equilibrium . When the relative error due to the memory effect is required to be below 0.05, the following relation derived from eqs. (3) and (11) should be satisfied for an ionization chamber with a copper electrode: Teg >140(Se/Ve)(Tag/Ha g ) [ Cl/m31 .

529

0

a0

600

1200 1800 2400 Time s Fig. 6. Simulation of the memory effect . Measurement of tritium from breeder material, however, may suffer from the memory effect because a low concentration of tritium with a small Hag/Tag ratio may be handled in a blanket tritium recovery system. In this case a measuring system with a lower or negligible memory effect is needed because changes of the background level or delays in the time response due to tritium adsorption make it difficult to get a quick measure of the tritium concentration in the gas stream although the analysis stated above does make it possible to quantify the transitional response curves . The results in this study suggest that a large Hag/Tag ratio and the existence of water vapor in the atmosphere of an ionization chamber can result in a small memory effect with a rapid response.

(15)

The critical tritium concentration using an ionization chamber with 120 m-1 for Se/V, is shown in fig. 7 where Se and Sa are the effective surface areas of the cathode and of the anode, respectively . This figure shows that an ionization chamber can be used with negligible relative error as a tritium monitor for the fueling cycle of a fusion reactor because more than several tens of thousands of Ci/m3 with an Hag/Tag ratio of 2 will be handled though the estimated critical tritium concentration by eq . (15) is about 7000 Ci/m3.

T ag t H ag Fig. 7. Critical tritium concentration and Hag /Tag for ionization chamber with copper electrodes.

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M. Nishikawa et al. / System to eliminate the memory effect of tritium

This concept is realized by introducing water vapor into the process gas at the ionization chamber as shown by dashed lines in fig. 2 where a Pt-SDB catalyst bed is used to convert gaseous hydrogen and oxygen into water vapor. Fig. 8 shows the change in output of the newly proposed ionization chamber system with time when the gas flow is used containing tritiated water of which the Hag/Tag ratio is the same as that used for fig. 4. Comparison of fig. 8 with fig. 4 shows that the memory effect is decreased significantly. Swamping with gaseous hydrogen can decrease the memory effect only to some extent because of a slower isotope exchange rate. Swamping with water vapor, however, can diminish the memory effect to such an extent that it can be ignored, as shown in fig. 9. The peak of the reading obtained when we changed from swamping with gaseous hydrogen to swamping with water vapor in this figure indicates the release of tritium attached on the inner surface of the ionization chamber during the swamping with gaseous hydrogen . We observed that all of the tritium adsorbed onto a copper surface can be recovered by a single purge. For stainless steel, however, more than one purge was necessary . Tritium was released from a stainless steel surface when the second purge was applied about twelve hours after an initial purge although the initial purge is continued until no tritium could be detected . Repetitive purging (three to five times) was required to recover almost all of the tritium from a stainless steel surface in this study. This could be due to the fact that the tritium transfer reaction from the oxide film to the surface is slower than that of the isotope exchange reaction between water vapor in the gas stream and tritium in the surface water. Quantitative tritium release

Ionization chamber 50cm 3 H2=33ppm, HT=0186ppm in N2+H20=1640ppm Hag/Tag =355-17989 (ionization chamber) Q=0.6 [/min HTO 1 HT-

time(min)

Fig. 8. Output of the newly proposed ionization chamber under the same conditions as shown in fig. 4.

Ionization chamber 50cm 3 H2=320ppm, HT=0.014ppm in N2 H a9 /Ta9=45700-667740 (ionization chamber) 0=0 .6 Omin

ô time(min)

Fig. 9. Output of the newly proposed ionization chamber with negligible memory effects .

from the copper surface is possible because no effective oxide film to trap tritium atoms is formed in the range of this study. Using a gold plated chamber [9] to decrease the memory effect utilizes this effect . More experiments are needed to clarify the behavior of tritium on a stainless steel surface. 5. Conclusions (1) The Ha g /Tag ratio determines the number of tritium atoms adsorbed . (2) The tritium level in the gas stream has no effect on the number of tritium atoms adsorbed but higher tritium levels give a shorter response time for the same Hag /Tag ratio in the gas stream . (3) A much larger tritium transfer rate is observed for tritiated water than for gaseous tritium in the gas stream, although the chemical form is not expected to have any effect on the number of tritium atoms adsorbed . (4) Chemical components in the gas stream, such as oxygen, nitrogen or inert gases, have no effect on the number of tritium atoms adsorbed though a small change in output reading is possible due to changes of the energy needed to generate ion pairs (W). (5) The temperature of the gas stream has a negligible effect over the range of this study. Possibly effects are to be expected, however, if higher temperatures are applied through changes in the amount of surface water. (6) The humidity of the gas stream can decrease the memory effect by reducing the Hag/Tag ratio and promoting the desorption of tritium through the isotope exchange reaction between tritiated water in surface water and water vapor in the gas stream .

M. Nishikawa et al. / System to eliminate the memory effect of tritium In the humidity range where the partial pressure exceeds several Pa, the amount of surface water on copper is constant, though it can be decreased at a lower partial pressure . (7) A modified ionization chamber with swamped water is proposed in this study to eliminate the tritium memory effect. (8) The use of oxygen-free, high conductivity copper for the electrodes of an ionization chamber is recommended in this study considering the simplicity in behavior of tritium on its surface. References [1] K. Munakata, M. Nishikawa, T. Takeishi and M. Enoeda, J. Nucl . Sci. Technol. 25 (1988) 383.

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[2] M. Nishikawa, K. Munakata, S. Izumi and T. Takeishi, J . Nucl. Mater., in print. [3] W.J . Price, Nuclear Radiation Detection (McGraw-Hill, New York, 1964) chap . 4. [4] C.A. Colmenares, Nucl. Instr. and Meth . 114 (1973) 269. [5] M. Nishikawa, T. Isayama and K. Shinnai, J. Nucl . Sci. Technol. 20 (1983) 145. [6] M. Enoeda, T. Higashijima, M. Nishikawa and N. Mitsuishi, J. Nucl . Sci. Technol. 23 (1986) 1083 . [7] T. Hirabayashi and M . Saeki, J. Nucl . Mater. 120 (1984) 309. [8] H. Okamoto and Y. Tuji, J. Phys . Soc. Japan 13 (1958) 649. [9] M. Matsuyama, K. Ichimura, K. Ashida and K. Watanabe, Fusion Technol . 8 (1985) 2461 .