Solid State lonics 23 (1987) 165-171 North-Holland, Amsterdam

APPLICATION OF SOLID ELECTROLYTE CELLS

UNDER THE INFLUENCE OF IONIZING IRRADIATION

Wolfgang RICHTER and Helmut ULLMANN Central Institute for Nuclear Research Rossendorf, Academy of Sciences of the GDR, Postfach 19, 8051 Dresden, German Democratic Republic

Received 1 October 1986

Ionizing irradiation can influence voltage measurements with solid electrolyte cells, which leads to errors in concentra- tion measurements.. In this work the influence of various types and dose rates of ionizing irradiation on solid electrolyte cells with gaseous or liquid metal electrodes were investigated theoretically and experimentally. Cell voltage and conduc- tivity measurements show that the main reason for cell polarization during irradiation consists of the electrical conductivity of ionized gases. The electrical conductivity of solid electrolytes increases only insignificantly also at high dose rates. Under the conditions of typical radioactive measuring agents in nuclear technology gamma radiation makes the highest contribu- tion to ionization in the cell. Measures are investigated to prevent the influence of ionizing irradiation on the voltage of solid electrolyte cells.

1. Introduction

The direct measurement of partial pressures or activities of chemical constituents in gases or melts have become a proven method in research and industry for the control of high-temperature processes. Reac- tions o f oxygen, carbon and hydrogen at high temper- ature take an important part also in nuclear technol- ogies. The application of solid electrolyte cells in nu- clear technology under conditions of remote control and high safety demands is promoted by their simple construction, reliable operation and the fact that they do not have to be calibrated.

Solid electrolyte cells have been applied to measure the quality of gases, liquids or melts in glove boxes, in cooling systems of reactors and to determine the oxygen stoichiometry of nuclear fuels.

Under certain conditions, measurements have to be carried out at considerable values o f ionizing irra- diation. Deviations of the measured values observed at these measurements in the literature have been as- cribed to the influence of irradiation. But there have not been any qualitative or quantitative evaluations of this influence. In this work, the influence o f ioniz- ing irradiation on solid electrolyte cells has been in- vestigated both theoretically and in model experi- ments [ 1 ].

0 167-2738/87/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Derision)

2. Type and values of radioactivity in agents of nuclear technology

The main part of energy released by nuclear fission in the form of kinetic energy of particles (fission prod- ucts and alpha particles) is absorbed at a very short distance in the nuclear fuel lattice to produce heat. Beta and gamma radiation show greater ranges. These two types of radiation escape primarily from the fission process and secondarily from the interaction of par- ticle radiation with matter. The energy of particles and quanta is many orders of magnitude higher than the energy of chemical bonding between the atoms and the energy of thermal excitation of the lattice sites (fig. 1). The range of the neutrons is particular- ly high, their absorption in matter depends on the ab- sorption cross sections of the atoms in the matter irradiated.

Table 1 shows the types of radiation, the dose rate and heat generation o f measuring media typical o f nuclear technology. In the primary system of power water reactors the layer of corrosion products, espe- cially 60Co and 54Mn at the pipe walls determine the gamma activity. In the primary system of breeder reactors the gamma activity of 24Na acts on the place

166 W. Richter, H. Ullman/Solid electrolyte cells under influence of irradiation

"

, , ,

LU I0 ~

I "6 ~ 1 3 10 0 R(m)

/ /

/ /

/ E B

Fig. 1. Energy, E, and range, R, of various types of radiations (E B energy level of chemical bonding, FP fission products).

of measurement. Samples of irradiated nuclear fuels have a high radioactivity due to the long-lived fission products even after long decay times.

processes, which result in a reversible or irreversible change of the characteristics of materials. The concen- tration of the defects generated by a particle or quan- tum of irradiation decreases in the sequence alpha-, beta-, gamma-irradiation. Consequently, under the in- fluence of irradiation one can expect an additional electronic conductivity in the solid electrolyte, with a deterioration of the quality of insulators, with elec- tric conductivity of gases in solid electrolyte cells. In agents of high radioactivity, especially due to alpha- radiation, intensive self absorption of radiation can lead to an uncontrolled temperature change [2]. These effects should be evaluated subsequently.

Alpha-radiation has a penetrat ion depth of some /am in solid material, beta-radiation one of about 1 mm. Gama- and n-radiaton penetrate solid materials up to a great depth. Therefore, mainly the gamma- and n-radiation have to be taken into consideration in evaluating the change of characteristics of solid elec- t rolyte cells.

3. Effect of ionizing irradiation on the constituents of solid electrolyte cells

Solid electrolyte cells consist of the solid electro- lyte as an ionic crystal, o f metallic electrodes, inorgan- ic insulator materials and gases as reference systems. By absorption in matter , ionizing irradiation generates ionic and electronic defects in the form of cascade

3.1. Change o f electrical characteristics o f solid elec- trolytes and ceramic insulators

Radiation-induced electrical conductivity arises from ionization only during the run of irradiation. The electrical charge carriers originate at the moment of irradiation, their concentrat ion depends on the rate of generation and lifetime.

At high-dose rates the radiation-induced electrical

Tabel 1 Type and values of radioactivity in various agents of nuclear technology.

Agent of measurement Type of radioisotope and radiation

Activity A, dose rate Px, n-flux ~o at the place of measurement

Heat generation by self absorption

sodium of the primary 24 Na system of a fast 7 = 2.75 MeV breeder reactor /3ma x = 1.39 MeV

delayed n = 0.2 ... 0.4 MeV

water of the primary corrosion products circuit of a thermal 6°Co, 54M_n and others reactor 7max = 1.7 MeV

spent nuclear fuel fission products 10% burnup, 7max = 2.5 MeV 100 d of cooling

Area x = 1.85 TBq/1 Px,max = 2.6 X 10 -3 A/kg _~ = 103 n/cm 2 s

Area x = 0.2 TBq/1 Px,max = 5 X 10 -7 A/kg

Ama x = 1.85 TBq/g Px,max = 2.6 X 10 -6 A/g (sample weight 0.5 mg)

max 1 W/kg

max 0.1 W/kg

max 100 W/kg

I¢. Richter, 11. Ullman/Solid electrolyte cells under influence of irradiation 16 7

conductivity is accompanied by structural changes. In this case, a reversible and an irreversible part of conductivity change can exist. The reversible part is caused by lattice defects, it anneals at elevated tem- peratures. Thermal annealing is particularly favoured in simply constructed lattices, e.g. the fluorite-type lattices of oxide electrolytes. The irreversible part of the conductivity change arises from the generation of new elements in the lattice by neutron capture. Alien atoms generate additional donator and acceptor levels for electrons.

During absorption of gamma-radiation in matter electrons of high energy will be released, their effect in the solid material is comparable with that of beta- rays. If we consider the absorption coefficient of gamma-radiation in solid electrolyte materials on the basis ofZrO 2 or ThO 2 we obtain an absorption of 1/10 to 1/5 of the applied gamma-dose rate, if the layer of the solid electrolyte amounts to some milli- meters. In the range of 10 to 1000 keV electron ener- gy the probability of the occurring defects spreads as follows [3] : About 60% electron excitation, about 35% ionization, about 5% elastic collisions, which results in the displacement of ions from their lattice sites. First of all, ionization contributes to an increase of electric conductivity. In oxides electrons are re- leased especially from oxygen ions. The free electrons will particularly occupy the positively charged oxygen vacancies in the lattice of oxide electrolytes. The for- mation of these "F-centers" becomes revealed by a colour change of Th-Y-oxide ceramics during irradia- tion at higher dose rates [4].

The neutrons cause elastic collisons in the crystal lattice (cross section trd), nuclear reactions to form new nuclides (tra), and in the case of thoria electrolytes

also nuclear fission occurs (of). During irradiation with fast neutrons elastic collision predominates, producing ionic disorder. During irradiation with thermal neutrons nuclear reactions predominate. The fission cross sections of thorium are small for fast and thermal neutrons, so that an influence of fission on the electrical characteristics can be neglected. The cross section values of the atomic constituents of the thoria and zirconia electrolytes are shown in table 2.

A quantitative evaluation of the parts of conduc- tivity generated by several kinds of irradiation is dif- ficult. It depends not only on the dose rate and the type of radiation but also on the energy of the radia- tion and on the atomic constituents of the material. A thermal neutron flux of 1011 n/cm 2 s generates about the same electron current as a gamma-dose rate of 10 R/s [2,5].

Under the conditions of the agents of measure- ment recorded in table 1, the influence of irradiation- induced conduction in the electrolyte on potentio- metric measurements with solid electrolyte cells is negligible. The electric characteristics should permit the application of solid electrolytes under relatively high dose rates of irradiation. This statement is like. wise valid for inorganic insulators.

3.2. Ionization o f gaseous media

The influence of ionizing radiation on gases gener- ates ionization. Free electrons and ions charged posi- tively or negatively (in gases of electronegative ele- ments 02, H2, halogen) were generated. The electrical conductivity of the gases is a function of the concen- tration and mobility of the charge carriers. In this case the concentration is determined by the equilibrium

Table 2 Cross section tr (in barns) of the atomic constituents of solid electrolytes for thermal and fast neutrons [7 ].

Atomic Thermal n Fast n (1 MeV) constituent

ad a) aa b) of c) o d o a of

Zr 6.2 0.185 Th 12.0 7.56 Y - 1.3 Ca 3.0 0.44 O 3.75 0.0002

- 6.59 0.01 - 3.9 X 10 -s 6.76 0.14 0.01

- 1 - . . 1 0 - -

- 3 . 2 - -

- 4 . 3

a) d = displacement; b) a = absorption; e) f = fission.

168 W. Richter, H. Ullman/Solid electrolyte cells under influence of irradiation

Table 3 Rate of formation and recombination, mobility of charge carriers in gases [5,6].

Gas Rate of formation of ions, relat, units

Recombination coeff., cm3/s

Mobility, cm2/V s

B+ B_

He high purity 1 Ar high purity 8.51 N2 high purity Ar techn. H 2 high purity 0.89 Air high purity 5.08 Air teehn. O2 high purity 5.82 H20(g) techn. CO2 "18.7

5.09 500 1.02 1.3 206 1.06 1.28 145

1.37 1.7 0.28 5.93 8.25 1.65 1.37 1.89

1.8 2.5 2.08 2 1.81

1.1 0.95

between the rates of formation and recombination of the charge carders. The rate of formation increases almost linearly with the density of the gas [5]. Among the charge carders, the electrons show the highest mobility. In electronegative gases, the electrons will toe captured predominantly by the formation of nega- tively charged gaseous ions. Therefore the general mobility of the charge carriers is much lower in these gases. In contaminated rare gases this effect is also re- markable (table 3). The radiation-induced current de- pends linearly on the dose rate. The specific electric current of air at normal conditions, 1 per volume V, is

I[V= 3.33 X 1 0 - 1 0 p D A/cm 3,

if the multiplication factor is neglected [2]. (P pres- sure in 105 Pa ,D dose-rate in R/s). The specific con- ductivities of rare gases and nitrogen can be higher by up to two or three orders of magnitude according to their purity.

The influence o f the electrical conduction in the gas on the voltage loss of solid electrolyte cells depends on the construction (arrangement of the electrodes) o f the ceils. Fig. 2 shows values of a 0.1% voltage loss for a cell with I cm 3 of air between electrode areas of I cm 2.

Ionizing radiation affects polarization of the elec- trode also in another manner: Kinetic blockage at the electrode can be reduced by ionization.

5810-5 2,58.113-4 D (k/kg) 2,58 10 -3

- \ 107 l - - - -

1°'o, 1 o (R/s) - ~ 10

Fig. 2. Vol tage loss 0.1% o f a cell w i th a 1 cm 3 o f air between n o n - i n s u l a t e d e l e c t r o d e a r e a s in d e p e n d e n c e o n t h e i n n e r r e -

s i s t a n c e R i and on thegamma-dose rate D.

4. Measurements on solid electrolyte cells under irra- diation

4.1. Experimental arrangements

A cobalt source and a X-ray source have been ap- plied as radiation sources. The dose rates were varied at the cobalt source by positioning the cell at various distances up to 6 R/s. At the X-ray source dose rates

W. Richter, H. Gilman/Solid electrolyte cells under influence of irradiation 169

radiation s o u r c e

thermo couple

gas outlel

heater ~

/ /

laS i n l e t

so/id e(ectrolyte -cel l

measurement of voltage or resistivity

a b d

1

c

- - 9

- - 4

S ~

e

Fig. 4. Cell arrangements with different free-gas paths between the electrodes. Relations a : b : c = 1 : 10 : 30; d : e = 1 : 100. 1 = PTFE stopper, 2 = ss tube, 3 = liquid sodium, saturated by oxide, 4 = solid electrolyte, 5 = Pt electrode, 6 = Pt wire, 7 = A1203 insulator tube, 8 = glazine layer, 9 = ceramic tube.

Fig. 3. Experimental arrangement for the voltage and resistiv- ity measurements of solid electrolyte cell under irradiation.

up to 10 R/s were adjusted by means of the electric current. The solid electrolyte cell has been arranged in an electric heater (fig. 3). The inner resistance o f the cell was varied by means o f the cell temperature. A gas stream through the cell was maintained.

The cells with the electrolyte Th0.sY0.201.9 were constructed in a different manner (fig. 4). The differ- ence essentially consisted o f the length o f the free path within the gas between the electrodes. In the cells b and c by application o f additional insulators the gas- eous path has been extended by a factor of 10 in b and 30 in cell c respectively as compared to the cell a.

4.2. Results o f voltage measurements

At the moment of switching on the radiation source the voltage o f the cell shown in fig. 4a decreases by a value of zaU. The value A U depends on the dose rate (fig. 5a). After switching off the radiation source the cell voltage gradually reaches the initial value. This transient effect is typical o f the behaviour of a cell after polarization by short-circuit.

The influence of the radiation on the cell voltage is the stronger the higher the inner resistance R i of the cell (fig. 5b). At a small inner resistance of the cell the influence o f radiation on the cell voltage can be neglected. In the ceils b and c of fig. 4 the voltage

1 I off

1800 J I ~ o n i r r a d l a [ i o n

1600 (

1400 ~ 0.,

1200 = ~ _ l~R/s

1000 0 10 20

E

30

ca t (min)-.

/,0

<3 I

150

100

5O

0 = - -

105 r o

o m

x

/,

106 107 108

Ri (.O.)

Fig. 5. (a) Transient effect of irradiation on the cell voltage at various dose rates; Co) Voltage losses & U of cells with differ- ent inner resistivities R i during gamma-irradiation by a dose rate of 10 R/s.

t 70 IV. Richter, H. Ullman/Solid electrolyte cells under influence of irradiation

eUo

U~, Ro

U Ro4- Ri

,AU U - U ~ . R o - I - R i - R o _ Ri

U U Rob R i Rot- Ri

Fig. 6. Scheme of a cell: (a) in absence of irradiation, (b) during irradiation, which causes a shortage by the outer resistance R a.

losses were lower at the same dose rate and with the same kind ,of gas than in cell a. This is caused by a partial short-circuit between the electrodes by the ionized gas, which has the same effect as a finite outer resistance R a between the electrodes (fig. 6). Than the voltage indicated under irradiation amounts to

U.~ = U 0 [R a/(Ra + Ri) ] , (1)

where U 0 is the cell voltage in the absence of radia- tion. From this follows the loss of voltage:

A U = U.r(Ri /Ra) . (2)

As outer resistance R a we apply the insulation resis- tance of the cell diminished by the radiation-induced conductivity oy to a value o fR . r. Then we obtain

R,~ = o~ 10/F), (3)

where 1 is the distance between the electrodes and F, the electrode area. From eqs. (1) - (3) we obtain

A U/U~/ = R i o. r 17/l . (4)

If the values ofRi , o~/and F have been kept constant during the experiment, the A U]U, r relations are pro- portional to the reciprocal values of the electrode distances, l[ l , as shown in eq. (4). The free paths in the gas between the electrodes of the cells shown in

fig. 4a, b, c behave towards each other as 5 to 50 to 150 mm. The relative voltage losses A U/U3, were in the relation 1 : 0.13 : 0.055, which is in good agree- ment with the relation of the reciprocal values of the electrode distance of 1 : 0.1 : 0.033. This dependence is shown in fig. 7a.

By applying different reference gases, the cell volt- age of cell 4a and the voltage losses under irradiation have been measured. The results (fig. 7b) demonstrate that the A U values for several gases agree satisfactori- ly in their graduation with those of the rates of forma- tion and the mobility of the charge carriers in the gases (table 3). It can be noticed that electrodes with

I(mm)

0.035 150 50 20 10 B 6

093

(I025

0,O2

o.ol 5

(1Ol

0905

0

j '

J

0.'0s o'.1 03s 0.2 1/= (~.~-~)

~ d

, 2

-2-8 HJH~0

b) 106 107 10e m (~z) =

Fig. 7. (a) Dependence o f the relation AU/U,y on the recipro- cal. value of the electrode distance I of the ceils; (b) depend- ence of the relation A U/U,y on the inner resistance R i of the cells during irradiation. The free-gas pa th between the elec- trodes is filled with different gases.

W. Richter, 1£. Ullman/Solid electrolyte cells under influence of irradiation 171

a hydrogen-containing reference gas show the lowest polarization by irradiation, followed by oxygen-con- taining gases.

ity around the samples. On the whole the values were ascribed to the solid state conductivity.

4.3. Results o f conductivity measurements 5. Condu~ons

The conductivity of the solid electrolyte Th0.sY0201.9 with and without irradiation has been determined either by means of the two-contact method at a 50 Hz ac frequency or by current inten- sity-voltage measurements [1 ]. Fig. 8 shows the results of other authors for several oxide ceramics together with our results. The experimental results are compared with theoretical evaluations (see section 3).

Comparing the curves 2 and 3, measured by means of the cells e and d, we observe that gas conductivity is the reason for the increasing conductivity by irra- diation of the cell e, whereas in the case of cell d conductivity through the gas path was practically ex- cluded. Here, we cannot observe any increase in con- ductivity by irradiation in comparison to conductivity without irradiation.

The conductivity results of other authors obvious- ly did not take into consideration the gas conductiv-

T (*C)

T

.£

L~

~,00 600 400 300 200 100 - B • ' \ . . . . .

~ k \ ~ Tho, sYo,201,

' \ t 'k''

' ~ " i s I ~ % soolt/,,

-12 ] Th 0~'--- k \ " F--~ / .

L i i i , , - -- 1D lb 1,8 2,2 2,6

1000 (K-') T

Fig. 8. Conductivity of some inorganic materials in the presence and in the absence of irradiation.

Ionizing irradiation causes transient effects in solid electrolyte cells. Cell voltage and conductivity measure- ments on solid electrolyte cells under ionizing irradia- tion show that the main reason for cell polarization during irradiation consists of the electrical conductivity of ionized gases.

The electrical conductivity of solid electrolytes in- creases only insignificantly also at high radiation dose rates. This effect on solid electrolyte cells can general- ly be neglected.

The experimental results are in good agreement with theoretical evaluations of the influence of ioniz- ing irradiation on the characteristics of solid electro- lytes, ceramic insulators and gases. Under the condi- tions of typical radioactive measuring agents in nuclear technology gamma radiation makes the highest con- tribution to ionization.

Measurements with solid electrolyte cells under ionizing irradiation can be performed without polar- ization after separation of the electrode spaces by ad- ditional ceramic insulation. Further measures to pre- vent the influence of radiation-induced gas conduc- tivity on cell voltage measurements are cell construc- tions with low inner resistance and with reference gases of a relatively low formation rate and mobility of charge carriers. Gases with an oxygen or hydrogen content should be prefered to rare gases.

References

[ 1 ] W. Richter, Dissertation (AdW der DDR, 1984). [2] J.N. Anne, Notes on radiation effects on materials

(Springer, Berlin, 1984). [3] V.I. Spitsyn and V.V. Gromov, Fiziko-khimicheskije

svoistva radioaktivnych tvjordich tel (Atomizdat, Moskva, 1973) [in Russian].

[4] T. Reetz, (1974) unpublished results. [5] N.S. Kostjukow et al., Radiacionneje electromaterialove-

denie (Atomizdat, Moskva, 1979) [in Russian]. [6] V. Kment and A. Kuhn, Technik des Messens radio-

aktivcr Strahlung, (Geest und Portig KG, Leipzing, 1963). [7] Group constants for nuclear reactor calculation

(Consultants Bureau, New York, 1964).