INFLUENCE OF PRELIMENARY RADIATION -OXIDIZING TREATMENT ON THECORROSION RESISTANCE OF ZIRCONIUM IN CONDITIONS OF
ACTION OF IONIZING RADIATION
A. A. Garibov, A. G. Aliyev, T. N. Agayev, G. Z. Velibekova
Institute o f the Radiation Problems o f Azerbaijan National Academy o f Sciences. A Z 1143 Baku,
Introduction
Today mainly water-cooled nuclear reactors predominate in atomic energetics. For safe work of nuclear reactors detection of accumulation process of explosives, formed during radiation and temperature influence on heat-carriers in contact with materials of nuclear reactors in normal and emergency regimes of work is of great importance.
The main sources of molecular hydrogen formation in normal and emergency regimes are the processes of liquid and vaporous water in vapometallic reaction [1-5].
At the result of these processes molecular hydrogen concentration in heat-carrier composition always exceeds theoretically expected concentration.
One of the main ways to solve the problem of water-cooled reactors safety is detection of possibilities to raise material resistance of fuel elements and heat carrier to joint action of ionizing radiation and temperature. The second way is inhibition of radiation-catalytic activity of construction materials’ surface during the process of water decomposition.
It’s been established, that one of the ways to raise resistance of zirconium materials to the influence of ionizing radiation is formation of thin oxide film on the surface of metals.
In the given work the influence of preliminary oxidizing treatment of zirconium surface on its radiation-catalytic activity during the process of water decomposition. With this aim zirconium is exposed to preliminary influence of gamma-quantum in contact with hydrogen peroxide at different meanings of absorbed radiation dose.
Experimental
Investigations were carried out in static conditions in special quartz ampoules with the volume V=1,0 cm3. Reactor zirconium with 99,9% of purity as thin strap was taken as the investigation object. Contacting surface of the samples was determined on the base of their geometric size and it made 34,6 cm2/g. With the aim of exclusion of influence of surface organic pollutions on the process of molecular hydrogen accumulation the samples had been preliminarily purified with the help of organic solvents - ethyl alcohol, acetone, and then were rinsed with distilled water. After that the samples were dried at the temperature 300^3209 in inert gas medium - argon. Dried samples were weighted to within ±5-10"5 g and were transferred into quartz ampoules. Ampoules with the samples were vacuumized up to P=10"3 Pa, firstly at the temperature T=300K, and then 473 K. Then hydrogen peroxide (CH2o 2= 9 mole/l) was added to samples until they were wholly covered with it. Ampoules were connected with special gasometer. The samples were exposed to preliminary radiation influence of gamma-rays (D&= 1.14Gy/s) at different times, then the samples were dried and weighted. After weighing the samples were transferred to the special ampoule for experiments of radiation-catalytic activity in processes of radiolytic decomposition of water. Necessary quantity of water was added into ampoule with samples by the way of water vapor condensation from graded volume of vacuum- adsorption plant. The accuracy of water injection into the ampoule with samples from vacuum- adsorption plant in investigated range of density meanings of water vapor is ±5%. The temperature during the experiments was maintained to within ±1K.
Radiation and radiation-thermal processes were carried out on isotopic source of y-radiation 60Co. Source dosimetry was carried out by means of chemical dosimeters - ferrosulfate, cyclohexane and methane. Re-calculation of adsorbed radiation dose meanings in investigated systems was made by comparison of electronic densities [11]. Gas products of the processes were transferred to special graded volumes and were analyzed by gas chromatograph method. At radiolytic process T=300K in structure of gas products except for H2 observed also O2, and at thermoradiolysis - H2. Zirconium materials’ corrosion was investigated by weight method.
For this purpose the initial metal samples and those ones exposed to the experiments dried in vacuum P«10"5Pa were weighted to within 10-5g.
Materials corrosion at the result of radiation-thermal and thermal processes was characterized according to samples overweight Am= mi- m0.
The speed of radiation components Wr.(H2) of radiation-thermal process of hydrogen accumulation was determined from speed difference of radiation-thermal and thermal processes:
Wr(H2) = Wrt(H2)-Wt(H2)
The impact of zirconium preliminary radiation treatment in H20 2 medium on electro physical properties has been investigated with the purpose of determination the mechanism of initial radiation treatment impact on corrosion resistance of zirconium materials. For this purpose after samples treatment the electrical resistivity and thermoelectromotive force were measured.
Measurement of samples resistivity was held with the help of four balloon-born point contacts by 1 U S
compensation method at direct current and by formula p = = • , where U -voltage drop
a A L
between two probes (2,3), A-current between probes (1,4), S-sample section. Resistivity inaccuracies didn’t exceed 2,7%. The following devices were used: power sources of TEC 41 type, universal voltmeter B7-21 and B7-21A for voltage drop measurement; for current measurement - combined digital device V,431.
During thermoelectromotive force (a) measurement, the absolute stationary method with compensation was used. Here temperature gradient AT=2 13° was created along the lamella and temperature was measured by differential med-constantan thermocouple and thermo electromotive force was determined by formula a= t - y, where y - thermocouple sensitivity [6-9]. During a measurement the inaccuracy made 6,0%.
After that the samples were placed in special ampoule with H202, were connected with gasometer and were exposed to gamma-irradiation impact at different exposure times.
After treatment the samples were dried and the measurement of electro physical properties was held in identical conditions.
Corrosion resistance and catalytic activity of previously treated samples was tested in radiation- thermal and thermal processes of water decomposition when T=673K, p(H20)= 5mg/cm3,
D&= 1 .1 4 G y /s, xtr= 30min. And with all this the kinetics of molecular hydrogen accumulation and metal oxidation as a result of radiation-thermal and thermal processes in contact with water had been studied.
Results and Discussion
For the purpose of detection the influence of metal materials on water radiolysis, the kinetics of H2 at radiolytic decomposition of water and water + reactor zirconium system at T=300K had been investigated.
Radiation-chemical outlet G(H2) at this made 0,44 and 0,54 molecules/100eV, accordingly.
G lH T i n » f e r T IO O rV
Observed growth of G(H2) meanings during H20 radiolysis in presence of metal Zr in comparison with radiolysis outlet of pure water may be explained by the contribution of emitted from metal under the influence of y-quantum 5-electrons and formation of additional active centers of water decomposition on metal surface.
The process speed at this makes AW=3,5T013 molecules/sec. In conditions of nuclear reactors work metal constructions are exposed to simultaneous influence of temperature and radiation in contact with heat-carrier.
That’s why kinetics of molecular hydrogen accumulation at radiation-thermal ad thermal processes in Zr contact with water at T= 673K and p=5 mg/ cm3 and preliminary metal irradiation. For the purpose of detection the contribution of radiation-heterogenic processes in radiation-thermal heterogenic processes, radiation-thermal and thermal processes of water decomposition were held in identical conditions (T=673K and p=5mg/cm3). On the basis of initial linear section of experimental kinetic curve Wrt(H2) and Wt(H2) speed meanings were determined.
In Fig. 1 and 2 typical kinetic curves of molecular hydrogen accumulation at thermoradiolysis and thermal processes of water decomposition in presence of metal zirconium. During preliminary radiation-oxidizing treatment of zirconium in presence of H202 formation of oxide film on metal surface may be observed.
Zr + H202—— Zr - Z r0x + H20 + H2
Fig 1. Kinetic curves o f molecular hydrogen
accumulation in radiation-thermal (1)
(D&= 1.14Gy / s) and thermal (2) processes of water decomposition in presence o f Zr samples, previously radiationally treated at D=20,52 kGy in presence o f H20 2.
Fig 2. Kinetic curves o f molecular hydrogen
accumulation in radiation-thermal (1)
(D&= 1 .1 4 G y /s) and thermal (2) processes of water decomposition in presence o f Zr samples, previously radiationally treated at D=287,28 kGy in presence o f H20 2.
Fig 3. Dependence o f radiation-chemical yield o f molecular hydrogen under radiation-thermal (1) ( # = 1.14Gy s ) and accordingly accumulation speed molecular hydrogen at thermal processes o f water decomposition in presence o f Zr (T=673K, p=5mg/cm3) from the time o f its preliminary radiation treatment in presence o f H20 2 at T = 300K.
These data will well be coordinated to results of research of processes of oxidation and deffect formation in previously radiationaly processed zirconium and at the further radiational-thermal processes in contact it with water. For the purpose of revealing a nature of active radiating defects in metal zirconium the physical properties previously radiationaly processed of metal zirconium were investigated at the presence of 30 % H20 2.
It’s known that dependence of hydrogen peroxide’s gamma-radiolytic decomposition speed from concentration of its water solutions passes maximum over the concentration range of hydrogen peroxide C(H202) «30 140% [12].
Under the influence of gamma-quantum as a result of electrons emission and relcase, positive states and coordinated unsaturated atoms are formed accordingly on surface of zirconium.
Zr ^ Zr + + Zr - (1)
The most possible hydrogen peroxide transformation processes are the following:
H 2 O 2 ^ 2 OH (2)
OH + H2O2 ^ H 3O + + HO2 (3)
HO2 + H 2O ^ H3O + + O2- (4)
H2O2 + O ^ HO" + OH + 1/2O 2 (5)
The process may pass under the influence of surface states of zirconium and decomposition products may interact with the surface:
Zr + OH ^ Zr - OH (6)
Zr + HO2 ^ Zr - O2H (7)
In both cases hydroxyl groups and partially protonated hydrogen are formed on the surface of zirconium. In such cases samples resistance is decreased due to hydrogen and OH-group existence. That’s why depending upon p = f (
D
) during the initial treatment times (D
< 20 -103Gy
) the minimum is observed (Fig.4).During the further decrease of treatment time and accordingly of radiation absorbed dose, radiation dehydroxylation and interaction with other H202 radiolysis products can be observed:
Zr - OH ^ Zr - O + H (8)
Zr + + HO“ Z r - O H (9)
Z r+ + O2 - ^ ZrO 2 (10)
As a result of these reactions, the oxide film that conditions decrease of samples’ electrical resistivity is formed on the surface of metal zirconium. During the following radiation dose increase creation of deficient states takes place in surface oxide phase.
As a result of radiation-heterogeneous processes 0-holes that migrate inside metal phase are formed in oxide phase. In consequence of these processes electrical resistivity of oxide phase is decreased under the law p
= f ( D 0 8
) . So, preliminary zirconium treatment in oxidizing medium (30%H20 2) under the impact of gamma radiation depending upon absorbed dose meaning leads to metal conductivity decrease and increase.Previously radiation-oxidationally treated samples were exposed to test in radiation-thermal and thermal processes in contact with heat carrier H20 at T=673K, p(H20)=5mg/cm3, D = 1 .1 4 G y /s, xtr=30min. Then electrophysical properties of tested samples were studied.
Dependences of electrical resistivity of samples that were exposed to radiation-thermal (2) and thermal (3) testings from prelimenary treatment times are given in Fig 4.
As it’s seen during radiation-thermal processes recovery of initial electrophysical state doesn’t pass wholly. During these processes in Zr - ZrOx - H 2O system oxidizing defectformation takes place in oxide phase.
Dehydroxylation and oxidation of zirconium surface in radiation-thermal processes pass with higher speed, than in thermal processes. That’s why during thermal processes recovery of highoxide state takes place in samples, treated during longer times x=50h, x=30h.
Electrical resistivity of samples after radiation-thermal and thermal processes is lower than in initial ones that may indicate high concentration of defect states in them.
That’s why in samples, previously exposed to radiation-oxidizing treatment at D>200kGy, in radiation-thermal processes during longer intervals of contact catastrophic oxidation area is observed (xcat>120min).
t » V H u m
I br
Fig 4. Dependence o f electrical resistivity of zirconium samples, previously irradiated at T=300K (1), thermally (2) and radiation- thermally (3) treated at T=673K (xtr=0,5 h).
For the purpose of determination the character of electroresistance dependence and radiation- catalytic activity from preliminary oxidizing processing of a metal surface the relative change of resistance and thermoelectromotive force of samples previously oxidizingly processed is investigated at times appropriate to a minimum (5 hours) dependence p (x).
In Fig.5 presents the dependences of relative Ap
resistivity change and thermoelectromotive
P
force (a) of radiation-thermally treated lamellas from radiation absorbed dose.
Ap
As it is seen from the figure at little doses grows intensively, but at comparatively high doses
P
3
Ap
(D> 3,5-10 Gy) growth of subsides and slowly approaches to saturation. In this area
P
thermoelectromotive force (a) meaning begins to subside in consequence with p change, but at low doses of a within the limits of experiment inaccuracy doesn’t almost change and is equal to
i mkv
(6 I 8) .
4 0 * o Tin-Q. ™ 10--io _ k B !§ t f a 4 İ A j -1C- . D , k G y
Fig. 5. Dependence o f ^ P = and a=D(D) P o
for metal zirconium (Zr) from absorbed dose. Resistivity change p depending upon the preliminary radiation treatment may be divided into four areas (Fig1):
1. When absorbed dose meaning is D=(0 120)-103 Gy p is abruptly decreased on 50% 2. When: D=(20 135)-103 Gy p meanings are increased.
3. When D=(50 1 70)-103 Gy - saturation range begins.
4. When D>70-103 Gy p meanings again subside slowly.
Similarly p meanings change for thermally and radiation-thermally treated samples. This is evidently connected with appearance of first point defects. In area D=(20 |35)-103Gr increase of p is connected with the fact that in parallel with point defects, vacancies begin to appear and to increase due to migration of internodal atoms.
The fact itself that p~m*-n-1 and a~m*3/2-n_1 means that here not only point defects, but vacancies, the concentration of which increase also are of great importance. This in its turn affects efficient masses of current carriers m* [10]. In area D=(50 1 70)-103 Gy p passes saturation range, where these two competing concentrations even. Originated vacancies come to zirconium surface and in interaction with water interact with oxygen. As a result of interaction Zr-ZrO2 is formed, localization of which passes up to the moment when the surface becomes saturated with them. Comparative resistivity in samples after radiation-thermal treatment grows slowly (Fig.5), but thermo electromotive force meanings decrease. As it is known thermo electromotive force (a) is very sensitive to metal Fermi- surface properties change and so on surface layer that had a contact with water the process passes very difficult. Here not only abovementioned two factors but also appearance of different bubbles, and also generation of different point defects and their recombination are of great importance.
So, change of meaning thermoelectromotive force (a) is connected not only from concentration of carriers of a charge (a~p-1) but also to effective weight of carriers (a~m32). Therefore in the field of saturation relative change depending on the absorbed doze (Fig. 5) in an interval D = 3^5,4 kGy reduces meaning thermoelectromotive force.
That is connected to change of concentration easy holes at a power level Fermi.
( f t gen= ' heavy holes + ' easy holes)
Basing upon abovementioned factors in processes that pass in concrete area of zirconium surface in contact with water, it’s necessary to evaluate surface chemisorption processes, though these processes at low absorbed doses of y-quantum are not very significant. But we are interested changes of surface electrophysical properties.
These properties to a greater or lesser extent are sensitive to adsorption processes that pass on zirconium surface. For qualitative evaluation on the base of current-voltage curve electrical resistivity (R) for all groups was determined: previously irradiated, radiation-thermally and thermally treated after preliminary treatment.
AR
The meanings of comparative electrical resistivity were determined and comparative R 0
Ap
resistivity changes were compared. Within the limits of experiment inaccuracy they coincided.
p
3
Ap
Nevertheless at large absorbed doses (D>60-10 Gr) change of meanings for radiation-thermally
p
treated samples is 2,4 times as much, than for thermally treated ones. It’s explained by the fact that at definite doses the relation of comparative resistivities of thermally treated
AP
P
and previously
Ap
irradiated samples makes
p
f
Ap'P f
ApP
P J
V P
J0.15
0.08 = 1.88, but at corresponding radiation doses for radiation-thermally treated samples this relation makes
f Ap
P
/f A pP
V P J
Ap
V p J
0.36 0.08 = 4.5: So, on the basis of achieved results the following conclusions can be drawn:
Radiation -oxidizing processing of zirconium in presence H20 2 at D=15^25kGy results to increase superficial catalytic activity during decomposition of water, that is connected that at small dozes in system Zr-ZrO2 oxide layers are formed defective condition and superficial free holes O"
■ The further increase of a doze of an irradiation results in radiation -heterogeneous processes of formation protective oxide layer, which causes increase of resistance r of samples up to the certain doze. The superficial condition of zirconium, formed at preliminary processing Zr at presence H202 in an interval of meanings of the absorbed doze of a scale - irradiation D = 120-200 kGy, are characterized low catalytic activity during radiating and thermal decomposition of water
■ Radiating processing of zirconium at presence of H20 2 at dozes D > 280 kGy results in increase of catalytic activity during radiation-thermal decomposition of water, and acceleration of approach of area of catastrophic oxidation of metal and accordingly to downturn of electroresistance of samples.
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