Physicochemical characteristics and radiation-chemical processes in the system PuO2-sorbed water

Article abstract of DOI:10.1134/S1066362206040175

The experimental data on formation of H₂ and O₂ during storage of PuO₂ samples containing sorbed water were analyzed. It was shown that the rates of formation of these gases, all other factors being the same, are governed

Full text of DOI:10.1134/S1066362206040175

ISSN 1066-3622, Radiochemistry, 2006, Vol. 48, No. 4, pp. 403 408. Pleiades Publishing, Inc., 2006.
Original Russian Text M.V. Vladimirova, 2006, published in Radiokhimiya, 2006, Vol. 48, No. 4, pp. 362 367.
Physicochemical Characteristics and Radiation-Chemical
Processes in the System PuO2 Sorbed Water
M. V. Vladimirova
Bochvar All-Russian Research Institute of Inorganic Materials, Federal State Unitary Enterprise,
Moscow, Russia
Received April 7, 2005
Abstract The experimental data on formation of H2 and O2 during storage of PuO2 samples containing
sorbed water were analyzed. It was shown that the rates of formation of these gases, all other factors being
the same, are governed by the procedure of water sorption: in sorption from the liquid phase these rates are
significantly higher than those in sorption from the gas phase. To elucidate the conditions of safe storage
of PuO2, it is necessary to obtain systematic quantitative data on the kinetics of H2 and O2 formation in
the system consisting of PuO2 and water sorbed from moist air. The optimized mathematical model of sorbed
water radiolysis adequately describing the experimental data on the formation of H2 and O2 at room tempera-
ture in the system consisting of PuO2 and water sorbed from the liquid phase is presented. The rate constant
of H2 and O2 recombination in the presence of PuO2 containing 2 3% water was found to be
1
10 mol 1 s . With the knowledge of the reaction rate constants, the model allows calculation of the
amounts of H2 and O2 and the pressure in the storage vessel depending on the amount of sorbed water, radia-
tion dose rate, and storage duration.
5
1
PACS numbers: 61.80.-x, 68.43.-h
DOI: 10.1134/S1066362206040175
Conversion of weapons-grade plutonium is an
urgent problem on the global scale. One of the ways
of solving this problem is conversion of weapons-
grade plutonium into dioxide by calcination of pluto-
nium oxalate whose production is well developed.
sorbed water and formation of H and O under given
2 2
conditions.
The stored plutonium dioxide can have different
specific surface areas (SSA) depending on the calcina-
tion temperature. The latter also determines the equi-
librium content of sorbed water. Plutonium dioxide
can sorb water from air or a liquid phase. Individual
The resulting PuO will be stored forever or for a
2
certain period of time with subsequent use for pro-
duction of mixed uranium plutonium oxide (MOX)
fuel.
batches of PuO can have different radiation dose
2
rates and can contain various impurities.
Plutonium dioxide is a hygroscopic substance. It
was shown previously [1] that, after calcination at 490
and 760 C and storage in air with a relative humidity
of 100% for several days, the water content in the
It is well known that the quantitative characteristics
of radiolysis for sorbed substances differ essentially
from those for free substances. Therefore, the data
obtained in study of -radiolysis of free water can-
not be used for evaluating the amount of H and O
PuO samples is 3.5 and 1.5%, respectively. Under
2
2
2
-radiation of Pu, the sorbed water decomposes to
form H , O , and H O [2]. Accumulation of H and
released in the radiolysis of water sorbed on PuO . In
2
2
2
2
2
2
addition, PuO has certain physicochemical features
O is a source of hazard in storage of both liquid and
2
2
complicating radiation-chemical processes and their
study. Plutonium dioxide is a chemically active sub-
stance. As shown previously [3 7], it reacts with
solid radioactive wastes containing water, due to
the possibility of formation of detonating gas and
the pressure increase in hermetically sealed vessels.
water to form H and superstoichiometric plutonium
2
Special containers will be designed for safe storage
dioxide and catalyzes the reaction of H and O .
2
2
and transportation of PuO . Design of containers is
2
Formation of H and O in storage of PuO con-
one of the main practical problems of PuO storage.
2
2
2
2
taining sorbed water is studied in the United States
and Russia.
For designing the containers it is necessary to have
data on the kinetic of radiolytic decomposition of
403

404
VLADIMIROVA
Table 1. Content of H2 in the gas phase upon keeping of
PuO2 samples for 7 days. Weapons-grade Pu [8]
Published data [8] presented in Table 1 show that,
at a water content of approximately 1% in the system
H O(total S.), hydrogen was not found in the gas
phase, and at a water content of 2.3 2.5% in this sys-
2
H2O(total S.)
H2O(outer S.)
tem, the H content is approximately 3 times lower
2
[H2O], %
H2, %
[H2O], %
H2, %
than that in the system H O(outer S.).
2
The systematic data on the kinetics of H and O
2
2
1.2
1.8
2.3
0.0
0.5
0.7
1.03
1.44
2.5
0.4
0.3
2.3
formation at room temperature in PuO containing
2
water sorbed from the liquid phase, i.e., in the system
H O(outer S.), were obtained in [9]. The content of
sorbed water was within 0.3 3%. The experiments
2
were mainly performed with PuO samples originat-
Table 2. Rates and radiation-chemical yields of H2 and
2
ing from nuclear power plants, and one of the samples
was of the weapons-grade; the dose rates were 11.6
O2 in the system PuO2 H2O(outer S.) (Pu from power
1
plants, D = 11.6 Gy s 1 g ; 22 2 C) [9]
1
and 1.9 Gy s per gram PuO , respectively. Data on
2
Rate,
Yield, molecules
per 100 eV
the influence of the dose rate are important, since in
conversion of weaponsgrade Pu its initial isotopic
composition and, hence, the dose rate of radiation will
change due to addition of Pu from power plants.
Form of Water
1
cm3 g 1 day
PuO2
content,
%
sample
H2
O2
G(H2) G(O2)
The amounts of H and O in the gas phase were
2
2
Pellet
Pellet
Pellet
Pellet
Pellet
Powder
Powder*
0.3
1.0
1.5
2.0
3.0
3.0
3.0
0.01
0.07
0.16
0.29
0.60
0.75
0.12
0.001
0.01
0.025
0.04
0.10
0.08
1.1
2.2
3.4
4.65
6.4
8.0
7.9
0.09
0.32
0.53
0.65
1.07
0.86
1.0
determined experimentally and the initial rates (expo-
sure time 10 30 days) and radiation-chemical yields
of their formation were evaluated. The results ob-
tained are presented in Table 2.
Table 2 shows that, at a given H O content, the ini-
2
0.015
tial rate of H and O formation is in direct proportion
2
2
to the dose rate of Pu radiation and does not depend
on the physical state of the sample (powder or pellet).
It is seen that, with increasing water content by a
factor of 10 (from 0.3 to 3%), the rate of formation
of the gases increases by a factor of 60 100. This is
caused by both an increase in the energy absorbed by
water and increase in the radiation-chemical yields of
H and O . It is seen that G(H ) and G(O ) do not
1
1
* Weapons-grade Pu, D = 1.9 Gy s
g .
Here we present the experimental data and op-
timized mathematical model developed for evaluation
of the amounts of H and O formed by radiolysis
2
2
of water sorbed on PuO .
2
2
2
2
2
EXPERIMENTAL DATA
noticeably depend on the radiation dose rate; with in-
creasing [H O] from 0.3 to 3% G(H ) increases from
2
2
The quantitative data on formation of H in the
2
1.1 to 6.4, and G(O ), from 0.09 to 1.1 molecules per
2
system PuO sorbed water are presented in [8 10].
2
100 eV. In -radiolysis of free water, G(H ) and
2
It was shown [8] that the percent content of H in the
2
G(O ) are 1.4 and 0.2 molecule per 100 eV, respec-
2
gas phase released in storage of moist PuO depends
2
tively [2].
on the procedure of water sorption on PuO , all other
2
It is well known that sorbed substances decompose
in radiolysis with significantly higher yields
than free substances. This is caused by a particular
mechanism of energy transfer of ionizing radiation
from a solid matrix to a sorbed substance. According
things being the same. In sorption from the liquid
phase, the percentage of H is significantly higher
2
than that in sorption of water from gas phase (moist
air). These results are very interesting and funda-
mental.
to published data [9], the rate of H formation in the
2
Apparently, water sorbed on PuO from the gas
2
system H O(outer S.) (3.0% H O) with weapons-
2
2
phase is uniformly distributed in the powder bulk over
the whole surface, whereas water sorbed from the
liquid phase forms a film at the outer surface of
the powder. We denote these two cases as follows:
H O(total S.) and H O(outer S.).
3
1
1
grade Pu is 0.12 cm g day . Data for the system
H O(total S.) with weapons-grade Pu are presented
2
in Table 3.
It was shown previously [10] in two experiments
2
2
RADIOCHEMISTRY Vol. 48 No. 4 2006

PHYSICOCHEMICAL CHARACTERISTICS
405
that, the higher temperature of PuO calcination, i.e.,
Table 3. Initial rates of H2 formation [V(H2),
2
1
cm3 g 1 day )]
the smaller its specific surface area, the higher the rate
of H formation. Table 3 shows that the rate of H
2
2
V(H2) at indicated temperature
formation increases with increasing amount of sorbed
water; in this case, the increase in the reaction rate is
significantly more pronounced than the increase in the
amount of sorbed water. Unfortunately, only few data
were obtained under comparable conditions. Published
data presented in [10] and our data differ essentially.
A comparison of our data shows that the rate of hy-
of PuO2 calcination,
C
[H2O],
%
450 [10]
600*
700 [10]
1.0
1.3
2.0
3.0
3.5
0.0012
0.00024
0.0019
0.0089
0.00078
0.0038
0.0055
drogen formation in the system H (outer S.) at a
0.0016
2
water content of 3% is higher than that in the system
H O(total S.) by a factor of 75.
2
* Unpublished data of Karnozov and Vladimirova.
Obtaining reliable quantitative data on the kinetics
of H and O formation in the most practically impor-
discrepancy in the experimental and theoretical rates
can be caused by the reaction of H with PuO and
2
2
tant system H O(total S.) is one of the urgent prob-
2
2
2
radiation-chemical and chemical recombination of H
lems related to prolonged storage of PuO .
2
2
and O , catalyzed by the PuO surface. Paffett and
2
2
Experiments with the system in which water was
Kelly [11] assumed that, at a water content of 0.5%
sorbed on PuO from the liquid phase gave maximum
2
(4 monolayers), H is formed by the radiolysis. How-
2
possible quantitative characteristics of radiolytic for-
mation of H and O .
ever, lower experimental rates in comparison with
theoretically possible rates are mainly caused by a de-
2
2
The following conclusions, extremely important
from practical and theoretical standpoint, can be made
crease in the rate of H formation in passing from
the first monolayer to the subsequent one, with an
increase in the distance of water from the emitting
2
from Tables 2 and 3: The rates of H formation and,
2
hence, the rates of water decomposition, other condi-
tions being the same, are significantly lower for the
PuO surface. It should be noted that the mechanisms
2
cited are hypothetical. In the corresponding papers and
reports, there are no quantitative data confirming one
or other mechanism, and the theoretically possible
system in which water was sorbed on PuO from the
2
gas phase, i.e., for the system H O(total S.). The rate
2
rates of H formation to which the experimental rates
of H formation for this system essentially depends on
2
2
were compared are not given.
the temperature of PuO calcination. A fundamental
2
question arises: why water sorbed on PuO from the
2
To elucidate the mechanism of physicochemical
liquid phase decomposes at a significantly higher rate
than water sorbed from the gas phase?
and radiation-chemical processes in the system PuO
2
H O(total S.), it is necessary to obtain systematic data
2
on the kinetics of H and O formation in the absence
It was suggested previously [8] that water sorbed
from the liquid phase behaves as free water, and
water sorbed from the gas phase, as sorbed water. I do
not agree with this opinion, since an increase in the
yields of H and O formation with increasing amount
2
2
and in the presence of air (O ) in the gas phase
2
depending on the specific surface area of the powder
and water content.
2
2
MATHEMATICAL SIMULATION
OF RADIATION-CHEMICAL PROCESSES
of water sorbed from the liquid phase and high yields
of H and O (Table 2) suggest that in this system
2
2
IN THE SYSTEM PuO SORBED WATER
2
the water behavior in radiolysis follows the laws of
sorbed states.
For practical purposes, to evaluate the pressure in
storage vessels, it is necessary to have data on the
kinetics of formation of radiolytic gases for a wide
range of time. There is no way to obtain experimental-
ly these data for weapons-grade plutonium, especially
The mechanisms of H formation in the system
2
PuO H O(total S.) were considered in a series of
2
2
studies carried out in the United States. The general
conclusion is that, at a water content of 0.5%, H is
formed not by the radiolysis but by the reaction of
2
for the system H O(total S.), since, as seen from
2
H O with PuO (PuO + H O
PuO
+ xH ); at
Table 3, the rates of H formation are extremely low
2
2
2
2
2+x
2
2
larger amounts of water, H is formed by the radioly-
and observations for many years are required. Data
can be obtained using mathematical simulation, which
also allows estimation of the process mechanism.
2
sis, but the rate of its formation is less than the
theoretically possible rate. It was concluded that the
RADIOCHEMISTRY Vol. 48 No. 4 2006

406
VLADIMIROVA
8
9
Initial Data for Development of Mathematical Model
K for weapons-grade Pu are 4.0 10 , 5.0 10 ,
3
8
and 3.5 10 , and for Pu from power plants, 2.5
10 , 2.9 10 , and 2.2 10 s , respectively.
7
8
7
1
It was shown previously [9] that, in storage of the
samples for 30 days, the rate of H formation de-
2
creases and its amount tends to a constant. At an H O
2
Evaluation of the H Amount
2
3
content of 3% (1.66 10 mol), the steady-state
3
amount of H is approximately 45 cm , which corre-
2
By including reactions (1) (3) with the correspond-
ing rate constants into the scheme, we evaluated the
3
sponds to 2 10 mol, i.e., the steady-state amount of
H is approximately equal to the initial water content.
2
amounts of H for various experiments with Pu from
2
Therefore, under the considered conditions, the radio-
lytic decomposition of sorbed water is complete. It
was concluded from these data that the kinetics of
power plants [12]. It was shown that the evaluated
amounts of H are significantly lower than the ex-
2
perimental values. The analysis of the rates of reac-
tions (1) and (3) showed that water is expended for
the formation of H and H O to approximately the
formation of H and other radiolysis products of
2
sorbed water are governed by the kinetics of radiolytic
decomposition of sorbed water, and the kinetics of
their formation are first-order with respect to water
concentration.
2
2 2
same extent, which results in underestimation of the
H amount in the calculations. It was concluded that
2
the decomposition of H O to water [12] should be
2
2
included in the scheme. Among possible reactions we
selected the reaction of H O with PuO , which yields
The reactions giving rise to the radiolysis products
can be presented in the form
2
2
2
water and superstoichiometric plutonium oxide:
H2O
H2O
H2,
O2,
(1)
(2)
(3)
PuO2(s) + H2O2(ads) PuO2 + x(s) + H2O(ads). (4)
Superstoichiometric plutonium oxide was found
experimentally in [4 7] in studying the reaction of
H2O
H2O2.
PuO with water. To elucidate the role of reaction (4)
2
The expressions for the initial reaction rates are
in the form
in the kinetics of H formation and to find its rate
2
constant, we performed the corresponding computer
simulations, which showed that satisfactory agreement
d[H2]/dt = K1[H2O],
d[O2]/dt = K2[H2O],
d[H2O2]/dt = K3[H2O].
(I)
(II)
with the experimental data is reached at K
2
4
3
1
1
10 mol
s
[12]. Thus, the minimum value of the
3
1
1
rate constant of reaction (4) is 2 10 mol s .
(III)
Scheme of Radiation-Chemical Reactions
Here and hereinafter, the brackets denote the per-
centage or molar amount of water and the amounts of
Previously [12 14] I suggested a mathematical
model of sorbed water radiolysis involving 5 primary
reactions giving the products of water -radiolysis
(H , O , H O , e , and OH), reactions of PuO with
3
H and O expressed in moles or in cm per gram
2
2
of PuO .
2
2
2
2
2
aq
2
The matter balance equation (in moles) is in the
form
H O and H O , and also 13 secondary reactions
2
2 2
involving molecular and radical products of water
radiolysis. Here I performed calculations which
showed that the rates of 13 secondary reactions are
significantly lower than the rates of formation of three
molecular radiolysis products: H , O , and H O .
[H2O]0 = [H2O]t + [H2]t + [O2]t + [H2O2]t. (IV)
In expressions (I) (III), K , K , and K are the rate
1
2
3
2
2
2 2
constants of formation of radiolysis products. K and
Based on this fact, here I present a model optimized
with respect to the amounts of primary radiolysis
products and reactions involving them. This model
does not include the reaction of PuO with H O, since
1
K were evaluated using the initial rates of H and O
2
2
2
formation presented in Table 2. To evaluate K , we
3
postulated that the ratio of K and K is equal to the
2
2
3
1
it was shown previously [4, 6] that the rate of this
reaction does not depend on the content of sorbed
water. It was found previously [6] that the rate of this
ratio of G(H O ) to G(H ) for free water in -radio-
2
2
2
lysis (0.89 [2]). The evaluated rate constants for the
system H O(outer S.) are given in [12 14]. It can be
2
1
1
suggested that, at a water content of 3%, K , K , and
reaction is 0.5 nmol h g .
1
2
RADIOCHEMISTRY Vol. 48 No. 4 2006

PHYSICOCHEMICAL CHARACTERISTICS
407
Reactions (1) (4) and also reaction (5) describing
recombination of H and O with formation of water
Table 4. Scheme of reactions included in the mathematical
model
2
2
were included in the optimized scheme (Table 4).
Recombination of H and O in the presence of PuO
2
Reaction
Rate constant
See [12 14]
2
2
was observed in [3, 4, 7]. In my papers [12 14] reac-
tion (5) was not included in the scheme.
H2O
H2O
H2O
H2
O2
(1)
(2)
(3)
To elucidate the role of reaction (5) and find its rate
constant, we evaluated the amounts of H and O for
H2O2
1
1
PuO2 + H2O2
PuO2 + x + H2O (4) 2.0 10 3 mol 1 s
[12]
2
2
[H O] = 2.0 and 3.0% at various K values. The cal-
2
5
H2 + 1/2O2
H2O
(5) 1.0 10 5 mol 1 s
(this work)
culations showed that good agreement of the calcu-
lated and experimental data for the system H O(outer
2
5
1
1
S.) is observed at K
1.0 10 mol s .
5
Table 5. Evaluated amounts of H2O, H2, O2, and H2O2
The quantitative data on recombination of H and
2
(moles) at various storage times of PuO2 from power
O in the presence of PuO at room temperature are
2
2
3
plants with an H2O content of 3% (1.66 10 mol).
presented in [4, 7]. It was found that, at the initial
4
System H2O(outer S.)
amounts of H and O in the gas phase of 1.9 10
2
2
5
and 8.37 10 mol and a time close to 0, 0.125, and
Time,
days
6
[H2O]
[H2]
[O2]
[H2O2]
1 day, the rates of water formation are 2.0 10 ,
7
7
1
1
8.0 10 , and 2.5 10 mol g day , respectively.
The decrease in the rate of water formation, i.e., in the
rates of H and O recombination, in time is attributed
3
3
3
3
4
4
5
5
6
6
6
5
4
4
4
3
3
3
3
3
3
3
6
5
5
5
4
4
4
4
4
4
4
5
1
3
5
1.6 10
1.5 10
1.4 10
3.5 10
1.0 10
1.6 10
3.1 10
1.0 10
1.3 10
1.43 10
1.46 10
1.47 10
1.47 10
1.47 10
4.3 10
1.2 10
2.0 10
3.8 10
1.1 10
1.5 10
1.47 10
1.45 10
1.3 10
1.2 10
1.1 10
2.2 10
4.0 10
4.3 10
4.1 10
2.0 10
7.0 10
1.1 10
4.1 10
7.3 10
3.0 10
2.7 10
5
5
5
5
6
6
7
7
7
7
2
2
in the papers to decreasing catalytic activity of the
PuO surface due to its moistening. At a rate of 2.0
10 1.27 10
2
6
1
1
3
1
1
50
100
150
200
300
400
500
5.0 10
1.6 10
5.0 10
2.3 10
8.7 10
6.7 10
5.8 10
10 mol g day , K = 1.46 10 mol s . This
value of the rate constant is maximum possible for dry
5
PuO . It is two orders in magnitude higher than the
2
maximum K found in this study for [H O] equal to
5
2
2 3%. It can be noted that the rate of reaction (8)
depends on the presence of impurities in PuO [5].
2
Evaluation of the H , O , and H O Amounts
2
2
2 2
Table 5 shows that, after the lapse of 100 days, the
The amounts of H and O were evaluated using
2
2
amount of O starts to decrease. Analysis of the calcu-
2
the scheme given in Table 4 and the corresponding
values of the rate constants. It was shown that the cal-
culated amounts of H and O reasonably agree with
lated data showed that this is due to reaction (5).
The calculations without consideration of reaction (5)
showed that the content of both O and H continu-
2
2
2
2
the experimental values for plutonium from power
plants at the content of sorbed water of 0.3 3%. This
fact proves the validity of the mathematical model
suggested and adequate accuracy of the rate constants
found.
ously increases in time [14].
It is seen from Table 5 that, when the steady-state
amount of H is reached, the sum [H ] + [O ] is prac-
2
2
2
tically equal to the initial water content. In this con-
nection, the equation of matter balance transforms
from expression (IV) to relationship (V):
The amounts of water radiolysis products and water
itself were evaluated. The results are presented in
Table 5; it is seen that the main amounts of H and O
[H2O]0 = [H2]st + [O2]st,
(V)
2
2
(80% of the maximum) are formed within 100 days,
and then [H ] slowly increases and [O ] decreases.
where the amounts of substances are expressed in
moles.
2
2
Thus, under these conditions, i.e., for a given amount
of sorbed water and a given radiation dose rate, the
calculations allow determination of the time interval
in which a gas pressure close to the maximum will
be reached and which should be under a particular
control.
Based on expression (V), we obtained the follow-
ing relationship between the initial amount of sorbed
water and steady-state amounts of H and O :
2
2
2
[H2]st + [O2]st = [H2O]0/(8 10 ).
(VI)
RADIOCHEMISTRY Vol. 48 No. 4 2006

408
VLADIMIROVA
1
Table 6. Rates of reactions (1), (2), and (5) (Vi, pmol s )
nificantly lower than that in the system H O(outer S.),
the role of reaction (5) will be significant at a short
2
3
at [H2O] = 2.5%, [O2] = 2.3 10
mol, and D =
1
2 Gy s 1 g . System PuO2 H2O(outer S.)
storage time of PuO samples also.
2
For a certain period of time depending on the radia-
Time, days
V1
46
40
34
V2
V5
tion dose rate and O content in the ampule, the ex-
2
pression for the rate of H formation has the form
2
7
50
5.4
4.7
4.0
2.9
1.2
0.64
0.45
0.07
0.47
0.9
1.7
2.7
d[H2]/dt = K1[H2O]
K5[H2][O2].
(VII)
100
200
500
800
1000
25
Thus, we developed a model allowing calculation
of the H and O amounts and the pressure in the
10.7
5.5
3.9
2.5
2.25
2
2
vessel for storage of PuO depending on the amount
2
of sorbed water, radiation dose rate, and storage time.
REFERENCES
In this relationship, [H ] and [O ] are expressed
2 st
2 st
3
2
in cm per gram PuO and [H O] , in %; 8 10 is
2
2
0
3
the scaling factor from cm to moles and from moles
1. Caldwell, C. and Menis, O., USAES Report, NUMEC
P-90, 1962, p. 13.
to per cents of H O.
2
Using expression (VI), it is possible to find the
2. Vladimirova, M.V., Radiatsionnaya khimiya aktinoi-
dov (Radiation Chemistry of Actinides), Moscow:
Energoatomizdat, 1983.
3
maximum amount of H and O in 1 cm under given
2
2
conditions and, by so doing, the maximum pressure of
gases. It should be emphasized that the maximum
amounts of H and O do not depend on the Pu form,
3. Haschke, J., Allen, T., and Stakebake, J., J. Alloys
Comp., 1996, vol. 243, p. 23.
2
2
i.e., on the radiation dose rate. The latter will affect
only the period of establishment of steady states, i.e.,
the period in which expressions (V) and (VI) are
valid.
4. Haschke, J. and Allen, T., Report of Los Alamos
National Laboratory, LA-13537-MS, 1999.
5. Morales, L., Allen, T., and Haschke, J., Proc. Int.
Conf. Plutonium Futures The Science, Pilay, K.K.S.
and Kim, K.C., Eds., Am. Inst. of Physics, 2000,
p. 114.
The calculations showed that, at the H O content
2
equal to 3%, the steady-state maximal amount of
3
superstoichiometric Pu oxide is 1.3 10 mol or
6. Haschke, J., Allen, T., and Morales, L., Science, 2000,
0.35 g per gram of PuO , which is 35% of the initial
vol. 287, p. 285.
2
amount of PuO . This amount is sufficiently large to
7. Haschke, J., Allen, T., and Morales, L., J. Alloys
Comp., 2001, vol. 314, p. 78.
2
be determined experimentally, provided that H O
actually decomposes to form water by reaction (4).
2
2
8. Report of Los Alamos National Laboratory, LA-
13781-MS, 2000, p. 32.
9. Vladimirova, M.V. and Kulikov, I.A., Radiokhimiya,
Calculation of the Rates of Individual Reactions
2002, vol. 44, no. 1, p. 83.
We calculated the rates of reactions (1) and (2) of
H and O formation and reaction (5) of their recom-
10. Duffey, J. and Livingstone, R., Abstracts of Papers,
5th Topical Meet. on Spent Nuclear Fuel and Fissile
Materials Management, Charlestone (the United
States), 2002.
2
2
bination in an air-containing ampule (initial content
3
of O 2.3 10
mol). The results are presented
2
in Table 6, from which it is seen that at a time of
200 days the rate of reaction (5) is significantly
11. Paffett, M and Kelly, D., Abstracts of Papers, 5th
Topical Meet. on Spent Nuclear Fuel and Fissile
Materials Management, Charlestone (the United
States), 2002.
lower than the rate of H formation, and at a time
2
longer than 500 days V approaches V . This is ex-
5
1
12. Vladimirova, M.V., Radiokhimiya, 2002, vo. 44,
plained by a decrease in the rate of reaction (1) due to
decreasing amount of sorbed water. Thus, our calcula-
tions showed that, in air-containing ampules, the
contribution of recombination of H and O to
no. 5, p. 455.
13. Vladimirova, M.V., J. Nucl. Sci. Technol., Suppl. 3,
2002, November, p. 379.
2
2
the apparent rate of H formation at a dose rate of
14. Vladimirova, M.V., Abstracts of Papers, 5th Topical
Meet. on Spent Nuclear Fuel and Fissile Materials
Management, Charlestone (the United States), 2002.
2
1
2 Gy s increases in time. Presumably, for the system
H O(total S.) in which the rate of H formation is sig-
2
2
RADIOCHEMISTRY Vol. 48 No. 4 2006