Development of Analysis and Estimation Technology
for Characterization of Fuel Debris(Development of Estimation Technology of Aging Propertiesof Fuel Debris)

Final report for FY 2019 and 2020

Project Background & Objectives
Data on degradation ofChernobyl NPP and
TMI-2 debris & vitrified
HLW

Exposure conditions• amount of oxygen in
the gas phase above
the contact solution,• dissolved oxygen• pH, temperature, etc

conduct
experiments inparallel on cold &hot samples

Zr / Fe / Sivariations

SNF
composition

Evaluating data on physical and chemical phenomenathat may contribute to aging of FD

knowledge
about changing
of FD
properties
caused by the
aging

knowledge
about FDparticles
release into
water/airenvironment

predictionof FD
properties
in the longtermdevelopcomprehensive
aging forecastof F1 FD

The results obtained in this work can be used for subsequent retrieval of FD from PCV/RPV of F1
NPS units, design and development of equipment for retrieval, treatment, transportation andstorage of FD. Predicted effects of aged FD could be used for safety assessment and radioactive
waste treatment for related research and development projects of the Mid-and-Long-Term Roadmap

2

Project Scope
Research program development:
•

•
•



Analyze data on fuel debris 
degradation:

Si-containing FD samples manufacturing method (including
number of “cold” and “hot” samples)
Accelerated aging (simulate FD before retrieval, during
retrieval & long-term storage in containers)
Assessment of factors affecting aging

Manufacturesamples:«Cold» samples

Development of computing
predictive model of FD aging

Perform calculations









Verify aging methodology

Carry out Tests

Analysis of experimental data:

Chernobyl NPP
TMI-2+
vitrified HLWManufacturesamples:«Hot» samples

Accelerated
aging of FD
samples••

concentration of dissolved nitrogen/oxygen,hydrogen peroxide and nitrogen-containing
products, pH, temperature and influencing
factors (alpha-emitters, degree of Uoxidation, concrete and corium content,
environmental conditions)

Development of acomprehensive predictive
forecast of F1 FD model
samples aging

Prediction model and input data for otherprojects for safe retrieval, treatment,
transportation and storage of FD3

2 PHASE

1 PHASE

Project Schedule 2019-2021
Research program development (2019)
 Analyze studies of vitrified HLW, real FD matrices chemical
stability after prolonged contact with aqueous solutions
 Develop research program based on impact factors

1
STAGE

2
STAGE

 Model samples manufacturing (2019) Cold / hot samples initial characterization
 Experiments infrastructure preparation

Tests implementation (2020)
Experimental data analysis
 Dissolution into aqueous solution under nitrogen & air environment
 Dry storage on air
 Leaching rates, XRD, SEM, pH
STAGE Environment parameters dependence of dissolved oxygen concentration, temperature,
pH, H2O2
Development of the mathematical prediction
aging model (2020-2021)
 Fuel debris components release into a contact solution
STAGE
 Secondary phases formation
 Chemical & diffusion model. Forecasts
4

3

4

Corium model
samplesmanufacturing

Samples Manufacturing
6 types of compositions of large-sized U-containing cold samples manufactured (> 300 g) 
Large-sized cold samples fragmented into small-sized “cold” samples

Sample characterization by
SEM / EDX, XRD, XRF
methods and chemical
analysis

Set of 45 “hot” samples 
manufactured by doping
cold samples with 238Pu
4% wt proportional to U
content of composition

C5





C4

Party of “cold’ smallsized samples
transported to RIAR

C3

790+ Cold samples manufactured (containing 238U)
chemical simulation of accelerated oxidation (by adding appropriate amount of
H2O2 and nitriding a contact solution) & leaching (accelerating dissolution
mechanism by increasing temperature and SA/V)

Set of 45 “hot” samples manufactured(containing 238U and 238Pu)
to verify the chosen method of aging for chemical similarity of aging
processes and to estimate the radiation factor impact

C2
C6
C2«hot»C1
C1 means Composition 1
C2…C6 means
Composition 2…Composition 6 etc

C3 «hot»

6

Compositions & preparation
In elements
Composition 1
Simulated
plant
U
Zr
Fe
Cr
Ni
Si
Ca
Al
O
Quench rate
Melt date
Facility

16.0
5.6
2.7
0.7
0.4
22.4
6.4
6.4
39.4
rapid
14/12/2019
RASPLAV-4

Composition 2

Composition 3

Composition 4

Fukushima Daiichi NPS(Chemical content changes gradually)
25.5
36.0
30.8
7.7
9.9
8.8
3.0
3.8
3.4
0.8
1.0
0.9
0.5
0.6
0.6
17.9
12.6
15.3
5.1
3.6
4.3
5.1
3.6
4.3
34.4
28.9
31.6
rapid
17/12/2019
RASPLAV-4

rapid
23/12/2019
RASPLAV-4

slow
12/11/2019
RASPLAV-2

Composition 5

Composition 6
TMI-2

36.0
9.9
3.8
1.0
0.6
12.6
3.6
3.6
28.9

65
15
7

rapid
23/10/2019
RASPLAV-2

rapid
26/12/2019
RASPLAV-3-

13

Composition 5

1 Heating and melting the charge material at a rate of 10 °С/s
2 Refilling the remaining charge material, adjusting theparameters of the inductor and the position of the crucible (slightoverheating over the liquidus T)

3 Homogenizing isothermal exposure of the melt (30 min)
4 Measurement of the depth of the molten pool, sampling
5 Cooling by switching off power of high-frequency generator (rateof 10 °С/s)

7Samples compositions
Total composition analysis
Сorium: concrete
mass ratio
Actual corium:
concretemass ratio
Cooling rate, °C/s

Composition 1

Composition 2

Composition 3

Composition 4

Composition 5

30 : 70

44 : 56

52 : 48

60 : 40

60 : 4026.4±1.3:73.6±4.0
15 ± 8(quenching)

37.1±0.8:62.9±2.0
12 ± 5(quenching)

43.7±0.5:56.3±0.9
10 ± 4(quenching)

50.5±6.8:
49.5±7.4
0.1 → 1.5
(slow cooling)

49.0±1.5:
51.0±5.2
12 ± 2(quenching)

3.77

3.95

4.78

4.89

4.90

Pycnometric density,
g/cm3

Mass, %End surface of the sample

Amorphous part

Reactor elements

Fuel elements

Composition 3
Amorphous silicate
phase (dark-grey)(U,Zr)O2 dendrites (light)

8

SEM/EDX analysis
Oxidic silicate-enriched sample specificmicrostructure (Composition 3)

Amorphous silicate phase(U,Zr)O2 dendrites

50 μm

10 μm

Composition 3

Oxidic-metallic sample specific microstructure(Composition 6)
Metallic
inclusions inc-(U1-x2,Zrx2)O2 ssoxidic part
Oxidicmatrixc-(U ,Zr )O ss
1-x1

50 μm

Oxidic
part

x1

BSE

5 μm

5 μm

Fe
Zr(U)Fe2+x2x1 < x2
10 μm

Metallicpart

U(Zr)Fe2+x
20 μm

5 μm

Topographic image

9

Cold Samples Manufacturing Results
Corium : Concrete ratio change
Composition 1

Composition 2

5 μm
Increasing of corium : concrete ratio
leads to increase size and quantity of(U,Zr)O2 inclusions:• In No. 1 - 100 nm sphericalnanoinclusions• In No. 2 – 1 μm spheroidal
dendrites• In No. 3 – 10 μm length dendrites•
In No. 5 – 100 μm length
dendrites• In No. 4 unhomogeneous ingot
with good crystallized phases

Composition 3

5 μm

Composition 4

5 μm

5 μm

Composition 5

5 μm

10

Hot samples
Phase study. Composition №3
Pores filledwith abrasive
Pores filled with
Wood’s Alloy(used formicrosection
preparation)

SE

Zone 3(U,Pu,Zr)O2
Amorphous
phase basedon SiO2

BSE

11

AgingExperiments

Accelerated Aging Methodology
AGING:••••

Factor of model FD elements release into a contact media
Secondary phases and dust particles generation
Phase content change
Factor of oxidation of U (IV)  U (VI)

Studies of 1F model samples in 3 different environments :(I)

Storage inside PCV — FD water cooling, nitrogen atmosphere (ratio N2:O2=100/1)(II)

Retrieval — water cooling and purge of nitrogen (ratio N2:O2=82/18)(III)

Long-term container storage of extracted FD — dry condition and atmosphere

(I)

(II)

N2

Air

T=25/50/90 °С
t=1,5-672 hours

T=25/50/90/120 °С
t=1,5-2016 hours

Monolith

H2O

(III)
Air
T=150 °С
t=2200 hours

H2O

Powdered13

Accelerated Aging Methodology
Dissolution experiment implementation scheme(I) Storage in water + N2 atmosphere(II) Storage in water + air atmosphere

Nitrated water

Deionized water

Initial characterization• Ox-Red state(Mossbauer Method)• Density• Porosity• Microhardness• Chemical elements
distribution(SEM/EPMA)• Phase composition(XRD)

Monolith samples
Sampling on 3, 7, 14, 21, 28 days
T=25, 50, 90 °C
SA/V = 3 cm-1
Extended tests: dissolution 56, 84 days with T=90, 120 °C

Powdered samples

H2O preparation• Demineralization• Deaeration (CO2)• Sealing• Cooling• N2 saturation (for (I)• Storage

Sampling: 0.5, 1.5, 4, 24, 168, 672 hrs
25, 50 °C
SA/V =10 and 100 cm-1

H2O2 mmol/l

pH
4,1

7

9,18

0,1

H2O characterization

0,3

Samples/Solution characterization:• Element composition (ICP-AES, ICP-OES);• Chemical elements distribution (SEM/EPMA);• Phase composition (XRD)

14

Accelerated Aging Methodology
Dry storage experiment implementation scheme(III) Storage air atmosphere
Small-sized samples

InitialCharacterization

Dry storage

OutputCharacterization

Stainless steel sealed container• Chemicalcomposition• Phase composition• Surface area
• Micro hardness• Morphology of thesurface• Porosity• Density• U(IV)/U(VI) ratio• Fe(II)/Fe(III) ratioAir

T: 150o C• Chemical composition• Phase composition• Surface area
• Micro hardness• Morphology of the surface• Porosity• Density• U(IV)/U(VI) ratio• Fe(II)/Fe(III) ratio

Sampling: 500, 800, 1200 and 2200 hours

15

Accelerated Aging
Leaching tests, mass loss
Composition 1. Deionized water, 28 days, T=90o•

The normalized mass losses ofspinelcomponents,
solid
solutionsofuraniumandzirconium (U, Zr, Fe, Cr, Ni) atelevated temperatures (90oC) arevery small, as well as the yield of
concretecomponents
attemperatures of 25 and 50oC•

For 1-5 compositions in an acidic
environment (pH = 4), themaximum mass loss is observed
for alkaline-earth Ca; in an
alkaline environment (pH = 9),
the maximum mass loss of Si andAl•

Normalized
lossesoftheelements of two parallel samples
have similar values

Composition 3. Deionized water, 28 days, T=90o

Figures show graphs of the dependence of the normalized mass losses from time of the glass phase components (Si, Ca, Al) duringleaching (T=90 °C) for two parallel samples (for pH 9 (blue), pH 4 (red), pH 7 (green).

16

Accelerated Aging
Leaching tests, pH, results
Composition 3. Deionized and nitrided water, 28 days, pH=4,7,9
T=25, 50,90o•

During the leaching of monolithic samples of 1-5 compositions in all the studied
conditions of the aqueous medium, the amorphous component dissolves and passes into
contact solutions of Si, Ca, Al. Normalized losses of the glass phase elements, i.e., Si,
Al, and Ca have the same tendency for all compositions•

Si is acidic by nature has large normalized losses in an alkaline environment, Ca is analkaline-earth element and leaches well in an acidic environment, Al is an amphotericelement and, according to our observations, leaches better in an alkaline environment
than in an acidic one.

•The effect of nitriding of water during leaching does not appear, the expected decreasein the yield of all elements in the solution under all conditions does not occur.

•

Normalized losses increases with an increase in the concentration of H2O2 for all
elements. The highest value of normalized Ca mass loss.

•

In nitrided water, the concentrations of Si and Al were below the detection limit, and thenormalized loss of Ca increased by 1 order of magnitude•

Concentrations of U, Zr, Fe, Ni, and Cr in the contact solutions under all leachingconditions (up to 28 days) were low•

No new crystal phases were detected on the surface of the samples after leaching at 90°C•

Phase content of samples of compositions 1-5 after leaching in all media for 28 days at atemperature of 50 ° C did not change

Graphs show the dependence of the normalized mass losses from time (days 1-28) of the glass
phase components (Si, Ca, Al) during leaching (T= 25, 50 and 90 °C) for (for pH 9 (blue), pH 4(red), pH 7 (green).

17

Accelerated Aging
Surface
pH=4, T=90oC

analysis. Composition 5, 28 days
pH=7, T=90oC

Conditions
Т, °С, рН,Concentration ofН2О2, mmol/L

Al2O3

SiO2

CaO

Cr2O3

FeO

NiO

Total

90°С, pH4

12

73

8

2

5

0

100.0

90°С, pH7

16

56

7

4

16

0

100.0

90°С, pH9

7

28

5

2

54

4

100.0

50°С, 0.3 mmol/L

13

69

8

3

8

0

100.0

Initial

13

65

9

3

10

1

100.0

• With pH=4.01 the Si content increases; the formation of ahydrolyzed silicate layer is possible
pH=9, T=90oC

CH2O2=0,3 mmol/l, T=50oC• When leaching in contact solutions with H2O2, no surfacechanges are detected after leaching• When leaching in contact solutions with pH=7 and 9.18, theiron content increases, and the formation of iron-containing
phases may occur.
•

Processes of formation of surface layers during leaching ofmonolithic samples for 28 days at different pH values insamples of compositions 1-5 are the same.

18

Accelerated Aging
Dissolution experiments results, T=90oC
Composition 1. Deionized water, CH202=0,3 mmol/l18 days, T=90o

Al2O3

SiO2

CaO

Cr2O3

FeO

NiO

ZrO2

UO2

∑

initial

8,7

64,5

5,6

0,0

2,6

0,0

5,5

13,2

100

after

5,0

65,0

3,1

0,0

3,5

0,0

7,3

16,1

100

Mass,%

Composition 3. Deionized water, CH202=0,3 mmol/l18 days, T=90o

Mass,%

initial
after

Al2O3

SiO2

CaO

Cr2O3

FeO

NiO

ZrO2

UO2

∑

6,7

47,3

4,5

1,2

4,2

0,0

9,5

26,7

99,6

4,0

44,3

2,4

0,0

6,0

0,0

11,9

31,4

100

•

Sample composition influence on dissolution results is
insignificant and could be averaged•

Observed changes in the surfaces of all samples•

Formation of a layer is observed, which covers the surface ofthe sample with crusts on samples of Compositions 1 and 2,
formed by poorly soluble Si and Fe compounds•

On the surfaces of samples of Compositions 3 to 5, a decreasein continuous layers is observed, it covers only specific areas•

Blockage of further leaching processes, which is explained byclogging of the pores of the surface layer by reprecipitated Si•

Composition 6 after leaching in some spectra is completely
covered with a layer of iron hydroxide, in other spectra only
uranium and zirconium oxides are present. This confirms theheterogeneity of the phase distribution in the Composition 6, andmay also indicate that iron is re-deposited on the surface of thesample forming a porous surface layer that does not prevent thefurther release of the components into the solution

19

Accelerated Aging
Extended dissolution tests. Composition 1
Composition 1. Deionized water, CH202=0,3 mml/l
56 days, T=90o

Ferruginous Chlorite
Fe4Al[Si3AlO10][OH]8 nH2O•

Secondary phases layer depth ~0,5 µm•

Ca, Zr, U are not detected in secondary phases•

Secondary phases are hydrated Fe-containing aluminosilicates

Hydrated Silica (opal)
SiO2·nH2O

20

Accelerated Aging
Extended dissolution tests. Composition 5
Composition 5. Deionized water, CH202=0,3 mml/l
56 days, T=90o•

Dissolution rates decreases and in balance on day 28 for all
glass phase components•

Secondary layer depth ~0,2 µm are zeolites (Fe-containing
aluminosilicates)•

The phases containing U and Zr are observed. Hypothesis: thesilicate-containing re-precipitated phases cover the crystals ofsolid solutions (U,Zr)O2 of variable composition•

The surface of the sample has become loose, there are shellsthat are presumably formed as a result of the dissolution of theglass phase (such as an etched surface)

Glass phase21

Accelerated Aging
Extended dissolution tests. Secondary phases•

Surface layer change occurs mainly due to the dissolution of theamorphous glass phase and the formation of new secondary
phases•Transition to the liquid phase of the most soluble components (Ca,Si, Al, Fe) leads to the appearance of a porous layer formed bypoorly soluble compounds of the solid solution (U,Zr)O2•

Secondary phases are formed by oxide compounds of Al, Si andFe with a small amount of Ni, Zr and Cr. Ca, U, and Zr were notfound in the secondary phases•

Main secondary phases are: hydrolyzed silicon oxide SiO2*H2O,
compounds of variable composition – ferruginous chlorite groups,
bertierine, crystals with a high content of Al and Fe - (AlxFe1x)O(OH).

•The thickness of the surface layer in the samples is proportional tothe amount of glass phase in the initial compositions. up to 500 nm(in different areas) in composition1, ≈300 nm for Composition 3•

Surface of composition 5 is changed due to the dissolution of theglass phase (by the type of etching), the secondary phases layer isminimalChlorite
Al, Feenriched
crystals

SiO2∙H2O

22

Accelerated Aging
Dissolution experiments results. Hot samples

g/sm2 *dayg/sm2 *day

Composition 3. Deionized water, CH202=0,3 mml/l
28 days, T=90oday

g/sm2 *dayg/sm2 *day

day

day

day

•

Dissolution rates decrease for allcomponents of hot samples. U, Zr,
Si, Fe release rates of Pu-samples
are higher than U-samples•

Ca, Al, Si release from amorphous
phase of ‘hot’ Pu-samples iscomparative to corresponding
components of ‘cold’ U-samples•

Hot samples has higher leachingrates of Fe, Cr, Ni, U, Zr, Pu, Am
but doesn’t depend on Pu
concentrations. Hypothesis: it is theresult of thermal treatment (doping)and higher porosity•

No secondary phases detected (28
days)

•

Surface remains unchanged, no
significate phase change (XRD)

23

Accelerated Aging
Dissolution experiments. General conclusions

Composition 1

Composition 3

Composition 5

Glass phase content decreases
Surface layer depth decreases
Mass loss decreases due protective layer•

Mass loss of all compositions arewithin similar ranges•

Composition 3 has higher mass loss•

Glass phase dissolves with Si and Al
re-precipitates on surface and Ca
releases into solution•

Fe, Ni and Cr supposedly dissolve
because found in surface secondary
phase layers•

U and Zr not found in secondary phase
but in solutions only

Mass loss decreases due glass phase content

24

Accelerated Aging
Dry storage experiments results
Composition 6. Dry storage 2200 h,
T=150oComposition 6. Dry storage 2200 h, T=150o
Brown rust generation•

Fe(OH)3, Fe2O3 brown rust generation•

Impurities of UO2, ZrO2

Composition 6. Dry storage 500h.
T=150o Dust generation

Composition 6. Dry storage 500h. T=150o

Composition 6. Dry storage 1200h. T=150o

25

Accelerated Aging
Dry storage results. Dust formation
Dry storage 2200h, T=150o

Total weight loss
mass.%

3,00E-01•

Dust fraction formation is less than 0.016% mass•

Fine fraction is less than 1% of total mass loss•

Fragments, initially exposed to contact with water, were dried,particles of secondary phases were observed out of thesamples. Hypothesis: the source of dust can be thesecondary phases formed during contact between debris andan aqueous solution

2,00E-01
1,00E-01
0,00E+00
1

2

3

4

5

6

Composition number
original samples

Loss of fine dispersed fraction

samples after leachingmass.%

Loss of coarse dispersed fraction
mass.%

3,00E-01
2,00E-01
1,00E-01
0,00E+00
1

2

3

4

5

Composition number
original samples

samples after leaching

6

7,00E-04
6,00E-04
5,00E-04
4,00E-04
3,00E-04
2,00E-04
1,00E-04
0,00E+00
1

2

3

4

5

6

Composition number
original samples

samples after leaching26

Accelerated Aging
Dry storage conclusions•

The process of dry storage at a temperature of 150 ° C for 2200 h of corium samples did not lead to significant changes in theirproperties•

No external changes in the surface of the MCCI samples were found. On the surfaces of composition 6, the presence of "brownrust" was observed.

•

No changes in the elemental and phase composition were found•Fragments, initially exposed to contact with water, were dried, particles of secondary phases separated from the samples. Thus
the source of dust can be the secondary phase formed upon contact between debris and an aqueous solution•

The entire volume of the hydrated layer formed during wet storage is potential volume of dust formation during further
treatment and dry storage (considered as prediction model assumption)

Dust fraction, g/cm2

Dry storage fine fraction release prediction with andwithout preliminary dissolution

years

27

Prediction model
development

Development of prediction model
Basic assumptions
FD properties change:
1) Estimation of the degradation (leaching) depth of the sample surface
2) Calculation of the volume of formation of secondary mineral phases on the surface

When the sample comes into contact with an aqueous solution, theprocess of leaching of components from the matrix is activated.
Sample properties change only in the near-surface layer
Oxide

glass

Secondary mineral phases –potential source of dust formationPoressamplesolution
Leaching zone29

Prediction model
Experimental program summary assumptions
1. Influence of the composition with the rate of release of the components into the solution
No effect of compositions 1-5 on the rate of release of the components
into the solution was found.
Experimental estimates of the rate of exit of components fromcompositions 1-5 can be averaged.
Composition 6 was considered separately from the others, since itconsists exclusively of hardly soluble oxides.

2. What components are leached from the samples into
the solution?
The components of the amorphous component (Ca, Si, Al) intensively pass into the aqueous solution. At the sametime, the oxide (crystal) does not actually dissolve even at higher temperatures.
The behavior of compositions 1-5 on contact with an aqueous solution can be considered as the behavior of a two phase system (highly-and low-soluble phases) .
Only Fe and U pass from composition 6 to the aqueous solution.

30

Prediction model
Experimental program summary assumptions
3. pH influence on the leaching rate

Mass flow

The total flow of the components into the aqueous solution increases at pH = 4 and pH= 9, compared to pH = 7.
Acid-base conditions determine the composition of the dissolved components: at pH =9, Al and Si are released into the solution, at pH = 4, Ca is dissolved.

Under different acid-base conditions, different secondary phases and different volumesof the dusty fraction can be expected to form.
4

7

9

pH

4. Temperature influence on the leaching rate

Mass flow

The total flow of components into the solution increases non-linearly with increasing
temperature.
The acceleration of aging was estimated by interpreting laboratory experiments andusing analytical dependencies. Acceleration: from 25 to 50 oC – 8 times; from 25 to
90 oC - 150 times, from 25 to 120 oC - 900 times.

31

25

50

90

120 T, оСPrediction model
Experimental program summary assumptions
5. Air atmosphere influence on the leaching rate
In experiments on the interaction of fuel-containing samples with aqueous solutions, it was found that the yield ofcomponents in nitrided solutions is slightly higher than in deoinized solutions.
A higher flow in a nitrided environment is formed by Ca. In this case, the Si and Al fluxes into the aqueous solution are thesame in the nitrided and deonized environment.

6. Secondary phases
On the surface of fuel-containing samples of compositions 1-5 exposed toaqueous solutions, the formation of a thin (several microns) layer of secondary
phases was recorded.

According to the results of thermodynamic calculations and SEM analysis, thesecondary phases are represented by zeolites and mica minerals.
A rapid formation of a brown secondary phase was recorded on the surface ofthe sample of composition 6. According to the calculated and SEM data, thephases are represented by iron oxides.

32

Prediction model
Experimental program summary assumptions
7. Aging of "cold” and" hot " samples: comparison
The rate of release of components (Ca, Al, Si) fromthe amorphous component of "hot" samples into an
aqueous solution is comparable to the flow of thesecomponents from non-draped ("cold") samples.

Composition 5“cold”“hot”

From the Hot samples there is an intensive release
into the solution of Fe, Cr, Ni, U, Zr, Pu, Am. Due tothis, the total rate of dissolution of the hot samples issignificantly higher than that of the cold.
It is assumed that the intensive dissolution of the hot
samples occurs due to the peculiarities of theirstructure and preparation modes.

33

Prediction model
Experimental program summary assumptions
8. Dissolution of powdered samples
The features of leaching of powdered samples and monoliths are similar. (Composition of leachable
components, relation to pH and temperature). Experiments with powders allowed us to observe theleaching process at normal temperatures (in experiments with monoliths at normal temperature, the yieldof the components was absent).

9. Dry storage
During dry storage, no changes were recorded on the surface of the original monolithic samples withcompositions 1-5. When drying the fragments initially exposed to contact with the aqueous medium, theparticles represented by the secondary phases were separated from the samples. Thus, the source ofdust can be a secondary phase formed during the contact of the debris-aqueous solution.
A brown coating (probably iron hydroxides) appears on the surface of composition 6.

34

Prediction model approach
Model 1: Thermodynamics+Kinetics(Chemical)
Multicomponent thermodynamic model of surface
dissolution
solution

Model 2: PhysicalOne-component balance model of surface
dissolution and solution diffusion into the sample
Glass matrix

Lab ScalePurpose:Processing laboratory
experiments and selectionof parameters forprediction

Long Term Scale
glass

Solutionoxideleaching zone
experiment duration

Purpose:Processing
laboratory
experiments.
Consideration of the influence of external factors (T,pH, composition, etc.). Calculation of the formation of
secondary phases on the sample surface

pores

glass

Border layer
oxide

pores

Purpose:Predicting the leaching layer
thickness of the sample
surface over long periods oftime

sampleLeaching zone

solution35

Multi-component layer dissolution model
Compositions 1-6
III. Kinetics of dissolution of PuO2

I. Kinetics of dissolution of oxides and spinelr  S A  Aj  e Ea , j / RTj

 a (1   )

pj qj

ni
i, j

R  k  S A0.33  Tc 1i

j - mechanisms corresponding to dissolution in acidic, neutral and alkaline conditions;SA - specific surface area of the phase per unit mass of water (m2/kg);
Aj - pre-exponential factor in the Arrhenius equation (mol/m2·s);
Ea- energy activation during the j-th reaction (J/mol);R - universal gas constant (8,31446 J/mol·К);
T - temperature (degrees Kelvin);ai - activity of the i-th water ion;
Ω – phase saturation ration фазы (the degree of deviation of the phase from the state of equilibrium in a
specific period of time - includes the equilibrium constant of dissolution)n, p, q - empirical coefficients.

k - reaction speed constant; SA – specific surface;Tс – grain sze;

IV. Kinetics of dissolution of FeR  S A  k  (1  )
k - reaction speed constant; SA – specific surface;

II. Kinetics of dissolution of amorphous partR  S A  k  kinh  e(  Ea / RT )  (1  )kinh 

1(1  k1   M i ) k2ik - reaction rate constant; kinh – deceleration ratio;
k1, k2 – selectable parameters; M – number of secondary phases formed;

36

Analytical model of dissolution from thesurface and diffusion deep into the sample
Asymptotic solution forconcentration in solution

responsible forthe diffusionfrom the surface

Diffusion + dissolution

Rate, mg/d cm2

4 Dn
Cb 
Ce f i S s t  Ce f i S s kbt
Change in flow rate over time

Diffusion only
Dissolution only

responsible fordissolving fromthe surface

At the initial moments of time, the main contribution
to the removal of components into the solution isplayed by diffusion.
Over time, the contribution of diffusion to the total
rate of dissolution decreases. The process of
dissolution begins to prevail.

Time, dCalculation of the equilibrium concentrations of components
T,
oC
90
50
25

Si
458.9
202.0
109.2

9
Ca
108.1
48.6
26.4

Al
111.1
49.2
26.6

Si
443.8
199.5
108.7

pH
7
Ca
102.6
48.1
26.3

Al
107.4
48.6
26.5

Si
442.1
199.0
108.6

4
Ca
102.2
47.9
26.2

Al
107.1
48.5
26.5

37

Interpretation of experiments T=50oC
Monolith samples
Compositions 1-5

pH9pH4
pH4, 7, 9

Model 1 "Chemical" (multicomponent kinetics and dissolution on the sample surface)
Model 2 “Physical " (kinetics and dissolution on the surface + diffusion into the sample)
Experimental

38

Hot sample No. 3 pH 7, T90oC
The rate of release of the components (Ca,Al, Si) from the amorphous component of
the "hot" samples into the aqueous solution
is comparable to the flow of thesecomponents from the "cold" samples.

SiO2 pH7 T90

CaO pH7 T90

However, unlike" cold " samples, Fe, Cr, Ni,
U, Zr, Pu, and Am are intensively released
into the solution. But no significant Pu
concentration impact observed between
different «hot» compositions
So, it is assumed that the intensive
dissolution of the hot samples occurs dueto the peculiarities of their structure andpreparation modes.

Al2O3 pH7 T90
Model 1 "Chemical" (multicomponent kinetics and dissolution on the sample surface)
Model 2 “Physical " (kinetics and dissolution on the surface + diffusion into the sample)
Experimental

Development of prediction model
FD aging: change of FD properties leading to radionuclide release from matrix to environment

FD considering as 2-phase system with different solubility

3 processes lead to FD aging and radionuclide release:1

Radionuclide release from amorphous phase because its
dissolution

2

Accumulation on the surface of hardly soluble dispersed
aggregates consisting of (U,Zr,Pu)O2 oxides and spinel;

3

Deposition of secondary phases and generation of dust fine
fraction which is risk factor for safe FD treatment;- Soluble amorphous phase- Low soluble oxide phase- Secondary phase40

Prediction of the leaching depth and accumulation of (U, Zr, Pu)O2
phase on the surface
Prediction model

PUZr  UZr V
M

M

i 1

i 1
M

V   Vi  Sl  S  li
N UZr  PUZr / S   UZr  lii 1

dl M f k i  i i bi ([Cei ]  [Ci ])dt glli (t )  l0  li (t ) 

M i [Cei ][1  exp( kbi f i S st )]S s  glfSiO2=0.46, fAl2O3=0.07, fCaO=0.05
Accumulation of (U, Zr, Pu)O2 phase on the surface
3

N UZr  UZr  li i 1

UZr 3
 M i [Cei ]{1  exp( kbi fi Sst )}S s gl i 1
Leaching depth

l 

N UZr
 UZr The surface transformation is characterized by theleaching of an amorphous glass phase (an alloy of Si, Al,
and Ca oxides), including insoluble agglomerations of theuranium-zirconium alloy (U, Zr)O2.
The loss of the glass phase mass during the transition of
Si, Al, and Ca compounds from the solid phase to thesolution leads to the formation of a mobile dissolution
front l(t) on which insoluble particles (U, Zr)O2 accumulate

41

Long term prediction results
Leaching depth(U, Zr)O2 phase accumulation speed on surface
3

N UZr  UZr  li i 1

UZr 3
 M i [Cei ]{1  exp( kbi fi Sst )}S s gl i 1

Prediction model parameters
pH
T C

90

50

25

Compone
nt
Si
Al
Ca
Si
Al
Ca
Si
Al
Ca

9.0
fi
0.46
0.07
0.05
0.46
0.07
0.05
0.46
0.07
0.05

7.0
Kbi, cm/d

7.93E-03
8.99E-02
1.83E-01
1.06E-03
4.47E-03
3.46E-02
2.26E-04
4.52E-04
9.77E-03

2.89E-03
7.98E-03
3.86E-02
3.78E-04
3.89E-04
7.02E-03
8.01E-05
3.90E-05
1.97E-03

Physical properties
9.0

7.0
Cei, mol/cm3

1.63E-05 1.58E-05 1.63E-05
2.13E-06 2.05E-06 2.13E-06
2.69E-06 2.57E-06 2.69E-06

7.19E-06 7.12E-06 7.19E-06
9.39E-07 9.39E-07 9.39E-07

Density (U, Zr)O2 8 g/sm3;
Amorphous phase density 2.5 g/sm3;(U, Zr)O2 Content
30%

Average volume fraction of components in thesample: SiO2 – 46%; Al2O3 – 7%; CaO – 5%External factors

3.88E-06 3.88E-06 3.88E-06

Temperature: 90, 50, 25 oC;
pH: 7 и 9

5.17E-07 5.17E-07 5.17E-07

Water flow scenario

1.22E-06

1.2E-06 1.22E-06

6.49E-07 6.49E-07 6.49E-07

I. Closed system: SA /V = 0.1 см-1;
II. Flow systeSA /V = 1E-04 см-1

42

Closed system

Flow system

RESULTS
The maximum accumulation of (U,Zr)O2 over 50 years will be observedin the conditions of the flow system
at T=90oC and pH=9.
Under these conditions, the glass
matrix leaching layer is 0.3 mm over
50 years. At the same time, up to 70
mg/cm2 (U, Zr)O2 accumulates on thesurface.
The chemical model shows that nosecondary phase is formed underdynamic conditions.
For static conditions at T=90oC and
pH=9, the etching depth will not
exceed 0.03 mm in 50 years. The
amount of (U, Zr)O2 phase will be 7
mg/cm2. In addition, according tocalculations based on the chemical
model, up to 360 mg / cm2 of thesecondary phase can be formed on
the surface.

(U,Zr)O2 accumulation on corium surface (solid curve)
Layer depth (dotted curve) depending on time
Refer to the equations shown on pages 41 and 42

43

Program code

Based on the proposed physicalmodel and conducted laboratory
experiments, a program code
was developed that allows to
calculate the change in theproperties of corium over time atdifferent phase ratios in the pH
range of 7-9 and T = 25-120 oC

On step 1
The compositions of the phases
and their ratio in corium are set.
The sources of the radioactive
output are also indicated.
Fuel debris initial composition settings menu

44

Program code
On step 2
According to the results oflaboratory experiments, therelationship between the rate ofrelease of individual components
into the solution with pH and
Temperature is determined.
In addition, based on the data ofthermodynamic modeling, therelationship between thetemperature and the equilibrium
concentration is set.

Leaching parameters and activity adjustment menu

45

Program code

Calculation results drop-down menu to choose
desired result data view: N, M, L or A

On step 3
The external conditions (pH, T)
and forecast time are set.
The conditions of water circulation
are also determined: static ordynamic.
Based on these parameters, theprogram calculates:

1. The accumulated mass of thepoorly soluble phase on thesurface of the debris, N[mg/cm2];
2. Accumulated mass loss during
glass dissolution, M [mg/cm2];
3. The thickness of the leachinglayer, L [cm];4. Accumulated activity in thedispersed fraction on thesurface of the debris, A[Bq/cm2].

Prediction model parameters and calculation results view46