2137-20
Joint ICTP-IAEA Advanced Workshop on Multi-Scale Modelling for
Characterization and Basic Understanding of Radiation Damage
Mechanisms in Materials
12 - 23 April 2010
Fission gas release
W. Wiesenak
OECD Halden Reactor Project
Halden
Norway
4
Fission gas release
Wolfgang Wiesenack
OECD Halden Reactor Project, Norway
Joint ICTP-IAEA Workshop on
The Training in Basic Radiation Materials Science and its
Application to Radiation Effects Studies and Development of
Advanced Radiation-Resistant Materials
Trieste, April 12-23, 2010
1
Overview
•
Why fission gas release calculation?
•
•
•
•
2
safety criteria
interaction with other phenomena
Basic mechanisms
Experimental data
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... but it may be more complicated
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Fuel behaviour codes must address and untangle interactions
that become more and more complex as fuel burnup increases
Fissions & fission products
Rim
formation
Conductivity
degradation
Fission gas
release
Fuel
swelling
Time
Enrichment
Fuel-clad
bonding
Gd
fuel
high B
high Li
|
Coolant chemistry
|
CRUD
|
AOA
Fuel
temperature
Rod pressure
Clad lift-off
Stored heat
PCMI
RIA
4
corrosion
LOCA
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Fission products
• The fission products range in
mass from around 72 to 161
(U-235) with peaks in the yield
distribution close to the
elements krypton and xenon
• The overall yield of these
so-called fission gases is
approximately 28 atoms per
100 fissions
• About 85% are Xe atoms,
about 15% are Kr atoms
(Kr yield is lower for Pu fiss.)
• Xe and Kr are noble gases with
low solubility in the fuel matrix
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Kr
Xe
There is no ideal place for fission gases
• If released, they can lead to
• thermal runaway (gap cond.)
• rod-overpressure
Æ restricted by safety criteria
• If retained, they can lead to
• large fission gas swelling and
thus pellet-clad interaction
• grain boundary embrittlement
and porosity with high pressure,
both contributing to fuel
fragmentation under accident conditions (RIA, LOCA)
• Safe reactor operation therefore requires a thorough
understanding of the behaviour of fission gas
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Fission gas release and safety criteria
• Fission gas generation is about 28 cm3/MWd
• A fraction is released from the fuel matrix to the free
rod volume, increasing the rod pressure
• all gas released Æ rod pressure 150 - 200 bar at room T
• Rod pressure is limited by safety criteria and must
therefore be calculated for the safety case
• Regulation differs from country to country and may
prescribe:
• rod pressure must not exceed system pressure
• a pressure limit which is above system pressure (e. g.
Belgium, 184 bar)
• pressure may exceed system pressure, but must not lead
to gap opening (no “lift-off” causing thermal feed-back)
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Rod pressure increase
• generated fission gas (easy)
• released fission gas (hard)
• change of free volume
• fuel densification and swelling
• clad creep and growth
• temperature of free volume
•
•
•
•
8
plenum
dishing, centre hole
pellet-cladding gap, chamfer
open fuel vol. (pores, cracks)
350
for 100% release
rod pressure at room T, bar
For rod pressure calculation,
fuel modelling codes have to
take into account
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300
7% free vol.
250
10% free vol.
13% free vol.
200
150
100
50
0
0
20
40
60
burnup, MWd/kg
80
Fission gas release
- basic mechanisms -
• Fission gas atoms diffuse from
within the fuel grain to the grain
surface (temperature driven)
• The fission gas atoms accumulate
at the grain surface in gas bubbles
• When the surface is saturated with bubbles, they
interlink and the gas is released out of the fuel matrix
• FGR depends on temperature and burn-up (time)
• FGR is <1% for temperatures below ~1000-1200oC
and ~10-20% at ~1500 oC
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Fission gas release
- phenomena involved 1.
2.
3.
4.
5.
6.
7.
Recoil
Knock-out & sputtering
Lattice diffusion
Trapping
Irradiation re-solution
Thermal re-solution
Densification
8.
9.
10.
11.
12.
13.
Thermal diffusion
Grain boundary diffusion
Grain boundary sweeping
Bubble migration
Bubble interconnection
Sublimation or vaporisation
For a detailed discussion of these phenomena (and more) see
FISSION GAS RELEASE, SWELLING & GRAIN GROWTH IN UO2
R. White, HPR-368, February 2008
(on Summer School CD)
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Recoil
Rrecoil # Surface / volume
11
• When a fission fragment is
close enough to a free
surface (< 6-9 m), it can
directly escape from the fuel
due to its high kinetic energy
(60-100 MeV)
• It only affects the geometric
outer layer of the fuel
Rrecoil ~ Fis.rate·Surface/Volume
• It is an athermal mechanism
(independent of temperature
and temperature gradient)
• Domain of application is for
T<1000°C
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Knock-out and sputtering
• Knock-out
• The interaction of a fission fragment, a collision cascade
or a fission spike with a stationary gas atom close enough
to the surface can cause the latter to be ejected
• Sputtering
• A fission fragment travelling through oxide looses energy
by interactions with UO2 at a rate ~1keV/Å
o high local heat pulse
o when it leaves the free UO2 surface, the heated zone will
evaporate or sputter
• Affect outer layer and open surfaces: (S/V)total
• Athermal; independent of T and temp. gradient
Release/Birth ~ (Surface/Volume)total
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Lattice diffusion, diffusion coefficient
D
Da Db Dc
• Thermal coefficient (T>1400oC)
Da
§ 'H ·
D0 exp ¨ ¸
© RT ¹
• Intermediate
Db
6.64 ˜10
25
§ 13800 ·
F exp ¨ ¸
T
©
¹
• Athermal coefficient (T<1000oC)
Dc
A ˜ F
• The release rate is enhanced by deviation
from stoichiometry (UO2+x) and by
impurities simulating non-stoichiometry
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cf. Turnbull et al
Fission gas release
- basic diffusion model -
• Fission gas release was observed very early on and
explained by diffusion out of the fuel grains (Booth)
wc
• Assumptions for (simple) model:
D'C S
wt
­C Rgr , t 0
°
® wc
0
°
¯ wr r 0
• atomic diffusion in hypothetical sphere
• grain boundary = perfect sink
• gas at grain boundary immediately
released (?)
• constant conditions of temperature
and fission rate
• Solution (Booth diffusion,
release rate)
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f (t )
C Rgr , 0 0
4
Dt
3 Dt
SRgr2 2 Rgr2
Observation: incubation threshold
1800
Original 1% data
Siemens 2% data
New 1% data
Empirical Halden threshold
Central temperature (°C)
1700
1600
1500
1400
1300
high gas release > 1%
1200
1100
1000
low gas release < 1%
900
800
0
10
20
30
40
Burnup (MWd/kgUO2)
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50
60
70
Fission gas release
- Bubble interconnection • Explains incubation
and onset of (stable) fission
gas release during normal
operation due to increase of
open surface (open tunnel
network)
• Explains burst release
(micro-cracking) during
abrupt power variations
requires precise
knowledge of local stress
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Classification of models
Mechanistic
Empirical
/ calibrated with specific data-base
/ many unknown parameters
/ limited application range
/ lack of detailed experimental data
- excellent performance
- physical description of phenomena
- inexpensive in use
- wide range of application
- efficient tool for fuel design
- possible to extend range
Focus on speed,
robustness, precision
17
Focus on (physical)
understanding
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Fission Gas Release models
• Large number of models
• Improvements
• numerical techniques
• new mechanisms
• Various applications: conditions, reactor types, …
• Large uncertainties
• (mechanistic) model parameters
• diffusion coefficient
• resolution, ...
• input parameters:
• temperatures
• hydrostatic stress, ...
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A simple model
(in use at Halden)
A simple, yet fairly successful model is in use at
Halden which considers the basic influences and
observations.
• Athermal release, always present
• Three-stage diffusion driven release process
1. intergranular diffusion to the grain boundary, Booth model
2. accumulation of gas atoms on the grain surface to a
concentration of 5·1015 cm-1 (not released to the free volume)
3. reaching the limit concentration signifies bubble interlinkage,
and from then on fission gas atoms arriving at the grain
surface are assumed to be released to the free volume
• Special procedure to apply the Booth solutions (which are
for constant conditions) to varying powers and temperatures
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ƒ An important result from the Halden
Programme is the 1% FGR
threshold established for UO2 up to
~30 MWd/kgUO2
ƒ The simple model is able to
reproduce the observations in the
low burnup range using a three-term
diffusion coefficient and fission gas
accumulation on grain boundaries
ƒ For higher burnup, a burnup
dependent diffusion coefficient is
required to maintain good agreement with the experimental data
20
Centre Temperature (C)
Comparison with FG release data
1600
Threshold for 1% FGR
Code calcs for 1% FGR
Experimental 1% data
1400
1200
1000
800
0
10
20
30
40
50
Burn-up (MWd/kgUO2)
60
Comparison of simple fission gas
release model with empirically
derived release threshold and
experimental data at high burnup
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Experimental
Measurements:
(see introductory lecture)
• Rod pressure
• Gamma spectroscopy
• Rod puncturing and gas analysis
Observations
• Release onset
• Interlinkage (Æ release onset)
• Release kinetics
• Grain size effect
• Gas mixing
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Centre Temperature (C)
Fission gas release onset
1600
Threshold for 1% FGR
Code calcs for 1% FGR
Experimental 1% data
1400
1200
.
1000
800
• Irradiation of fresh and high
burnup fuel (segments from
LWRs)
0
10
20
30
40
50
Burn-up (MWd/kgUO2)
60
new
point
• lnstrumentation:
- fuel thermocouple
- rod pressure sensor
• Stepwise power / temp.
increase to establish onset of
fission gas release
• Simultaneous measurement
of fuel temperature (most
important parameter) and
pressure
22
Release onset
Temperature history and measured
rod pressure (fission gas release)
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Gas flow system
(radioactive fission product analysis)
To off gas system
Gas flow path
Flow meter
P215
He Ar
Cold trap bank
7
8
9
10
11
12
Rods 7,8,9,10 UO2
11,12 MOX
J Detector
P216
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Gas flow analysis
Evaluation method
Isotopic release
rate from Jspectrometry
R
B
S
V
D
D R
O B recoil
Isotopic birth rate
from fission yield
calculations
Surface area to
volume ratio *
square root of
diffusion coefficient
In start-up diffusivity
analysis S/V fixed
24
Recoil release to
birth rate
Square root of precursor
enhancement factor / decay
constant
In first cycle analysis
D fixed
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Gas flow analysis
Assume S/V and derive diffusion coefficient
1.E-18
S/V = 70 cm-1
temperatures varied by
changing power and fill gas
Diffusion coefficient, m2/s
1.E-19
330 W/gHM
1.E-20
80 W/gHM
1.E-21
10 W/gHM
IFA-563
IFA-558
IFA-504
IFA-655 large grain UO2
IFA-655 standard grain UO2
IFA-655 heterogeneous MOX
IFA-655 homogeneous MOX
1.E-22
1.E-23
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1000 / Fuel temperature, K-1
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1.3
1.4
1.5
1.6
1.7
Gas flow analysis
Assume diffusion coefficient and derive S/V
250
Rod 7 (standard grain UO2)
Rod 8 (large grain UO2)
Rod 9 (standard grain UO2)
Rod 10 (large grain UO2)
Rod 11 (homogeneous MOX)
Rod 12 (heterogeneous MOX)
S/V, cm-1
200
150
100
50
0
0
26
5
10
15
Rod average burnup, MWd/kgOxide
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20
25
Through-life gas flow measurements
No change in recoil
R/B as HBS is formed
Interlinkage due to
temperature excursion
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Fission gas release kinetics
• Steady state power
(temperature) for longterm kinetics
• For high burnup fuel,
power dips are
necessary in order to
obtain communication
with the plenum (tight
fuel column)
Temperature and FGR history
28
• Envelope of release
curve indicates diffusion
controlled release
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Influence of grain size on gas release (I)
• According to diffusion models, an increased grain size
will in general result in reduced fission gas release
• The onset of fission gas release occurs at about the
same time since the inner (grain) surface varies with
1/Rgrain and thus compensates for fewer atoms arriving
at the grain surface
• Larger grains are produced by using additives which
may influence (increase) the diffusion coefficient
• Concerning gas release properties, the effect of
increased diffusion length (lower FGR) therefore
competes with the effect of increased diffusion
coefficient (higher FGR)
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Influence of grains size on gas release (II)
• higher power and
FGR >10%
• high burnup
• Satisfactory
prediction with
diffusion-based
FGR model
(Turnbull)
30
12
fission gas release, %
• Grain size increase
is less effective at
grain size 8.5 μm
10
grain 22 μm
grain 8.5 μm
predicted
8
6
4
grain size 22 μm
2
0
52
53
54
Burn-up (MWd/kgUO2)
55
Measured and predicted FGR for
UO2 fuel with different grain sizes
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Gas mixing in a fuel rod (I)
• Released fission gas has to move (diffuse) to the
rod plenum
• A temporary strong dilution of the fill gas in the fuel
column may result …
• ... and cause increased fuel temperatures and
positive feedback on fission gas release.
• Gas mixing behaviour and feedback has been
investigated in different ways:
• Injection of argon at one end and tracing the equilibration
• Cause FGR and monitor the temperature response
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Gas mixing in a fuel rod (II)
• Argon injected in lower
loweer plenum
plenum,
• Diffusion to upper ple
enum, equilibration
• Temperature responsee measured
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Related: gas flow through the fuel column
- hydraulic diameter measurements 4
• D = mean fuel diameter
• DH = hydraulic diameter
• p1 = gas pressure – supply side
(high pressure = rod pressure)
• p2 = gas pressure – return side
(low pressure = rig pressure)
• Ha = Hagen number
• = dynamic viscosity
• L = flow channel length
• R = universal gas constant
• T = gas temperature
33
Hydraulic diameter (μm)
DH
0.021 ˜ K ˜ L ˜ R ˜ T ˜ n
S ˜ D ˜ p12 p 22 250
gap closure with 0.7 ΔV/V
swelling per 10 MWd/kgU
initial gap
200
150
100
50
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0
Burnup (MWd/kg UO2)
0
10
20
30
40
50
60
Related: gas flow through the fuel column
- delayed fission gas release 17
Internal rod pressure (bar)
16
15
14
steady
state
13
12
11
10
9
8
When the accommodation
power level is approached, the
hydraulic diameter decreases at
a higher rate and the fuel
column becomes very tight.
34
0
5
10
15
20
Average heat rate (kW/m)
25
The fuel column becomes
permeable after a limited power
reduction. The release of the
inner overpressure to the fuel
rod plenum is detected then.
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30
Delayed fission gas release
• Delayed fission gas release means that the rod
pressure, as detected by a pressure transducer
located in the rod plenum, increases when power is
reduced and a path to the plenum is opened
• The phenomenon is regularly observed when fuel
burnup exceeds 40-50 MWd/kg
• It cannot be diffusion driven since the pressure step
is also observed after a reactor scram (the fuel is
cooled down by several hundred degrees within a
few seconds, and any diffusion is effectively stopped)
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The END
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