Impact of americium on safety performance
of liquid metal cooled reactors
Janne Wallenius
Professor
Reactor Physics, KTH
Generation IV reactors
Background
Radiotoxic inventory [Sv/g]
100
10
By recycling of Am & Cm (in addition to Pu)
one may achieve
TRU
240
Pu
239
1
Pu
238
243
Am
0.1
Pu
100 times less long lived high level waste
241
Am
Storage time of residual waste < 1000 years
242
Pu
0.01 Unat
237
Np
101
102
103
104
105
Generation IV reactors
t [y]
6
10
Increase capacity of geological storage by
factor 3-6
Why fast reactors?
Fissionprobability
238
Pu
239
Pu
240
Pu
241
Pu
242
Pu
241
Am
243
Am
244
Cm
245
Cm
246
Cm
247
Cm
Recycling of Am & Cm in thermal
spectrum increases Cf-252
inventory by factor ~ 1000,
compared to fast spectrum recycle
Fast reactors permit breeding of
Pu, increasing fuel resources by a
factor 100
0.2
0.4
0.6
Generation IV reactors
0.8
1
Impact of americium on safety
of fast reactors
Introduction of americium will lead to
Reduction of Doppler feedback
Increase in coolant temperature coefficient
Reduced delayed neutron fraction
Degraded performance under transients
Generation IV reactors
Impact on Doppler feedback
800
Doppler constant [pcm]
600
2 years delay between reprocessing
and fuel loading yields 1% Am
400
200
0
Pu fraction in MOX fuel ~ 18 % yields
conversion ratio ~ 1.0 in a typical
sodium fast reactor (SFR)
Am [%]
0
5
10
15
Fuel: (U0.82-x,Pu0.18,Amx)O2
Generation IV reactors
In order to burn the Am production from
one LWR of same power, an SFR fuel
load with 3% Am is necessary.
Americum is detrimental for Doppler
feedback!
Physics of Doppler feedback
Doppler feedback mainly derives
from neutron captures occuring
below 100 keV
Doppler
contribution
Capture
spectrum
E [keV]
0.1
1
10
100
Generation IV reactors
1000
The lower the energy of the resonance
where capture occurs, the more
efficient is the Doppler feedback
In an SFR with standard MOX fuel, 65%
of the Doppler feedback derives from
neutron captures below 3 keV,
constituting only 15% of all captures!
Why does americium kill Doppler?
Capture cross section [b]
The capture cross section of 241Am
is ten times larger than that of 238U
at E > 100 keV.
2.5
2.0
1.5
Increasing Am inventory, fewer
neutrons will reach the region
where Doppler feedback is
functional.
241Am
1.0
0.5
238U
E [MeV]
0.2
0.4
0.6
Generation IV reactors
0.8
1
Even with 70% uranium in the fuel,
the Doppler feedback may
become negligible!
Coolant temperature feedback
1
Fission probability
Fission cross section of 241Am
exceeds capture cross section
already at 0.7 MeV.
0.8
0.6
241
Am
0.4
The more americium, the more
sensitive is the fuel to changes
in coolant density
238
U
0.2
En [MeV]
0.5
1.0
Generation IV reactors
1.5
2.0
Delayed neutron fraction
10
8
Yield [%]
Increasing mass of mother nuclide shifts
mass of the lighter fission product
235
U
6
245
Cm
4
Reduced yield of delayed neutron
emitter bromine
2
A
90 100 110 120 130 140 150 160
Nuclide
νtot
νd/νtot
238U
2.53
1.89%
239Pu
3.02
0.22%
241Am
3.37
0.13%
244Cm
3.42
0.13%
Generation IV reactors
Increasing neutron number of mother
nuclide increases prompt neutron yield
Fraction of delayed neutrons emitted
from fission products decreases with
mass of the mother nuclide.
Effective delayed neutron fraction
Probability [MeV-1]
Delayed neutron spectrum is softer than
prompt neutron spectrum
1.5
1.0
0.5
0.0
0.0
βeff is smaller than β in a fast spectrum
Delayed
Probability of capture by americium
larger than for prompt neutrons
Prompt
E [MeV]
0.5
1.0
1.5
Generation IV reactors
2.0
2.5
βeff/β decreases with concentration of
americium
Impact of americium: Summary
Fuel
(U0.8,Pu0.2)O2
(U0.7,Pu0.2,Am0.1)O2
KD
550 pcm
230 pcm
αNa
+0.2 pcm/K
+0.4 pcm/K
βeff
390 pcm
330 pcm
Reduces Doppler constant,
Increases coolant
temperature coefficient
Decreases βeff
Assume MOX fuel average temperature is 1500 Kelvin
αD = -KD/T ≈⋲ -0.37 pcm/K (no americium): |αD/ αNa| > 1!
αD = -KD/T ≈⋲ -0.15 pcm/K (10% americium): |αD/ αNa| < 1!
Generation IV reactors
Response to transient over-power
Fuel
(U0.8,Pu0.2)O2
(U0.7,Pu0.2,Am0.1)O2
αD
-0.4 pcm/K
-0.2 pcm/K
αNa
+0.2 pcm/K
+0.4 pcm/K
βeff
390 pcm
330 pcm
Assume that 1$ of reactivity is inserted (withdrawal of control rod)
Assume Δ∆TNa ≈⋲ 0.1 x Δ∆TMOX
How much have coolant & fuel temperatures increased when the total
negative reactivity feedback reaches 1$?
Generation IV reactors
Additional temperature feedbacks
Axial expansion of the fuel: response time ~ speed of sound x fuel length
Radial expansion of the assembly support grid plate.
Fuel
(U0.8,Pu0.2)O2
(U0.7,Pu0.2,Am0.1)O2
αax
-0.3 pcm/K
-0.5 pcm/K
αR
-0.6 pcm/K
-1.2 pcm/K
Data for 1500 MWth
SFR.
Geometry effects are
large in small cores.
Influence of americium is positive, but effect is smaller in cores with CR = 1
Generation IV reactors
Transient overpower accident
in a sodium fast reactor with MOX fuel
3200
Tfuel [K]
1 $ reactivity insertion simulated for
MOX fuelled SFR with SAS4A/
SASSYS.
3100
3000
Adding 1% Am increases maximum
fuel temperature by 75±15 K
Tfail
2900
2800
0
Am [%]
1
2
3
4
Fuel: (U0.8-x,Pu0.2,Amx)O2
Generation IV reactors
5
Fuel failure limit (solidus) reached
for 2.5% americium
Failure margin can be maintained if
nominal power is reduced by 6% for
each percent Am added to the fuel.
Metallic versus oxide fuels
1500
Metallic alloy fuels feature a much lower Δ∆T
from core inlet to fuel maximum temperature
Tfuel [K]
1450
Δ∆Toxide ~ 1500 K (failure margin ~ 700 K)
1400
1350
Δ∆Tmetal ~ 200 K (failure margin ~ 400 K)
Tfail
Similar increase in relative power consumes
less of failure margin in a metal fuel
1300
1250
1200
0
Am [%]
1
2
3
4
5
Fuel: U0.88-x,Pu0.12,Amx-10Zr
Adding 1% Am increases Tmax by 25±5 K
Power reduction to maintain ultimate failure
margin under a UTOP:
Δ∆P = -3% per percent Am
Generation IV reactors
Dense ceramic fuels
30
Thermal conductivity [W/m/K]
25
UN
Nitride fuels feature a unique combination
of high thermal conductivity and high
failure temperature
20
PuN
15
10
AmN
Δ∆Tnitride ~ 500 K (failure margin > 1500 K)
5
0
T [K]
600
800
1000
1200
1400
Generation IV reactors
Carbide fuels today can not be fabricated
without evaporating americium!
1600
Δ∆P = -2.7% per percent Am
Linear rating
50
Permitted linear rating [kW/m]
40
30
For similar SFR geometries with
conversion ratio ~ 1.0,
Nitride
Nitride fuels allow largest linear power
during normal operation
Metal
20
Oxide
10
Am [%]
0
0
2
4
6
Generation IV reactors
8
10
Nitride fuels allow to accommodate
more americium at the lowest power
penalty.
Summary & discussion
Americium is detrimental for
Doppler feedback, coolant temperature coefficient & effective delayed
neutron fraction
Power penalty of Am in MOX fuels: Δ∆P = -6% per percent Am
Power penalty of Am in metallic & nitride fuels: Δ∆P = -3% per percent Am
Major factor to improve safety and maximise americium burning rates:
Ratio of margin to failure to total Δ∆T under nominal operating conditions.
Nitride fuels provide best possible performance, thanks to combination of
high thermal conductivity and high failure temperature.
Generation IV reactors