Working Paper IANUS 2/2013 - Technische Universität Darmstadt

Working Paper
IANUS 2/2013
Moritz Kütt
Fast Reactors fueled with WPu-MOX
IANUS
Interdisziplinäre Arbeitsgruppe
Naturwissenschaft Technik
und Sicherheit
Die Interdisziplinäre Arbeitsgruppe Naturwissenschaft, Technik und Sicherheit (IANUS) beschäftigt
sich seit über 20 Jahren mit naturwissenschaftlich orientierter Friedensforschung. Ein Schwerpunkt
ist dabei die Analyse nuklearer und biologischer Forschung, Technologien und Materialien im
Hinblick auf mögliche Waffenanwendungen und effektive Kontrollmöglichkeiten, um dies zu verhindern. Dieses Spektrum hat sich deutlich durch Fragen nach der Gestaltung ambivalenter Forschung
und Technologie über die zivil-militärische Dual-use-Problematik hinaus erweitert. Hinzugetreten
ist die Erarbeitung konzeptioneller Ansätze (z.B. präventive Rüstungskontrolle, prospektives Technology Assessment, nuklearwaffenfreie Welt) sowie grundsätzlicher Überlegungen zur adäquaten
Wahrnehmung heutiger Wissenschaft und ihrer Verantwortbarkeit. Über den interdisziplinären
Studienschwerpunkt NaG ist IANUS auch Teil der Lehre an dieser Universität.
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Moritz Kütt. Fast Reactors fueled with WPu-MOX. Arbeitsbericht 02/2013, Interdisziplinäre Arbeitsgruppe Naturwissenschaft Technik und Sicherheit (IANUS).
Copyright (c) 2013 by Moritz Kütt
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Namensnennung – Nicht-kommerziell – Weitergabe unter gleichen Bedingungen 3.0
(CC BY-NC-SA 3.0)
Abstract
In the light of the Plutonium Management and Disposition Agreement between Russia
and the United States it was agreed that 34 metric tons of Russian weapon grade
plutonium will be burned in the BN-600 and BN-800 fast reactors. Verification of the
process is for-seen and the resulting spent fuel should not be reprocessed.
Using a similar fast reactor model based on the German SNR-300 project, the change
in plutonium isotopic vector for WPu fueled fast reactors are calculated. The fuel
would reach burnup in the range of 60-80 MWd/kg HM. Starting with a Pu-239 content of 93.8 wt % there will still be more than 90 % Pu-239 in the mixture up to
a burnup of 40 MWd/kg HM and 86.7 wt% at a burnup of 75.2 MWd/kg HM. In
the reactor core, the total plutonium content will be reduced by about 14 % at this
burnup.
Contents
1 Introduction
2
2 Reactor Model
2
3 Results
4
4 Conclusion
6
1 Introduction
Large stockpiles of military plutonium exist worldwide. With ongoing efforts of nuclear disarmament, parts of the plutonium become surplus for military purposes.
With the Plutonium Management and Disposition Agreement between Russia and
the United States (concluded in 2000), an amount of 34 tons of military stockpiles
were declared excess for military use in each country. Plans were made and discussed how to make this material inaccessible. After discussing several options over
the years, the current amendment (concluded in 2010) for-sees the elimination of
American stockpiles as mixed oxide fuels (MOX) in existing Light Water Reactors.
The construction of the MOX fuel fabrication facility is ongoing, however it has been
slowed down recently due to budget restrictions. According to [9], the United States
again reconsider different options.
On the Russian side, it has been agreed that the material will be eliminated in the two
fast reactors BN-600 and BN-800. The first has been operational since many years
using Highly Enriched Uranium as fuel, the second is currently under construction.
According to [9] it is for-seen that both reactors will operate as plutonium burners
and the IAEA will verify the process. No reprocessing of the spent MOX fuel produced
with military excess plutonium should take place.
While reprocessing is not for-seen, it is still interesting to estimate the isotopic vector
of the plutonium of the spent WPu MOX. To estimate the development during reactor
operation, a reactor burnup code is used. The code MCMATH has been developed
at IANUS over several years [3, 7, 8, 6, 1]. It uses and iterative way to calculate the
change of the isotopic composition over time. The Monte Carlo Code MCNP(X) is
used to calculate one-group cross sections for a given reactor model, Mathematica
solves the burnup equations for each step and controls the whole process.
This working paper uses a Fast Reactor model to give estimates on the change of
the isotopic vector. As fuel, WPu MOX using an isotopic composition as defined in
Table 1 has been used. It is difficult to access detailed design studies and plans for
the two Russian fast reactors. Because of this, an existing and known similar model
was used for the calculations based on plans for the German prototype fast reactor
“Schneller-Natriumgekühlter Brüter 300” (SNR-300) [5].
Pu-238
Pu-239
Pu-240
Pu-241
Pu-242
Am-241
0.01
93.8
5.8
0.13
0.02
0.22
Table 1: Sample isotopic vector of Weapon-grade plutonium, values in wt% [2].
2 Reactor Model
In the 70s and 80s, Germany pursued a liquid sodium cooled fast reactor program.
The main goal was to build and operate a 300 MWe reactor, the SNR-300 in Kalkar, in
the West of Germany. The reactor was built and filled with sodium, but never became
critical. A more detailed description can be found in [5], including detailed references
for the core layout.
The reactor has a thermal power of 762 MWth using hexagonal fuel elements with a
diameter of 115 mm. Different core configurations were discussed at the time. The
one chosen for this paper is the “Mark Ia” core configuration (cf. left side of 1). This
configuration has 193 fuel elements. In axial direction, the elements have a central
fuel part (driving region) and lower and upper part containing fertile material (axial
breeding region). A radial breeding region is formed by 96 elements surrounding the
fuel elements (a rather small fertile area compared to other breeder reactors). The
2
Contents
100
50
50
cm
cm
100
0
0
-50
-50
-100
-100
-100
-50
0
50
100
cm
-100
-50
0
50
100
cm
Con trol Elemen t
Fuel Elemen t LE
Sodium Blin d Elemen t
Reflector Elemen t
Fuel Elemen t HE
Breedin g Elemen t
Figure 1: Layout of the original SNR-300 Mark Ia Core in xy-plane (left image, used for
WPu-Breed) and the design modified to form the core for WPu-Burn.
core has an overall height of 135 cm and a diameter of 220 cm (including breeding
blankets). The driving region of the core has a diameter of about 190 cm and a height
of 95 cm.
12 additional elements are placed in the inner core for controlling purposes (primary
and secondary control elements and boron carbide elements to reduce reactivity of
fresh fueled cores). It was for-seen that a small, changeable number of sodium blind
would be introduced in the core for reactivity adjustments. The core modeled in this
paper used six such elements.
The fuel elements were based on MOX fuel using reactor grade plutonium from British
MAGNOX reactors and German Light-Water Reactors. The driving region is subdivide
in two parts. In the inner part, a lower enrichment of plutonium (24.5 wt %, Fuel
Element LE) is used for flux flattening, the outer part uses an enrichment of 36.2
wt % (Fuel Element HE). The fertile material in the breeding elements is natural
uranium.
In case of criticality, the reactor configuration should operate for a total burnup time
of 441 Full Power Days (FPD). An average burnup of fuel elements of 57 MWd/kg
HM was estimated by [4].
Using the described model, the plutonium isotopic vector in the driving regions (both
parts) was replaced by weapon grade plutonium as in 1. All control elements are
left in place. One model keeps the radial and axial blanket region as in the SNR-300
MarkIa (WPu-Breed). Considering that the Russians plan to use their fast reactors as
plutonium burners, a second model is introduced replacing all blanket regions with
sodium (WPu-Burn). For simplicity, the plutonium enrichment in MOX has been kept
similar to the SNR-300.
There is high confidence that the model will yield valid results regarding the isotopic
vector of plutonium. There are differences to the BN-600 and BN-800 designs, but
their effects should be small. The model has not been optimized specially with regard
to WPu in the MOX fuel. Instead the fuel in both core types has been burned until
reactivity dropped below zero (without control elements), ke f f < 1.
2 Reactor Model
3
3 Results
1.15
Keff
1.1
1.05
1
0
20
40
60
80
100
Bu rn u p H MWd  kg HML
Figure 2: Calculated ke f f for WPu-Burn and WPu-Breed over the burnup of the driving
region of the reactor.
Before the resulting isotopic vector is shown, some results are present to discuss effects of the breeding regions on the reactor core and especially the driving region. As
can be seen in Figure 2, differences exist with regard to ke f f between the core with
blanket (WPu-Breed) and without blanket (WPU-Burn). At high burnups, the breeding region in WPu-Breed adds significantly to the overall reactivity of the reactor.1 .
Because of this, the core with fertile material can reach a slightly higher burnup of
88.5 MWd/kg HM before reactivity drops below zero, while the burner core reaches
75.2 MWd/kg. Both cores would meet the original goal for the SNR-300 of running
for 441 FPD.
1
Diffe re n ce in Prop ortion H wt %L
Pu - 239 Prop ortion H wt %L
94
92
90
88
86
0.5
0
-0.5
-1
0
20
40
60
Bu rn u p H MWd  kg HML
80
100
0
20
40
60
80
Bu rn u p H MWd  kg HML
Figure 3: Pu-239 content (wt %) in the Figure 4: Difference of Pu-239 and Pu-240
driving region of WPu-Breed and
between WPu-Burn and WPuWPu-Burn.
Breed.
Considering that the reactor should be run in a burning mode, it is interesting to
estimate the effect of the breeding region on the buildup/destruction of different
plutonium isotopes in the driving region. Figure 3 shows the proportion of Pu-239
in the driving regions of WPu-Breed and WPu-Burn. Figure 4 shows the difference
between the two driving regions (WPu-Breed - WPu-Burn) for Pu-239 and Pu-240,
respectively. It can be concluded that there is no large difference in the isotopic
change over burnup while a breeding blanket is present or not.
1
4
Only the burnup for the driving region of WPU-Breed is shown. The breeding region is not included
in the Heavy Metal total for burnup.
Contents
Would one include the breeding region in the described analysis, there would be a
larger difference. In the breeding region no plutonium is present at the beginning,
hence there is a large change during burnup. The exact composition is not considered
in this paper.
As both models are very similar and plans are made for a Russian plutonium burner
reactor, in the following only results for WPu-Burner are shown. Figure 5 and Table
2 show the isotopic vector of the reactor operation. In the table, the composition at
Begin-Of-Life and End-Of-Life (75.2 MWd/kg HM) are shown, as well as intermediate
steps. Up to a burnup of 40 MWd/kg HM the Pu-239 content is still bigger than
90 wt %, and even after the relatively long burnup of 75.2 MWd/kg HM there remain
87.2 wt % Pu-239 in the mixture, and only 11.8 wt % Pu-240.
Often Russian burnup of fast reactors is given in % h.a., the percentage of heavy
atoms fissioned in the reactor over life time. Calculating this value for the presented
calculations, one can see that 10 MWd/kg HM equal a burnup of about 1% h.a.
100
Prop ortion H wt %L
10
1
0.1
0.01
0.
20.
40.
60.
80.
Bu rn u p H MWd  kg HML
Figure 5: Isotopic composition of plutonium in WPu-Burn core (in wt%). Colors: Pu238, Pu-239, Pu-240, Pu-241, Pu-242.
Pu-238
Pu-239
Pu-240
Pu-241
Pu-242
Am-241
BOL
0.01
93.8
5.8
0.13
0.02
0.22
10 MWd/kg HM
20 MWd/kg HM
30 MWd/kg HM
40 MWd/kg HM
50 MWd/kg HM
60 MWd/kg HM
70 MWd/kg HM
75.2 MWd/kg HM
0.012
0.015
0.019
0.024
0.029
0.036
0.043
0.046
92.9
92.1
91.1
90.2
89.2
88.2
87.2
86.7
6.62
7.45
8.3
9.16
10.
10.9
11.8
12.3
0.181
0.239
0.304
0.376
0.455
0.543
0.637
0.689
0.022
0.025
0.029
0.034
0.039
0.045
0.053
0.057
0.217
0.215
0.213
0.212
0.21
0.21
0.21
0.21
Table 2: Change in the isotopic vector of plutonium used as MOX fuel in a fast reactor
(All values in wt%).
WPu-Burn also would aim to reduce the total amount of plutonium. While still a
big part of the MOX fuel contains fertile uranium, some plutonium is also produced
during reactor operation. The question is mainly if the net balance of produced and
fissioned plutonium atoms is negative. Figure 6 shows the relative balance of plutonium in WPu-Burner. At End-of-Life, the initial plutonium content has been reduced
3 Results
5
by 14 %. Besides fissioning the initial plutonium, more has been constantly produced
by neutron captures of the U-238 present in the core.
0
Re la tive Ba la n ce H %L
-2
-4
-6
-8
- 10
- 12
- 14
0
20
40
60
80
Bu rn u p H MWd  kg HML
Figure 6: Relative balance of Pu-239 in WPu-Burn.
4 Conclusion
This paper analyzed the depletion of WPu MOX fuel in Fast Reactors. Based on a
model for the German Fast Reactor SNR-300, calculations were carried out using the
computer system MCMATH. The result shows that even after relatively high burnups,
still approx. 90 wt% of the plutonium in the reactor driving region is Pu-239. If
reprocessed, the spent fuel the resulting plutonium would be nearly weapon grade.
The effect on the total amount of plutonium is small: The initial plutonium content
in fuel would only be reduced by 14 %.
Taking these results into account, one can look at the case of Russia using their WPu
in their BN-600 and BN-800 reactors. Their spent fuel would contain nearly weapongrade plutonium, and not much of the excess plutonium would be burned.
Further analysis should be carried out using a BN-800 model, it would also be possible
to study different scenarios of reactor fueling - HEU fuel, and mixed HEU and MOX
fuel.
6
Contents
References
[1]
Matthias Englert. “Neutronenphysikalische Simulationsrechnungen zur Proliferationsresistenz nuklearer Technologien”. PhD thesis. Technische Universität
Darmstadt, 2009.
[2]
National Research Council Panel on Reactor-Related Options for the Disposition
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9780309051453. URL: http://www.nap.edu/openbook.php?record_id=4754.
[3]
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[4]
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[5]
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[6]
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Darmstadt, 2007.
[7]
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[8]
Christoph Pistner. “Neutronenphysikalische Untersuchungen zu uranfreien
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[9]
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References
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