Complex Approach to Study Physical
Features of Uranium Multiple
Recycling in Light Water Reactors
A.A. Dudnikov, V.A. Nevinitsa,
A.V. Chibinyaev, V.N. Proselkov,
Russian Research Center “Kurchatov Institute”
A.Yu. Smirnov, G.A. Sulaberidze
Moscow Engineering Physics Institute
(National Nuclear Research University)
International Conference on Management of Spent Fuel from Nuclear Power Reactors,
IAEA, Vienna, Austria, 31 May – 4 June 2010
Introduction



Irradiated uranium fuel contains more then 90%
of uranium, but at the present time only small
number of states has the experience in using of
recycling uranium and this experience is limited to
one recycle.
At the same time multiple recycling decreases the
need in uranium mining and improves the
utilization of uranium resources.
Calculations show that in VVER-1000 irradiated
fuel residual U-235 concentration remains more
than in natural uranium up to burnup level ~ 60
MW·days/kg of heavy metals (h.m.).
2
Fuel with Recycled Uranium (1/2)


Utilization of reprocessed uranium as a source of fuel is more
complicated due to U-232 and U-236 isotopes presenting in
irradiated fuel. Some other uranium isotopes effect on the fuel
reprocessing and fabrication is significantly less.
U-236 is a parasitic neutron absorber and to compensate this
effect fuel with recycled uranium must be enriched more than
that free from U-236. The expression
∆C5 = K236·C6
is usually employed to determine the additional U-235
enrichment ∆C5 required compensate for the reactivity penalty
due to U-236 concentration C6. Reactivity compensation factor
K6 can be defined from the requirement to keep the same fuel
campaign for reprocessed fuel.
3
Fuel with Recycled Uranium (2/2)




Analysis of calculations shows that for the VVER-1000
fuel with recycled uranium compensation factor
K236 = 0.3+0.05 can be recommended
U-232 effect on neutron physical parameters is negligible
due to very small concentration in reprocessed uranium.
However introduction of this isotope may lead to an
increase radiation dose rate to personnel because of the
hard gamma rays from its decay daughters.
To limit this dose rate at the fuel fabrication plant U-232
concentration in fuel with recycled uranium was
restricted at the level 2·10-7 wt. %.
Taking into account the enhanced technologies of fuel pin
fabrication this restriction can become softer.
4
Multi-Recycled Modeling (1/4)



In present paper the combined approach was
implemented on the base of simultaneous
modeling of neutron-physical processes and
processes of cascade isotope separation for
analysis of physical problems of usage multirecycled uranium for LWR fuel cycle.
The goal of the investigation was evaluation of
relative consumption of natural uranium for
additional uranium enrichment depending on
U-232 concentration in recycled uranium.
The parasitic neutron absorption in U-236 was
compensated by additional U-235 enrichment.
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Multi-Recycled Modeling (2/4)
1.
2.
An initial loading is supposed enriched and fabricated from
natural uranium only. Fresh fuel isotope composition
obtained after gas centrifuges enrichment was calculated in
following assumptions:
ideal cascade was implemented without mixing on relative
concentrations of 235UF6 and 238UF6;
stage separation factor was equal to 1.2 .
Then fuel is irradiated in reactor of VVER-1000 type up to
given burnup level and after during 10 years is cooled and
reprocessed. Uranium is separated and turns to fresh fuel
fabrication.
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Multi-Recycled Modeling (3/4)

Equivalent mass concentration of U-235 in the
enrichment product stream is obtained from
equation
P
P
P
C235

C

К

C
eq
235nat
236
236


Then new fresh fuel is irradiated, cooled,
reprocessed and enriched as it is described above
Heterogeneous model of pin fuel cell was used
for neutron-physical calculations to obtain
neutron induced reaction rates. Calculations were
performed with the MCNP5 Monte Carlo code.
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Multi-Recycled Modeling (4/4)
For re-enrichment purpose quasi-ideal cascade is
implemented containing natural uranium feed (F)
and additional reprocessed uranium feed (E) .
The scheme of the separating cascade with an
additional flow of a feed
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Scenario of Uranium Multiple Recycling
Cycle 1
Low Enriched
Uranium
Natural Uranium
Cascade
Enrichment
of Natural
Uranium
Cascade
Enrichment
Natural Uranium
Recycled
Uranium
Recycled
Uranium
Cascade
Enrichment
Irradiation
Spent Fuel
Cooling and
Reprocessing
Recycled
Uranium
Cycle 2
Cycle 2
Low Enriched
Uranium
Irradiation
Spent Fuel
Cooling and
Reprocessing
Recycled
Uranium
Spent Fuel
Cooling and
Reprocessing
Recycled
Uranium
Cycle3....Cycle N
Cycle N
Low Enriched
Uranium
Irradiation
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Scenario of Uranium Multiple Recycling
The following set of uranium recycling scenarios is
considered:
 With restrictions on U-232 in product flow,
=2∙10-7%.
 With restrictions on U-232 in product flow,
=5·10-7%.
 Without restrictions on U-232.
Two burnup levels were considered:
48 MW·days/kg (h.m.) and 60 MW·days/kg (h.m.)
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Natural uranium relative consumption, recycled
uranium relative consumption and relative separation
work consumption in dependence on recycle number.
Initial enrichment is 4.0% wt, burnup is 48 MW
days/kg (h.m)
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Natural uranium relative consumption, recycled
uranium relative consumption and relative separation
work consumption in dependence on recycle number.
Initial enrichment is 4.4% wt, burnup is 60 MW
days/kg (h.m)
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Specific natural uranium consumption and separation
work for five consecutive recycles, with and without
account of U-236 compensation
Recycle number,
48 MW days/kg
Specific natural uranium consumption
Specific excess
consumption of
natural uranium
for U-236
compensation, %
With U-236
compensation
Without U-236
compensation
1
5.343
5.157
3.52
2
5.373
5.075
5.55
3
5.370
4.986
7.15
4
5.379
4.934
8.29
5
5.393
4.904
9.08
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Specific natural uranium consumption and separation
work for five consecutive recycles, with and without
account of U-236 compensation
Recycle number,
60 MW days/kg
Specific natural uranium consumption
Specific excess
consumption of
natural uranium
for U-236
compensation, %
With U-236
compensation
Without U-236
compensation
1
6.273
6.076
3.14
2
6.308
6.005
4.81
3
6.306
5.923
6.07
4
6.312
5.876
6.91
5
6.327
5.856
7.44
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Problem of U-236


Besides the fact that the presence of U-236 in fuel makes
additional fuel enrichment necessary in order to reduce
the parasitic neutron absorption, this isotope is also one of
the factors causing the growth of U-232 concentrations in
spent fuel during the next recycles.
This is due to the fact that in reactor irradiation conditions
this isotope (via the short-lived U-237) is the precursor of
Np-237, which, in its turn, precedes U-232 through the
chain of radioactive transformations
Np-237 (n,2n) Np-236m 0.48(-) Pu-236 () U-232.
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U-232 concentration during irradiation in
VVER-1000 fuel cell, with and without account
of Np-237 (n,2n) Np-236m reaction
16
Fragment of chains of nuclear reactions at an
irradiation of fuel in a core
17
There is a tendency
to growth of mass
concentrations of
U-232 and Np-237
in spent
fuel with growth of
recycle number in
case of absence of
restriction on U-232
in fresh fuel.
These factors result
to problems with
recycled uranium
usage because of
increase of hard
gamma rays emitters
concentration.
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Implementation
of
restriction
on
U-232
concentration in fresh fuel,
made
from
recycled
uranium, due to dilution of
U-236 in fresh fuel, allows
to moderate growth of
U-232 concentration in
spent fuel and makes it
possible
to
reach
equilibrium
of
U-232
and
Np-237
concentrations in spent fuel
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U-232 and U-236 Concentration in Fuel Recycling
in Dependence of Recycle Number (48 MW Days/kg)
The existence of isotope correlation between U-232 and
Np-237 is the reason of the existence of isotope
correlation between U-232 and U-236 in fresh and spent
fuel.
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CONCLUSIONS




Analysis of calculated results allows the following conclusions to be
formulated in connection with the scenarios considered
Multiple usage of recycled uranium without any limitations imposed on the
content of U-232 isotope allows natural uranium consumption to be reduced by
about 16% and 10% for the respective burnup rates of 48 and 60 MWday/kg.
In case the content of U-232 is limited (by 2∙10-7 mass %), natural uranium
economy reduces to 7% and 3.5% for the respective burnup rates of 48 and 60
MWday/kg.
Identified conditions of achieving equilibrium concentrations of U-232 show
that this would require the contents of U-236 in fresh fuel to be limited (in the
given study this limitation occurred naturally as a result of recycled uranium
dilution to admissible U-232 concentrations).
Excess amounts of natural uranium spent on U-236 compensation, depending
on the specific recycle number, lie between 3.5 and 9% for the burnup of 48
MWday/kg, and between 3 and 7%  for the burnup of 60 MWday/kg
Results of this study lead to the conclusion that development of U-236
separation technology could reduce accumulation of U-232 in the LWR fuel
cycle and improve natural uranium economy.
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