# Wastes Management Through  Transmutation in  an ADS Reactor

Authors: Barbara Calgaro; Barbara Vezzoni; Nicola Cerullo; Giuseppe Forasassi; Bernard Verboomen
Journal: Science and Technology of Nuclear Installations
 (2008)

---

## Abstract

The main challenge in nuclear fuel cycle closure is the reduction of the potential radiotoxicity, or of the time in which that possible hazard really exists. Probably, the transmutation of minor actinides with fast fission processes is the most effective answer. This work, performed inSCK⋅CEN (Belgium) and DIMNP Pisa University, is focused on preliminary evaluation of industrial scale ADS (400 MWth, 2.5 mA) burning capability. An inert matrix fuel of minor actinides, 50% vol. MgO and 50% vol. (Pu,Np,Am,Cm)O1.88, core content, with 150 GWd/ton discharge burn up, is used. The calculations were performed using ALEPH-1.1.2, MCNPX-2.5.0, and ORIGEN2.2. codes.


---

## Body

## 1. Introduction

This work aims to open a discussion about dedicated transmuters,
connected to “partitioning and transmutation (P&T) strategy”, namely,
a subcritical reactor such as accelerator driven system (ADS).In particular, in this paper, the preliminary burning capability
efficiency of an industrial scale ADS (400 MWth, 2.5 mA) is evaluated [1].Results obtained, with special regard to neutronic
computational issues, have its basis in the contest of multipurpose hybrid
research reactor for high-tech applications project (MYRRHA), see [2],
experimental ADS design in development in Studiecentrum voor Kernenergie-Centre
d'Étude de l'Energie Nucléaire, SCK·CEN, Belgium.The starting point of this work is the present Belgian situation, where
with seven PWR installed (5800 MWe), Belgian government decided (2003) to phase
out nuclear energy production in the next 25 years, despite the opportunity of
guarantying energy national independence and the engagement to respect the Kyoto agreement.This was the incentive to propose an alternative scenario: the substitution of LWRs, the consequent installation
of European pressurized reactor (EPR) in synergy with industrial scale ADS to
support energy demand and, in the same time, to burn a consistent amount of HLW, could be a good connection to the foreseen possible fast reactors (Generation IV) [3].Jointly, to reach an appropriated knowledge of transmutation strategy, the state of art
of internal dosimetry in term of preliminary research was been done.


## 2. General Aspects

Global demand of energy increasing, oil and gas rise in prices, and significant
climate changes are the majors issues of present century. To contain these
phenomena, in order to obtain a sustainable energetic development, a critical and an integrate approach
and the social acceptability of nuclear energy are nowadays necessary conditions.Some strategies and technical solutions to minimize the quantities of long life
radionuclides, as complementary approach to geological disposal of spent nuclear
fuel (SNF), exist and are(1)
Pu recycling (MOX) in LWRs and FRs;(2)
MA and LLFP recycling in dedicated facilities as ADS;(3)
generation IV fast
reactors able to burn the small, self-produced MAs and LLFPs amount. In the Integrated Project
IP-EUROTRANS (EURATOM 6th Framework Programme—6th FP [4]), the preliminary
concept of “European facility for industrial transmutation” (EFIT), can be
found. His structure is based on the experiences gained during the 5th FP, where three experimental X-ADS designs
(SCK·CEN, ANSALDO, and FRAMATOME) are defined.


## 3. Computational Tools

To evaluate an industrial ADS burning efficiency, ALEPH-1.1.2
code [5, 6] was used. It is a Monte Carlo burnup code,
under development at SCK·CEN in the framework of MYRRHA project, able to couple any version of
MCNP [7] or MCNPX [8] with ORIGEN2.2 [9], for evolution calculation, using
NJOY99.112 [10], for processing nuclear data in ENDF format (JEFF-3.1 [11]).
MCNPX-2.5.0 and ORIGEN2.2 codes were used also separately from ALEPH-1.1.2, as
explained further on.


## 4. Nuclear Waste Inventory Evaluation

Doel-3 (Belgium), a 1000 MWe PWR, is
currently the reference reactor chosen in this work. Material discharged by
this plant loaded by 4.3% enriched inU235 fuel was assumed to
perform the calculations.ALEPH-1.1.2 [5] code and JEFF-3.1 “evaluated data library” [11] were used to evaluate the inventory, considering a 50GWd/ton
burnup and an irradiation cycle of 4.5 years.Those specifications permitted to obtain
weight fractions, in good agreement with literature values [12], referred to 1
ton of initial heavy metal (IHM), as 12.067 kg of Pu, 0.811 kg of Np, 0.282 kg of Am, and 0.128 Kg of Cm. The
same values expressed as percentage are 1.06%wt. Pu/SNF, 0.072%wt. Np/SNF,
0.025%wt. Am/SNF, and 0.011%wt. Cm/SNF.Noted that the quantities written above do
not seem to play a significant role in health hazard and the choice to confine
them in a geological disposal is the main solution adopted nowadays (once through
scenario). But, also, nuclides half-life is related to social acceptability
because these elements contribute to radiotoxicity for hundreds of thousands years.Initial ADS fuel composition definition was performed on the basis of the
waste inventory after the reactor full life, considering single batches decay:
this value tries to represent a real PWR inventory composition.So, we chose a 58.5 years of full life, in
the context of PLEX strategy (Plan Lifetime EXtension). We simulated that any
single batch undergoes 4.5 years of irradiation cycle (an ALEPH-1.1.2 code calculation),
and we built a matrix in which every 1.5 years (length of internal fuel cycle)
each batch is taken out from reactor and starts natural decay. In this way,
hypothesizing constant working features during reactor full life, we estimated
weight fractions upon the whole of minor actinides (Table1).Table 1
Fractions of Np, Am, and Cm upon the whole of minor actinides after a 1000 MWe PWR full life (58.5 years,
under constant conditions).
ElementWeight fraction [%]Np0.315Am0.675Cm0.010


## 5. Radiotoxicity Aspects

Radiotoxicity data from different
sources imply often difficulties in understanding values and in comparing
results among working groups; critical approach is then necessary to erase all
uncertainties.In this work, the radiotoxicity unit chosen is the “Sv/GWe-y,” a suitablereference that allows to plot diagrams for a power plant radiotoxicity
assessment independent from country law annual limit of intake for members of public.Radiotoxicity coefficients data-bases
referred to ICRP72 [13], that enable us to use an updated version of ORIGEN2.2
and ALEPH-1.1.2 codes, were created and considered as a further result of the described activity.Also reference level (RL) needs a best
estimated evaluation. We simulated (ORIGEN2.2 code calculation) the decay
chains of natural uranium isotopic composition (g/cm3) till to reach
the secular equilibrium. We supposed that 1 ton of U 4.3%-enriched needed 8
tons of natural uranium, and we obtained a value of 3.07 106 Sv/GWe-y that corresponds to
1.61 105 Sv/ton
IHM [1], in perfect agreement with international literature (NEA-OECD
Report [12]).


## 6. Theoretical Partitioning & Transmutation

The assessment of theoretical partitioning and transmutation efficiency is the
preliminary step to burning capability evaluation. In fact, it becomes a term
of comparison.Some definitions help to better understand this analysis:(i)
“Transmutation efficiency” defines the fraction of nuclides in SNF transformed
in ones with shorter half-lives.(ii)
“Transmutation cycle” defines the irradiation period inside reactor; a decay
period follows. Within
a single cycle, a transmutation efficiency close to unity is impossible to
reach and it will be necessary an adequate study and an optimisation of
subsequent cycles.Acceptable time to reach reference level
has been evaluated considering Pu a resource and not a waste and its
partitioning efficiency. Separation of 99.9%wt. of Pu permits the achievement
of RL in 2-3×104 years,
considering the PUREX Process efficiency 99.9%wt [14].After this evaluation, we assumed
that a suitable time to reach RL, in order to consider the P&T acceptable,
has to be 2, max 3,×103 years (Figure 1). In terms of MAs residual
concentration left in wastes, the acceptable value obtained is about 27%wt.
Theoretical transmutation efficiency, that is the reference value to tend
forwards for ADS minor actinides burning capability, becomes equal to 73%wt.
for all of minor actinides, in addition to 99.9%wt. Pu partitioning.Figure 1
Theoretical transmutation
efficiency of minor actinides: SNF (red line), SNF without 99.9% of Pu, and the reference levels are also represented.


## 7. Industrial ADS Burning Capability

Fuel isotopic composition definition and core
configuration are the first steps to calculate burning capability
efficiency of an industrial scale ADS. We underline that this work is devoted
to a preliminary evaluation and does not perform a full-scale calculation.Industrial ADS structure (400 MWth, 2.5 mA,
600 MeV proton beam) modelled is pool-type reactor (Figure2(a)), cooled by
lead, with a diameter approximately 3 times bigger than height, characterized
by keff=0.97, 150 GWd/ton discharge burnup and loaded with CERCER fuel.Conceptual configuration of EFIT (a).
Simplified design of ADS fuel pin (b). Simplified design of ADS assembly (c).
(a)(b)(c)We considered an inert matrix fuel of minor
actinides 50%vol. MgO and 50%vol. (Pu,Np,Am,Cm)O1.88. In particular, the
industrial size ADS studied consists in a hexagonal core and a central
spallation loop separated from the core. Target, inside the spallation loop,
consists of a windowless Pb free surface (design by MYRRHA project [2]). This
layout allows a correct coupling with LINAR accelerator. The fuel assembly is
hexagonal composed by a triangular lattice of cylindrical pins (Figures 2(b) and
2(c)). Basic data on the reactor facility, presented as EFIT development
in 6th FP, have been furnished to the authors by MYRRHA project SCK·CEN
research team (design data referred to March 2006).Isotopic composition of ADS fuel was determined by successive steps.(i)
Fixing fractions of Np, Am, Cm on the whole of MA equal to the
fraction calculated with accumulation during a PWR full life (Table1). 20
years of natural decay, since the last batch has been extracted from reactor at
the end of life, were taken.(ii)
Considering first generation plutonium in agreement with Belgian
scenario chosen. This condition was dictated by Belgian government decisions: 2
reactors only, among 7, and just for a limited period of time (1995–2005), were
loaded with MOX fuel.(iii)
Considering the nonstoichiometric ratio inside MA and Pu oxides
equals to 1 : 1.88 instead of 1 : 2. Some other steps became necessary to reach the suitable fuel
composition, hypothesizing fuel density as 90% of theoretical density: at
first, the calculation of ideal fuel solid solution within spots of (Np,Am,Cm)O1.88;
and second, the subsequent fuel and matrix ideal solid solution calculation.At last, the quantity of first generation
plutonium loaded in ADS fuel was minimized, in order to consider it a resource
for future efficient development of fast reactors.Different criticality calculations by
MCNPX-2.5.0 [8] were then performed to respect the fixed keff=0.97, where Pu
concentration was a free parameter.Taking into account these specifications, a
composition of 37%at. Pu (atomic fraction) in 50%vol.
fuel was obtained. Core was thought to be loaded with 324 homogeneous
assemblies (Figure3).Figure 3
ADS MCNPX core model with 324 homogeneous fuel
assemblies. Horizontal section (Section A-A). Particulars: assembly upper
matching nozzle,z=92 cm (Section B-B); fuel pin lower plenum in z=−90 cm (Section
C-C).The originality of this
work consists in considering the addition to Am and Cm of Np isotopes (Np-237
witht1/2=2.144⋅106 y) inside ADS fuel.The simulated time period consists of 3 irradiation cycles
(ALEPH-1.1.2 code calculation) of 2394 effective full power days (efpd) each,
spaced out by decay periods of 3 years. 2394 efpd value is built on burn-up
value, on thermal power of reactor and on quantity (ton) of initial heavy metal
loaded (1). In particular, values chosen as reference are, respectively, 150 GWd/ton IHM, 400 MWth, and 6385 tonIHM, where(1)efpd=(burn-up)⋅(tonIHM)Pth 2394 efpd, equal to 6.6 years, was
considered acceptable and in literature the same order of magnitude can be
found, if referred to fast reactors [12].This study was subsequently focused on the
optimization of geometry and irradiation time parameters. Results obtained
after nearly 2 months of parallel calculation in 4 CPU Dual Xeon 3 GHz are
shown in Tables2 and 3.Table 2
Burning capability for an industrial ADS: MA elements evolution during 3 transmutation cycles.
(BOC: begin of cycle, EOC: end of cycle, and ETC: end transmutation cycle).
ElementMass/initial massBOC1EOC1EOC2ETCNp1.0000.6900.4430.249Am1.0000.6930.4540.271Cm1.0002.4783.2023.592Total MA1.0000.7100.4770.296Table 3
Burning capability for an industrial ADS: some isotopes evolution.
IsotopeMass/initial massBOC1EOC1EOC2ETCNp2371.0000.6900.4430.249Am2411.0000.6450.3910.211Cm2441.0002.8773.5943.831The brief overview of
data written into tables makes possible to notice that, after the first cycle,
transmutation efficiency (ϵ) is 29%wt. At the end of transmutation cycles
(EOC3), if we consider total amount of MAs, a transmutation efficiency equal to
72% is achieved (Figure 4(a)). This value is in agreement with those values
found with theoretical transmutation analysis described above.Industrial ADS burning capability during 3 transmutation cycles: mass to
initial mass of total MAs evolution (a); evolution of single elements (b);
evolution of Pu isotopes (c); evolution of Cm isotopes (d) [mass to initial
mass plus decay].
(a)(b)(c)(d)In any case, the
obtained values do not consider effective MAs separation efficiency (in
particular the Cm one) necessary to prepare fuel for ADS.Such preliminary remarks
underline that a good result in transmutation and, then, in radiotoxicity
reduction could be obtained just after more than 3 cycles inside ADS.On the whole, total
amount of MAs decreases during the burning processes; but some isotope content
does not. Curium is an example. In order to describe
better single elements amount during the transmutation period, that grow out of
transmutation and decay chains contributions, mass to initial mass plus decay
curves are presented (Figures4(b), 4(c), 4(d)).Curium inventory,
accumulated in a single batch or during a LWR full life, is not too much (about
0.11 Kg Cm for each ton of SNF produced by LWR), but problems rise because of curium decay chain:
it decays into Plutonium and increases total radiotoxicity.About this last aspect, Cm-244 (Table3),
about 81.5% of total Cm with half-life of 18.1 years, increases at about 280%
its initial quantity at the end of transmutation cycle (ETC). The Cm-244, in
particular, decays into Pu-240, with half-life of 6563 years. So, Pu-240 inventory
increases during transmutation (Figure 4(c)) and it brings a significant
contribution to whole long time radiotoxicity.


## 8. Conclusions

Present LWRs and Generation IV fast reactors could be joined together by
few ADS transmuters, built in limited number, in order to consider Pu a
resource and not a waste.In particular, the substitution of LWRs, the
consequent installation of a new generation of passive safe reactors and
industrial scale ADS to support energy demand and, in the same time, to burn a
consistent amount of HLW, could be a good connection to
Generation IV, in particular to fast reactors.The definition of possible isotopic
compositions of fuel and possible core configuration is the first step in order
to calculate burning capability efficiency of an industrial scale ADS.Moreover, creation of radiotossicity
coefficients data-bases improves a more efficient and flexible use of
ALEPH-1.1.2 and ORIGEN2.2 codes.



---

*Source: 756181-2008-06-09.xml*