How to meet requirements for different evolutions of nuclear energy

How to meet requirements
for different evolutions of nuclear energy
M. Salvatores
Senior Scientific Advisor
CEA, Nuclear Energy Division (France) and
Idaho National Laboratory (USA)
The role of fast reactors
Fast reactors and close fuel cycles
Fuel cycle scenarios and issues
Potential benefits of fuel cycles with P&T
Technological challenges:
 Safety
Actinide Chemistry
Advanced simulation,uncertainty/sensitivity analysis,V&V
 Nuclear energy can be a powerful asset for future generations in order to meet
growing energy demand
 However, new goals, as formulated within Gen-IV, should be accounted for:
 Safety and Reliability
 Sustainability and Waste Minimisation
 Enhanced Economics
 Proliferation resistence and physical protection
 Innovative reactor systems should be conceived from the very beginning to meet
the possible evolution of needs and objectives and/or the emergence of new
technologies, without leaving the burden of legacy waste inventories, both of
radioactive materials and potentially associated to proliferation risks
 Fast reactors and adapted fuel cycles can represent a flexible and credible long
term solution
The role of fast reactors
As first discovered by Enrico Fermi back in 1944, the nuclear characteristics of
Transuranics (TRU) cross sections in a fast neutron spectrum allow a unique
Breed (i.e. Conversion Ratio, defined as the ratio of the fissile produced to
the fissile destroyed, CR>1)
Burn (TRU or Minor Actinides, MA), i.e. CR<1.
Breed (e.g. Pu) and burn (MA)
CR~1: Self-sustaining fast reactor.
Wide coolant and fuel type choice according to the objective, e.g. for
short Doubling Time* DT: Na coolant and dense (e.g. metal) fuels
* time
required for a specific reactor to produce enough fissile material in excess
of its own fissile inventory to fuel a new, identical reactor
A wide range of MA content and different Pu vectors or TRU
compositions can be handled, according e.g. waste minimization or
Pu management strategies
The fission/absorption ratios are consistently higher for the fast spectrum reactors.
Thus, in a fast spectrum, actinides are preferentially fissioned, not transmuted into
higher actinides
New challenges within reactor physics and fuel cycle areas
 Physics features of the cores and associated innovative fuel cycles are in principle well understood.
 However, if the current knowledge in reactor physics allows to explore wide ranges of parameters, there are challenges associated to several specific core and fuel cycle design features.  This is essentially due to the requirement to reduce uncertainties for safety assessment, the reduction of margins (economy) and for overall design optimization.
 Typical examples are:
• Cores with low reactivity loss during the cycle (combination of effects with different signs due to isotopes build‐up and depletion),
• Cores with near zero void reactivity coefficients (leakage and spectrum components compensation, local effects)
• Increased inventory of Minor Actinides (MA) in the fuel (waste management)
• Cores with no uranium blankets (reflector effects)
• Fuel cycles with large amounts of higher TRU: high decay heat and neutron sources at irradiated fuel unloading, reprocessing and fabrication steps.
Fast Reactors and close fuel cycles
Critical FRs together with close cycle are a unique technology
Possibility to vary the CR from <0.5 to >1.5, without degrading the Pu vector and
without building up large amounts of Cm and other higher mass actinides (up
to Cf isotopes)
 Burning ~70% of the max theoretical TRU burning rate, whatever the Pu
vector and whatever the MA/Pu ratio (for P&T, Partitioning and
Transmutation, strategies)
 Self-sustained recycle of Pu and MA burning, either in the core as
homogeneous fuel (not-separated TRU) or in specific S/A as heterogeneous
recycle of MA targets;
 Doubling times of <10 years with dense fuels (e.g. metal) and Na coolant.
FRs have a unique potential to keep a large range of fuel cycle options open
without leaving a legacy of highly radiotoxic and radioactive material.
Fuel cycle issues should however be carefully analyzed
The flexibility of a fast reactor allows to consider very different cases,
associated to different strategies. Three examples:
1) Break-even FRs: CR~1 (or TBG~0)
At present, this option is a reference for FRs in some OECD countries with
non-proliferation concerns (i.e. no Pu-producing blanket). Break-even FR
can handle Pu and avoid its build-up.
Alternative: LWRs with CR~1 (lower moderation with tighter lattice and
consequently harder spectrum).
 Many studies in the past. Difficulty to overcome problems of positive
void coefficient, in particular for degraded Pu vectors.
 Still concerns remain on the safe operability of this system.
2) Fast reactors with CR>>1 (or TBG>>0)
Numerous studies (IAEA, IEA, etc) have made predictions of worldwide energy demand and nuclear energy share. Among the most recent ones (IPCC, IIASA):
World: from ~3000TWhe in 2010 to ~24,000 TWhe in 2200
Uranium resources could be under potential stress if only U-LWRs are deployed
The deployment of FRs with short doubling times (here indicated as CDT), i.e
with CR>>1, can help.
FR with CR>>1
An extra significant
gain if CDT~8 y
Fast reactors and multiple recycle allow sustainability in terms of
optimal utilization of resources and waste minimization.
Uranium utilization without reprocessing has been envisaged
since an early proposal by Teller, and successively by Van Dam
and by Sekimoto (the CANDLE concept) and more recently by the
Travelling Wave Reactor proposal of Terra Power
However, no miracle solution can be found with any once-through
3) Burner fast reactors CR<1 (or IBG<<0):
According to the fractional amount of the fertile material (e.g. Uranium) one can have a breeder, a break‐even or a burner core
Varying the ratio TRU/(TRU+U) one can reach the maximum theoretical consumption of TRU
Relation between TRU consumption rate and TRU fraction in critical Advanced
Burner Fast Reactors:
70-80% of max.
theoretical consumption
can be obtained with
TRU/(U+TRU) ~0.4-0.6
both for metal or oxide
fuelled cores and for a
wide range of Pu/MA
Normalized TRU consumption rate
Metal, MA/Pu~1 feed
Oxide, MA/Pu~1 feed
Metal, LWR-TRU feed
Oxide, LWR-TRU feed
TRU fraction
Alternatives: the ADS
Potential safety problems in the case of a critical core loaded with only
TRU and with a high content of Minor Actinides.
In these types of cores, the absence of Uranium produces both a very low
fraction of delayed neutrons and a very low Doppler reactivity coefficient
(in general, mostly due to U-238 capture).
Moreover, a high content of Minor Actinides like Am isotopes and Np
induces a deterioration of the void reactivity coefficient (in case of liquid
metal coolants).
Sub-critical systems (or Accelerator Driven Systems ADS), were
“rediscovered” (~1985), since they provide a possible way out from these
potential difficulties.
Example: Case of burning out a legacy inventory (e.g. ~150 t Pu+ 30 t MA)
Comparison of the potential performances of a critical low CR FR and of an ADS
Pu/MA ratio
Adapted to German needs (Pu
and MAs burning)
TRU Burning rates
TRU burned / TRU loaded per
Irradiation time
(with 100% load factor)
Estimated number of units
needed to burn-out the TRU
inventory in the SNF
TRU cumulative losses after 150
years of operation (tons)
MgO or
Mo (natural)
400 MWth
~ 70/30
~ 1200 MWth
~ 80/20
40-45 kg/TWhth
~ 10%
14-16 kg/TWhth
~ 13%
3 years
5 years
7-8 units working for 150 y
7-8 units working for 150 y
~ 1.8
~ 1.8
Alternatives (other than ADS):
Deep burners: HTRs; IMF-LWRs and Increased Moderation LWRs:
In all cases, the neutron spectrum can be considerably softer than
a standard PWR, and consequently higher capture-to-fission
ratios for « gateway » isotopes (Pu-242, Cm-244 etc) will induce
very significant build-up of higher mass actinides (up to Cf
As a reminder, the Cf-252 production during the irradiation of TRU fuel in a
standard PWR (need for very long out-of-pile cooling times):
Cf-252 inventory in the core. Case of full TRU
multirecycle in a LWR
Cf-252 inventory in the core. Case of full TRU
multirecycle in a FR
Fuel cycle scenarios and issues
Today options for the
nuclear fuel cycle:
Spent fuel disposal
Spent fuel
reprocessing and Pu
A general scheme for
advanced fuel cycles (with
Partitioning and
Transmutaion, P&T ):
Geological Disposal
Direct Disposal
Temporary Storage
for heat decay
Cs, Sr
Spent Fuel
from LWRs
Stable FP, TRU losses
Stable FP, TRU losses
Dedicated Fuel
LLFP target
Dedicated Fuel
LLFP Target
LLFP: Long lived fission products (Tc -99, I -129, Se -79, ...); MA: Minor Actinides (Am, Np, Cm)
Three major scenarios with advanced fuel cycles (and P&T)
Sustainable development of nuclear energy with waste minimisation.
One type of reactor, one fuel type, one reprocessing process
(homogeneous TRU recycling). FR with CR≥1
b) „Double strata“ fuel cycle: 1) commercial reactors with Pu utilisation
2) separate MA management. Two separate fuel cycles.
FR with CR<1 or ADS with CR=0
Scenarios a) and b) imply the continuous use of nuclear energy, the
stabilisation of the TRU stocks in the fuel cycle and the minimisation
of wastes in a repository.
Reduction of TRU stockpiles (e.g. as a legacy from the past
operation of power plants: potential change of energy mix or even
phase-out). FR with CR<1 or ADS with CR=0
All three scenarios go beyond the strategy of „once-through“
(„open“) fuel cycle (i.e. the final storage of irradiated fuel), and imply
fuel reprocessing.
Feasibility issues of the different fuel cycle options indicated previously can arise
when considering not only the core feasibility but also fuel cycle performances.
Impact expected at fuel fabrication, reprocessing and transportation.
Case of decay heat and neutron production after post‐irradiation cooling (at fuel fabrication):
Fuel type
(Pu only,
reference) recycle
Homog. TRU Homog. TRU Homog.TRU
CR=1 and
CR=0.5 and CR=0.5 and
MA/Pu~0.1 MA/Pu~0.1
Potential benefits of fuel cycles with P&T
In principle, P&T offers significant potential benefits to any of the fuel cycles
described previously:
- Reduction of the potential source of radiotoxicity in a deep geological
storage („intrusion“ scenario)
- Reduction of the heat load and high level waste volumes: larger
amount of wastes can be stored in the same repository
- If TRU are not separated (e.g. in the homogeneous recycling in a Fast
Neutron Reactor), improved proliferation resistance is expected
 Results of impact studies in the USA, in Japan and in Europe
 However, still a debated issue between P&T and Waste Management
Communities: which are the “good” metrics?
 A comparative analysis has been completed within the OECDNEA
Potential benefits of P&T for advanced fuel cycles
MA and FP
A. Porracchia, CEA-France
Heat load reduction. A US study
Plutonium, americium,
caesium, strontium, and
curium are primarily
responsible for the decay
heat that can cause
repository temperature limits
to be reached
Large gains in repository
space are possible by
processing spent nuclear fuel
to remove those elements
Potential volume
reduction factors
(R.Wigeland et al.)
Similar studies in Europe and Japan
with consistent conclusions
The impact of P&T: Major Findings of the NEA Task Force
The analysis performed has pointed out that most recent studies have
demonstrated that the impact of P&T on geological disposal concepts
can be significant even if not overwhelmingly high.
In fact, by reducing waste heat production a more efficient
utilization of repository mines is likely. In practice, the reduction of the
thermal output of the High Level Wastes by a factor of ~3 can reduce the
needed repository gallery length by a factor ~3 and the repository footprint
up to a factor 9.
A clear reduction of the actinide inventory in the Highly Active
Waste (HAW) reduces risks arising from less probable evolutions of
a repository, i.e. increase of actinide mobility in certain geochemical
situations and radiological impact by human intrusion.
 P&T can reduce the importance of uncertainties both in normal
evolution and in particular those related to hypothetical disruptive
scenarios that can bring man in direct contact with the disposed waste.
By introducing P&T in the fuel cycle, the impact of new waste forms (e.g.
long-lived low- and intermediate-waste from reprocessing) should be
accounted for.
The existence of previously vitrified wastes can reduce the full potential
benefit from P&T
 In summary, the introduction of P&T could increase the options for
optimised waste forms since it provides a much needed flexibility in
front of possible storage options, regulatory evolutions and the
handling of uncertainties.
Technological challenges
 Safety
 Fuels
 Materials
 Actinide Chemistry
 Advanced simulation, uncertainty/sensitivity analysis, V&V
 and…Economics!
International collaboration plays a key role
In what follows, a few examples
Safety: avoid
recriticality in FRs
Y. Tobita
Fuels: ultra-high burn-ups
Materials: high doses and low swelling
Actinide Chemistry: MA recovery, hydro and pyro-processes
Very few full process (fuel fabrication, irradiation, reprocessing and new fuel
fabrication) demonstrantions. One example for metal fuel: the METAPHIX
experiment (JRC-CRIEPI-CEA)
Advanced simulation, uncertainty/sensitivity analysis, V&V
 Modeling and Simulation have become key issues for development:
“...the Modeling and Simulation hub will focus on providing
validated advanced modeling and simulation tools necessary to
enable fundamental change in how the U.S. designs nuclear
power and fuel cycle technologies. This has the potential to
improve the performance and reduce the costs of nuclear
technologies.” (DOE Statement when established that specific
 However, full advantage from advanced simulation only if an effective
validation strategy is applied
 Uncertainty/sensitivity analysis plays a key role in this context
 Modern sensitivity tools are available in some fields (e.g. neutronics)
but more efforts needed
 Also, sound uncertainties (variance-covariance data) should be
developed (here again pioneer work in neutronics)
 New paradigms in experiment conceptual design.
1- V&V: Thermal-hydraulics and separate effects validation
Strategy to assess turbulent heat transfer in liquid metals (R. Stieglitz, KIT-Germany).
2- V&V: New smart experiments in neutronics
New “Smart” Validation Experiment: MANTRA
(Measurements of Actinide Neutronic Transmutation Rates with Accelerator Mass
• Samples of actinides irradiated at ATR (INL) and analyzed at ATLAS (ANL) with AMS (Accelerator Mass Spectrometry).
Samples irradiation in ATR
• With AMS extreme accuracy on actinides densities are achievable with only days of irradiation.
• Neutronic filters are used to modulate different types of spectra, e.g., fast, epithermal, and thermal.
• New information on capture cross sections of actinides will be obtained down to Bk, Cf.
3- Science-driven validation is a « grand challenge » in the field of
materials (and fuels):
 Empiricism is not practical, not possible for experiments to cover all materials
and exposure conditions required
 Development and application of sound, physically based models will help plan
and interpret the results of experiments:
- allow interpolation within and extrapolation beyond discrete data set to
complete domain of required exposure conditions
-provide information for conditions that are too costly or impossible to reach
(from “Radiation Effects in Fuel Cladding and Structural Alloys” by R. E. Stoller,
BES Workshop: Basic Research Needs for Advanced Nuclear Energy Systems,
Bethesda, July 31-August 2, 2006)
Nuclear energy development: if sustainability is a key
goal, it is essential to keep options open (i.e. evolution of
role of NE in the energy mix)
Nuclear system: need to consider reactor plus fuel
FR with close fuel cycle and MA management are a
most flexible long term option
Impact of different options should be carefully
evaluated: not only from a technical point of view (e.g. on
the fuel cycle), but also on societal issues (e.g.
acceptability etc)
Technological challenges suggest the need of strong
international cooperation
« Our simple, or simplified, illustrations carry
us but a little way, and only half prepare us
for much harder things. »
D’Arcy Thompson, On Growth and Form