Eleonora Bomboni - Academia.edu (original) (raw)

Papers by Eleonora Bomboni

This paper presents a critical review of the recent improvements in minimizing nuclear waste in t... more This paper presents a critical review of the recent improvements in minimizing nuclear waste in terms of quantities, long-term activities, and radiotoxicities by innovative GCRs, with particular emphasis to the results obtained at the University of Pisa. Regarding these last items, in the frame of some EU projects (GCFR, PUMA, and RAPHAEL), we analyzed symbiotic fuel cycles coupling current LWRs with HTRs, finally closing the cycle by GCFRs. Particularly, we analyzed fertile-free and Pu-Th-based fuel in HTR: we improved plutonium exploitation also by optimizing Pu/Th ratios in the fuel loaded in an HTR. Then, we chose GCFRs to burn residual MA. We have started the calculations on simplified models, but we ended them using more "realistic" models of the reactors. In addition, we have added the GCFR multiple recycling option using k eff calculations for all the reactors. As a conclusion, we can state that, coupling HTR with GCFR, the geological disposal issues concerning high-level radiotoxicity of MA can be considerably reduced.

ABSTRACT Preliminary analyses already performed showed that innovative GCRs, both thermal and fas... more ABSTRACT Preliminary analyses already performed showed that innovative GCRs, both thermal and fast, are very promising candidate to reach the Gen-IV sustainability goal. The integrated LWR-HTR-GCFR basically aims at closing the current nuclear fuel cycle: in principle, thanks to the unique characteristics of Helium coolant reactors, LWR SNF along with DU become valuable material to produce energy. Additionally, burning HMs of LWR SNF means not only a drastic reduction in the Unat demand but also a remarkable decrease in the long-term radiotoxic component of nuclear waste to be geologically stored. This paper focuses on the analyses of the LWR-HTR-GCFR cycle performed by the University of Pisa in the frame of the EU PUMA project (6th FP). Starting from a brief outline of the main characteristics of HTR and GCFR concepts and of the advantages of linking LWR, HTR and GCFR in a symbiotic way, this paper shows the integrated cycle involving a typical LWR (1000 MWe), a PBMR (400 MWth) and a GCFR-“E” (2400 MWth). Additionally, a brief overview of the main technological constraints concerning (Pu+MA)-based advanced fuels is given, in order to explain and justify the choices made in the framework of the considered cycle. Thereafter, calculations performed and results obtained are described.

Problems about future energy availability, climate changes, and air quality seem to play an impor... more Problems about future energy availability, climate changes, and air quality seem to play an important role in energy production. While current reactor generations provide a guaranteed and economical energy production, new nuclear power plant generation would increase the ways and purposes in which nuclear energy can be used. To explore these new technological applications, several governments, industries, and research communities decided to contribute to the next reactor generation, called "Generation IV." Among the six Gen-IV reactor designs, the Gas Cooled Fast Reactor (GCFR) uses a direct-cycle helium turbine for electricity generation and for a CO 2 -free thermochemical production of hydrogen. Additionally, the use of a fast spectrum allows actinides transmutation, minimizing the production of long-lived radioactive waste in an integrated fuel cycle. This paper presents an analysis of GCFR fuel cycle optimization and of a thermal-hydraulic of a GCFR-prototype under steady-state and transient conditions. The fuel cycle optimization was performed to assess the capability of the GCFR to transmute MAs, while the thermal-hydraulic analysis was performed to investigate the reactor and the safety systems behavior during a LOFA. Preliminary results show that limited quantities of MA are not affecting significantly the thermal-fluid-dynamics behavior of a GCFR core.

Science and Technology of Nuclear Installations, 2009

Nowadays nuclear is the only greenhouse-free source that can appreciably respond to the increasin... more Nowadays nuclear is the only greenhouse-free source that can appreciably respond to the increasing worldwide energy demand. The use of Thorium in the nuclear energy production may offer some advantages to accomplish this task. Extensive R&D on the thorium fuel cycle has been conducted in many countries around the world. Starting from the current nuclear waste policy, the EU-PUMA project focuses on the potential benefits of using the HTR core as a Pu/MA transmuter. In this paper the following aspects have been analysed: (1) the state-of-the-art of the studies on the use of Th in different reactors, (2) the use of Th in HTRs, with a particular emphasis on Th-Pu fuel cycles, (3) an original assessment of Th-Pu fuel cycles in HTR. Some aspects related to Thorium exploitation were outlined, particularly its suitability for working in pebble-bed HTR in a Th-Pu fuel cycle. The influence of the Th/Pu weight fraction at BOC in a typical HTR pebble was analysed as far as the reactivity trend versus burn-up, the energy produced per Pu mass, and the Pu isotopic composition at EOC are concerned. Although deeper investigations need to be performed in order to draw final conclusions, it is possible to state that some optimized Th percentage in the initial Pu/Th fuel could be suggested on the basis of the aim we are trying to reach.

The Generation IV Initiative aims to realizing the complete sustainability of nuclear power. At t... more The Generation IV Initiative aims to realizing the complete sustainability of nuclear power. At the moment, long-lived radioisotopes in nuclear waste are one of the most challenging problems to solve. Additionally, the uranium resources are badly exploited, because the current fuel cycle uses only 1% of the mineral resources. Closing the nuclear fuel cycle would mean both exploiting better the uranium resources and reducing drastically the long-term radiotoxicity of the final waste. Particularly, actinides are recoverable material to produce energy by fission and, on the other hand, they are very toxic and radiotoxic elements for hundreds thousand years. Common LWRs tend to increase the global amount of plutonium, neptunium, americium and curium. Indeed, their fuel has a very short metallurgical burnup (less than 50 GWD/tU) and that implies what follows:

Nuclear science and engineering: the journal of the American Nuclear Society

The pebble bed gas-cooled reactor is one of the most promising concepts among the Generation III�... more The pebble bed gas-cooled reactor is one of the most promising concepts among the Generation III� and Generation IV reactors. Currently, the pebble bed modular reactor (PBMR) design, both U and Pu and minor actinides fueled, is being developed. Modeling the arrangement of coated particles (CPs) inside a spherical region like a pebble seems to be an important issue in the frame of calculations. To use the (relatively) old Monte Carlo codes without any correction, some approximations are often introduced. Recent Monte Carlo codes like MCNP5 and some new original subroutines that we have developed allow the possibility of obtaining more detailed and more physically correct geometrical descriptions of this kind of system. Some studies on modeling pebbles and pebble bed cores have already been carried out by other researchers, but these works are substantially limited to AVR-type UO2-fueled pebbles. However, the impact of approximated models on fuel mass, reactivity, and reactor life pre...

Nuclear Engineering and Design, 2010

The double-heterogeneity characterising pebble-bed high temperature reactors (HTRs) makes Monte C... more The double-heterogeneity characterising pebble-bed high temperature reactors (HTRs) makes Monte Carlo based calculation tools the most suitable for detailed core analyses. These codes can be successfully used to predict the isotopic evolution during irradiation of the fuel of this kind of cores. At the moment, there are many computational systems based on MCNP that are available for performing depletion calculation. All these systems use MCNP to supply problem dependent fluxes and/or microscopic cross sections to the depletion module. This latter then calculates the isotopic evolution of the fuel resolving Bateman's equations.

Fourth International Topical Meeting on High Temperature Reactor Technology, Volume 2, 2008

ABSTRACT The HTR pebble fuel experiment HFR EU1bis was irradiated in the High Flux Reactor, Pette... more ABSTRACT The HTR pebble fuel experiment HFR EU1bis was irradiated in the High Flux Reactor, Petten, The Netherlands, in 2004 and 2005. It consisted of five fuel pebbles from the German HTR program (GLE4 type, UO2 fuel, 16.75% enrichment) and six minisamples (UO2 fuel, 9.75% enrichment). Its instrumentation included three flux monitor sets. The experiment was loaded in a REFA-170 rig, surrounded by a strongly moderating filler element. The central fuel temperature was held at 1250oC during the irradiation. In the framework of the European RAPHAEL project, Post Irradiation Examination (PIE) has been done at NRG in Petten, The Netherlands and at JRC ITU in Karlsruhe, Germany. In Petten, flux monitor analysis has been done, whereas in Karlsruhe, a quantitative evaluation of γ-emitters was used to make a burn-up determination. A benchmark description based on this experiment has been written by NRG. Until now, five RAPHAEL project participants have modeled the experiment, each with their own neutronics code system. Participating codes are three versions of MONTEBURNS (MCNP with ORIGEN), MURE/MCNP and OCTOPUS (MCNP with FISPACT). The pebble burnup and isotopic inventories (Bq/gram initial HM) of selected fission products and actinides in the fuel pebble samples are both calculated and determined by gamma spectrometry, mass spectrometry and ion chromatography by JRC-ITU. Additionally, two participants calculated the flux monitor activities that were measured by NRG. A burnup measurement of 11.0 %FIMA by gamma spectrometry could be confirmed by calculation. Differences between the various modeling approaches and the experimental burn-up determination will be discussed.

Annals of Nuclear Energy, 2012

ABSTRACT This paper aims at comparing some simplified models to simulate irradiation cycles of Pu... more ABSTRACT This paper aims at comparing some simplified models to simulate irradiation cycles of Pu fuelled pebble bed reactors with Monteburns2.0� code. As a reference core, the PBMR-400 (proposed in the framework of the EU PUMA project, where this kind of core fuelled by a Pu and Pu–Np fuel has been studied) was taken into account. Pebble-bed High Temperature Reactor (HTR) cores consist of hundreds of thousands pebbles arranged stochastically in a cylindrical or annular space and each pebble is a single fuel element, and it is able to reach ultra-high burn-ups, i.e. up to 750 GWd/tHM (for Pu-based fuels). Additionally, pebble-bed cores are characterised by a continuous recirculation of pebbles from the top to the bottom of the core. Modelling accurately with current computer codes such an arrangement, in order to predict the behaviour of the core itself, is a very difficult task and any depletion code specifically devoted to pebble-bed burn-up calculation is not available at the moment. Because of limitations of the most common current MCNP-based depletion codes as well as huge calculation times, simplified models have to be implemented. After an analysis of the literature available on pebble-bed models for criticality and burn-up calculations, a preliminary assessment of the impact of different kind of simplified models for a Pu-Np fuelled Pebble-Bed Modular Reactor (PBMR), proposed in the framework of the EU PUMA project, is shown, particularly as far as burn-up prediction with Monteburns2.0� code is concerned.

ABSTRACT The PUMA project, a Specific Targeted Research Project (STREP) of the European Union EUR... more ABSTRACT The PUMA project, a Specific Targeted Research Project (STREP) of the European Union EURATOM 6th Framework Program, is mainly aimed at providing additional key elements for the utilisation and transmutation of plutonium and minor actinides (neptunium and Americium) in contemporary and future (high temperature) gas-cooled reactor design, which are promising tools for improving the sustainability of the nuclear fuel cycle. PUMA would also contribute to the reduction of Pu and MA stockpiles and to the development of safe and sustainable reactors for CO2-free energy generation. The project runs from September 1, 2006 until August 31, 2009. PUMA also contributes to technological goals of the Generation IV International Forum. It contributes to developing and maintaining the competence in reactor technology in the EU and addresses European stakeholders on key issues for the future of nuclear energy in the EU. An overview is presented of the status of the project at mid-term.

The long-term radiotoxicity of the final waste is currently one of the main drawback of nuclear p... more The long-term radiotoxicity of the final waste is currently one of the main drawback of nuclear power. Indeed, isotopes of Neptunium and Plutonium along with some long-lived fission products are dangerous for more than 100000 years. Actually, 96% of the spent Light Water Reactor (LWR) fuel consists of actinides, hence it is able to produce a lot of energy by fission if recycled. The effective exploitation of Uranium resources is intrinsically connected with an effective actinides burning. At the moment, it is clear that these goals can be achieved only by combining different concepts of nuclear cores in a "symbiotic" way, as suggested in the frame of the Generation IV Initiative. Light-Water Reactor -(Very) High Temperature Reactor ((V)HTR) -Gas Cooled Fast Reactor (GCFR) symbiotic cycles have good capabilities as far as the integral actinide exploitation is concerned. Particularly, HTR fuelled by Plutonium oxide is able to reach an ultra-high burn-up and to burn Neptunium and Plutonium effectively. In contrast, not negligible amounts of Americium and Curium build up in this core, although the total mass of Heavy Metals (HM) is strongly reduced. Americium and Curium are characterized by an high radiological hazard as well. Nevertheless, at least Plutonium from HTR (which is rich in nonfissile nuclides), Neptunium and, if appropriate, Americium can be used as a "driver" fuel for the GCFR along with large amounts of Depleted Uranium (DU): that is feasible with this kind of core thanks to its very good neutron economy. This paper focuses on the potentialities of the LWR-HTR-GCFR fuel cycle, highlighting also the challenges (both from the technological and neutronic points of view) to face with while realizing such a cycle. On the basis of the main technological constraints three possible (original) LWR-HTR-GCFR are proposed and their capabilities in actinides burning are assessed as well. Finally, some hints about designing an Am-Cm dedicated assembly are supplied.

The paper deals with the use of a symbiotic cycle in order to minimize the LWR waste radiotoxicit... more The paper deals with the use of a symbiotic cycle in order to minimize the LWR waste radiotoxicity, improving, on the same line, previous work . The obtained results could be considered rather positive. We will show what is possible to do in this field using new original symbiotic cycles, remarkably improving the previous results. To reach this goal, we investigated innovative fuel cycles by using the gas cooled reactors (both thermal and fast). Their very favourable neutronic economy, supported by an appropriated spectrum, allows to transmute/fission actinides, in particular transuranic ones. In this frame, we developed a strategy based on an original symbiotic fuel cycle. We assume to begin using, as normal, enriched uranium in LWRs. The second step deals with burning all the actinides recovered from LWRs spent fuel in HTRs. One of the major innovative results after this irradiation consists in the strong reduction of the neptunium which represents one of the greatest concerns in long term disposal. The last one consists in adding, as fuel in GCFRs, depleted uranium together with all the residual actinides of HTR spent fuel. As final result we obtain a reduction of the Level Of Mine Balancing Time (LOMBT) from 250000[11] (LWR once through) to about 200 years (proposed symbiotic cycle). This research has to be considered in progress and needs of further confirmation mainly by technological point of view.

This paper presents a critical review of the recent improvements in minimizing nuclear waste in t... more This paper presents a critical review of the recent improvements in minimizing nuclear waste in terms of quantities, long-term activities, and radiotoxicities by innovative GCRs, with particular emphasis to the results obtained at the University of Pisa. Regarding these last items, in the frame of some EU projects (GCFR, PUMA, and RAPHAEL), we analyzed symbiotic fuel cycles coupling current LWRs with HTRs, finally closing the cycle by GCFRs. Particularly, we analyzed fertile-free and Pu-Th-based fuel in HTR: we improved plutonium exploitation also by optimizing Pu/Th ratios in the fuel loaded in an HTR. Then, we chose GCFRs to burn residual MA. We have started the calculations on simplified models, but we ended them using more "realistic" models of the reactors. In addition, we have added the GCFR multiple recycling option using k eff calculations for all the reactors. As a conclusion, we can state that, coupling HTR with GCFR, the geological disposal issues concerning high-level radiotoxicity of MA can be considerably reduced.

ABSTRACT Preliminary analyses already performed showed that innovative GCRs, both thermal and fas... more ABSTRACT Preliminary analyses already performed showed that innovative GCRs, both thermal and fast, are very promising candidate to reach the Gen-IV sustainability goal. The integrated LWR-HTR-GCFR basically aims at closing the current nuclear fuel cycle: in principle, thanks to the unique characteristics of Helium coolant reactors, LWR SNF along with DU become valuable material to produce energy. Additionally, burning HMs of LWR SNF means not only a drastic reduction in the Unat demand but also a remarkable decrease in the long-term radiotoxic component of nuclear waste to be geologically stored. This paper focuses on the analyses of the LWR-HTR-GCFR cycle performed by the University of Pisa in the frame of the EU PUMA project (6th FP). Starting from a brief outline of the main characteristics of HTR and GCFR concepts and of the advantages of linking LWR, HTR and GCFR in a symbiotic way, this paper shows the integrated cycle involving a typical LWR (1000 MWe), a PBMR (400 MWth) and a GCFR-“E” (2400 MWth). Additionally, a brief overview of the main technological constraints concerning (Pu+MA)-based advanced fuels is given, in order to explain and justify the choices made in the framework of the considered cycle. Thereafter, calculations performed and results obtained are described.

Problems about future energy availability, climate changes, and air quality seem to play an impor... more Problems about future energy availability, climate changes, and air quality seem to play an important role in energy production. While current reactor generations provide a guaranteed and economical energy production, new nuclear power plant generation would increase the ways and purposes in which nuclear energy can be used. To explore these new technological applications, several governments, industries, and research communities decided to contribute to the next reactor generation, called "Generation IV." Among the six Gen-IV reactor designs, the Gas Cooled Fast Reactor (GCFR) uses a direct-cycle helium turbine for electricity generation and for a CO 2 -free thermochemical production of hydrogen. Additionally, the use of a fast spectrum allows actinides transmutation, minimizing the production of long-lived radioactive waste in an integrated fuel cycle. This paper presents an analysis of GCFR fuel cycle optimization and of a thermal-hydraulic of a GCFR-prototype under steady-state and transient conditions. The fuel cycle optimization was performed to assess the capability of the GCFR to transmute MAs, while the thermal-hydraulic analysis was performed to investigate the reactor and the safety systems behavior during a LOFA. Preliminary results show that limited quantities of MA are not affecting significantly the thermal-fluid-dynamics behavior of a GCFR core.

Science and Technology of Nuclear Installations, 2009

Nowadays nuclear is the only greenhouse-free source that can appreciably respond to the increasin... more Nowadays nuclear is the only greenhouse-free source that can appreciably respond to the increasing worldwide energy demand. The use of Thorium in the nuclear energy production may offer some advantages to accomplish this task. Extensive R&D on the thorium fuel cycle has been conducted in many countries around the world. Starting from the current nuclear waste policy, the EU-PUMA project focuses on the potential benefits of using the HTR core as a Pu/MA transmuter. In this paper the following aspects have been analysed: (1) the state-of-the-art of the studies on the use of Th in different reactors, (2) the use of Th in HTRs, with a particular emphasis on Th-Pu fuel cycles, (3) an original assessment of Th-Pu fuel cycles in HTR. Some aspects related to Thorium exploitation were outlined, particularly its suitability for working in pebble-bed HTR in a Th-Pu fuel cycle. The influence of the Th/Pu weight fraction at BOC in a typical HTR pebble was analysed as far as the reactivity trend versus burn-up, the energy produced per Pu mass, and the Pu isotopic composition at EOC are concerned. Although deeper investigations need to be performed in order to draw final conclusions, it is possible to state that some optimized Th percentage in the initial Pu/Th fuel could be suggested on the basis of the aim we are trying to reach.

The Generation IV Initiative aims to realizing the complete sustainability of nuclear power. At t... more The Generation IV Initiative aims to realizing the complete sustainability of nuclear power. At the moment, long-lived radioisotopes in nuclear waste are one of the most challenging problems to solve. Additionally, the uranium resources are badly exploited, because the current fuel cycle uses only 1% of the mineral resources. Closing the nuclear fuel cycle would mean both exploiting better the uranium resources and reducing drastically the long-term radiotoxicity of the final waste. Particularly, actinides are recoverable material to produce energy by fission and, on the other hand, they are very toxic and radiotoxic elements for hundreds thousand years. Common LWRs tend to increase the global amount of plutonium, neptunium, americium and curium. Indeed, their fuel has a very short metallurgical burnup (less than 50 GWD/tU) and that implies what follows:

Nuclear science and engineering: the journal of the American Nuclear Society

The pebble bed gas-cooled reactor is one of the most promising concepts among the Generation III�... more The pebble bed gas-cooled reactor is one of the most promising concepts among the Generation III� and Generation IV reactors. Currently, the pebble bed modular reactor (PBMR) design, both U and Pu and minor actinides fueled, is being developed. Modeling the arrangement of coated particles (CPs) inside a spherical region like a pebble seems to be an important issue in the frame of calculations. To use the (relatively) old Monte Carlo codes without any correction, some approximations are often introduced. Recent Monte Carlo codes like MCNP5 and some new original subroutines that we have developed allow the possibility of obtaining more detailed and more physically correct geometrical descriptions of this kind of system. Some studies on modeling pebbles and pebble bed cores have already been carried out by other researchers, but these works are substantially limited to AVR-type UO2-fueled pebbles. However, the impact of approximated models on fuel mass, reactivity, and reactor life pre...

Nuclear Engineering and Design, 2010

The double-heterogeneity characterising pebble-bed high temperature reactors (HTRs) makes Monte C... more The double-heterogeneity characterising pebble-bed high temperature reactors (HTRs) makes Monte Carlo based calculation tools the most suitable for detailed core analyses. These codes can be successfully used to predict the isotopic evolution during irradiation of the fuel of this kind of cores. At the moment, there are many computational systems based on MCNP that are available for performing depletion calculation. All these systems use MCNP to supply problem dependent fluxes and/or microscopic cross sections to the depletion module. This latter then calculates the isotopic evolution of the fuel resolving Bateman's equations.

Fourth International Topical Meeting on High Temperature Reactor Technology, Volume 2, 2008

ABSTRACT The HTR pebble fuel experiment HFR EU1bis was irradiated in the High Flux Reactor, Pette... more ABSTRACT The HTR pebble fuel experiment HFR EU1bis was irradiated in the High Flux Reactor, Petten, The Netherlands, in 2004 and 2005. It consisted of five fuel pebbles from the German HTR program (GLE4 type, UO2 fuel, 16.75% enrichment) and six minisamples (UO2 fuel, 9.75% enrichment). Its instrumentation included three flux monitor sets. The experiment was loaded in a REFA-170 rig, surrounded by a strongly moderating filler element. The central fuel temperature was held at 1250oC during the irradiation. In the framework of the European RAPHAEL project, Post Irradiation Examination (PIE) has been done at NRG in Petten, The Netherlands and at JRC ITU in Karlsruhe, Germany. In Petten, flux monitor analysis has been done, whereas in Karlsruhe, a quantitative evaluation of γ-emitters was used to make a burn-up determination. A benchmark description based on this experiment has been written by NRG. Until now, five RAPHAEL project participants have modeled the experiment, each with their own neutronics code system. Participating codes are three versions of MONTEBURNS (MCNP with ORIGEN), MURE/MCNP and OCTOPUS (MCNP with FISPACT). The pebble burnup and isotopic inventories (Bq/gram initial HM) of selected fission products and actinides in the fuel pebble samples are both calculated and determined by gamma spectrometry, mass spectrometry and ion chromatography by JRC-ITU. Additionally, two participants calculated the flux monitor activities that were measured by NRG. A burnup measurement of 11.0 %FIMA by gamma spectrometry could be confirmed by calculation. Differences between the various modeling approaches and the experimental burn-up determination will be discussed.

Annals of Nuclear Energy, 2012

ABSTRACT This paper aims at comparing some simplified models to simulate irradiation cycles of Pu... more ABSTRACT This paper aims at comparing some simplified models to simulate irradiation cycles of Pu fuelled pebble bed reactors with Monteburns2.0� code. As a reference core, the PBMR-400 (proposed in the framework of the EU PUMA project, where this kind of core fuelled by a Pu and Pu–Np fuel has been studied) was taken into account. Pebble-bed High Temperature Reactor (HTR) cores consist of hundreds of thousands pebbles arranged stochastically in a cylindrical or annular space and each pebble is a single fuel element, and it is able to reach ultra-high burn-ups, i.e. up to 750 GWd/tHM (for Pu-based fuels). Additionally, pebble-bed cores are characterised by a continuous recirculation of pebbles from the top to the bottom of the core. Modelling accurately with current computer codes such an arrangement, in order to predict the behaviour of the core itself, is a very difficult task and any depletion code specifically devoted to pebble-bed burn-up calculation is not available at the moment. Because of limitations of the most common current MCNP-based depletion codes as well as huge calculation times, simplified models have to be implemented. After an analysis of the literature available on pebble-bed models for criticality and burn-up calculations, a preliminary assessment of the impact of different kind of simplified models for a Pu-Np fuelled Pebble-Bed Modular Reactor (PBMR), proposed in the framework of the EU PUMA project, is shown, particularly as far as burn-up prediction with Monteburns2.0� code is concerned.

ABSTRACT The PUMA project, a Specific Targeted Research Project (STREP) of the European Union EUR... more ABSTRACT The PUMA project, a Specific Targeted Research Project (STREP) of the European Union EURATOM 6th Framework Program, is mainly aimed at providing additional key elements for the utilisation and transmutation of plutonium and minor actinides (neptunium and Americium) in contemporary and future (high temperature) gas-cooled reactor design, which are promising tools for improving the sustainability of the nuclear fuel cycle. PUMA would also contribute to the reduction of Pu and MA stockpiles and to the development of safe and sustainable reactors for CO2-free energy generation. The project runs from September 1, 2006 until August 31, 2009. PUMA also contributes to technological goals of the Generation IV International Forum. It contributes to developing and maintaining the competence in reactor technology in the EU and addresses European stakeholders on key issues for the future of nuclear energy in the EU. An overview is presented of the status of the project at mid-term.

The long-term radiotoxicity of the final waste is currently one of the main drawback of nuclear p... more The long-term radiotoxicity of the final waste is currently one of the main drawback of nuclear power. Indeed, isotopes of Neptunium and Plutonium along with some long-lived fission products are dangerous for more than 100000 years. Actually, 96% of the spent Light Water Reactor (LWR) fuel consists of actinides, hence it is able to produce a lot of energy by fission if recycled. The effective exploitation of Uranium resources is intrinsically connected with an effective actinides burning. At the moment, it is clear that these goals can be achieved only by combining different concepts of nuclear cores in a "symbiotic" way, as suggested in the frame of the Generation IV Initiative. Light-Water Reactor -(Very) High Temperature Reactor ((V)HTR) -Gas Cooled Fast Reactor (GCFR) symbiotic cycles have good capabilities as far as the integral actinide exploitation is concerned. Particularly, HTR fuelled by Plutonium oxide is able to reach an ultra-high burn-up and to burn Neptunium and Plutonium effectively. In contrast, not negligible amounts of Americium and Curium build up in this core, although the total mass of Heavy Metals (HM) is strongly reduced. Americium and Curium are characterized by an high radiological hazard as well. Nevertheless, at least Plutonium from HTR (which is rich in nonfissile nuclides), Neptunium and, if appropriate, Americium can be used as a "driver" fuel for the GCFR along with large amounts of Depleted Uranium (DU): that is feasible with this kind of core thanks to its very good neutron economy. This paper focuses on the potentialities of the LWR-HTR-GCFR fuel cycle, highlighting also the challenges (both from the technological and neutronic points of view) to face with while realizing such a cycle. On the basis of the main technological constraints three possible (original) LWR-HTR-GCFR are proposed and their capabilities in actinides burning are assessed as well. Finally, some hints about designing an Am-Cm dedicated assembly are supplied.

The paper deals with the use of a symbiotic cycle in order to minimize the LWR waste radiotoxicit... more The paper deals with the use of a symbiotic cycle in order to minimize the LWR waste radiotoxicity, improving, on the same line, previous work . The obtained results could be considered rather positive. We will show what is possible to do in this field using new original symbiotic cycles, remarkably improving the previous results. To reach this goal, we investigated innovative fuel cycles by using the gas cooled reactors (both thermal and fast). Their very favourable neutronic economy, supported by an appropriated spectrum, allows to transmute/fission actinides, in particular transuranic ones. In this frame, we developed a strategy based on an original symbiotic fuel cycle. We assume to begin using, as normal, enriched uranium in LWRs. The second step deals with burning all the actinides recovered from LWRs spent fuel in HTRs. One of the major innovative results after this irradiation consists in the strong reduction of the neptunium which represents one of the greatest concerns in long term disposal. The last one consists in adding, as fuel in GCFRs, depleted uranium together with all the residual actinides of HTR spent fuel. As final result we obtain a reduction of the Level Of Mine Balancing Time (LOMBT) from 250000[11] (LWR once through) to about 200 years (proposed symbiotic cycle). This research has to be considered in progress and needs of further confirmation mainly by technological point of view.