[en] The increasing availability of fissile material and the worldwide perception of the need to reduce or control such material make a new approach towards nuclear reactors necessary. The Nuclear Fission Division at ENEA is presently interested in evaluating various options for the burning of excess plutonium, among which the LWR reactors using rock-like inert matrix fuel in a once-through cycle and fast reactors with recycling. This paper deals with the latter option. A fast system, rich in neutrons, can produce plutonium by breeding, or it can burn it (together with Minor Actinides) with a high degree of effectiveness: this flexibility makes the fast system worth considering. In the framework of a co-operation project between GE-USA and ENEA, the PRISM MOD D oxide-fuelled reactor (small size, 840 MW) was studied as a burner. A complete set of studies was carried out covering several fields: neutronics, thermohydraulics, dynamics and safety. A negative void coefficient was achieved thanks to a particular arrangement of the fuel subassemblies (leaky core) and to the enrichment distribution. The central zone, loaded with shielding subassemblies, accounts for a negative void coefficient even in the inner zone. Moreover, this zone could become a suitable place for burning long-lived fission products. As a burner, using a standard fuel, the expected performance is a consumption of about 60-70 kg Pu/TWhe. ULOF and TOP analysis show a begin transient evolution. Only half of the GEMs are sufficient to close the ULOF transient; the reactor power reaches its maximum at 1.35 of the nominal figure during a TOP accident. A method of detecting a failure in a subassembly and identifying the subassembly concerned was developed. It was demonstrated that it is possible to create a sufficient number of tags for them to be recognised as different by the detector system, by using 'cocktails' of only four tag gases. The total gas required to overcome the uncertainties is less than 15 ncm³ of gas/pin. This method assumes that the detector system is unable to measure the absolute quantities, but only the mutual rations. By processing the results of the detector system with a simple code, the effectiveness of the method can be significantly increased. Moreover, experimental measures can be used to reduce the required quantity of gas. (author)

[en] Italian past activities on FRs have produced a significant amount of knowledge and know-how. With respect to the objectives of FRKP Initiative, the Italian position is to contribute to: => reduce the risk and mitigate the consequences of the loss of knowledge and know-how on past activities on FRs in Italy; => preserve and make accessible (as appropriate) the existing information on FRs within an agreed framework (e.g. the FRKPI)

[en] The implementation of safeguards at an early stage of the design of a given component of the fuel cycle will contribute significantly to the improvement of the proliferation resistance behaviour. The complete fuel cycle should be considered in the assessment of the proliferation resistance also during the design phase, although designs of nuclear reactors or fuel cycle elements are not complete and not all information and specifications on fuel cycle, installations/plants, location, etc are available. From the reactor designers point of view, the proliferation resistance, is not among the main technical parameters or constraints to be considered in the early conceptual design stage: in order to avoid intervening too late, after the optimisation interactions have frozen the neutron design, a clear and self-consistent set of proliferation resistance requirements should be given at a very early stage, as design constraints. This will allow to efficiently improve the proliferation resistance at reactor core design level. A possible approach to help the core designer to enhance the proliferation resistance characteristics will be discussed. (author).

[en] A specific aim of the EUROTRANS Integrated Project, partially financed in the European 6. Framework Program (by EU), is to demonstrate the conceptual feasibility of high rate Minor Actinides transmutation in an Accelerator Driven System. This concept reactor, called European Facility on Industrial scale Transmuter (EFIT), is fuelled by U-free (Pu, MA)O_2-x + MgO innovative Cer-Cer fuel type with high MA content and is cooled by lead. The design of the sub-critical core is based on the so called '42-0' transmutation rate approach, characterized by zero Pu net balance and maximum MA transmutation rate of 42 kg/TWh_th together with reduced BU reactivity loss (∼200 pcm/year). An essential requirement for the EFIT reactor is that it remains sub-critical in any plant condition. To avoid unwanted returns to criticality, the sub-criticality level and the reactivity coefficients should be known with high accuracy. In this paper, the sensitivity of the K_eff and of the reactivity coefficients from the nuclear data was studied in stochastic approach. Results of these analyses show the need of additional work for uncertainties reductions in order to obtain recommended nuclear data for the neutron core design. (authors)

[en] In the recent years a variety of small (≤ 300 MWe) and simpler reactors for electricity generation by nuclear power and for heat processing have been proposed. Integrated systems, compact configurations, long-lived cores, etc. under various technologies: LWR, FNR and graphite-moderated HTR, are developed. The better neutron economy of the FNRs allows the use of the energy potential of uranium, while their high power density facilitates the development of compact reactor. Compact cores allow a facile transportation and modularity, while long-lived compact cores allow also the improvement of the proliferation resistance behaviour. In the same time the long-lived cores size is conditioned firstly by the burn up reactivity loss and it can be also conditioned by the fuel burn up limit and the neutron damage limits for both fuel and cladding. In order to meet the goals of the long life core option, an appropriate approach to define the size of the core has been proposed. The impact on the criticality level and on the neutron performances for FNR with long life core is discussed. The neutron parameters dependency on the fuel irradiation cycle length has been also highlighted. The feasibility and the success of the long-lived cores are strongly dependent on the new materials availability and on the high accuracy of the nuclear data. (author).

[en] In the frame of the last EURATOM-FP7 Program, a large sized Sodium-cooled FR (SFR) has been studied. Mixed carbides fuel (U, Pu)C has been adopted for the backup core solution and important work has been also performed in order to obtain an “optimised” backup configuration “close” to the reference one, which is fuelled by mixed oxides fuel (U, Pu)Ox. The peculiarity of both core designs (the reference configuration and the optimised backup configuration) is the adoption of a 60 cm Plenum zone in the upper part of each fuel assembly (FA), that is filled by coolant, in order to mitigate (when emptied) the core positive coolant void effect. This paper presents some results of a detailed study of the coolant void effect for the above SFR with mixed carbides core. Many aspects, like geometric heterogeneity, the burn up state, the operating conditions, etc., have been taken into consideration in order to obtain information about the “propagation” and the behaviour of the coolant void effect itself. The performed study investigates also the coolant void effect consequences on some reactivity coefficients, which are important for a safe behaviour of the reactor. The investigation consisted in the steady state simulations of the reactor on different operating conditions by using the Monte Carlo approach. (author).

[en] In the frame of the last EURATOM-FP7 Program, a large sized Sodium-cooled FR (SFR) has been studied. Mixed carbides fuel (U, Pu)C has been adopted for the backup core solution and important work has been also performed in order to obtain an ''optimised'' backup configuration ''close'' to the reference one, which is fueled by mixed oxides fuel (U, Pu)Ox. The peculiarity of both core designs (the reference configuration and the optimised backup configuration) is the adoption of a 60 cm Plenum zone in the upper part of each fuel assembly (FA), that is filled by coolant, in order to mitigate (when emptied) the core positive coolant void effect. This paper presents some results of a detailed study of the coolant void effect for the above SFR with mixed carbides core. Many aspects, like geometric heterogeneity, the burnup state, the operating conditions, etc., have been taken into consideration in order to obtain information about the ''propagation'' and the behaviour of the coolant void effect itself. The performed study investigates also the coolant void effect consequences on some reactivity coefficients, which are important for a safe behaviour of the reactor. The investigation consisted in the steady state simulations of the reactor on different operating conditions in Monte Carlo approach. (authors)

[en] European R and D for Accelerator Driven System (ADS) design and fuel development in the 6th EC Framework Programme is driven by the Integrated Project EUROTRANS [1]. In EUROTRANS two ADS design routes are followed, the XT-ADS and the EFIT. The XT-ADS is designed to demonstrate the concept of ADS with a subcritical core combined with an accelerator. The longer-term EFIT development (European Facility for Industrial Transmutation) aims at a generic conceptual design of a full transmuter. This paper will concentrate on the EFIT core, which has a thermal power of about 400 MWth. The main goal of the EFIT design is to achieve an effective transmutation rate of the Minor Actinides (MAs) and respect important operational requirements as e.g. a low reactivity swing, a low power peaking, reasonable beam requirements and guarantee a high safety level. In order to have a sufficient subcriticality level, EFIT design is postulated to have a keff of 0.97. The risk of an accidental over-(the maximum) current does not exist in EFIT because, due to the very small burn-up reactivity swing, EFIT will work in the whole cycle with the maximum current allowed by the accelerator. A so-called 42-0 approach [2] is finally proposed by ENEA and adopted in the EFIT core design. With this 42-0 strategy, the EFIT core will be able to transmute about 42 kg/TWhth of MAs and keep a near zero net mass balance of Pu. The current EFIT core is loaded with a CERCER U-free fuel with MgO as the matrix. The 9Cr1MoVNb (T91) steel is used for the clad, which has a maximum temperature limitation of 550 C at the normal full power operation condition. Lead is used as the core coolant. It has an inlet temperature of 400 C and an outlet temperature of 480 C [3]. The temperature of 400 C at core inlet provides a margin to avoid lead freezing, and the temperature of only of 480 C at the core outlet offers many advantages in terms of reduced structure corrosion rates, improvement of the mechanical characteristics (making negligible creep of the structures), and reduces thermal shocks at transient conditions. Moreover at this 480 C nominal average core outlet temperature, the fuel clad temperature can be maintained below the limit of 550 C during the normal operation condition. Some core design data will be presented in the following section. For EFIT safety studies, the defence-in-depth concept has been applied [4]. The demonstration of the adequacy of the design with the safety objectives is structured along three basic conditions: (1) The Design Basis Conditions (DBC - structured into 4 Categories). The design of the plant results essentially from the analysis of these events. It must be shown that their consequences are very limited and, in any case, the risk of a whole core accident initialed by these events is very low. (2) Design Extension Conditions (DEC - limiting events, complex sequences and severe accidents) evaluated for licensing purposes independently of their occurrence frequency. The consequences of these accidents are analyzed and their consequences in the environment have to be demonstrated to be lower than the limiting release targets. (3) Residual Risk situation. The consequences of these situations are not analyzed since they are postulated to be unacceptable. The prevention measures regarding their occurrence have to be demonstrated to be sufficient. The safety principles and safety guidelines have been elaborated For EFIT within EUROTRANS and a comprehensive and representative list of transients has been established to test the safety behavior of the reactor plant. For innovative reactors such as EFIT, cliff-edge effects should be identified and excluded. For safety analyses, fuel parameter limits related to the different accidental categories have been determined on the basis of recent experimental evidences. Due to existing uncertainties, fuel melting or disintegration may only be allowed in the DEC category. In this paper, based on the current EFIT core design, a first transient analysis of the Unprotected Loss of Flow (ULOF) accident will be reported together with the steady state analysis performed by the SIMMER-III code [5, 6]. SIMMER-III is a two-dimensional, multi-velocity-field, multi-phase, multicomponent, Eulerian, fluid-dynamics code system coupled with a structure model including fuel-pins, hexcans etc., and a space-, time- and energy-dependent transport theory neutron dynamics model. The overall fluid-dynamics solution algorithm is based on a time-factorization approach, in which intra-cell interfacial area source terms, heat and mass transfers, and the momentum exchange functions are determined separately from inter-cell fluid convection. In addition, an analytical equation-of-state (EOS) model is available to close and complete the fluid-dynamics conservation equations. The code has originally been allocated in the severe accident domain of fast sodium cooled reactors. However, the philosophy behind the SIMMER development was to generate a versatile and flexible tool, applicable for the safety analysis of various reactor types with different neutron spectra and coolants, up to the new accelerator driven systems (ADS) for waste transmutation. (orig.)

[en] Accelerator-Driven-Systems represent one of the possible future strategies for transmuting minor actinides. EFIT, the conceptual industrial burner designed in EUROTRANS IP, is an ADS of about 400 MW_th, fuelled by MA and Pu in inert matrix, cooled by lead (673-753 K) and sustained by a 800 MeV proton of some 15 mA. It features the MA fission (42 kg/TWh_th) while maintaining a zero net balance of Pu and a negligible k_eff swing during the cycle. Three radial zones, differing in pin diameter or in inert matrix percentage have been defined in order to maximize the average power density together with the flattening of the assembly coolant outlet temperatures. Thermal-hydraulic analyses have been performed and show acceptable maximum temperatures: 1672 K peak fuel temperature (disintegration at 2150 K) and 812 K peak cladding temperature in nominal conditions (max 823 K). The behaviour of the core power, the temperature and the reactivity during the Unprotected Loss Of Flow transient (ULOF) has been studied as well by obtaining: a peak fuel temperature of 1860 K, a peak cladding temperature of 1030 K, a power increase of 2% removed by natural circulation. (authors)

[en] The Collaborative Project for a European Sodium Fast Reactor (CP ESFR) is performed (2009-2012) in the 7th European Framework Programme. It is devoted to the identification and study of innovations to be considered for the future in the core design, safety, reactor architecture, components and the dissemination of knowledge related to this technology among young European professionals. (author)