Luigi Mansani

THE EUROPEAN LEAD-COOLED EFIT PLANT: AN INDUSTRIAL-SCALE ACCELERATOR-DRIVEN SYSTEM FOR MINOR ACTINIDE TRANSMUTATION: I

In order to reduce the volume and the radiotoxicity of the nuclear waste coming from the operation of existing pressurized water reactors, accelerator-driven systems (ADSs) have been envisioned. The Lead-Cooled (Pb) European Facility for Industrial-Scale Transmutation (EFIT) (Pb-EFIT) plant is the first ADS design that has been going into a rather detailed engineering level. It is a lead-cooled, 385-MW(thermal) ADS prototype for minor actinide (MA) transmutation designed to achieve an optimal MA destruction rate of [approximately]42 kg/TW·h(thermal). The spallation target unit is located in the center of the diagrid where 800-MeV protons from the accelerator impinge on a free surface of lead exposed to vacuum. The core inlet temperature was set at 400°C to assure a sufficiently large safety margin to lead freezing, and the core outlet temperature was limited to 480°C to allow acceptable corrosion. The ferritic-martensitic 9% Cr steel T91 protected against corrosion with alumina FeCrAlY [GESA (Gepulste Elektronen Strahl Anlage) treatment]. The primary circuit is designed for effective natural circulation, i.e., relatively low pressure losses, and the design offers good protection for a heat removal system in case of a blackout accident. The EFIT plant is designed to have a low likelihood and a low degree of core damage, to eliminate the need for off-site emergency responses in case of a severe accident, to use an extensively reliable passive safety system to fulfill the safety functions, and to eliminate the need of alternating-current safety-grade power (no safety-grade diesel generator). Three systems contribute to the decay heat removal (DHR) function of Pb-EFIT: the steam generators, the direct reactor cooling system, and the isolation condenser system. The EFIT plant exhibits four primary pumps; eight steam generator units, each rated at 52 MW, provide heat removal under normal operation. On the secondary side, the water steam ensures a thermal efficiency of [approximately]40% with the superheated vapor secondary circuit, taking into account the electricity required by pumps (from both the primary circuit and the secondary circuits) but without deducing the power required for the accelerator. An estimate of the Pb-EFIT plant cost has been performed based mainly on experience and engineering judgment. A best estimate (base cost and contingency) of about €1890 million, with an overall uncertainty of 22%, has been found.

Design of a Decay Heat Removal System for EFIT Plant

The Integrated Project EUROTRANS, funded by the European Commission as part of the 6th European framework program, provides the advanced design of a multi purpose research oriented Accelerator Driven System (ADS), called XT-ADS (eXperimenTal-ADS), and the preliminary design of an industrial scale ADS, called EFIT (European Facility for Industrial Transmutation) [1]. EFIT [2] [3] is fuelled with Minor Actinides (U-free fuel) and its main design options overcome the limits of previous ADS projects, particularly as concerns the economic aspects, maintaining meanwhile the high safety level, the high reliability and low investment risks. EFIT is endowed with three different systems to remove decay heat, this paper deals with the Decay Heat Removal System 2 (DHR2). DHR2 mainly consists of an isolation condenser (IC) connected to the secondary side of the Steam Generators (SGs). DHR2 is a passive safety system: system operation is based on condensation of the steam coming from the SG unit and gravity head injection of the condensate back to the SG inlet. The system has been designed to satisfy the following two requirements: • to evacuate the decay power maintaining acceptable values of the vessel and fuel temperature; • to avoid lead solidification also in the configuration of maximum efficiency.Copyright

Preliminary Accident Analysis to Support a Passive Depressurization System Design

The new generation of evolutionary nuclear power plants, e.g., the Westinghouse AP600 and the General Electric simplified boiling water reactor, relies on a full reactor coolant system (RCS) depressurization to allow gravity injection from an in-containment tank and thereby assure long-term core cooling. Studies performed to support the licensing process and design of both evolutionary and innovative reactors have shown that cold water injection may, under particular plant conditions, induce a large plant depressurization. Preliminary studies have been performed to support the design of a passive injection and depressurization system (PIDS) based on the idea of depressurizing the RCS by mixing cold water with the RCS hot water and inducing steam condensation in the primary system. The analyses, performed with the RELAP5/MOD3 computer code, show the response of a typical midsize pressurized water reactor plant [two loops, 600 MW (electric)] equipped with the PIDS. Different RCS injection locations including pressurizer, vessel upper head, and hot leg, and actuation at different residual reactor coolant masses have been investigated. The PIDS performance has also been verified against the following reference severe accident scenarios: (a) complete station blackout event, and (b) a small-break loss-of-coolant accident and concomitant station blackout event.

Lead Fast Reactor Sustainability

The electricity production systems, especially those based on nuclear fission, are increasingly facing more tight constraints and are subjected to more deep analyses based on the three aspects of economical sustainability, environmental sustainability and social sustainability. Nuclear Reactors future development has been outlined in the framework of the GIF (Generation IV International Forum), where the Lead Fast Reactor (LFR) is placed among the most promising innovative solutions. Many aspects of LFR offer a huge improvement from different points of view. The non pressurization of the system and the absence of sources of hazardous chemical potential energy enhances consistently its safety aspects, improving the perception of inherent safety of the Generation IV (G4) reactors in the public opinion. At the moment, due to the abundance of the new fossil resources, the competitiveness of Nuclear Power Plants is severely challenged, this aspect representing the most difficult to manage, besides the public acceptability. Moreover, for G4 reactors, an additional “cost premium” associated with the innovative technological concept has to be taken into account. Conversely, looking at the mid-term future, the real economical comparison has to be performed considering as competing sources, according to the IPCC recommendations and constraints enacted by the European Community, only CO2 free sources. In this context, economical competitiveness could be regained depending on the “cost premium” to be added to fossil fuels to become CO2 free, through the improvement of the carbon separation and storage techniques. The intrinsic lead properties (e.g.: low absorption cross section) permit to easily design LFR flexible cores, optimized with respect to a number of possible goals, as a long-lived core with minimal reactivity swing intended for battery concepts, or what is called an “adiabatic” core, where the entire Pu and MA inventory in the spent fuel can be indefinitely reused in a closed fuel cycle. The latter option allows to limit the waste throughput to the fission products only (along with the — unavoidable — losses from fuel reprocessing), and to benefit of natural resources minimization. These are both specific Generation IV goals envisioned to reach nuclear energy sustainability. An overall fuel cycle balance in a scenario with a step by step introduction of LFR reactors fleet grown in a specific geographical area, is in details analyzed in [1] and presented in this conference.Copyright

A Case History of CFD Support to Accelerator Driven System Plant Design

The Integrated Project EUROTRANS, funded by the European Commission in the VI European framework program, was aimed at providing the advanced design of a multi purpose research oriented Accelerator Driven System (ADS), called eXperimenTal-ADS (XT-ADS), and the preliminary design of an industrial scale ADS, called European Facility for Industrial Transmutation (EFIT). One contribution of CRS4 (Centro di Ricerca, Sviluppo e Studi Superiori in Sardegna) has been to provide support to the overall plant design by means of Computational Fluid Dynamics (CFD) simulations. The simulations were required by the designer either for basic checking or in case of doubts on the validity of some technical options. We present four series of simulations which lead to the detection of unsatisfactory plant behaviour, related design modification and eventually control of the variant behaviour correctness. The first three simulation series deal with the EFIT design while the forth one deals with the XT-ADS design. In the first case, the simulation put in evidence a large recirculation zone under the reactive core that had to be removed for oxygen control concern. The recirculation zone is suppressed by modifying the shape of the core support grid. In the second case, we put in evidence a recirculation zone at the entrance of the pumping system above the core. This recirculation zone can lower the pump efficiency. The entrance shape was modified to eliminate the recirculation zone. In the third case, we check the behaviour of the passive Decay Heat Removal (DHR) heat exchanger. We show that while the primary coolant flow is globally organized as expected, some flow mixing limits the efficiency of the system. The system efficiency is restored by increasing its passive pumping strength. This is performed simply extending the Heat Exchanger shroud a half-meter in the bottom direction. In the last case, we investigate the capability of an external DHR system to withstand a long complete plant shutdown. The simulation encompasses about 6 hours of physical time, enough to understand the critical trends and infers that the DHR system may be not sufficient for its purpose. This result has suggested some modification to the design (i.e. surface treatment to improve metal wall emissivity) as well as to the accident management (i.e. restart primary pumps to eliminate fluid stratification). All these design improvements have been obtained in a reasonable amount of time thanks to the continuous collaboration and exchange of information between the CFD engineer and the designer.© 2009 ASME