International Conference on the Physics of Reactors “Nuclear Power: a sustainable Resource”




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International Conference on the Physics of Reactors “Nuclear Power: A Sustainable Resource”

Casino-Kursaal Conference Center, Interlaken, Switzerland, September 14-19, 2008

Minor actinide transmutation in ADS: the EFIT core design


C. Artiolia,*, X. Chenb , F. Gabriellib , G. Glinatsisa, P. Liub, W. Maschekb,

C. Petrovicha, A. Rineiskib , M. Sarottoa, M. Schikorrb


a ENEA, Via Martiri di Montesole 4, IT-40129 Bologna, Italy

b Forschungszentrum Karlsruhe (FZK), P.O. Box 3640, D-76021 Karlsruhe, Germany


Abstract

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 MWth, 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/TWhth) while maintaining a zero net balance of Pu and a negligible keff 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.


1.Introduction


The sustainability and the public acceptance of nuclear energy production can be improved by the minimization and reduction of nuclear waste. The Minor Actinides (MA) have a long-term radio-toxicity and one of the possible future strategies for transmuting them is represented by the use of Accelerator Driven Systems (ADS), which allow a higher MA content in the fuel. On the other side, the cost/benefit ratio of such innovative systems has to be evaluated and challenging coordinated R&D is necessary.

Within the 6th Framework Program, the European Community has funded, besides other projects supporting partitioning and transmutation, a conceptual design of an ADS (Domain DM1 of the Integrated Project EUROTRANS). This project is called EFIT (European Facility for Industrial Transmutation) and investigates the feasibility and the potentiality of such systems (Knebel, 2006). The design will be worked out to a level of detail which allows a cost study estimate. EFIT, of about 400 MWth, is loaded with MA and Pu in a CERCER U-free fuel. The core coolant, allowing a fast spectrum, is pure lead, as well as the windowless target for the 800 MeV proton beam. The reference sub-critical level has been postulated to be keff=0.97, figure that has to be confirmed by the full safety analysis (Rimpault, 2006).

This paper deals with the neutronic and thermal-hydraulic design of the EFIT core (Artioli et al., 2007a; Barbensi et al., 2007). The core has been conceived with the aims of: maximizing the fissions of MA, achieving a negligible keff swing during the cycle (to keep the proton current rather constant in order to avoid an oversizing of the target and of the accelerator), maximizing the average power density (i.e. the volume density of MA transmutation), while keeping low the coolant pressure drop.

For EFIT, as for any kind of reactors, the defence-in-depth concept has been applied. The demonstration of the adequacy of design with the safety objectives is structured along three kinds of basic conditions: The Design Basis Conditions (DBC–structured into 4 Categories), Design Extension Conditions (DEC–limiting events, complex sequences and severe accidents) and Residual Risk Situations. For the EFIT the safety principles and safety guidelines have been defined within EUROTRANS and a comprehensive and representative list of transients has been defined to test the safety behaviour of the reactor plant. For innovative reactors such as the EFIT ADT cliff-edge effects should be identified and excluded. For a safety classification fuel limits related to the different safety categories have been defined based on recent experimental evidence. Due to the existing uncertainties, fuel melting or disintegration should only be allowed in the DEC category. Important boundary conditions to be taken into account in the safety evaluation are the significant positive void worth, the missing of the Doppler prompt reactivity feedback, the very low delayed neutrons effective fraction (Artioli et al. 2007a) and the strong production of He via the transmutation process.

While coolant boiling processes can be excluded because of the high boiling point of lead coolant, pin failures could lead to a gas blow-down from the plena, to local voiding and reactivity addition. From the list of transients some representative ones, which are also traditionally investigated in fast reactor systems, have been chosen for the current paper, as the unprotected loss of flow (ULOF).
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