Nureth-15 paper

The 15th International Topical Meeting on Nuclear Reactor Thermal - Hydraulics, NURETH-15 Pisa, Italy, May 12-17, 2013

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THE ADVANCED LEAD FAST REACTOR EUROPEAN DEMONSTRATOR (ALFRED) M. Frogheri, A. Alemberti and L. Mansani Ansaldo Nucleare, Corso Perrone 25, 16152 Genova, Italy [email protected], [email protected], [email protected]

ABSTRACT The Generation IV International Forum (GIF) member countries, identified the six most promising advanced reactor systems and related fuel cycle as well as the R&D needed to establish the feasibility and performance capabilities of the next generation nuclear energy systems, known as Generation IV. Among the promising reactor technologies for fast reactors (Sodium and Lead Fast Reactors) being considered by the GIF, the LFR has been identified as a technology with great potential to meet the goals of increased safety, improved economics for electricity production, reduced nuclear wastes for disposal and increased proliferation resistance. Ansaldo Nucleare, with its past experience on fast reactors, is promoting research and development of a pure Lead cooled fast reactor, as coordinator of the LEADER project (Lead-cooled European Advanced DEmonstration Reactor), funded by the European Commission in the frame of the seventh framework program. The LEADER project aims to the development of an industrial size plant to a conceptual level (ELFR) and of a scaled demonstrator of the LFR technology, called ALFRED (Advanced Lead Fast Reactor European Demonstrator). To this aim, the issues and the safety concerns emerged during previous Lead cooled reactor projects, have been analyzed and a set of design options and safety provisions proposed. The paper presents, after a summary of the LEADER project, the main design features of ALFRED facility. In particular, since Ansaldo Nucleare is in charge of the design of the main components/systems, relevance is given to the description of the Reactor Vessel, the Steam Generators, the Primary Pumps and the Decay Heat Removal System design. 1.

INTRODUCTION

Concerns over energy resource availability, climate change, air quality, and energy security suggest an important role for nuclear power in future energy supplies. While the current Generation II and III nuclear power plant designs provide an economically and publicly acceptable electricity supply in many markets, further advances in nuclear energy system design


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can broaden the opportunities for the use of nuclear energy. To explore these opportunities, worldwide governments, industries, and research centres started a wide-ranging discussion on the development of next-generation nuclear energy systems known as “Generation IV.” In Europe. the EC organized the Sustainable Nuclear Energy Technology Platform (SNETP) that, through its Strategic Research Agenda promoted the development of fast reactors with closed fuel cycle The Roadmap proposed by the European Sustainable Nuclear Industrial Initiative (ESNII), includes the lead-cooled fast reactor as an alternative technology to be developed in parallel with the sodium-cooled fast reactor. The LFR system features a fast-neutron spectrum and a closed fuel cycle for efficient conversion of fertile uranium and management of actinides. A full actinide recycle fuel cycle with central or regional fuel cycle facilities is envisioned. The LFR can also be used as a burner of actinides from spent fuel by using inert matrix fuel. During the 1970’s and 80’s, considerable experience was developed in Russia in the use of LeadBismuth Eutectic (LBE) for reactors dedicated to submarine propulsion and to develop new reactor designs based on both LBE (i.e. the SVBR reactor) and lead (i.e. the BREST reactor) as primary coolants. In the USA, in the past considerable effort was devoted to investigations of lead corrosion and materials performance issues as well as system design of the SSTAR reactor, while more recently the focus has included the development of the desired characteristics and design of a technology pilot plant or demonstrator reactor. More recently, an extensive R&D program on Lead technology was initiated in Europe and is still ongoing. In particular, the R&D activities are focused on suitable materials, surfaces coatings (oxide layers, alluminization, tantalum, etc.) and lead chemistry (corrosions inhibitors). In the period 1999-2001, Ansaldo Nucleare, within a group of Italian organizations, worked out a first configuration design of a Lead-Bismuth Eutectic cooled Experimental Accelerator Driven Systems (ADS). The activity continued within the 5th Framework Program (FP) of the European Commission (EC), that funded a project named PDS-XADS (Preliminary Design Studies of an Experimental Accelerator), where Ansaldo Nucleare was the coordinator of the studies related to the Primary System and to the Core. In the frame of the project IP EUROTRANS of the 6th FP of the EC, 51 European Organizations had the strategic R&D objective to pursue forward an European Transmutation Demonstrator (ETD) in a step-wise manner. The aim of the project was twofold: develop the conceptual design of an European Facility for Industrial Transmutation (EFIT) with a pure lead-cooled reactor; carry out the detailed design of the smaller eXperimental Transmutation in an ADS (XT-ADS) as irradiation facility and for demonstration of key features of EFIT. The first step in the development of a Lead Cooled Critical Fast Reactor in Europe started in 2006, when EURATOM decided to fund ELSY (European Lead cooled SYstem). The ELSY project, coordinated by Ansaldo Nucleare, developed a very innovative pre-conceptual design of an industrial plant for electricity production able to close the fuel cycle.


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The 7th FP funded the CDT project, aimed to further develop an experimental device based on the resulting MYRRHA/XT-ADS facility of the FP6 EUROTRANS project that may serve both as a test-bed for transmutation and as a fast spectrum irradiation facility, operating as a subcritical (accelerator driven) system, and as a critical reactor. Finally, after the end of ELSY project in February 2010, the LFR development continued with the LEADER project (Lead-cooled European Advanced DEmonstration Reactor), started in April 2010 in the frame of EC 7th FP. 2.

LEADER PROJECT OVERVIEW

The LEADER project is based on the EC call for a conceptual design of a Lead Fast Reactor and it takes into account the indications emerged from SNETP, as well as the main goals of ESNII. The LEADER project is strongly committed to the conceptual design of a scaled/demonstrator reactor (European Lead Fast Reactor Technology Demonstrator Reactor – ETDR) to be constructed in the relatively short term. The ETDR, during the LEADER project kick off meeting, was named ALFRED (Advanced Lead Fast Reactor European Demonstrator). The focus of the first part of the LEADER project activities has been the resolution of the key issues emerged in the frame of the previous projects (e.g. core not enough constrained, floating of fuel assemblies, risk of core bypass caused by not fixed connection between pump ducts and Inner vessel, too complicated FA upper restrains), to reach a new configuration. With reference to this updated configuration of an industrial size Lead Fast Reactor (ELFR), the design of a scaled, fully representative (ALFRED) has been performed. The objectives of the project activities for ALFRED are: - to define the main suitable characteristic and design guidelines for the facility; - to use components/technologies already available in the short term to be able to proceed in the near future to a detailed design followed by the construction phase; - to evaluate safety aspects and perform a preliminary safety analysis; - to minimise the cost of the demonstrator. Moreover, the demonstrator shall confirm that the newly developed and adopted materials, both structural materials and innovative fuel, are able to sustain high fast neutron fluxes and high temperatures. The LEADER project involves 17 partners from Industry, research organisations and universities. The total effort is 502 person-months over a period of 36 months. 3.

ALFRED REFERENCE CONFIGURATION

The configuration of the primary system is pool-type. This concept permits to contain all the primary coolant within the Reactor Vessel, thus eliminating all problems related to out-of vessel circulation of the primary coolant (Figure 1). The primary coolant is molten lead, which presents several favourable characteristics, such as:


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a very high boiling point (1745°C) and very low Partial Pressure. For this reason, there is no need to pressurize the plant primary side, resulting in a easier design of the reactor vessel. Moreover, lead boiling scenario is practically impossible and this, together with proper Reactor vessel design, drastically reduces core voiding risk. It is chemically inert with air and water: this allows the elimination of any intermediate loop and the installation of the Steam Generator Unit inside the Reactor Vessel.

Figure 1 - ALFRED 3D sketch favorable neutronic characteristics (it is a low moderating medium and has a low absorption cross-section). This gives the possibility to have a fast neutron flux even with large amount of coolant in the core; there is no need of compact Fuel Assemblies and it is possible to have relatively large spacing among the fuel rods, with reduced core pressure losses and thus enhanced natural circulation capability. density is slightly higher than that of the oxide fuel, thus, in the LFR, fuel dispersion dominates over fuel compaction, reducing considerably the likelihood of the occurrence of severe re-criticality events in the case of core disruption. In fact, the dispersion of fuel and molten clad from core active zone is enhanced resulting in a self-reducing extension of core degradation. As for all Gen IV advanced systems, the development of the technology is associated with research challenges. In the case of the LFR, these challenges include: •

molten lead interacts with structural materials, mainly with the mechanisms of corrosion at high-temperature and erosion. The provisions that can be adopted to improve the compatibility of lead and steels are:


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- Operate at low temperature range (400 °C - 480°C) and maintain a controlled amount of oxygen dissolved in the coolant. - Select material, such as Austenitic low-carbon steels (e.g. AISI 316L), ferriticmartensitic steels (e.g. T91), 15-15/Ti steel. - Utilize surface coatings, e .g. by alluminisation of surfaces (with Fe-Cr-Al-Y) and surface treatment by electron beam (GESA treatment). - Limit coolant flow velocity to a value that cause a negligible erosion (typically 2– 3 m/s). •

lead has a high melting point (327.4°C) and this can result in a risk of coolant freezing, although this is not considered to be a safety issue but an investment protection issue. To decrease this risk, LFR is equipped with active systems to keep the lead molten during any normal operational conditions (e.g. during planned shutdown). In case of accidental emergency conditions, a sufficient grace time (more than 30 min) is available for the operator to reduce the heat removed by the Decay Heat Removal System (e.g. shutdown one DHR loop by manually closing one valve). However, promising activities are on going on the DHR design, in order to have the reduction of the removed heat by passive means.

•

lead is opaque and this makes the in-service inspection and handling of fuel assemblies more difficult. For this reason, each component inside the Reactor vessel is removable and the fuel assemblies upper end extends beyond the lead free surface in the cover gas for refueling without the need of in-vessel machines.

3.1

Primary system arrangement

The Reactor assembly (Figure 2) presents a simple flow path of the primary coolant, with a Riser and a Downcomer. The heat source (the Core), located below the Riser, and the heat sink (the Steam Generators) at the top of the Downcomer, allow an efficient natural circulation of the coolant. The primary coolant moves upward through the pump impeller to the vertical shaft, then enters the SG through the lead inlet holes, flows downwards on the shell and exits the steam generator. The free level of the hot pools inside the Steam Generator & Primary Pump units is higher than the free level inside the Inner Vessel, the different heads depending on the pressure losses across component parts of the primary circuit. The volume between the primary coolant free levels and the reactor roof is filled by a cover gas plenum. The Reactor Vessel (RV) is cylindrical with a torospherical bottom head. It is anchored to the reactor cavity from the top, by means of a vessel support. The upper part is divided in two branches by a “Y” junction: the conical skirt that supports the whole weight and the cylindrical one, that supports the Reactor Cover. A cone frustum, welded to the bottom head, has the function of bottom radial restraint of Inner Vessel. A steel layer covering the reactor pit, constitutes the Safety Vessel (SV). The dimensions of gap between the safety vessel and the reactor vessel is sufficient to the space for reactor vessel ISI tools. The safety vessel is cooled by the same system that cools the concrete of cavity walls. This


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system is inserted inside the concrete and is independent from the reactor cooling systems. This design solution mitigates the consequences of through-wall cracks with leakage of lead: any reactor vessel leakage is discharged into the Safety Vessel. The RV and the SV are arranged in such a manner that in case of a reactor vessel leak, the resulting primary coolant always covers the SG inlet and the lead flow path is indefinitely maintained.

Figure 2 - Reactor block vertical sections: 01) Fuel assembly ;02) Inner vessel;03) Core lower grid; 04) core upper grid; 05) Reactor vessel; 06) Reactor cover; 07) Steam Generator; 08) Vessel support; 09) Primary pump; 10) Reactor FAs cover) The Inner Vessel (IV), Figure 3, has two main functions: Fuel Assemblies support and separation between hot plenum and cold plenum. It is fixed to the cover by bolts and is radially restrained at bottom. Lead flow is guided from the FAs outlet towards the PP inlet pipes by a toroidal halfring. Moreover, the pipes that connect the hot zone with the inlet of PP are integrated in the Inner Vessel. The cylindrical IV has a double wall shell: the outer thick wall has a structural function, while the inner thin wall follows the core section profile. The Core Lower grid is a box structure with two horizontal perforated plates connected by vertical plates. The plates holes are the housing of FAs foots and the plates distances must be sufficient to assure the verticality of FAs. The diagrid is mechanically connected to the inner vessel with pins (possible removal/replacement during reactor lifetime). The Core Upper grid is a box structure like the lower grid but more stiffer. It has the function to push down the FAs during the reactor operation. A series of preloaded disk springs press each FA on its lower housing. A hole is present for each disk to allow the passage of instrumentation (i.e. thermocouples).


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Upper grid

Cylinder

Pin Lower grid

Figure 3 - Inner Vessel: 3-D sketch and axial section with FAs inside 3.2

Core

The adopted core configuration is constituted by wrapped Hexagonal Fuel Assemblies. It utilizes MOX as fuel and uses hollow pellets, a low active height and a large fuel rod pitch (thanks to the lead physical properties) in order to improve the natural circulation. The total power is 300 MWth. The core scheme (Figure 4) is made of 171 Fuel Assemblies (FAs), 12 CR (Control Rods) and 4 SR (Safety Rods), surrounded by 108 Dummy Elements (ZrO2-Y2O3) shielding the Inner Vessel.

Figure 4 - ALFRED core configuration Each Fuel Assembly (Figure 5) is about 8 m long and consists of 127 fuel pins, fixed to the bottom of the wrapper and restrained sideways by grids (the wires can not be adopted due to the selected fuel pitch). Tungsten deadweight (Ballast) prevents buoyancy forces in lead. Upper elastic elements (cup springs) prevent lifting induced by hydrodynamic loads and accommodate


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axial thermal expansions. The FAs upper end extends beyond the lead free surface in the cover gas for easy inspection and handling. In this way it is possible to make the refuelling without the need of in-vessel refuelling machines.

Figure 5 - Fuel Assembly geometry ALFRED is equipped with two diverse, redundant and separate shutdown systems (adapted from the one that is under investigation in the frame of the CDT-MYRRHA project): (1)

CR (Control Rod) system, used for both normal control of the reactor (start-up, reactivity control during the fuel cycle and shutdown) and for SCRAM in case of emergency. The Control rods are extracted downward and rise up by buoyancy in case of SCRAM. The control mechanism pushes the assembly down with a ball screw, placed, with its motor and resolver atop the cover (at cold temperature (