PEBBLE BED MODULAR REACTOR (PBMR) - A POWER GENERATION LEAP INTO THE FUTURE ABSTRACT

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1 PEBBLE BED MODULAR REACTOR (PBMR) - A POWER GENERATION LEAP INTO THE FUTURE Mr Thinus Greyling, Pebble Bed Modular Reactor (Pty) Ltd, South Africa ABSTRACT The development, procurement and construction of largescale high technology equipment always have an uplifting impact on industry as a whole. The South African energy sector is poised to embark on a massive development programme and the pebble bed modular reactor will become an important element in the nation s energy delivery make-up. Since its launch, the pebble bed modular reactor technology is providing a positive impact on industry and skills development, but also promises to become a world leader for safe and clean power. The pebble bed modular reactor is classified as a Generation IV nuclear power plant that will ensure safety by passive means, and is modular in construction, proliferation resistant and cost effective. This paper presents the safety features of the design, plant layout and some of the construction challenges when one gets involved with the future nuclear high technology environment. It will also highlight the significant application of steel design in the pebble bed modular reactor implementation including an overview of design methodology and materials selection. The opportunities for local industries to get involved in one of the most exiting nuclear projects in the world today are described, and it is all happening right here on our South African doorstep. 1. INTRODUCTION South Africa is yet again stepping up on the world front with the development of a gas-cooled nuclear reactor with some very unique design characteristics that will engrave a path for the future nuclear reactors to come. The project goal is to have a demonstration plant constructed of the pebble bed modular reactor by 2011 at the current Koeberg site in the Western Cape. The pebble bed modular reactor is a new generation nuclear power plant expected to achieve the very important environmental and safety expected goals from the public, to be exactly that which is demanded i.e. to be safe, efficient, environmentally friendly and economically viable. It is a modular design that can be constructed in individual units and linked to form a multi module plant, to produce electrical power in modules of 400MWth/165MWe output, and also has the capability to be transferred into a heat processing plant without major modifications. The plant layout criteria is to divide the plant into functional areas so that the migration from delivering power by means of using a closed loop Brayton cycle to a steam secondary cycle producing electricity or heat, is possible. The pebble bed modular reactor originates from the German HTR-Module (300MWe) reactor and the 15MWe AVR research reactor which operated in Germany for 21 years, as an indirect cycle between the reactor unit operating in helium, and then transferring heat to a secondary steam cycle driving the turbine and generator set. The major technology shift from the German system to the pebble bed modular reactor, was to move from the indirect cycle between helium and steam, to a close-loop direct helium cycle, i.e. drive the turbine directly by means of the helium flowing through the core at a higher temperature and pressure referred to as the Brayton thermodynamic closed loop cycle. Additional to the indirect Brayton cycle, a further design objective was to design the control rods into the reflector instead of directly into the core of pebbles (which previously had caused major damage of fuel pebbles in the German AVR). The core changed from a mixed pebble bed between fuel and graphite spheres to an annular core cavity consisting only of fuel spheres where the graphite spheres are principally replaced by a graphite central reflector. This change enforced three core unloading devices instead of one due to the complexity to support the annular column and being possible to have a central de-fuelling chute at the same time. The fuel handling system is a similar system as employed in the HTR-Module, except for the three (instead of one) de-fuelling mechanisms at the bottom of the reactor with return lines to the top of the reactor. 2. MAIN SYSTEMS DESCRIPTION Main Power System: The Main Power System (MPS) of the pebble bed modular reactor module consists of the following main subsystems: Reactor Unit (RU) Power Conversion Unit (PCU) Pressure Boundary (PB) System Reactor Support Systems Core Conditioning System (CCS) Core Barrel Conditioning System (CBCS) Fuel Handling and Storage System: The function of the Fuel Handling and Storage System (FHSS) is to perform all the required fuel manipulations required during the entire life cycle of the pebble bed modular reactor. These include: Initial loading of the core of the reactor with graphite spheres. Replacing the graphite spheres with fresh fuel spheres intermixed with graphite spheres during initial start-up. Gradually changing the start-up core composition of graphite and fuel to a fuel only composition, and then to a core consisting of fuel to be used in the equilibrium state. Loading and unloading the fuel into and from the reactor core while the reactor is operating at power Spent fuel discharge to spent fuel tanks. Loading of fresh fuel to compensate for spent fuel discharges. Page 1 of 5

2 The fuel spheres are circulated by means of a combination of gravitational flow and pneumatic conveying processes using helium at Main Power System (MPS) operating pressure, as the transporting gas. The FHSS consists of the following subsystems: Core Loading Subsystem (CLS) Sphere Storage Subsystem (SSS) Sphere Circulation Subsystem (SCS) Sphere Replenishment Subsystem (SRS) Fuel Handling Control Subsystem (FCS) Circulating Gas Subsystems (GCS) Sphere Decommissioning Subsystem (SDS) Auxiliary Gas Subsystem (AGS) High-level Waste Handling Subsystem (HLWHS) Helium Inventory Control System: The primary functions of the HICS are: Control of the helium mass within the MPS. Storage of the helium of the MPS and FHSS during a maintenance outage. The components of the HICS are the following: Inventory Control Storage Vessels Control Valves Capacitance Mass Isolation Valves Pressure Relief Valves Bursting Discs Main Compressor Multi-purpose Compressor Piping Buffer Tanks Civil Building (Nuclear and Conventional Islands): The functions of various sections of the Nuclear Island are categorised below. The reactor building is defined as the entire structure that houses the PPB and its ancillary systems. The reactor building is designed to withstand loads and missile impacts due to events induced either by man-made or natural sources external to the building. These sources typically include earthquakes, aircraft impact, tornadoes, loads induced by accidents at nearby industrial facilities or transportation routes, extreme winds and environmental temperatures, and flooding. However, these sources specifically exclude sabotage, terrorist attacks and the effects of war. The reactor building is operated at a pressure less than atmosphere so that leaks occur into the reactor building and not visa versa. Within the reactor building, the citadel surrounds and supports the Reactor Pressure Vessel (RPV) and Power Conversion Unit (PCU). That part of the citadel, which houses the RPV and Reactor Cavity Cooling System (RCCS), is referred to as the Reactor Cavity (RC). The primary function of the citadel is to form a second barrier to externally generated Design Basis Accidents (DBAs). In the case of internally generated DBAs, the RC constitutes the primary barrier to the RPV. The RC also provides the seismically qualified base for the support of the RPV and the RCCS. An additional function of the citadel is to enclose the high radiation area around the RPV and the PCU, and provide radiation protection for the plant personnel. The Generator House performs the following primary functions: Provides access to the generator during operation and maintenance. Houses the ancillary plant serving the generator e.g. breaker, Static Frequency Converter. Houses the generator transformer and unit transformer bus bars. Houses the two redundant trains of electrical systems interfacing with the Nuclear Island. Houses the two lube oil systems serving the turbine, compressors and generator in the controlled and non-controlled areas. Auxiliary Systems: This paper only concentrates on the main auxiliary system consisting of a number of systems supporting the main power system and main support systems. They are: Active Cooling Systems, Heating, Ventilation and Cooling Systems, Reactor Cavity Cooling System. Site Layout: The Environmental Impact Assessment (EIA) for the construction of the pebble bed modular reactor has approved the existing Koeberg site as the feasible site for the construction of the pebble bed modular reactor. Koeberg is situated about 40km away from Cape Town on the West Coast road R CONSTRUCTION METHODOLOGY AND TECHNIQUES Advanced construction techniques are to be used for the pebble bed modular reactor and will be demonstrated and evaluated during the construction of the demonstration plant. Concepts to achieve this goal are developed and to be demonstrated to guarantee significantly shorter schedules and ultimately cost, while ensuring high-quality and world class design and construction techniques, and reducing rework to a minimum. Actual construction experience and statistics shall be accumulated for refining future pebble bed modular reactor construction multi module methods. To achieve this goal, PBMR is using advanced software to simulate the construction schedule with the 3D model. A 4D model is created by integrating the construction schedule in Primavera with the 3D CAD model to simulate the sequence with time visually. The main purpose of the 4D model is to identify the site work congestion during construction and to Page 2 of 5

3 optimise the sequence to reduce construction time as far as possible before getting to site. It will also identify what change implications will have during construction. It also mitigates the risks during construction visually. Excavation: The bulk excavation for the reactor building will be a battered excavation which will require dewatering throughout the construction period. At the level of the rock head, the base of the excavation will extend approximately 10m beyond the footprint of the structure. On completion of the substructure, the excavation will be backfilled with the excavated sand. Advanced excavation techniques are currently been evaluated to optimise alternative design solutions for the dewatering effects of the excavation site. The concept is to make use of diaphragm walls around the excavation area to control the underground water flow and enable a dry and stable excavation area which will speed up the time to excavate considerably. Civil construction optimisation: The Nuclear and Conventional islands are currently constructed by means of stick building techniques, which is time consuming and conservative. More advanced techniques like slip forming and jump forming are being evaluated to reduce construction time, due to the high construction costs per day. Installation of systems: The current installation method is to install systems as modular factory assembled units as far as possible, to benefit in time and economics through well established process learning curves for the demonstration plant. These modular plants will be installed as the civils progress per floor, especially for the heavier equipment based at the bottom of the plant. Additionally, smaller units can be installed by means of the internal plant equipment handling systems, i.e. the power conversion unit lay down crane can receive equipment through the loading bay hatch, and either install them through the lay down area, or move them between floors by using the 40t equipment hoist serving each level of the plant. A 30t lay down crane for installing equipment above the reactor can be utilised to its maximum in this area. The bottom line is schedules are short, costs are lower and more predictable. Heavy lift and transportation: The pebble bed modular reactor plant has a few abnormal load challenges to be transported and installed to and at site. An extensive heavy lift and transportation study has being conducted to prove the viability to move this equipment from port to site and install it within the shortest time durations. The study was conducted around the reactor pressure vessel, at a mass of 900 tonnes, and the core structure assembly at tonnes. return temperature of 500 C and a pressure of 9 MPa of helium gas coolant, with a peak fuel temperature of C, for which existing materials will be used. The selection and the qualification of material grades are key issues for meeting the requirement of pebble bed modular reactor normal and off-normal operating conditions: Graphite for the reactor core and internals. High-temperature metallic materials for internals, piping, valves, high-temperature heat exchanger, gas turbine sub components. Ceramics and composites (C-C, SiC-SiC ) for control rod cladding and other specific reactor internals, as well as for intermediate heat exchangers and gas turbine components for veryhigh-temperature conditions. Characterisation tests in relevant service conditions are set up in a data base, under quality assurance processes, on thermo-mechanical properties under irradiation for the graphite in the core, as well as corrosion resistance. Design and construction methodologies need to be addressed for key components of the system such as the reactor pressure vessel, core, internals, blowers, valves, hot ducts, heat exchangers, turbine, reactor cavity cooling system (RCCS) and other sub systems. In particular, automatic welding techniques are used to weld and inspect the hot ducts outlet to the reactor, and dedicated test loops need to support these techniques and component designs. 4. MATERIALS USED IN THE PBMR Materials development and qualification, design codes and standards require new investigations for the design and construction of the key components for the pebble bed modular reactor. The service conditions considered correspond to a core coolant outlet temperature of 900 C, a Page 3 of 5

4 4.1 Assessment of materials used in the PBMR Plant wide COMPONENTS MASS (METRIC TONNE) MATERIAL TYPE MAX WALL THICKNESS [MM] FOR 3 REACTORS PER YEAR FOR 30 REACTORS OVER 10 YEARS Reactor plate 500 SA533 Type-B Cl Reactor plate 250 SA533 Type-B Cl Reactor forgings( rings, nozzles heads, flanges) 285 SA508 Gr-3 Cl Balance of Pressure Boundary (forged) 300 SA508 Gr-3 Cl Balance Pressure Boundary (other) 500 SA533 Type-B Cl Core Barrel 255 SA H Core Barrel (forged part) 70 SA336 Gr F316H HICS Tanks (6-off) ASTM A516 GR Fuel handling (forged) 121 SA 336 F Fuel handling piping 21 SA 335 P Fuel storage tanks (12-off) 670 SA 387 Grade Miscellaneous piping and others 603 Various Types RCCS pipe work 80 ASTM A106B RCCS standpipes 168 ASTM A106C RCCS tanks 51 AISI 316L Rebar for civil structures Grade 450 MPa Total approximate weight of steel Graphite Very-high-temperature graphite is used for the reflectors and support structures in the core. The reference material for the permanent side reflector support blocks at the hot duct entrance and selected core support post blocks is SGL NBG- 18 graphite grades. There are approximately graphite blocks in the reactor, and in total including dowels and fixation parts. The graphite blocks weigh up to kg High-temperature metallic materials They will be needed for: The Reactor Pressure Vessel Ferritic-martensitic steels with sufficient chromium to allow elevated temperature service and stabilise the microstructure from irradiation damage at temperature < 300 C. The material to be used is Mod 9CR1Mo (SA 508 Grade 3 within ASME code class 1 allowable). Mass 950 tonnes. Core Barrel Assembly Nickel base alloys are used for the Core Barrel Assembly at temperatures up to 800 C. The material to be used is Type 316H. Mass 280 tonnes. Control rods There are 24 control rods. The material to be used is Inconel 800H. Hot pipes Nickel base alloys are used for the hot ducts at temperatures up to C. The material to be used is CFRC. The hot pipes are manufactured and transported in sections weighing up to 100 tonnes. The Recuperator Page 4 of 5

5 The two-off recuperators are printed circuit heat exchangers. The material to be used is Inconel 617. Mass per recuperator 230 tonnes Civil construction material Rebar Grade 450 MPa diameter 32mm ~ tonnes. Concrete 35 or 50 MPa. Some areas are high density concrete for shielding purposes. Excavation m 3 sand, m 3 bedrock. 5. CONCLUSION The pebble bed modular reactor is a first-of-a-kind demonstration plant with attributes that make it ideally suited for short, competitive and cost effective construction times through advanced world class construction techniques. The challenge now is to build it within cost and time, and make it work. 6. ACKNOWLEDGEMENT The author wishes to express his appreciation to PBMR (Pty) Ltd for allowing publication of this document. Page 5 of 5