Reactor System Configuration of
3. Specific Design Features
Design PhilosophyThe PBMR®-400 is a high-temperature helium-cooled, graphite moderated pebble bed reactor with a multi- pass fuelling scheme. The design objectives and features mean that the reactor can be deployed close to the end user since there shall be no design base or credible beyond design base event that would need anyone living near the site boundary to take shelter or be evacuated. To achieve this objective there shall be no need for engineered or moving mechanical components to ensure this objective is met while the exposure to plant personnel will be significantly lower than today’s best international practice.
Reactor Core and Fuel Characteristics
The core neutronic design is an annular core with an outer diameter of 3.7 m and an inner diameter of 2 m shaped by the fixed central reflector. The effective cylindrical core height is 11 m. In steady state (equilibrium core) operation the fuel sphere power (maximum 2.7 kW per sphere) and operational temperatures (<11000C) fulfil the design criteria set. The core contains ~452 000 fuel spheres or ‘pebbles’ with a packing fraction of 0.61. The fuelling scheme employed is a continuous on-line multi-pass system. Fresh fuel elements are added to the top of the reactor while used fuel pebbles are removed at the bottom to keep the reactor at full power.
On average fuel spheres are circulated six times through the reactor. This reduces power peaking and maximum fuel temperatures in normal operation and loss of coolant conditions.
The coated particle pebble fuel used is shown below. The fuel kernel (UO2) is coated first by a porous layer of pyrocarbon, followed by a dense layer of pyrocarbon, a silicon carbide layer and an outer dense layer of pyrocarbon. About 15 000 of these coated particles and graphite matrix material are made into an inner fuel zone and surrounded by a 5 mm outer fuel free zone to make up the 6 cm diameter fuel sphere or pebble.
Power Conversion System
The Brayton cycle power conversion cycle with direct gas turbine is adopted. It is a closed cycle where the helium coolant is used to transport heat directly from the core to the power turbine. The design incorporates turbine, compressors and power generator in a single shaft. The flow of the Helium is depicted in the figure below. The direct gas cycle is attractive since it promises the benefits of simplification, with the potential of lowering the capital and operational costs. Due to the high outlet gas temperatures one will also expect a substantial increase in the overall system efficiency.
Reactivity Control
Excess reactivity is limited by continuous refuelling while adequate passive heat removal ensures an inherent safe design with no event with significant fission product release being possible. Adequate reactivity control and long-term cold shutdown capability is provided by two separate and diverse systems while the overall reactivity temperature coefficient is negative over the total operational range. The reactivity control system facilitates load following between 40% and 100%.
Fuel Handling System
Fuel spheres are circulated in the online handling system by means of a combination of gravitational flow and pneumatic conveying processes using helium at system operating pressure, as the transporting gas. The system functions as an online fuel replenishing system. This involves fuel unloading, discharging spent and damaged/worn fuel to used fuel vessels, reloading fresh fuel and fuel that can be returned back to the reactor.
Reactor Pressure Vessel and Internals
The average core height is 11 m and the annulus thickness is fixed at 0.85 m. The centre reflector diameter is 2 m and contains eight borings for the Reserve Shutdown System, consisting of borated graphite spheres of 10 mm diameter. The centre and side reflectors are manufactured from nuclear grade graphite blocks that are stacked in columns to make up the geometry of the core. The side reflector columns have borings for the control rods, as well as riser channels for the incoming coolant gas. All the blocks are connected with graphite keys to prevent diversion of the coolant flow. The whole of these ceramic core internals is housed in a stainless-
steel Core Barrel that is supported on the bottom of the Reactor Pressure Vessel.
PBMR®-400 power conversion unit components and layout, and He flow.
4. Safety Features
The safety philosophy for modular HTRs has been described a number of times in the past 30 years and has been adopted with a few modification by PBMR®. Its basis is that an accident equivalent to severe core damage must be inherently impossible by limiting reactivity increases and ensuring that decay heat can be removed passively after a loss of coolant event. The PBMR® has a simple design basis, with passive safety features that require no human intervention and that cannot be bypassed or rendered ineffective in any way. If a fault occurs during reactor operations, the system, at worst, will come to a standstill and merely dissipate heat on a decreasing curve without any core failure or release of significant radioactivity to the environment.
Engineered Safety System Approach and Configuration
The PBMR® nuclear reactor system is designed to derive maximum safety benefits from its inherent passive safety characteristics which are; designed to rule out core melt, all ceramics fuel, coated particle provides excellent containment for the fission product activity, large negative temperature feedback, Helium coolant is chemically inert (single phase), large thermal capacity lead to slow thermal transients, no common mode failure in the core (a single fuel failure does not lead to additional failures), ingress of water into core eliminated by design and air ingress limited.
Decay Heat Removal/ reactor Cooling Philosophy
The Reactor Cavity Cooling System provides a means to remove residual heat passively for a defined time, and indefinitely with the use of an active system after refilling the cooling system. For this to work, the Reactor Pressure Vessel and the core need to be long and slender. The belt region of the RPV is not insulated to allow heat radiation and convection to the water filled cavity cooler. In the event of the loss of active core cooling by the main circulation system, the cavity cooler and/or the building structural materials are able to limit the increase in fuel temperature in the most affected region of the core to below the allowable fuel temperature limit.
Containment Function
The most important barriers to fission product release are the coatings of the fuel particles. A second barrier is provided by the Helium Pressure Boundary. A third barrier is the confinement building. The vented confinement is designed for very low leakage at low pressure, and to prevent damage to components important to safety, as well as to contain the build-up of higher activity gas in the delayed phase of a depressurisation event. Depending on the size of a pressure boundary break the system may be vented and then closed again with the released gas filtered as required.
5. Plant Safety and Operational Performances
The PBMR®-400 safety does not rely on engineered systems that may fail but on the inherent design and the laws of physics. The risk metrics core damage frequency and large early release frequency are not applicable,
but the same concepts are reflected in the immediate and delayed release category definitions. The design of the PBMR® represents a significant advancement in plant safety with an estimated delayed release category frequency of 1.0 x 10-5 per reactor year while maintaining an expected capacity factor of 95%.
6. Instrumentation and Control Systems
The PBMR® system consists of an inherently stable and slow acting heat source (Reactor Unit), due to its large thermal capacity, which makes it nearly self-regulating, coupled to a fast-acting power conversion machine.
The Power Conversion Unit therefore require active control to remain stable under all anticipated operating scenarios. The reactor power is adjusted by changes in the helium mass flow rate in the power conversion unit.
The helium inventory system is used to change the pressure (mass adjusted through changes in density) and power control is subsequently performed in combination with a bypass valves.
7. Plant Arrangement
PBMR®-400 building layout.
8. Design and Licensing Status
The Reactor Plant Preliminary Design was completed and demonstration of key technologies were underway when the project was terminated in 2010.
9. Fuel Cycle Approach
Once through uranium cycle was planned and analysed; pebble bed reactors are flexible to accommodate other fuel cycles (plutonium or thorium) too.
10. Waste Management and Disposal Plan
The Waste Handling System is designed to handle, store and discharge low- and medium-level liquid and solid radioactive waste generated during normal operation, maintenance activities, and upset conditions of the PBMR®-400; including preparation for the final disposal. During final decommissioning, the spent fuel spheres are removed from the Spent Fuel tanks and conveyed to a point where they can be loaded into the Spent Fuel Transport Casks suitable for final disposal at a designated site.
11. Development Milestones
1993 The South African utility Eskom identifies PBMR as an option for new generating capacity.
1995 Start of the first pre-feasibility study.
1999 Design optimization: PBMR®-268 with dynamic central column.
2002 Design changed to PBMR®-400 with fixed central column.
2002 The Pebble Bed Micro Model (PBMM) demonstrated the operation of a closed, three shaft, pre- and inter- cooled Brayton cycle with a recuperator.
2004 Vertical layout of turbo machines changed to conventional single horizontal layout.
2006 Commissioning of Helium Test Facility for full scale system and component tests.
2006 Tests starts in the Heat Transfer Test Facility.
2007 Advanced fuel coater facility commissioned.
2009 Coated particles sent for irradiation testing at INL.; Alternative process heat markets and designs explored.
2010 Project closure.
2018 Project in care and maintenance.
Reactor System Configuration of AHTR-100
MAJOR TECHNICAL PARAMETERS
Parameter Value
Technology developer, country of origin
Eskom Holdings SOC Ltd.,