Reactor System Configuration of AHTR-100
MAJOR TECHNICAL PARAMETERS
Parameter Value
Technology developer, country of origin
Eskom Holdings SOC Ltd.,
detailed application to High Temperature Reactor (HTR) designs would require an industrial-scale reactor plant to prove their suitability for a commercial reactor. In order to implement these advances in a demonstration plant, the AHTR-100 was conceptualised by Eskom Holdings SOC Ltd. – PBMR SOC Ltd. is wholly owned by Eskom Holdings SOC Ltd.
With an output of temperature of 1200°C the AHTR-100 is classified as a Very High Temperature Reactor (VHTR). Specific demonstrated nuclear technologies, such as the fuel design, will however remain the same as that of the PBMR®.
2. Target Application
The AHTR-100 can produce electricity at high efficiency via a combined direct helium Brayton cycle and Rankine bottoming cycle with an intermediate heat storage for load following or process heat applications or a bottoming steam cycle, as depicted in below Figure.
AHTR-100 schematic with topping, bottoming cycle and energy storage unit.
3. Specific Design Features
Design PhilosophyAs in the PBMR®, the AHTR-100 is a high-temperature helium-cooled, graphite moderated pebble bed reactor but with a once-through fuelling scheme. The design safety targets and features means 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 goal there shall be no need for engineered or moving mechanical components to ensure this target is met while the exposure to plant personnel shall also be significantly lower than today’s best international practice.
Reactor Core and Fuel Characteristics
The core neutronic design results in a small cylindrical core with a diameter of 2.6 m. The effective cylindrical core height is 9.35 m. In steady state (equilibrium core) operation the fuel sphere powers (average 0.91 kW) and operational temperatures (1200oC) fulfil the design criteria. The core contains ~110 250 fuel spheres or
‘pebbles’ with a packing fraction of 0.61. The fuelling scheme employed is the continuous on-line once- through method. 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.
The fuel kernel (UO2) is coated by a first porous layer of pyrocarbon, followed by a dense layer of pyrocarbon, a silicon carbide layer and an outer dense layer of pyrocarbon. About 13 330 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
A Brayton power conversion with direct gas turbine is adopted as topping cycle. It is a closed cycle where the helium coolant is used to transport heat directly from the core to power turbine. The design incorporates a single shaft for the turbine, the compressors and the power generator. Heat exchangers (up to 3) to remove heat to the bottoming cycle is included.
From the reactor unit the hot helium enters directly to the turbine where energy is used to drive the shaft and therefore the electric generator and compressors. From the turbine the helium then passes consecutively through the primary side of the first high temperature heat exchanger, then the pre-cooler, the low pressure compressor, intercooler, high pressure compressor and then on to the high-pressure intercooler before re- entering the reactor unit.
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.
This primary cycle will operate in baseload maximum capacity continually and provide 30% of the total plant electricity. This limits reactor operating transients to startup, full load operation, and shut down.
The heat exchangers contain molten salt coolant in the secondary side removing heat from the primary circuit and storing it for use in a load following Rankine cycle.
Reactivity Control
Excess reactivity is limited by once through, continuous refuelling cycle 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 are provided by two separate and diverse control rod and small absorber sphere (SAS) systems while the overall negative reactivity temperature coefficient is negative over the total operational range.
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 fresh fuel replenishment, fuel unloading, and discharging used fuel to the used fuel vessels.
Reactor Pressure Vessel and Internals
The average core height is 9.35 m and the reflector thickness 0.95 m. The 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 prestressed concrete reactor pressure vessel.
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 modifications by AHTR-100 in the same manner as with the 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 AHTR, like 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 through a heat pipe system on a decreasing curve without any core failure or release of radioactivity to the environment.
(a) Engineered Safety System Approach and Configuration
The AHTR builds on the PBMR® nuclear reactor system that 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, the 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.
(b) Decay Heat Removal/ reactor Cooling Philosophy
The Reactor Cavity Cooling System (RCCS) is a means to remove residual heat passively for a defined time, and indefinitely with the use of a passive heat pipe system. The use of a pre-stressed concrete pressure vessel in effect insulates the core from the atmosphere and as a result, the system requires passive heat removal by the heat pipe system. In the event of the loss of active core cooling by the main circulation system, the heat pipe system is activated automatically through the temperature rise and are able to limit the increase in fuel temperature in the most affected region of the core to below the allowable fuel temperature limit.
(c) Containment Function
As with the PBMR®, 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
As in the PBMR®, the AHTR-100 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 AHTR based on the PBMR® represents a significant advancement in plant safety with an estimated delayed release category frequency of 1.0E-5 per reactor year while maintaining an expected capacity factor of 95%.
The AHTR concept is directed to be a simplistic design, exhibit inherently safety characteristics and high operational efficiency. The operating modes, states and transitions are under definition but it is specified that the unit is able to shutdown with no human intervention requirements, in the event of LOFC.
6. Instrumentation and Control Systems
As in the PBMR®, the AHTR 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. Design and Licensing Status
The design basis for the proof of concept machine has been completed for a direct cycle machine. The intent is to test and proof several aspects of the technology prior to implementation in the commercial power plant.
The layout for the overall plant is being developed with operating modes, states and transitions progressively defined.
The licensing framework for the proof of concept is also complete and the nuclear regulator is appraised on the effort of the developments in the project. Reactor Plant Conceptual Phase has been completed with key R&D work continuing in the field of qualifying materials and deisgn and construction of demonstration components.
8. Fuel Cycle Approach
Once through uranium cycle is planned and analysed; pebble bed reactors are flexible to accommodate other fuel cycles (plutonium or thorium) too.
9. Waste Management and Disposal Plan
The Waste Handling System would be based on the PBMR®-400 experience and designed to handle, store and discharge low- and medium-level liquid and solid radioactive waste generated during normal operation, maintenance activities, and upset events; including preparation for the final disposal. During final decommissioning, the spent fuel spheres are removed from the interim storage and conveyed to a point where they can be loaded into the Spent Fuel Transport Casks suitable for final disposal at a designated site.
10. Development Milestones
2010 PBMR® Project in care and maintenance since 2010.
2016 AHTR-100 R&D activities commence.
2017 AHTR-100 Version 1 concept completed.
2018 R&D activities continue.
2019 R&D put on hold pending funding availablility.
MAJOR TECHNICAL PARAMETERS
Parameter Value
Technology developer, country of origin
STL Nuclear, South Africa
Reactor type HTGR (Pebble Bed)
Coolant/moderator Helium/graphite
Thermal/electrical capacity, MW(t)/MW(e)
100 / 35 single module plant Primary circulation Forced circulation
NSSS Operating Pressure (primary/secondary), MPa
4 / - Core Inlet/Outlet Coolant
Temperature (oC)
250 / 750
Fuel type/assembly array TRISO particles in pebbles:
LEU, Th/LEU, Th/HEU or Th/Pu
Number of fuel assemblies in the core
~ 150 000 pebbles; around 125 to 150 pebbles/day throughput Fuel enrichment (%) Various, see description below Core Discharge Burnup (GWd/ton) 80-90
Refuelling Cycle (months) Online fuel loading
Main reactivity control mechanism Absorber rods in the reflector Approach to safety systems Passive
Design life (years) 40 full power years Plant footprint (m2) 5000 buildings only
RPV height/diameter (m) 15.7 /5.6 (flange outer diameter) RPV weight (metric ton) 155
Seismic Design (SSE) 0.3g for generic site; (0.5g under consideration)
Fuel cycle requirements / Approach Various options – see text below Distinguishing features No core meltdown, no active
engineered safety systems, spent fuel in acceptable form
Design status Conceptual design
1. Introduction
The HTMR100 (High Temperature Modular Reactor) pebble bed reactor is a high temperature gas cooled reactor, graphite moderated and cooled by forced helium flow. The existing design of the module is to produce high quality steam which is coupled to a steam-turbine/generator system to produce 35 MW(e). The steam can be used in a wide range of cogeneration applications. The reactor is also suitable to provide direct high temperature energy for process heat. The design of the reactor is based on proven technology and therefore no new basic technology development is needed. The size of the reactor and the fuel cycle were chosen to simplify the design and operation of the module. The approach to small intrinsic safe modular units ensures continuous production, easy road transportability, skid mounted sub systems, wider range of manufactures, fast construction and an enhanced licensing process.
2. Target Application
The HTMR100 can supply electric power to any distribution grid and to standalone or isolated electricity users.
It can be deployed as single modules or multi-module plants as well as for medium temperature process heat applications (later also upgradable to very high temperature). The HTMR100 is a perfect fit for clients who want to progressively extend their generating capability. The unique safety characteristics make it possible to introduce and construct these plants to non-nuclear countries. First-world countries that want to utilize their stock of Plutonium for peaceful applications are also markets for HTMR100 reactors.