Vulcan
General Characteristics:
Type:Mobile nuclear reactor
Physical Characteristics:
Length:12.19 m
Width:2.44 m
Height2.87 m
Mass:37,500 kg
Reactor Characteristics:
Thermal power:14 MWth
Power output:5 MWe
Cold Startup Time:<8 hours to full rated power
Load Following Range:25–100% power, 5% per minute ramp rate
Operating lifetime:30 years
Overview:
SDI Power System's Vulcan is transportable nuclear power system intended to provide reliable, fuel-independent electrical power to remote and expeditionary Forward Operating Bases (FOBs). The Vulcan reactor is an advanced helium gas-cooled Very-High-Temperature Reactor (VHTR) utilizing highly enriched QUADRISO fuel elements which directly drives a closed-cycle helium gas turbine operating on the Brayton thermodynamic cycle, eliminating the intermediate heat exchanger and steam generator found in conventional nuclear plants and dramatically reducing system mass and complexity. The entire Vulcan reactor module is designed to fit within a standard 40-foot ISO container which lets it be easily transported by ship, rail, truck, or air to remote locations to provide power for airbases, naval installations, and other military or civilian facilities located in remote areas.
Design & Construction:
The Vulcan reactor module structural frame is based on a standard 40-foot ISO intermodal container frame. The internal structure is a custom-designed heavy-duty spaceframe fabricated from high-strength low-alloy (HSLA) steel to support the concentrated loads of the reactor pressure vessel, turbomachinery, and heat exchangers during all transport modes, including the dynamic loading environment of airlift and ship motion at sea. The module is divided into three compartments along its length: the reactor compartment (approximately 8 m), housing the RPV and its support structure; the turbomachinery compartment (approximately 3 m), housing the turbo-compressor-generator assembly; and the heat exchanger compartment (approximately 1.5 m), housing the recuperator, precooler, and intercooler core blocks. The remaining length accommodates structural end frames, piping, valves, and the coaxial duct connection between the RPV and turbomachinery.
Reactor core:
The Vulcan reactor core is a right-circular annular prismatic block designed to optimize the ratio of fuel volume to neutron leakage while maintaining a geometry amenable to passive decay heat removal. The active core consists of an annular region of hexagonal graphite fuel blocks surrounding a central graphite reflector column. The annular core ensures that no fuel location is more than approximately 25 cm from a reflector boundary, providing an inherently short thermal conduction path for passive heat removal during loss-of-coolant scenarios. The active core region contains 66 fuel columns arranged in a single annular ring of hexagonal positions, with each column consisting of 8 stacked fuel blocks for an active height of 3.2 meters. The central reflector consists of 7 hexagonal columns of solid graphite. An outer radial reflector ring of 36 solid graphite columns surrounds the fuel annulus. Top and bottom axial reflectors of 0.8 m each complete the neutron reflector envelope. The Vulcan reactor employs employs QUADRISO (QUADruple-layer Isotropic) coated fuel particles which add a fourth ceramic barrier layer to the standard TRISO architecture, providing additional fission product retention capability at the extreme temperatures and burnup levels demanded by the long-core-life design. Each QUADRISO particle, which is approximately 650 microns in diameter, consists of a spherical kernel of uranium oxycarbide (UCO) fuel surrounded by four concentric ceramic coating layers. The UCO kernel composition is selected over uranium dioxide (UO2) to limit the buildup of carbon monoxide gas pressure at high burnup, which can cause coating failure in pure oxide kernels. The kernel is surrounded by a porous carbon buffer layer that provides void volume for fission gas accommodation and attenuates fission fragment recoil, an inner pyrolytic carbon (IPyC) layer that provides structural support and acts as a diffusion barrier, a silicon carbide (SiC) layer that serves as the primary metallic fission product diffusion barrier and pressure vessel, a zirconium carbide (ZrC) layer which provides a supplementary high-temperature metallic fission product barrier with superior performance above 1600°C where SiC begins to degrade, and an outer pyrolytic carbon (OPyC) layer that provides additional structural support and protects the ZrC layer during fuel compact fabrication. The uranium enrichment is set at 19.5 wt% U-235. The QUADRISO particles are overcoated with a graphite matrix material and pressed into cylindrical fuel compacts approximately 12.5 mm in diameter and 50 mm in length. The compacts are loaded into blind fuel holes drilled in the hexagonal graphite moderator blocks. Each fuel block contains 210 Zircaloy-4 lined fuel channels and 108 helium coolant channels in a regular triangular pitch array. The graphite blocks are fabricated from nuclear-grade IG-110 graphite, used for its high thermal conductivity, low neutron absorption cross-section, and excellent irradiation dimensional stability.
The reactor coolant is helium gas at 7.0 MPa pressure which enters the core at 460 °C and exits at 850 °C with a total coolant flow rate through the core of 6.5 kg/s. The primary control system consists of 18 control rod drive mechanisms (CRDMs) operating 800H nickel-iron-chromium alloy clad B4C control rods in channels located in the outer radial reflector. The three power changing control rods are actuated by a closed-loop hydraulic control circuit which insert and retract them from core while the fifteen safety rods are normally held outside of the reactor core under spring pressure by a series of clock-type springs wound by a hydraulic motor which actuates through an electromagnetic clutch, ensuring the rods will spring back into the reactor and initiate scram if the reactor suddenly loses power or if the scram control is remotely activated. The control rods provide sufficient negative reactivity for cold shutdown with the highest-worth rod stuck out, satisfying the traditional N-1 shutdown criterion. The Vulcan core exhibits a strong negative temperature coefficient of reactivity, dominated by the Doppler broadening of U-238 resonance absorption cross-sections. The overall isothermal temperature coefficient is approximately -7 pcm/°C, ensuring that any power excursion is self-limiting. The moderator temperature coefficient is also negative due to the under-moderated core design. These inherent feedback mechanisms ensure that the reactor will passively shut itself down and reach a safe, stable state following any credible reactivity insertion event without operator action. The reactor pressure vessel (RPV) is a cylindrical shell with hemispherical heads, fabricated from modified 9Cr-1Mo-V steel. The RPV is designed to withstand the 7 MPa helium pressure at a cold-side wall temperature maintained below 400°C by the returning cold helium flow in the coaxial duct annulus and by a dedicated vessel cooling flow path. The vessel design incorporates provisions for thermal expansion, seismic restraint within the emplacement bunker, and through-wall penetration nozzles for the coaxial hot duct, control rod drives, instrumentation, and the reserve shutdown system hoppers. Core shielding designed to protect the compressor, turbine, and electrical equipment from radiation damage consists of side and axial composite shields 5.0 centimeters thick made from boron carbide (B4C) and grade 316 stainless steel. To save weight and allow the reactor to be more easily transported the reactor does not include any biological shield and should be employed with a 300 meter personnel exclusion zone while in operation.
Turbomachinery & Power Conversion Equipment:
The Vulcan reactor employs a closed-cycle brayton power cycle with the reactor essentially functioning as a large nuclear-powered single-spool turboshaft engine. The primary helium circuit is a single integrated loop that passes through the reactor core and directly into the gas turbine power conversion system without an intermediate heat exchanger. This direct-cycle configuration is a key design decision that eliminates the mass, volume, cost, and thermal efficiency penalty of a secondary loop. The helium flows downward through the reactor core (inlet plenum at top, outlet plenum at bottom), exits the reactor pressure vessel through a coaxial hot duct, passes through the turbomachinery, and returns to the reactor vessel through the annular cold return path of the coaxial duct. A helium purification system (HPS) continuously processes a slipstream of the primary coolant to remove chemical and radioactive impurities. The HPS includes a high-temperature metallic filter for particulate removal, a copper oxide bed oxidizer for converting CO and H2 to CO2 and H2O, a molecular sieve dryer, an activated charcoal adsorber bed cooled by liquid nitrogen for noble gas (Kr, Xe) removal, and a final getter bed for oxygen and nitrogen. The HPS processes approximately 5% of the total helium inventory per hour, maintaining impurity levels below specification limits necessary to prevent graphite oxidation and metallic component corrosion.
Helium working fluid at 35 °C and 2.75 MPa is first compressed by the low pressure compressor, exiting at 95°C and 4.5 MPa. The helium then flows through the intercooler and through the high pressure compressor, exiting at 115°C and 7.15 MPa pressure. The Helium is then passed through the cold side of the recuperator where it is preheated by the turbine exhaust and then passed into the reactor core inlet plenum at 490 °C and 7.05 MPa pressure. The helium working fluid then absorbs heat from the nuclear fuel elements and leaves the core at a 850°C and 6.95 MPa. The helium is then expanded through the power turbine, performing work on the turbine rotor. The helium leaves the turbine at 510°C and 2.85 MPa where the partially expanded helium then passes through the hot side of the recuperator (a compact counter-flow heat exchanger), transferring residual thermal energy to the cold-side helium entering the reactor core. The helium then passes through the precooler, a series of helium-to-air heat exchanger running along the top of the reactor module that rejects waste heat to the environment, returning the helium to a temperature of 35 °C and pressure of 2.75 MPa where it then again enters the compressor. The turbomachinery consists of a single-shaft arrangement comprising the power turbine, the low-pressure compressor, the high-pressure compressor, and the electrical generator, all mounted on a common shaft supported by active magnetic bearings. The use of magnetic bearings eliminates the need for lubricating oil systems, which would be incompatible with the high-purity helium environment and would introduce an unacceptable contamination risk. The magnetic bearing system includes backup catcher (touchdown) bearings for safe coast down in the event of a bearing control system failure or loss of power. The turbine is a six stage axial-flow design with nickel-based superalloy blading. At the 850°C turbine inlet temperature, uncooled metallic blades are feasible due to the benign (non-oxidizing) helium environment, avoiding the complexity and efficiency penalty of blade cooling systems required in open-cycle combustion gas turbines. The compressors are also multi-stage axial-flow machines with titanium alloy blading for the lower-temperature stages. The entire rotor assembly is approximately 3.5 m in length and has a design rotational speed of 15,000 RPM, with the generator producing high-frequency AC power that is converted to 60 Hz grid-quality power via a power electronic frequency converter. The electrical generator is a permanent-magnet synchronous machine directly coupled to the turbomachinery shaft, producing three-phase AC power at approximately 750 Hz (15,000 RPM, 6-pole). The generator employs a rare-earth permanent magnet rotor and a fully encapsulated stator with hermetically sealed stator winding cooled using bleed helium from the high pressure compressor. The high-frequency output is converted to 60 Hz, 4,160 V three-phase power by a solid-state frequency converter based on insulated-gate bipolar transistor (IGBT) technology. The frequency converter also provides a means of precise speed control for the turbomachinery shaft, supporting load-following operations. A step-down transformer in the switchgear module converts the output to the required distribution voltages (480 V or 208 V) for FOB loads.
The recuperator is a Printed Circuit Heat Exchanger (PCHE) fabricated from Alloy 617 by chemical etching and diffusion bonding. The PCHE technology provides a very high surface area density (approximately 2,500 m²/m³), enabling the achievement of 95% effectiveness in a compact and lightweight unit. The precooler and intercooler are also compact heat exchangers. For FOB applications with limited water availability, the baseline design employs forced-air cooling using helium-to-air fin-fan heat exchangers, with the secondary (air) side connected to the module via insulated ducting through the bunker wall. In coastal or riparian environments, a helium-to-water option reduces the air-side heat exchanger size and improves cycle efficiency in hot climates.
Control:
The Vulcan instrumentation and control (I&C) system is a fully digital, triple-modular redundant (TMR) architecture implementing both the reactor protection system (RPS) and the plant control system (PCS) on qualified programmable logic controllers. The I&C system is housed primarily in the control module (a separate 20-foot ISO container) connected to the reactor module by redundant fiber optic and hardwired safety cables routed through conduits in the bunker access tunnel. The control module is located outside the exclusion zone and serves as the primary operator interface. Core nuclear instrumentation consists of ex-vessel fission chambers and compensated ion chambers located in the outer reflector region for neutron flux measurement over the full operating range (source, intermediate, and power). Core outlet temperature is measured by redundant Type N thermocouples in the outlet plenum. Helium pressure, flow, and purity are monitored by pressure transducers, differential pressure flow elements, and online mass spectrometry in the helium purification system. Turbomachinery health monitoring includes shaft vibration proximity probes, magnetic bearing position sensors, and bearing load cells. The generator output is monitored by the power electronics frequency converter, which also serves as the primary means of turbomachinery speed and power control. The PCS is designed for autonomous steady-state operations with load-following capability, requiring operator intervention only for startup, shutdown, and off-normal events. The system implements automatic load-following by adjusting control rod positions and turbomachinery speed in response to grid frequency or load demand signals from the FOB power distribution system.
Operation:
The Vulcan reactor omits an integral biological radiation shield from the reactor module. The design is instead intended to rely on site-provided shielding in the form of a below-grade or partially buried reinforced concrete bunker with compacted earth overburden, combined with a large standoff distance (exclusion zone) to reduce dose rates to acceptable levels in personnel-occupied areas. The emplacement bunker is a reinforced concrete cut-and-cover structure designed to be constructed by organic military engineer units. Minimum bunker internal dimensions are 13.5 m length by 3.5 m width by 4.0 m height, with walls and roof constructed from >1,500 kg/cm2 strength Ultra-high-performance concrete (UHPC) with a minimum wall thickness of 0.6 meters with a floor consisting of a 0.3 m reinforced concrete slab on prepared subgrade. The bunker should then be covered 2.0 meters (minimum, compacted fill) of earth overburden on the roof and 3.0 m meters (minimum, compacted fill) on the sides. The combination of 0.6 m concrete plus 2 to 3 meters of compacted earth (density ~1,800 kg/m³) provides a total areal density of approximately 5,000–7,000 kg/m² in the radial direction, more than sufficient to attenuate the reactor radiation field to below 2 mrem/hr at the outer earth surface directly above the reactor. At the 300 m exclusion zone boundary the calculated dose rate is below 0.25 mrem/hr (below natural background). At the 100 m occupational boundary (where continuously manned guard posts or perimeter sensors would be located), the dose rate is below 0.5 mrem/hr, satisfying occupational dose limits for radiation workers with large margin.
The Vulcan is designed as a sealed, non-refuellable reactor with a 10-year full-power core life. At the end of core life, the entire reactor module is shut down, allowed to cool for a minimum decay period (nominally 30 to 90 days), extracted from the emplacement bunker, and transported as a sealed unit to a nuclear facility for fuel removal, spent fuel disposition, and module refurbishment or decommissioning. No fuel handling operations are performed in the field, eliminating the need for fuel handling equipment, hot cells, or special nuclear material accountability infrastructure at the FOB. At end of core life, the spent QUADRISO fuel remains within the graphite blocks inside the sealed RPV. The coated particle fuel form is inherently robust for long-term storage and is an acceptable waste form for direct geological disposal without reprocessing. The spent fuel retains its fission products within the ceramic coatings even under repository conditions. Decommissioning of the emplacement bunker involves radiological survey, decontamination if necessary (expected to be minimal given the sealed primary system), removal of non-contaminated concrete and earth for conventional disposal, and site restoration. The bunker concrete may be slightly activated by neutron exposure and may require management as low-level radioactive waste.
