Each subcritical power module rejects thermal energy as a result of the combined Brayton and Rankine power cycles operations. In the Holos design, the unavoidable thermal rejection to the environmental air (Ultimate Heat Sink – UHS) occurs in two steps: The Brayton cycle components transfer the rejected thermal energy to a closed-loop Organic Rankine Cycle (ORC) integrated and thermally coupled with the subcritical power module reflectors and shield; The ORC condenser is then passively coupled to environmental air through heat exchangers thermally coupled to dedicated ISO container heat exchange surfaces. In this manner, the total amount of thermal energy rejected to the environment is reduced and the thermodynamic efficiency is increased (from 45% to 60%). The closed-loop integral ORC system captures a portion of the waste thermal energy rejected by the Brayton cycle and converts it into conditioned electricity. As the subcritical power module is positioned to execute a temporary or permanent shutdown, its fuel cartridge continues to naturally produce decay heat. The electrical power rate produced under shutdown is proportional to the power history prior to shutdown and the time elapsed from shutdown. Passive natural convective air-cooling maintains adequate fuel cartridge cooling even under Loss Of Coolant Accident (LOCA) scenarios.

Fully operational Holos Quad generators with power ratings up to 13 MWe are completely encased in a single transport container with subcritical power modules comprising fuel cartridges lasting 12-20 Effective Full Power Years (EFPY) between refueling. Holos EFPY is based on applications requirements under the constraints of enrichment and generator duty cycle. For the Holos Quad 13 MWe configuration, each subcritical power module develops 5.5 MW-thermal. Each Holos Quad subcritical power module can be transported individually or already coupled and in compliance with transport ISO container dimensional requirements. For power ratings higher than 13 MWe, Holos generators can be formed by larger subcritical power modules coupled by combining multiple transport containers. In these configurations, each module is positioned near one another to satisfy core geometry and mass requirements.

For applications requiring power ratings in excess of 60 MWe, a single Holos Titan generator becomes more cost-effective when compared to the cost of 5x smaller Holos Quad generators clustered and coupled to match a similar power rating (e.g. 5x 13 MWe = 65 MWe). The Holos Titan configuration is formed by larger subcritical power modules. As for the Holos Quad configuration, each Titan subcritical power module satisfies the dimensional constraints represented by standard transport containers. Once at the delivery site, the larger subcritical power modules are positioned to execute thermal and neutron coupling to produce electricity and process heat. The Holos Quad and Titan configurations are powered by fuel cartridges that last 5-12-20 years between refueling (depending on low-grade enrichment and application requirements). For Holos Titan configurations, the ORC components are housed within independent retrofitted transport containers. As for the Quad configuration, the ORC modules are thermally coupled to the subcritical power modules without BoP equipment and convert thermal energy discharged by the Brayton power conversion equipment into electricity.

As each subcritical power module houses only a fraction of the fuel forming the whole core, and represents relatively small dimensions (comparable to aviation jet engines), full-scale testing to validate and monitor safety performance can be cost-effectively executed at the factory. This reduces costs and expedites construction and licensing procedures. Holos total number of Systems, Structures and Components (SSCs) forming the generator’s modules is substantially reduced. As a result, full-scale testing can be executed on individual subcritical power modules or on the whole generator to satisfy and monitor safety performance standards. Because of these features, factory certification can follow regulatory processes and quality assurance standards applied to aviation jet engines. Overall, the generator leaves the factory fully tested, operational and certified for deployment at any site (no site-specific stressors constraints).

When the fuel cartridges are replaced at the end of their fuel cycle, Holos power conversion components can be reconditioned and the generator can be re-licensed to resume operation for a total operational life of 60 years (two additional total fuel cartridges replacement per refueling after the first fuel cycle of 20 years). To substantially reduce decommissioning cost, Holos fuel cartridges and power conversion components are designed to fit within licensed canisters for temporary or long-term storage. As portions of the power components (mainly the electric motors representing the generator and recirculator) are removed from the subcritical power module, the fuel cartridges remain sealed within their reinforced structure during decommissioning activities and all the way to the welding of the storage cask lid. Depending on applications, the ORC components continue to produce electricity at power rates proportional to the decay rate. As a portion of the decay heat energy is converted into electricity, Holos fuel cartridges represent a lowered thermal loading for the dry cask and for underground, unventilated repositories with no active cooling. Fuel cartridge extraction, lifting and repositioning within licensed casks can be executed with conventional hydraulic lifting equipment retrofitted with shields and remotely operated. Alternatively, the fuel cartridges can be lifted with bridge cranes and positioning within dry casks follows procedures similar to those adopted for refueling and storage of conventional light water reactor fuel bundles (no extra shielding required).

Holos enhanced safety is inherently achieved by a highly simplified and integrated design with passive redundant and diverse safety features. Holos combined relatively low power rating and components size support low-cost full-scale testing to simulate design and beyond design basis accident scenarios to validate safety performance. Low-cost full-scale testing further supports the implementation of quality assurance programs during components manufacturing, testing and maintenance programs while in operation to assure safety performance remains unaffected over the operational life of the generator. Holos design’s safety features factor lessons-learned from severe nuclear power plant accidents and operational-experience. Modern melt-resistant fuels already offer significant benefits in terms of sealing radioactive volatiles within the fuel itself under all operational and off-normal conditions. To further enhance safety, Holos fuel cartridges seal the melt-resistant fuel within multi-layered and reinforced structures that are passively cooled by environmental air. Holos safety features include engineered safety systems addressing Design Basis Threat/Attack scenarios.

Holos core mass and geometry are actively matched to follow electric demand by regulating the coupling of multiple independent subcritical power modules. The total amount of nuclear fuel sealed within the reinforced fuel cartridges represents substantially smaller radionuclide sources when compared to nuclear cores conventionally equipping operational and small modular reactor designs. These special features enable a substantial reduction of the Evacuation Planning Zone (EPZ) from several miles, as currently required for operational reactors, to a few hundred feet. Computer Fluid Dynamic, Mass and Heat Transfer analyses show that even under total loss of coolant the fuel cartridges temperatures remain significantly below safety thresholds with passive cooling. The melt-resistant fuel loaded within Holos fuel cartridges has been tested above safety temperature thresholds and demonstrated no release of volatile radionuclides up to extremely high temperatures (that cannot be reached via passive cooling). Each fuel cartridge segregates the melt-resistant fuel and provides mechanical and hydrostatic features that further minimizes migration of radionuclides. The small radioactive sources represented by the fractioned Holos core and the diverse and redundant inherent passive and engineered safety features make the fuel cartridges a substantially reinforced multi containment system. These structures represent multiple pressure boundaries when compared to licensed nuclear fuel transport canisters, transported across multiple countries without requiring evacuation planning zones.

As shown in the project organigram under Manufacturability , Stages II-VI comprise full-scale testing of all components through multi MegaWatt low-cost test rigs developed for waste heat recovery systems. Testing includes subcritical power module safety performance validation under design basis scenarios, beyond design basis and design attack/sabotage basis scenarios. Full-scale testing is executed by replacing the fuel cartridges with surrogate thermal cartridges (e.g. based on non-nuclear heat sources) providing thermal energy for the subcritical power module turbomachinery to operate at full power. Under these low-cost testing configurations, subcritical power modules can be coupled to subsystems and operated under all credible operational, off-normal and sabotage scenarios. As part of full-scale testing activities, the reinforced fuel cartridge structure can be subjected to mechanical stressors involving pressure, vibrational, structural and thermal-cycling to validate technical and safety performance. Similarly, testing of the AMPS, and power electronics and controls can be executed at full power with dynamic electric loads to simulate unstable electric grids and validate the near real-time high-resolution load following features offered by the design.

Operational multi MegaWatt test-rig hardware and power systems developed to validate performance of Waste Heat Recovery Systems (WHRSs) under transients and at steady-state can be adapted to provide thermal energy to support Holos surrogate fuel cartridge. The power source utilized to validate WHRSs generates in excess of 5MW-thermal with near real-time adaptability as it is represented by a modified aviation turbo-jet engine. Megawatt-class compact heat exchangers and closed-loop thermal-to-electricity power conditioning systems are tested under all operational and off-normal conditions by controlling the test-rig power source. Individual Holos Quad subcritical power modules are designed to produce approximately 5.5MW-thermal at maximum power rating. By leveraging the currently operational WHRSs test-rig infrastructure, integration of specialized heat exchangers with the power source and transport of thermal energy from the power source to Holos surrogate fuel cartridge can be accomplished in 6 months. Accordingly, transients, normal and off-normal operational tests inclusive of Holos power module testing under design basis accident scenarios, beyond design basis accident scenarios and design basis attack scenarios can be safely executed. These activities support design fine-tuning with full-scale test validations of technical and safety performance.