US company X-energy on 16 March announced the start of work on the conceptual design of its Xe-100 high temperature gas-cooled pebble bed modular reactor. This followed an evaluation by an external panel of industry experts from Southern Nuclear, Burns & McDonnell and Technology Insights. The review validated the baseline design parameters, preparatory documentation, analysis tools, scope of the proposed conceptual design phase (including all planned deliverables), management processes and overall team readiness to proceed to the next phase of reactor development. X-energy plans to deploy the Xe-100 within 10 years. CEO Kam Ghaffarian said moving the reactor into conceptual design put the company "well on the way" towards its goal.

US research into high-temperature gas-cooled reactors (HTGRs) began in the 1940s at Oak Ridge National Laboratory (ORNL), and X-energy's reactor design is based on earlier Department of Energy (DOE) studies. Demonstration HTGR plants of various designs have operated in Germany, the UK and the USA, and two are currently operable – Japan's HTTR, and China's China's HTR-10.

The Xe-100 is a 200MWt (75MWe) reactor, which X-energy plans to build as a standard four module plant, which would generate about 300MWe. The plant will use fuel containing Triso (tristructural-isotropic) fuel particles. Each Triso particle has a kernel of uranium oxycarbide (uranium dioxide) enriched to 10% uranium-235, encased in carbon and ceramic layers. About 25,000 Triso particles, each about 0.4 millimetres in diameter, are embedded in graphite to produce spherical fuel pebbles. About 17,000 pebbles will be used in each reactor.

Triso fuel's carbon and ceramic layers prevent the release of radioactivity, providing each particle with its own independent containment system, while the graphite surrounding the particles moderates the nuclear reaction. Such fuel cannot melt down and X-energy described the reactor as "walk-away" safe in the event of a loss of coolant. All of the plant's components will be road-transportable, streamlining construction by enabling the plant to be installed, rather than constructed, at the project site, the company said.

In January, X-energy was awarded cost-shared funding of $53m over five years from the DOE to support the development of the Xe-100, working in partnership with BWX Technology, Oregon State University, Teledyne-Brown Engineering, SGL Group, Idaho National Laboratory and ORNL. At the same time, the DOE selected Southern Company Services, a subsidiary of Southern Company, to receive similar funding to develop its Molten Chloride Fast Reactor in partnership with TerraPower, Electric Power Research Institute, Vanderbilt University, and ORNL. In August last year, X-energy and Southern Nuclear Operating Company signed a memorandum of understanding to collaborate on development and commercialisation of their respective advanced reactor designs.