Interestingly enough, I founded a company in 1993 called Adams Atomic Engines, Inc. based on a design for a closed cycle gas turbine using a pebble bed reactor, nitrogen as the working fluid and a simple Brayton cycle using compressors and turbines that were designed for the combustion turbine market.
These machines could be made available in a variety of power outputs by choosing the appropriate reactor internal volume, filling it with the appropriate number of pebbles, and choosing compressors and turbines that were already being manufactured, thus avoiding the costs of design and creating whole new supply chains for unique machines run on some kind non standard gas like helium.
It is a well proven and testable fact that pebble bed reactors using TRISO fuel particles can be made so that they can withstand a complete loss of cooling without scram and still maintain all fuel temperatures below the temperature that will damage any of the elements. That statement holds true up to a thermal power level of about 300 MW, so it is not terribly interesting to people who think in terms of reactors with thermal power of 3,000 to 5,000 MW.
However, since large combustion turbines are generally in the power range of 100-150 MWe, it is a very useful characteristic for someone interested in closed Brayton cycles that are intended to compete against combustion turbines and diesel engines.
Anyway, you can find out more by perusing the documents available at http://atomicengines.com and http://atomicengines.blogspot.com.
Adams Atomic Engines, Inc. by the way, was never successful in raising enough capital to move off of our computers and into actual machinery. Part of the problem was that the TRISO fuel particles that we initially thought had been proven to be good enough were not available anymore. There was, and remains, a very credible fuel manufacturing and testing program that is designed to meet NRC expectations being run by the DOE and the NGNP partnership. That program is scheduled to have licensable fuel ready by 2021 or 2022. We couldn't wait that long.
Rod Adams, Publisher, Atomic Insights