The low power means very little of the uranium is burned up. Therefore, the fuel does not swell and releases very little gas.
The kilopower reactor running for 15 years would have 0.12% swelling. This is less than 10% of swelling from the heat generated.
Radiation damage is also so little it can be ignored.
Generating heat at 1200K can be used to efficiently convert the heat to electricity with a Stirling engine. The core power would be limited to 4 kWt.
Beryllium Oxide surround the core to reflect the neutrons back so that the system is only critical within the reflectors for safety and to keep the core smaller. 300 kilograms of low enriched uranium (7%) is enough to make the reactor work.
Los Alamos National Research lab is helping to design small compact fast reactors like KiloPower and Megapower. They are designed to maximize mechanisms so the reactors are totally self-regulating. The Los Alamos objective is to design-in self-regulation as the front-line feature in order to minimize technical and programmatic risk and to demonstrate via testing that self-regulation is both reliable and repeatable.