Project Pele

Project Pele is a project of the US Department of Defense to build a deployable nuclear power reactor for use in United States Armed Forces remote operating bases.
In 2020 the project was listed as relevant to lunar and Mars missions. presumably for surface operations rather than rocket propulsion.
Initial contracts.
On 9 March 2020, the Department awarded three development contracts, to:
The two-year engineering design competition is for a small nuclear micro-reactor in the 1-5 megawatt (MWe) power range.
Development.
When the three designs are matured, in about 2022, one will be selected to be built as a prototype.
The envisaged reactor is intended to be deployable by road, rail, aircraft, or sea. It will be capable of quickly being brought on load, and be inherently safe.
The project may not proceed past developed design, or prototyping, if the design is not achievable.