In this SBIR, USNC will develop an accident tolerant reactor shut-down system for Nuclear Thermal Propulsion (NTP) systems that will guarantee sub-criticality in the event of a water submersion accident. Reactor shut down during a water submersion accident is a crucial issue that must be addressed in NTP systems. The technology that USNC will develop in this SBIR is a low risk design feature that has notable advantages over existing reactor shut-down systems during water submersion accidents. USNC's accident tolerant reactor shut-down system will consist of enhanced control drums with significantly more criticality worth. USNC's enhanced worth control drums are a low risk modification to the traditional control drum where a small amount of fuel is added opposite to the neutron absorber. In addition, USNC's enhanced worth control drums will be moved deeper into the active core to further enhance the criticality worth of the control drums. The combination of adding fuel and moving the drums deeper into the active core will substantially increase the shutdown margin of the control drums and will be sufficient to maintain sub-criticality in the worst case water submersion accidents. Neutronic analysis codes such as MCNP6 and Serpent 2 will be utilized.