Francisco J. Arias

Heat Removal System for Shutdown in Nuclear Thermal Rockets and Advanced Concepts

A study was conducted to derive a first estimate of how much hydrogen would be needed to prevent the core from overheating after shutdown and, from this, be able to assess the advantages of using a dedicated decay heat removal system to reduce or eliminate the amount of hydrogen needed to prevent the core from overheating after shutdown. It was demonstrated that the use of such a heat removal system could be needed by certain special nuclear thermal propulsion concepts, such as the fission fragment rocket or the recently proposed pulsed nuclear thermal rocket where significant amplification of specific impulse, along with thrust could be obtained by the direct use of fission fragments or by pulsing the nuclear core.

The behavior of radiogenic particles at solidification fronts

The thermal behavior of insoluble radiogenic particles at the solid-liquid interface of an advancing solidification front and its significance with regard to environmental impact are discussed. It is shown that, unlike classical particles, where the most probable behavior is engulfing by the solidification front, radiogenic particles are more likely to be rejected by the solidification front. Utilizing a simplified physical model, an adaptation of classical theoretical models is performed, where it is shown that, unlike classical particles, for radiogenic particles the mechanism is thermally driven. An analytical expression for the critical velocity of the solidification front for engulfing/rejection to occur is derived. The study could be potentially important to several fields, e.g. in engineering applications where technological processes for the physical removal of radionuclide particles dispersed throughout another substance by inducing solidification could be envisaged, in planetary science where the occurrence of radiogenic concentration could result in the possibility of the eruption of primordial comet/planetoids, or, if specific conditions are suitable, particle ejection may result in an increase in concentration as the front moves, which can translate into the formation of hot spots.