The objective of this effort was to design, fabricate, integrate, and fly a nuclear thermal rocket without having to build massive ground test facilities. Furthermore, this nuclear rocket would be inspected internally without having to disassemble it. Another aim here was to predict the reliability for a 15-to-20-year lifetime for a surface nuclear power plant essential for supplying power to explorers on the Moon or Mars, all without having to build and test a full-scale reactor. If the performance of these systems could be predicted, one could establish the impact of manufacturing tolerances on the operations of these systems.
The Little Prairie Services (LPS) team, the Center for Space Nuclear Research (CSNR), and Sandia National Laboratories (SNL) collaborated on this project to investigate the feasibility of developing integrated multiphysics codes to do just that. This software program integrates high-level multiphysics packages to solve problems specific to nuclear thermal propulsion systems. It identifies promising techniques and technologies to reduce liquid hydrogen propellant boiloff, and reviews and identifies techniques for nuclear rocket engine decay heat removal. The program shows how fiber optics can be used to measure important nuclear system parameters such as strain, temperature, elongation, and vibration to monitor the health status of a nuclear rocket engine.
The LPS team started with some existing codes developed by the CSNR called IROC, the Integrated Rocket Optimization Code. This code is used to generate a nuclear thermal rocket design from a nuclear perspective. It can generate results that predict the way power is generated within the nuclear core. A separate code was used to perform preliminary thermal hydraulics; that is, the way the nuclear generated heat was removed from the core.
The codes were not integrated; in other words, it was not possible to feed back information from the nuclear power portion to modify the physics of how power was generated once heat was removed. Further, although the models had the ability to perform changes to a given design, that feature was not automated. During this effort, these issues were meliorated. The code is now more transparent and can perform in a multiphysics closed loop, as shown in Figure 1.
One of the most important aspects of operating a nuclear power system in space is launch safety. Launch safety for a nuclear reactor is primarily concerned with a launch incident that might result in the reactor becoming inadvertently critical; that is, generating nuclear energy when it is not supposed to. One of the most significant concerns is what happens if a launch or pad accident submerges the reactor in water or the nuclear system is dropped onto a solid surface. In these processes, the reactor can be deformed, sometimes significantly. Consequently, being able to predict how the criticality of the reactor is modified through impact-induced deformations is of critical importance.
Historically, linking codes such as PRESTO, a Lagrangian-Eulerian hydrocode with neutron multiplication codes such as MCNP, was a painstaking and manual process. LPS determined that taking a reactor model and generating it in a code such as AutoCAD Inventor and subsequently exporting it in a .stl file to PRESTO would allow a direct conversion of the MCNP input deck from IROC to a PRESTO input file. Generating a 3D CAD file is an essential step in other multiphysics models as well. The real breakthrough was generating a script that translated the Exodus II output file from PRESTO into a file that a mesh-based transport code, Serpent 2, could interpret. The LPS team was able to accomplish this. When compared to existing manual routines, the LPS approach reduced the number of independent, multi-user steps from 21 to 5.
LPS created a simplistic three-concentric-cylinder model. This model is used to provide a simple way to test multiple impact conditions with results that can be cross-checked with legacy codes such as MCNP. SNL computed the results of an impact with a concrete surface at 250 m/s, as shown in Figure 2.
The upper graphic shows the results with a metallic outer shell; the lower with a ceramic shell. The results are dramatically different. LPS also took an existing impact model, a long-rod penetrator made of tungsten impacting a steel plate. The tungsten rod was replaced with UO2 and the steel with water. The researchers were able to predict a very high neutron multiplication factor consistent with what would be expected under these circumstances.
The objective with this analysis is to be able to initially screen for impact conditions that would result in a critical assembly, and then mitigate any condition where keff ≥ 1. As a result, thousands of potential conditions could be screened within weeks, where such an undertaking would have taken years in the past.
This work also identified mechanisms to limit hydrogen boil-off in long-term space storage conditions, and reviewed and identified techniques for nuclear rocket engine decay heat removal. The program shows how fiber optics can be used to measure important nuclear system parameters such as strain, temperature, elongation, and vibration to monitor the health status of a nuclear rocket engine.
During a proposed next phase of the effort, the LPS team will complete a 3D CAD model of the entire engine and core, and use computational fluid dynamics codes to predict flow under normal and anomalous conditions; determine manufacturing tolerance conditions that would lead to unacceptable performance, and continue the impact modeling.