Some blasts and impacts have the potential to breach the multiple layers of protection. Analyses are performed to prove effectiveness and efficacy of the materials and design used when engineering the radioactive power system. For example, the simulation below shows that due to the numerous layers of containment and protection, no radioactive material is released upon impact at a velocity of 100m/s, a much greater velocity than 60m/s – terminal velocity for this example system.
Some blasts and impacts can change the potential for criticality for a system. Analyses are performed to characterize this change in the potential for criticality during launch accidents to inform the source terms of the probabilistic risk assessment. For example, the simulation below was used to determine that an impact on a hard surface would not increase the potential for criticality of the generic reactor.
Material and Model Calibration
Many material models are based on relatively basic test data, and available data for nuclear materials and integrated nuclear systems is often limited. In contrast, other (non-nuclear) materials may have more extensive datasets and more mature, sophisticated modeling approaches. Where key material properties are uncertain or unavailable, analysts often use iterative calibration methods to infer unknown properties and refine model inputs.
Objects tested and modeled subjected to fire
Sandia works to determine the effect of potential fire environments on the radioactive material incorporating vaporization and condensation processes. Liquid propellant fire temperatures can exceed radioactive material vaporization temperatures and solid propellant fire temperatures can exceed material melt temperatures and radioactive material vaporization temperatures.
The Sandia Fire Model (SFM) code is used to determine the effects of liquid propellant fireballs. Additionally, Sandia’s Plutonium Entrainment and Vaporization after a Coincident Impact (PEVACI) code is a Sandia-developed model that is used to determine fire response for released radioactive material or cladded material to solid propellant fires. Sandia Fire and Thermal test facilities can be used to conduct physical testing of components, assemblies, and whole vehicles in fire environments to complement, verify, and validate modeling.
Propellant fire modeling includes the following effects:
- Liquid propellant vaporization of radioactive material that is enclosed in system hardware or exposed from previous insults
- Solid propellant vaporization of radioactive material that is enclosed in system hardware or exposed from previous insults.
- Highly complex interactions beneath and surrounding burning solid propellant fragments, including burning the several chemical constituents, gaseous heat convection, thermal radiation, droplet impingement, slag buildup, and geometric feedback effects.
Clad and aeroshell simulated with SINDA – Systems Improved Numerical Differencing Analyzer
Modeling reentry effects
Atmospheric reentry environments can lead to breakup of the launch vehicle, space vehicle, and/or the space nuclear system. The analyses evaluate how the range of reentry conditions affects the configuration, including resulting trajectories, component heating, and material ablation (erosion). These types of analysis uses a suite of Sandia-developed codes, with each tool addressing a specific part of the problem. Trajectory Analysis and Optimization Software (TAOS) is used to generate the space nuclear system trajectories. Aerodynamic heating along those trajectories is estimated with a Python tool that implements the relevant correlations. The Sandia Parallel Aerodynamics and Reentry Code (SPARC) is then used to compute material thermal response. Although SPARC is primarily a computational fluid dynamics code, it also includes a thermal solver. Its “Strand” capability enables efficient analysis of complex 3-D objects by creating 1-D through-thickness “strands” at each face of a surface mesh and simulating the thermal response for each strand as an uncoupled 1-D calculation.
Situations leading to reentry of the spacecraft into Earth’s atmosphere prior to insertion into the mission’s interplanetary trajectory are grouped as follows:
- Suborbital – Reentry due to accidents that occur after achieving 100,000 ft altitude and prior to the attainment of the nominal Earth parking orbit
- Circular Orbit Decay – Reentry from circular orbital decay. (All orbits eventually become circular in response to atmospheric drag)
- Powered, Elliptic Delayed, and Elliptic Prompt Reentry – These are reentry modes associated with misdirection of thrust. Powered Reentry results when the misdirection forces the spacecraft into the atmosphere while the attached upper stage is still thrusting. Reentry at velocities higher than orbital reentry is possible, but very unlikely.
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