Subscale Active Filtration of Exhaust (SAFE)

Many different options have been suggested over the years as to testing options. The simplest is to fire the rocket with its’ nozzle pointed into a deep borehole at the Nevada Test Site, which has had extensive geological work done to determine soil porosity and other characteristics that would be important to the concept. Known as Subsurface Active Filtration of Exhaust, or SAFE, it was proposed in 1999 by the Center for Space Studies, and continued to be refined for a number of years.

SAFE schematic

SAFE concept, Howe 2012, image courtesy NASA

In this concept, the engine is placed over an already existing (from below-ground nuclear weapons testing) 8 foot wide, 1200 foot deep borehole, with a water spray system being mounted adjacent to the nozzle of the NTR. The first section of the hole will be clad in steel, and the rest will simply be lined with the rock that is being bored into. The main limiting consideration will be the migration of radionucleides into the surrounding rock, which is something that’s been modeled computationally using Frenchman’s Flat geologic data, but has not been verified.

SAFE injector model

SAFE injection system model, Howe 2012

The primary challenges associated with this type of testing will be twofold: first, it needs to be ensured that the fission products will not migrate into groundwater or the atmosphere; and second, in order to ensure that the surrounding bedrock isn’t fractured – and therefore allows greater-than-anticipated migration of fission products to migrate from the borehole – it is necessary to prevent the pressure in the borehole from reaching above a certain level. A sub-scale test with an RL-10 chemical rocket engine and radioisotope tracers was proposed (this test would have a much smaller borehole, and use known radioisotope tracers – either Xe or Kr isotopes – in the fuel to test dispersion of fission products through the bedrock). This test would provide the necessary migration, permeability, and (given appropriate borehole scaling to ensure prototypic temperature and pressure regimes) soil fracture pressures to ensure the full filtration of the exhaust of an NTR.

The advantage to doing this test at Frenchman’s Flat is that the ground has already been extensively tested for the porosity (35%), permeability (8 darcys), water content (initial pore saturation 30%), and homogeneity (alluvium, so pretty much 100%) that is needed. In fact, a model already exists to calculate the behavior of the soil to these effects, known as WAFE, and the model was applied to the test parameters in 1999. Both full thrust (73.4 kg/s of H2O from both exhaust and cooling spray, and 0.64 kg/s of H2) and 30% thrust (20.5 kg/s H2O and 0.33 kg/s of H2) were modeled, both assuming 600 C exhaust injection after the steel liner. They found that the maximum equilibrium pressure in the borehole would reach 36 psia for the full thrust test, and 21 psia in the 30% thrust case, after about 2 hours, well within the acceptable pressure range for the borehole, assuming the exhaust gases were limited to below Mach 1 to prevent excess back-pressure buildup.

P-Tunnel setup

Other options were explored as well, including using the use of the U-la facility at the NNSS for horizontal testing. This is an underground set of tunnels in Nevada, which would provide safety for the testing team and the availability of a hot cell for reactor disassembly beside the test point (the P-tunnel facility is also cut into similar alluvial deposits, so primary filtration will come from the soil itself, and water cooling will still be necessary).

INL geology 2

INL geological composition, image courtesy DOE

Further options were explored in the “Final Report – Assessment of Testing Options for the NTR at the INL.” This is a more geologically complex region, including pahoehoe and rubble basalt, and various types of sediment. Another complication is that INL is on the Snake River plain, and above an aquifer, so the site will be limited to those places that the aquifer is more than 450 feet below the surface. However, the pahoehoe basalt is gas-impermeable, so if a site can be found that has a layer of this basalt below the borehole but above the aquifer, it can provide a gas-impermeable barrier below the borehole.

A 1998 cost estimate by Bechtel Nevada on the test concept estimated a cost of $5M for the non-nuclear validation test, and $16M for the full-scale NTR test, but it’s unclear if this included cost for the hot cell and associated equipment that would need to be built to support the test campaign, and I haven’t been able to find the specific report.

However, this testing option does not seem to feature heavily in NASA’s internal discussions for NTR testing at this point. One of the disadvantages is that it would require the rocket testing equipment, and support facilities, to be built from scratch, and to occur on DOE property. NASA has an extensive rocket testing facility at the John C. Stennis Space Center in Hancock County, MS, which has geology that isn’t conducive to subterranean testing of any sort, much less testing that requires significant isolation from the water table, and most NASA presentations seem to focus on using this facility.

The main reasons given in a late 2017 presentation for not pursuing this option are: Unresolved issues on water saturation effects on soil permeability, hole pressure during engine operation, and soil effectiveness in exhaust filtering. I have been unable to find the Bechtel Nevada and Desert Research Institute studies on this subject, but they have been studied. I would be curious to know why these studies would be considered incomplete.

One advantage to these options, though, which cannot be overstated, is that these facilities would be located on DOE land. As was seen in the recent KRUSTY fission-powered test, nuclear reactors in DOE facilities use an internal certification and licensing program independent of the NRC. This means that the 9-10 year (or longer), incredibly expensive certification process, which has never been approved for a First of a Kind reactor, would be bypassed. This alone is a potentially huge cost savings for the project, and may offset the additional study required to verify the suitability of these sites for NTR testing compared to certifying a new location – no matter how well established it is for rocket testing already.


Ground Testing a Nuclear Thermal Rocket: Design of a sub-scale demonstration experiment, Howe et al, Center for Space Nuclear Research, 2012

Subscale Validation of the Subsurface Active Filtration of Exhaust Approach to NTP Ground Testing, Marshall et al, NASA Glenn RC, 2015 (Conference Paper) and (Presentation Slides)

Final Report – Assessment of Testing Options for the NTR at INL, Howe et al, Idaho NL, 2013