Space Fission Power Post #7:Entry-Level Option: Low Power Space Reactor (LPSR) Systems

A space fission system that utilizes a low power space reactor (LPSR) meets the two primary criteria of an entry-level space fission system: 1) a near-term space exploration need and 2) an affordable development approach. The need for a LPSR system is driven by NASA’s need to continue and expand its robotic exploration of the solar system. NASA has dependably relied on radioisotope systems to power science missions for many decades; however, there is no current supply of Pu-238 to meet NASA’s near-term exploration goals.  Because of this, NASA prompted DOE to restart production of Pu-238, although the present effort will produce only a small fraction of NASA’s anticipated needs, and not for several years.  Even if an option existed that could produce all the Pu-238 that NASA would like, the integrated cost of producing the power sources would inhibit the number/level of missions that NASA could pursue. To mitigate the Pu-238 supply/cost issue, NASA has invested in Stirling conversion technology, which can provide ~4 times the electrical output per gram of Pu-238 as compared to thermoelectrics.  This approach appears to be on a successful track, but does not solve the underlying problem.  Even with Stirling engines, a long-term supply of Pu-238 to meet NASA’s exploration needs will require substantial investment, and the program would be threatened continuously by shifting political and programmatic forces.  Furthermore, all of this effort would simply maintain current exploration capabilities, whereas NASA’s historical charter has been to expand our ability to explore space.

As an alternative, a simple fission system based on existing technology could provide robust, long-lived power at 0.5 kWe, 1 kWe, or whatever might be optimal for a specific exploration goal.  The mass of an LPSR system would be similar to a radioisotope system, but more importantly it would likely be of lower cost.  The fission system would be fueled with enriched U-235, which currently exists in substantial quantity and if needed can be made relatively inexpensively. In addition to solving the Pu-238 cost issue, the fission system would also provide NASA with a technology that could grow into a much more ambitious space program; e.g. orders of magnitude more science/communications, outer planet orbiters, Mars surface power, nuclear propulsion, etc.  Fission systems have other advantages, in that they do not have to be cooled during all pre-deployment operations (fabrication, assembly, transport, storage, on the launch pad, etc.), they can load follow power demand, and the lack of radioactivity prior to deployment eliminates the need to mitigate the risk of transport/launch dispersal accidents.

Since the 1970s, there have been several failed US space reactor programs, primarily due to the high cost, risk, and time required for nuclear development and testing. The thermal power of a ~1 kWe system is so low that it can introduce three major advantages.  First, existing technology can be used for all components (and the irradiation environment will be benign).  Second, the burnup of fuel is so small that the loss of reactivity is negligible even over a 10+ year mission, which means that no reactor control is required after initial startup (this improves reliability and decreases development cost).  Even without reactor control thermal power will passively follow the demands of the power conversion system.  Third, the thermal power is low enough to allow a unique testing approach that is not afforded by higher power systems – a low-cost nuclear-powered system test.   This last advantage has the potential to reduce the cost of demonstrating nuclear-powered operation of a space fission power system from ~$1B to <<$100M.  Zero-power critical testing is considered an essential element of all envisioned space reactor programs, as it provides valuable nuclear data at relatively low cost and risk.  The costs of these tests are generally 2 orders of magnitude lower than those estimated nuclear-powered tests of typical space power reactors. At the low thermal power of a LPFS, nuclear-powered system operation (e.g. system startup to full power) might fall within the scope of existing critical test facilities and procedures.

A design approach was developed for LPSR concepts based on anticipated requirements and uses, which is referenced in LPSR_Design_Approach. Science mission power reactors will likely be coupled to either thermoelectric or Stirling power conversion systems. The choice of power conversion might be driven by the “usual” parameters – mass, heritage, reliability, etc., but in this case there might be a strong desire to use Stirling engines in order to keep the thermal power within the scope of critical testing. A concept study of a 1-kWe thermoelectric LPFS was conducted in 2009 by NASA/DOE. The system uses a solid core of U-MO fuel that conducts fission power to Na heat pipes.  The heat pipes travel through the shield to thermoelectric generators. The identical reactor core could also be attached to Stirling engines to produce 3 to 5 kWe. This concept is referenced in LPSR_Reference_System and LPSR_Reference_reactor.

An even lower power reactor concept that provides 500 We via Stirling engines has been developed because a <2 kWt reactor is almost certain to fall within the test envelope of existing facilities. This system directly meets the power needs for near-term NASA space exploration missions, and may be the simplest and most affordable. This concept, referred to as the very low power fission system (VLPFS) is discussed in the LPSR_VLPSR reference.

The only factor that could complicate the development of a LPFS would be a requirement to reduce mass below what could be obtained with existing technology.  In addition to keeping the power system mass low, there are likely to be given stringent dose requirements, which will drive the design to low-mass shielding materials, tight shadow-shielded geometries, and large payload separation distances.  The table below shows the mass of two Pu-238 systems along with 3 possible first generation LPSR systems; the mass in parenthesis includes a shield that reduces dose to allow the use of off-the-shelf electronics (25 kRad).

Engineered System	Alpha (kg/kWe)
120 We Pu-238 thermoelectric (MMRTG)	410
140 We Pu-238 Stirling (ASRG)	180
500 We 1st generation LPSR-Stirling	tbd
1 kWe 1st generation LPSR-Thermoelectric	~420 (770 with COTS electronics shield)
10 kWe 1st generation LPSR-Stirling	~100 (170 with COTS electronics shield)

In conclusion, if the thermal power level can be kept low, and the mass requirement allows existing technology to be used, then a LPSR system offers the lowest cost, shortest path to a nuclear-powered system demonstration (realistically <3 years for <$100M).  A LPSR system also has the nearest term potential usage by NASA, with missions identified that could use the system as soon as it is available.