For more than a decade, the Particle Bed Reactor (PBR) has been a capability in search of a mission. The nascent PBR technology promises higher operating temperatures than those of conventional solid core reactors such as were developed in the 1960s under the NERVA program, which can translate into a more efficient power generator, or a more capable propulsion system.
Despite these claimed advantages, proponents have failed to identify a high priority mission that would justify the expense of resolving the many remaining technical uncertainties. The technical risks of the PBR include:
+ Challenges in fabricating the high temperature fuel particles that are the key to this technology -- efforts to date have failed to conclusively demonstrate that fuel particles can withstand the rigors of the reactor operating environment;
+ The low thermal capacity of the reactor core increases the risk of thermal damage to the core in off-normal conditions, or during reactor cool-down;
The particle bed reactor has been evaluated for applications that range from multi-megawatt burst-mode electric power supply for space-based weapons, to nuclear rocket engines for strategic defense interceptors and piloted interplanetary missions. Nearly $200 million has been spent by the Defense Department for development of this technology for military propulsion applications as part of the highly classified Timberwind program.
Beginning in 1990 the Defense Department, and the Strategic Defense Initiative Organization which sponsored Timberwind, began seeking support from other agencies for broader applications for this technology. In particular, efforts were made to apply the PBR technology to NASA's new Space Exploration Initiative. This process culminated in the establishment by the Air Force of the Space Nuclear Thermal Propulsion program in late 1991, which assumed much of the work previously conducted under the Timberwind program.
Although military proponents continue to assert significant military missions for this technology, the strong drive to gain broader institutional support for this project suggests that even proponents recognize the marginal contribution of PBR systems to current or projected national security requirements. Indeed, a close review of such applications suggests that the military utility of the PBR technology is at best marginal. Whether other civil space requirements will justify further development of the PBR engine remains to be seen. But the current low levels of funding for other technologies related to the Space Exploration Initiative makes it unlikely that the PBR will be selected as the propulsion concept of choice at this early stage of SEI development. While continued low level funding of this technology is likely, in order to avoid excessive loss of momentum, it is doubtful that large scale testing of PBR propulsion systems will occur in the absence of more clearly defined and broadly supported applications. This appears unlikely in the near term.
A - PROGRAM HISTORY
Active interest in particle bed reactor systems dates back to 1982. The development of this technology over the past decade has proceeded through five phases:
Initial discussions of the particle bed reactor in the early 1980s focused on its potential for supplying high levels of burst mode electric power. These discussions did not respond to specific mission applications, but identified the PBR as a potentially attractive candidate should requirements for such capabilities develop.
An effort by the Air Force in the mid-1980s to characterize high-leverage future military technologies, Project Forecast II, identified the PBR as a potentially attractive candidate for orbital transfer vehicle applications;
The Strategic Defense Initiative considered PBR systems as one potential candidate for satisfying its Multi-MegaWatt (MMW) burst-mode power requirement in support of space-based directed and kinetic energy weapon systems.
In late 1987 the Strategic Defense Initiative Organization established a highly classified program, code named Timberwind, to evaluate the application of PBR rocket engines for propelling long-range anti-missile interceptors.
In early 1991, much of the work under the Timberwind program was declassified, and the technology was evaluated for a wider range of applications, including space launch vehicles and piloted interplanetary missions.
These various mission requirements have resulted in a rapid evolution of reactor design concepts. For each of these applications and reactor configurations, a number of advantages have been asserted for the Particle Bed Reactor.(1)
+ Interest in the PBR is predicated on the characteristic relationship between operating temperature and specific impulse for nuclear thermal propulsion systems (Figure IV-1 - the two lines are for nozzle expansion ratios ( E ) of 100 and 500, which bound current practice). The assumption is that the advanced fuels used in the PBR concept, coupled with the inherent operational characteristics of the PBR, would result in a significantly higher performance than solid core reactors (Figure IV-2), with operating temperatures as much as 500o K higher.
+ The concept promises significant reductions in system mass over solid-core reactors, made possible by the significant increase in the heat transfer surface area of the particle fuel elements (a 10 K difference between fuel and coolant temperatures). The PBR has superior heat removal characteristics, given the twenty-fold greater surface area to volume ratio of the fuel particles versus the solid matrix fuel used in NERVA. The NERVA fuel elements have about 4.7 cm2 surface area per cm3, while the PBR fuel particles have about 97.5 cm2 surface area per cm3.
+ Thermal transport characteristics are superior, with less than half the temperature drop required to drive the heat out of the fuel into the propellant. The temperature drop in the PBR fuel element is about 300 K, while the NERVA temperature drop is about 600 K.
+ Most of the reactor has relatively low operating temperatures, simplifying reactor component design and fabrication. Only 20% of the entire reactor operates at high temperature, and only the hot frit and a portion (30%) of the fuel particle bed experience temperatures above 2000 K. Thus the PBR also may be capable of higher temperatures (3000 K) and higher burnup fraction. The higher power density results in a smaller reactor, which can reduce shielding mass requirements to half the mass of a shield for a comparable NERVA reactor.
+ This relatively benign thermal environment permits the choice of moderators based on mission requirements rather than material thermal properties. Thus highly efficient hydrogenous moderators such as lithium hydride can be used, which results in significantly more compact reactor designs, and thus higher thrust-to-weight ratios.
+ PBR has simpler construction and lower thermal stress than NERVA-derived systems. NERVA required a complex support structure to accommodate the large thermal expansion and stress on the solid graphite fuel elements. In contrast, the PBR experiences much lower structural stress, since the PBR fuel particles are loosely packed, and the hot frit is not axially constrained.
+ The PBR has lower core pressure drop and coolant velocity than the NERVA-derived system, easing structural design and increasing reliability. The PBR core pressure is in the range of 1 to 2 atm, compared with over 10 atm for NERVA, while the exit Mach number for the PBR is about 0.2, compared with 0.4 in NERVA.
+ The propellant flow path results in moderator controls and most structural components operating at propellant inlet temperature, permitting selection of a broader range of materials and improving reliability. In addition, the small size of the fuel elements would greatly reduce sensitivity to thermal shock.
+ Performance is further enhanced by matching propellant flow to fuel element power distribution. The fuel element is configured to produce the highest power where the coolant is at the lowest temperature and to provide less power in those regions of the fuel element where the propellant temperature is approaching the desired maximum.
+ The reactor can be brought to full power much more quickly than is the case with NERVA type systems. Computer simulations of SNTP PBR operations by Sandia suggest that the reactor can be ramped from 3 kWt to 50 MWt in less than three seconds.
+ A variety of safety advantages have also been claimed.(2) The compact size of these reactors permits reduced shielding mass, which could be as little has 50% of a comparable NERVA-derived reactor. Low engine mass also permits
the option of multiple engine redundancy, which may be more readily neutronically decoupled. Containment and confinement of the fuel particles can be facilitated by system redundancies, as well as the fact that most of the particles are relatively cool. Most core materials are not at elevated temperatures, and component thermal gradients are relatively low, permitting light-weight structural elements which minimize radiation heating. Launch safety options include a large loading of neutron poison, such as B4C in the core cavity.
+ The primary advantage of the SNTP technology for propulsion applications is a specific impulse approximately twice that of LH2/LOX propellants with a comparable engine thrust-to-weight ratio. PBR thrust-to-weight ratios are related to specific impulse, with low-pressure PBR engines exhibiting a wide range of values for both figures of merit (Figure IV-3).
However, significant operating risks require further clarification:
+ One of the main unresolved technical risks associated with this concept is the problem of debris clogging of the very small (micron size) holes in the frits supporting the particles. This would result in localized redistribution of propellant flow, which would lead to thermal anomalies which could damage fuel particles and other structural elements.
+ Additional understanding is required of anomalous propellant flow and thermal profile conditions. It has been suggested that the propellant flow instabilities resulting from the development of local hotspots in the fuel particle bed might constrict propellant flow, leading to further localized heating, and ultimately to fuel particle and structural damage or failure.
+ Another significant risk of the PBR system derives from the low heat capacity of the particle bed. This low heat capacity results in one of the more attractive characteristics of the PBR, the potential for more efficient thermal transport from the fuel to the propellant. But it also creates a significantly increased risk of melting of fuel particles and other low temperature core elements in the event of over-temperature or other accidental events.
+ The low heat capacity of the PBR core requires very stringent reactivity control to avoid power overshoot during reactor ramp-up. Failure to prevent overshoot could result in excessive structure material temperatures and possibly catastrophic fuel particle melting. Closed-form reactor controllers, driven by rates of change in reactivity with very rapid control feedbacks, have the potential for over-controlling reactors. Avoiding this problem requires a core configuration with an inherently very high negative coefficient of reactivity.
+ Tests to date have demonstrated fuel element power densities that are low relative to those of operational systems. Resolution of these concerns will require much more extensive testing at more representative power densities.
These issues have increasingly dominated the debate over the merits of the PBR technology.
1 - Project Forecast II
Project Forecast II was a major Air Force wide effort to identify new technologies with potentially large impacts on military capabilities. One of the 70 technology areas identified by late 1986 as having the potential to significantly enhance future Air Force capabilities is direct nuclear propulsion, identified in Project Forecast II as Project PT-2 Safe Compact Particle Bed Nuclear Propulsion.(3) The study concluded that technology development priorities included (in order of priority) the hot frit, bed power density, fuel particles, fuel
element end regions, and start-up and control (Figure IV-4).
As part of the Air Force Project Forecast II evaluation of nuclear propulsion for Earth Orbit transfer vehicles, EG&G conducted an analysis for the Air Force Astronautics Lab which includes an extensive discussion of testing facility requirements.(4) The report noted:
"The open air engine tests used to develop the NERVA fuel and engine system are not environmentally acceptable today. A proposed alternate fuel development approach is to screen candidate fuel systems and conduct failure tests in test reactors. Such testing facilities are designed for testing of closed loop systems. Failure tests on full scale core segments could also be conducted closed loop in larger facilities. A full scale engine system qualification could be accomplished but the facility must be designed to shut down immediately at the first detection of fission product release...
"It is proposed that candidate fuel systems be screened and failure tests conducted in test reactors such as the Advanced Test Reactor (ATR) at INEL... Failure tests on full scale core segments could also be conducted closed loop in larger facilities such as the LOFT containment facility at INEL... (which) has the capability of removing up to 60 MW of heat from within the containment without external circulation of primary loop coolant."
According to this analysis, the NERVA reliability goal of 0.995 included an estimated probability of three catastrophic failures per 1,000 engine cycles. The report observed that one of the disadvantages of nuclear space propulsion is the fact that:
"Adverse public reaction to flying/launching/orbiting anything with "nuclear" components may exist."
The report also noted that, compared with the Particle Bed Reactor:
"Generally, the NERVA concept will be easier to test, because of the lower power density in the fuel elements (3.6 kW/cm3 versus 8.2 kW/cm3), but both systems can be tested."
Thus, Air Force interest in the Particle Bed Reactor for propulsion applications antedates the genesis of the Timberwind program by several years.
2 - SDI Multi-Megawatt Program
The particle bed reactor was one of a number of concepts evaluated under the SDI Multi-Megawatt Power project. But a review of gas-cooled reactor concepts for SDI burst-mode and steady state power production applications concluded:(5)
"The NERVA derivative reactor appears to have a substantial lead over the other concepts. The NERVA reactor has demonstrated successful operation for operating conditions in excess of the expected SDI requirements in the NRX A-6 and XE' reactor demonstration tests... Years of technology and engineering development work would be required for other burst mode concepts to reach this stage... the concepts had to show some benefit to justify the risk and cost of a revolutionary concept, and these did not."
Given prior Air Force interest in the PBR, and the negative conclusion concerning its applicability to the multi-megawatt mission, other applications would be needed if the concepts development was to be continued.
As previously discussed, it is probably not coincidental that the formation in late 1987 of the Timberwind program came only months after this report was completed.
1. Powell, J.R., et al, "Nuclear Propulsion Systems for Orbit Transfer based on the Particle Bed Reactor," chapter 28 in Space Nuclear Power Systems 1988, (Orbit Book Company, Malabar, FL, 1989), pages 185-198.
2. Ludewig, H., "Particle Bed Reactor," NASA Nuclear Propulsion Workshop, NASA Lewis Research Center, Cleveland OH, July 10-12, 1990.
3. Beyers, Dan, "US Air Force Targets Three Project Forecast II Systems for Inclusion in Budget," Defense News, 27 October 1986, page 14.
4. Ramsthaler, J.H., et al, "Safe, Compact, Nuclear Propulsion -- Solid Core Nuclear Propulsion Concept," Final Report AFAL-TR-88-033, October 1988.
5. Marshall, A.C., "A Review of Gas-Cooled Reactor Concepts for SDI Applications," SAND87-0558, (Sandia National Laboratory, Albuquerque, NM, August, 1989). The initial draft of this report was released in August 1987.