The idea for the SNAP-2 reactor originally came from a 1951 Rand Corporation study, looking at the feasibility of having a nuclear powered satellite. By 1955, the possibilities that a fission power supply offered in terms of mass and reliability had captured the attention of many people in the USAF, which was (at the time) the organization that was most interested and involved (outside the Army Ballistic Missile Agency at the Redstone Arsenal, which would later become the Goddard Spaceflight Center) in the exploitation of space for military purposes.
The original request for the SNAP program, which ended up becoming known as SNAP 2, occurred in 1955, from the AEC’s Defense Reactor Development Division and the USAF Wright Air Development Center. It was for possible power sources in the 1 to 10 kWe range that would be able to autonomously operate for one year, and the original proposal was for a zirconium hydride moderated sodium-potassium (NaK) metal cooled reactor with a boiling mercury Rankine power conversion system (similar to a steam turbine in operational principles, but we’ll look at the power conversion systems more in a later post), which is now known as SNAP-2. The design was refined into a 55 kWt, 5 kWe reactor operating at about 650°C outlet temperature, massing about 100 kg unshielded, and was tested for over 10,000 hours. This epithermal neutron spectrum would remain popular throughout much of the US in-space reactor program, both for electrical power and thermal propulsion designs. This design would later be adapted to the SNAP-10A reactor, with some modifications, as well.
Initial Testing and Prototyping of SNAP-2 Core
SNAP-2’s first critical assembly test was in October of 1957, shortly after Sputnik-1’s successful launch. With 93% enriched 235U making up 8% of the weight of the U-ZrH fuel elements, a 1” beryllium inner reflector, and an outer graphite reflector (which could be varied in thickness), separated into two rough hemispheres to control the construction of a critical assembly; this device was able to test many of the reactivity conditions needed for materials testing on a small economic scale, as well as test the behavior of the fuel itself. The primary concerns with testing on this machine were reactivity, activation, and intrinsic steady state behavior of the fuel that would be used for SNAP-2. A number of materials were also tested for reflection and neutron absorbency, both for main core components as well as out-of-core mechanisms. This was followed by the SNAP-2 Experimental Reactor in 1959-1960 and the SNAP 2 Development Reactor in 1961-1962.
The SNAP-2 Experimental Reactor (S2ER or SER) was built to verify the core geometry and basic reactivity controls of the SNAP-2 reactor design, as well as to test the basics of the primary cooling system, materials, and other basic design questions, but was not meant to be a good representation of the eventual flight system. Construction started in June 1958, with construction completed by March 1959. Dry (Sept 15) and wet (Oct 20) critical testing was completed the same year, and power operations started on Nov 5, 1959. Four days later, the reactor reached design power and temperature operations, and by April 23 of 1960, 1000 hours of continuous testing at design conditions were completed. Following transient and other testing, the reactor was shut down for the last time on November 19, 1960, just over one year after it had first achieved full power operations. Between May 19 and June 15, 1961, the reactor was disassembled and decommissioned. Testing on various reactor materials, especially the fuel elements, was conducted, and these test results refined the design for the Development Reactor.
For more information on the SNAP-2 Experimental Reactor, including a list of primary sources on the experiments and results, check out the SER page here.
The SNAP-2 Development Reactor (S2DR or SDR, also called the SNAP-2 Development System, S2DS) was installed in a new facility at the Atomics International Santa Susana research facility to better manage the increased testing requirements for the more advanced reactor design. While this wasn’t going to be a flight-type system, it was designed to inform the flight system on many of the details that the S2ER wasn’t able to. This, interestingly, is much harder to find information on than the S2ER. This reactor incorporated many changes from the S2ER, and went through several iterations to tweak the design for a flight reactor. Zero power testing occurred over the summer of 1961, and testing at power began shortly after (although at SNAP-10 power and temperature levels. Testing continued until December of 1962, and further refined the SNAP-2 and -10A reactors.
Fhttps://beyondnerva.com/snap-2-development-reactor-s2dr/or additional information about the SNAP-2 Development Reactor and its experimental results, check out the S2DR page, available here.
Other Test Stands and Experiments
A third set of critical assembly reactors, known as the SNAP Development Assembly series, was constructed at about the same time, meant to provide fuel element testing, criticality benchmarks, reflector and control system worth, and other core dynamic behaviors. These were also built at the Santa Susana facility, and would provide key test capabilities throughout the SNAP program. This water-and-beryllium reflected core assembly allowed for a wide range of testing environments, and would continue to serve the SNAP program through to its cancellation. Going through three iterations, the designs were used more to test fuel element characteristics than the core geometries of individual core concepts. This informed all three major SNAP designs in fuel element material and, to a lesser extent, heat transfer (the SNAP-8 used thinner fuel elements) design.
Extensive testing was carried out on all aspects of the core geometry, fuel element geometry and materials, and other behaviors of the reactor; but by May 1960 there was enough confidence in the reactor design for the USAF and AEC to plan on a launch program for the reactor (and the SNAP-10A), called SNAPSHOT (more on that below). Testing using the SNAP-2 Experimental Reactor occurred in 1960-1961, and the follow-on test program, including the Snap 2 Development reactor occurred in 1962-63. These programs, as well as the SNAP Critical Assembly 3 series of tests (used for SNAP 2 and 10A), allowed for a mostly finalized reactor design to be completed.
CRU-V: Mercury Rankine Power Conversion System
The power conversion system (PCS), a Rankine (steam) turbine using mercury, were carried out starting in 1958, with the development of a mercury boiler to test the components in a non-nuclear environment. The turbine had many technical challenges, including bearing lubrication and wear issues, turbine blade pitting and erosion, fluid dynamics challenges, and other technical difficulties. As is often the case with advanced reactor designs, the reactor core itself wasn’t the main challenge, nor the control mechanisms for the reactor, but the non-nuclear portions of the power unit. This is a common theme in astronuclear engineering. More recently, JIMO experienced similar problems when the final system design called for a theoretical but not yet experimental supercritical CO2 Brayton turbine (as we’ll see in a future post). However, without a power conversion system of useable efficiency and low enough mass, an astronuclear power system doesn’t have a means of delivering the electricity that it’s called upon to deliver.
A page on the CRU program will be available in the future, for now a literature review of original source material is available here.
Reactor shielding, in the form of a metal honeycomb impregnated with a high-hydrogen material (in this case a form of paraffin), was common to all SNAP reactor designs. The high hydrogen content allowed for the best hydrogen density of the available materials, and therefore the greatest shielding per unit mass of the available options.
Testing on the SNAP 2 reactor system continued until 1963, when the reactor core itself was re-purposed into the redesigned SNAP-10, which became the SNAP-10A. At this point the SNAP-2 reactor program was folded into the SNAP-10A program. SNAP-2 specific design work was more or less halted from a reactor point of view, due to a number of factors, including the slower development of the CRU power conversion system, the large number of moving parts in the Rankine turbine, and the advances made in the more powerful SNAP-8 family of reactors (which we’ll cover in the next post). However, testing on the power conversion system continued until 1967, due to its application to other programs. This didn’t mean that the reactor was useless for other missions; in fact, it was far more useful, due to its far more efficient power conversion system for crewed space operations (as we’ll see later in this post), especially for space stations. However, even this role would be surpassed by a derivative of the SNAP-8, the Advanced ZrH Reactor, and the SNAP-2 would end up being deprived of any useful mission.
The SNAP Reactor Improvement Program, in 1963-64, continued to optimize and advance the design without nuclear testing, through computer modeling, flow analysis, and other means; but the program ended without flight hardware being either built or used.
Initial planning for the SNAP-2 offered many options, with communications satellites being mentioned as an option early on – especially if the reactor lifetime could be extended. While not designed specifically for electric propulsion, it could have utilized that capability either on orbit around the Earth or for interplanetary missions. Other options were also proposed, but one was seized on early: a space station.
At the time, most space station designs were nuclear powered, and there were many different configurations. However, there were two that were the most common: first was the simple cylinder, launched as a single piece (although there were multiple module designs proposed which kept the basic cylinder shape) which would be finally realized with the Skylab mission; second was a torus-shaped space station, which was proposed almost a half a century before by Tsiolkovsky, and popularized at the time by Werner von Braun. SNAP-2 was adapted to both of these types of stations. Sadly, while I can find one paper on the use of the SNAP-2 on a station, it focuses exclusively on the reactor system, and doesn’t use a particular space station design, instead laying out the ground limits of the use of the reactor on each type of station, and especially the shielding requirements for each station’s geometry. It was also noted that the reactors could be clustered, providing up to 11 kWe of power for a space station, without significant change to the radiation shield geometry. We’ll look at radiation shielding in a couple posts, and look at the particulars of these designs there.
Since space stations were something that NASA didn’t have the budget for at the time, most designs remained vaguely defined, without much funding or impetus within the structure of either NASA or the US Air Force (although SNAP-2 would have definitely been an option for the Manned Orbiting Laboratory program of the USAF). By the time NASA was seriously looking at space stations as a major funding focus, the SNAP-8 derived Advanced Zirconium Hydride reactor, and later the SNAP-50 (which we’ll look at in the next post) offered more capability than the more powerful SNAP-2. Once again, the lack of a mission spelled the doom of the SNAP-2 reactor.
The SNAP-2 reactor met its piecemeal fate even earlier than the SNAP-10A, but oddly enough both the reactor and the power conversion system lasted just as long as the SNAP-10A did. The reactor core for the SNAP-2 became the SNAP-10A/2 core, and the CRU power conversion system continued under development until after the reactor cores had been canceled. However, mention of the SNAP-2 as a system disappears in the literature around 1966, while the -2/10A core and CRU power conversion system continued until the late 1960s and late 1970s, respectively.
References and Additional Reading
Preliminary Results of the SNAP-2 Experimental Reactor, Hulin et al 1961 https://www.osti.gov/servlets/purl/4048774
SNAP-2 RELIABILITY PROGRAM, Burgess 1963 https://www.osti.gov/servlets/purl/4048742
BOOMER – A DIGITAL PROGRAM FOR EVALUATING THE THERMAL AND KINETICS RESPONSE OF A SNAP 2/lOA REACTOR, Winston 1964
SNAP 2 SUMMARY REPORT, Jarrett 1973 https://www.osti.gov/servlets/purl/4430852
Application of the SNAP 2 to Manned Orbiting Stations, Rosenberg et al 1962 https://www.osti.gov/servlets/purl/4706177
APPLICATIONS OF SNAP REACTOR SYSTEMS TO COMMUNICATIONS SATELLITES 1962 https://www.osti.gov/servlets/purl/4782601
TECHNOLOGICAL IMPLICATIONS OF SNAP REACTOR POWER SYSTEM DEVELOPMENT ON FUTURE SPACE NUCLEAR POWER SYSTEMS, 1970s(?)
System Design and Development
THERMAL EXPANSION OF SNAP MATERIALS 1961
SNAP 2 STRUCTURAL AND DYNAMIC ANALYSIS, 1964
SNAP 2 PERFORMANCE ANALYSIS, 1964 https://www.osti.gov/servlets/purl/4659359
Nuclear Design and Development
Four Goup Cross Sections, Colston 1962 https://www.osti.gov/servlets/purl/4741378
KINETICS, STABILITY, AND CONTROL A SELECTED BIBLIOGRAPHY 1963
SAFETY ANALYSIS REPORT – SNAPTRAN 2/lOA-3 WATER IMMERSION TESTS 1964 https://www.osti.gov/servlets/purl/4048379
METHODS FOR CALCULATING FAST-NEUTRON LEAKAGE FROM THE SNAP-TSF REACTOR AND PRELIMINARY RESULTS 1967
Fuel Element Design and Development
DEVELOPMENTAL TESTING SNAP 2 FUEL ELEMENTS 1964
SNAP TECHNOLOGY HANDBOOK VOLUME II HYDRIDE FUELS AND CLADDINGS 1964 https://www.osti.gov/servlets/purl/4485359
EFFECTS OF VIBRATION AND SHOCK ON HYDROGEN PERMEATION OF SNAP 2/lOA DEVELOPMENTAL FUEL ELEMENTS 1963 https://www.osti.gov/servlets/purl/4474942
ASYMMETRIC HEAT TRANSFER IN A SNAP 2 FUEL ELEMENT 1964
Transverse Rupture Tests on Modified SNAP Fuel 1964
IRRADIATION PERFORMANCE OF A FULL-SCALE PROTOTYPE SNAP 2 REACTOR FUEL ELEMENT, 1967 https://www.osti.gov/servlets/purl/4333278
Primary Coolant Loop Design and Development
Evaluation of Nak as the Primary Coolant for the SNAP II System, Wallerstedt 1959 https://www.osti.gov/servlets/purl/1023272
SNAP 2 Primary System Test – Objectives, System Description and Procedures, Kikkin 1961 https://www.osti.gov/servlets/purl/4806915
Velocity Deviations in a SNAP Reactor Cooling Channel, 1962
SNAP TECHNOLOGY HANDBOOK VOLUME I LIQUID METALS 1964
SODIUM PUMP DEVELOPMENT AND PUMP TEST FACILITY DESIGN 1963
SPACE NUCLEAR SYSTEM THERMOELECTRIC NaK PUMP DEVELOPMENT SUMMARY REPORT 1973 https://www.osti.gov/servlets/purl/4450502
SNAP Fuel Temperature Peaking with Cusps in Coolant Channel 1963
Hot Channel Effect on Fuel Temperature 1964 https://www.osti.gov/servlets/purl/4635983
Primary Loop Startup Capabilities for an Advanced 20 KWe Mercury Rankine System 1964 https://www.osti.gov/servlets/purl/4648895
SNAP 2/10A HYDRAULIC STUDIES, Thomasson 1964 https://www.osti.gov/servlets/purl/4071156
ANALYSIS OF SNAP REACTOR COOLANT CROSS-FLOW IN THE SNAP-2 REACTOR, Montgomery 1964 https://www.osti.gov/servlets/purl/4005294
Power Conversion System Design and Development
DYNAMIC ANALYSIS, 1960 https://www.osti.gov/servlets/purl/4166808
ROTATIONAL SPEED CONTROL, Dauterman et al 1962 https://www.osti.gov/servlets/purl/4065680
CRU DESIGN AND DEVELOPMENT 1962 https://www.osti.gov/servlets/purl/4709935
Transient Thermal Start-up Analysis for CRU-V 1963 https://www.osti.gov/servlets/purl/4648932
Parasitic Radial Magnetic Forces in the CRU-V Alternator 1963
SNAP MERCURY RANKINE PROGRAM SNAP 2 STRUCTURAL EVALUATION PROTOTYPE SYSTEM (PSM-2) 1964 https://www.osti.gov/servlets/purl/4683532
High Power CRU Scaling Analysis and Pre-Conceptual Designs 1964
CRU V DESIGN AND DEVELOPMENT 1967 https://www.osti.gov/servlets/purl/4720495
Component Design and Development
PRELIMINARY STUDY OF THE LIQUID METAL LOOP AND TEST RIG FOR PHASE II OF THE INVESTIGATION OF LXQUIP METAL LUBRICATED BEARINGS AND ROTOR-BEARING DYNAMICS 1965
Materials Design and Development
BEARING DESIGN & DEVELOPMENT, 1960 https://www.osti.gov/servlets/purl/4063342
TURBINE DESIGN AND TESTING 1960 https://www.osti.gov/servlets/purl/4178248
MERCURY MATERIALS EVALUATION AND SELECTION FY-1962
MERCURY MATERIALS EVALUATION AND SELECTION FY-1963 https://www.osti.gov/servlets/purl/4457277
Mercury Pump Degradation, 1963 https://www.osti.gov/servlets/purl/4648930
CORROSION PRODUCTS IN THE SNAP 2/MRPCP CRU V TEST SYSTEM , 1967
Hg Boiler Development
MERCURY BOILING RESEARCH 1962 https://www.osti.gov/servlets/purl/4733677
Spiral Boiler Evaluation, 1962 https://www.osti.gov/servlets/purl/4747775
BOILER CONDITIONING PHASE I RESULTS, 1966
MERCURY BOILER DEVELOPMENT ON THE SNAP 2/MRPC PROGRAM Ziobro et al 1968 https://www.osti.gov/servlets/purl/4510637
BOMPUP – SNAP 2 Thermoelectric Boiling Mercury Pump Model, 1965
MERCURY CONDENSING EXPERIMENTS 1964 https://www.osti.gov/servlets/purl/4014817
DEVELOPMENT OF LIQUID-MERCURY-LUBRICATED BEARINGS
VOLUME I ANALYTICAL DESIGN APPROACH AND STATUS OF BEARING SYSTEMS (WITH APPENDIXES) https://www.osti.gov/servlets/purl/4482772
VOLUME II PLAIN BEARING EXPERIMENTAL RESULTS https://www.osti.gov/servlets/purl/4475183
VOLUME III TILTING-PAD BEARING EXPERIMENTAL RESULTS https://www.osti.gov/servlets/purl/4647320
VOLUME IV THREE-SECTOR BEARING EXPERIMENTAL RESULTS, 1966 https://www.osti.gov/servlets/purl/4583209
VOLUME V THREE-PAD BEARING EXPERIMENTAL RESULTS
VOLUME VI SPIRAL-GROOVE THRUST BEARING EXPERIMENTAL RESULTS
SNAP-2, FY 1963, CRU-IVM TEST HISTORY, 1963 https://www.osti.gov/servlets/purl/4626083
SNAP SYSTEMS IMPROVEMENT PROGRAM MERCURY RANKINE PROGRAM APRIL – JUNE 1964 https://www.osti.gov/servlets/purl/4471035
FINAL SUMMARY REPORT – SNAP 2/MERCURY RANKINE PROGRAM REVIEW VOLUME 1 Wallerstedt et al 1967 https://www.osti.gov/servlets/purl/4642824
Component Design and Development
Structural Analysis of SNAP 2. Reactor Vessel Top Head, 1962
Equilibrium panel surface temperatures in the SNAP-2 instrument compartment, Greshko 1962 https://www.osti.gov/servlets/purl/6129861
R/C and PCS Components and Structures (?) Prior to Injection, 1963
Startup and Shutdown Transients for the SNAP-2 R/C and PCS Components, 1964 https://www.osti.gov/servlets/purl/4647202
Thermo-Physics Technical Note No. 55: Thermal and Hydraulic Analysis of the Tower Shield Facility Experiment Heat Rejection System 1965
THE SNAP 2 RADIATIVE CONDENSER ANALYSIS (unknown year)
Thermal Analysis of an Anodized Reflector for SNAP 2, 1964
THE DEVELOPMENT AND QUALIFICATION OF THERMAL CONTROL COATINGS FOR SNAP SYSTEMS 1965 https://www.osti.gov/servlets/purl/4623831
SNAP REACTOR CONTROL-DRUM DRIVE 1964 https://www.osti.gov/servlets/purl/4480131
Radiation Damage Study on the Lithium Hydride SNAP Shield 1961
SNAP SHIELD TEST EXPERIMENT REACTOR PHYSICS TESTS 1962
COMPARISON OF MONTE CARLO CALCULATIONS WITH MEASUREMENTS OF FAST-NEUTRON DOSE TRANSMITTED FROM A BEAM SOURCE THROUGH A SNAP-2 LiH SHIELD V. R. Cain and K. D. Franz, 1968 https://www.osti.gov/servlets/purl/4841060
MEASUREMENT OF THE FAST-NEUTRON DOSE RATE TRANSMITTED THROUGH A IJE SHIELD WHEN USED AS A WINDOW IN lRt)N-OIL SHIELD MOCKUPS 1969 https://www.osti.gov/servlets/purl/4765356
The ORNL-SNAP Shielding Program, Mynatt et al 1971 https://www.osti.gov/servlets/purl/4045094
DEVELOPMENT OF A LARGE METAL ULTRAHIGH VACUUM SIMULATION CHAMBER 1961 https://www.osti.gov/servlets/purl/4773830
TECHNICAL DESCRIPTION OF A SODIUM-COMPONENT TEST INSTALLATION 1961 https://www.osti.gov/servlets/purl/4781951
SODIUM-TO-AIR COOLING SYSTEM 1969
Progress Reports and Auditing
SNAP 2/10 REACTOR PROGRESS REPORT
APRIL-JUNE 1961 https://www.osti.gov/servlets/purl/4474972
JULY-SEPTEMBER 1961 https://www.osti.gov/servlets/purl/4462624
JANUARY – MARCH 1962 https://www.osti.gov/servlets/purl/4482703
SNAP 2 NUCLEAR APU DEVELOPMENT PROGRESS REPORTS
APRIL-JUNE 1961 https://www.osti.gov/servlets/purl/4467133
JULY-SEPTEMBER 1961 https://www.osti.gov/servlets/purl/4464950
JULY-SEPTEMBER 1962 https://www.osti.gov/servlets/purl/4471169
SNAP SUPPORTING R& D
JANUARY-MARCH 1962 https://www.osti.gov/servlets/purl/4571425
Safety and Flightworthiness
TSF-SNAP REACTOR SAFETY ANALYSIS REPORT, Lewin 1967
SAFETY ANALYSIS REPORT SNAPTRAN 2/10A-1 AND -2 SAFETY TESTS, 1965
BIBLIOGRAPHY SNAP AEROSPACE NUCLEAR SAFETY PROGRAM REPORTS, 1973
Nuclear Criticality Safety Experiments, Calculations, and Analyses-1958 to 1982 https://www.osti.gov/servlets/purl/6489025
Decommissioning and Remediation
Aa Evaluation of the Techniques for End-of-Llfe Shutdown of Orbiting SNAP Reactors, 1963 https://www.osti.gov/servlets/purl/6528458
SNAP Re-entry Orbit^- Comments on the Atmospheric Entry and Discussion of a Proposed Test 1962 https://www.osti.gov/servlets/purl/6387230