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

S2 Critical Assembly
SNAP Critical Assembly core, image DOE

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.

SNAP-2 Experimental Reactor (SER, S2ER)

S2ER Cross Section
SNAP-2 Experimental Reactor core cros section diagram, image DOE

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.

S2ER Schedule and Timeline

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.

SNAP-2 Development Reactor (S2DR)

S2DR Core Xsec
SNAP 2 Development Reactor core cross section, image DOE

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.

S2DR Development Timeline

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

S2 PCS Cutaway Drawing
CRU mercury Rankine power conversion system cutaway diagram, image DOE

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.

s10 FSM Reactor
SNAP-2/10A FSM reflector and drum mechanism pre-test, image DOE

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.

S2 Program History Table

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.

S2 Cylinder Station
Cylindrical space station, image DOE

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.

s2 Toroidal Station
Hexagonal/Toroid space station. Note the wide radiation shield. Image DOE

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.

Hg Rankine Cutaway Drawing
Power conversion system, SNAP-2

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

Overall Program

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
https://www.osti.gov/servlets/purl/4048602

SNAP 2 SUMMARY REPORT, Jarrett 1973 https://www.osti.gov/servlets/purl/4430852

Mission Proposals

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(?)
https://www.osti.gov/servlets/purl/5445023

System Design and Development

THERMAL EXPANSION OF SNAP MATERIALS 1961
https://www.osti.gov/servlets/purl/4019446

SNAP 2 STRUCTURAL AND DYNAMIC ANALYSIS, 1964
https://www.osti.gov/servlets/purl/4049604

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
https://www.osti.gov/servlets/purl/4720086

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
https://www.osti.gov/servlets/purl/4169957

Fuel Element Design and Development

DEVELOPMENTAL TESTING SNAP 2 FUEL ELEMENTS 1964
https://www.osti.gov/servlets/purl/4487035

SNAP TECHNOLOGY HANDBOOK VOLUME II HYDRIDE FUELS AND CLADDINGS 1964 https://www.osti.gov/servlets/purl/4485359

Fuel Matrix

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
https://www.osti.gov/servlets/purl/4626080

Fuel Clad

Transverse Rupture Tests on Modified SNAP Fuel 1964
https://www.osti.gov/servlets/purl/1471195

Post-Irradiation

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
https://www.osti.gov/servlets/purl/4748631

SNAP TECHNOLOGY HANDBOOK VOLUME I LIQUID METALS 1964
https://www.osti.gov/servlets/purl/4038102

Pump Development

SODIUM PUMP DEVELOPMENT AND PUMP TEST FACILITY DESIGN 1963
https://www.osti.gov/servlets/purl/4101282

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
https://www.osti.gov/servlets/purl/966133

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

System 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
https://www.osti.gov/servlets/purl/4659314

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
https://www.osti.gov/servlets/purl/4647201

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
https://www.osti.gov/servlets/purl/4600110

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
https://www.osti.gov/servlets/purl/4463091

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
https://www.osti.gov/servlets/purl/4506588

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
https://www.osti.gov/servlets/purl/4523196

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
https://www.osti.gov/servlets/purl/4446506

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
https://www.osti.gov/servlets/purl/4626154

VOLUME VI SPIRAL-GROOVE THRUST BEARING EXPERIMENTAL RESULTS
https://www.osti.gov/servlets/purl/4649046

Status Reports

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 Reports

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
https://www.osti.gov/servlets/purl/4095745

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
https://www.osti.gov/servlets/purl/4626082

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
https://www.osti.gov/servlets/purl/7116801

THE SNAP 2 RADIATIVE CONDENSER ANALYSIS (unknown year)
https://www.osti.gov/servlets/purl/4098479

Thermal Analysis of an Anodized Reflector for SNAP 2, 1964
https://www.osti.gov/servlets/purl/4048698

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

Shielding

Radiation Damage Study on the Lithium Hydride SNAP Shield 1961
https://www.osti.gov/servlets/purl/6388475

SNAP SHIELD TEST EXPERIMENT REACTOR PHYSICS TESTS 1962
https://www.osti.gov/servlets/purl/4796589

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

Facilities Requirements

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
https://www.osti.gov/servlets/purl/4719063

Program Costs

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
https://www.osti.gov/servlets/purl/4745789

SAFETY ANALYSIS REPORT SNAPTRAN 2/10A-1 AND -2 SAFETY TESTS, 1965
https://www.osti.gov/servlets/purl/4597269

BIBLIOGRAPHY SNAP AEROSPACE NUCLEAR SAFETY PROGRAM REPORTS, 1973
https://www.osti.gov/servlets/purl/4564693

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