Space Nuclear Power

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Space Nuclear Power

• Benjamin Amiri
• Brian Ade
• David Dixon
• Space Nuclear Power Group
• Los Alamos National Lab

2
Purpose of This Course

• To overview the basic physics underlying fission
  reactors
• To convey an understanding of the design
  approaches and considerations for fission reactor
  systems
• To convey an understanding of design and
  integration issues related to space fission
  systems
• To convey an understanding of special issues
  related to space fission systems (e.g., Special
  Nuclear Materials safeguarding and transport)?

3
Questions Faced by Teachers for Studentswho are
tomorrows adults

• What are the properties of nuclear materials?
• What is radiation?
• How to produce, use and dispose of nuclear
  materials?
• What is a nuclear reactor?
• What is the difference between an open and
  closed fuel cycle?
• What is transmutation?
• What is a nuclear weapon?
• What is environmental management
• What are nuclear programs of tomorrow?
• What are nuclear technical challenges of
  tomorrow?

4
Some Historical Perspective

• Start with Oklo, Gabon, rich uranium deposits and
  groundwater seeped
• 2 billion years later
• 1905 Einstein develops Nobel Prize winning
  explanation of the Photoelectric Effect-
  Einsteins Theory of Special Relativity - energy
  and mass are equivalent.
• 1924 Prince Louie de Broglie- particles can
  behave as waves
• 1925 1926 Schrodinger and Heisenberg formulate
  quantum mechanics
• In 1930, W. Bothe and H. Becker highly
  penetrating radiation was emitted when beryllium,
  boron or lithium were bombarded by alpha
  particles from Po
• In 1932, Irene Currie and her husband protons
  were produced when striking hydrogen containing
  substances with this newly discovered penetrating
  radiation
• James Chadwick particle is neutral and weighs
  the same as proton, named this new particle
  neutron
• In 1934, Enrico Fermi irradiated uranium with
  neutrons trying to produce the first transuranic
  elements however, he accidentally achieved the
  world's first nuclear fission. In 1938, he
  receives the Nobel Prize in Physics
• In 1939, Hans and Strassmann Reactants from
  fission included elements in the medium mass
  region. The presence of these medium mass region
  nuclides suggested that the nucleus had split.
  The fact that the sum of these medium mass
  nuclides did not add to the sum of the initial
  parent suggested that some of the mass was
  converted into energy.
• L. Meitner and O. Frisch termed this process
  fission and also calculated the energy released
  during fission of a U-235 nuclide to be 200 MeV
• Albert Einstein then wrote his famous letter to
  President Franklin Roosevelt
• Enrico Fermi and Leo Szilard proposed placing
  uranium in a matrix of graphite. On December 2nd
  1942, the first controlled self-sustaining chain
  reaction was achieved in a squash court under the
  University of Chicagos Stagg Field
• Little Boy, August 6 1945, Hiroshima killing over
  100,000 people, Fat Man, August 9 1945, Nagasaki
  killing 75,000

5
Reactor History

• In 1946, Enrico Fermi published a scheme for
  outlining the future uses of nuclear energy
• September 5, 1953 First peacetime research
  reactor critical in Raleigh, NC.
• December 8, 1953 US President Dwight D.
  Eisenhower, Atoms for Peace speech to the United
  Nations
• December 2, 1957 Duquesne Light Company --
  Shippingport Atomic Power Station (PWR).
• August 3, 1957 Argonne National Laboratory and
  General Electric -- GE Valacitos
• First commercial BWR to be licensed by the US AEC
• The progress of these commercial power systems
  later lead to the development of other competing
  reactor systems
• CANDU, GCFR, LMFBR, PBMR, MSR

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Fission is a Well-Developed,Extensively Utilized
Technology

• Fission power systems have been operating safely
  and reliably since 1942
• Fission reactors are used by governments,
  industry, utilities, and universities
• Existing nuclear fission power plants are the
  least expensive source of electricity in the US
  public acceptance may be increasing
• Several countries have plans for development
  and/or utilization of advanced reactors.

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What is the structure of an atom

• Neutron 1u, charge is neutral /-
• Proton 1u, charge is positive
• Electron 0.0005u, charge is negative -
• Atomic number, Z of protons in atom
• Atomic mass neutrons Z
• Nucleus contains neutrons and protons
• Electrons are in orbit around nucleus
• Isotopes of the same element have the same Z
  number, but different nuclear mass

8
Common Sources of Radiation

• Common Sources of Radiation Include
• Solar radiation
• H-3 , Be-7, C-14
• Concrete/Building Materials (lt10 mrem)?
• Uranium is found in ppb in concrete and decays to
  radon gas which emits alphas/gammas
• Yourself
• K-40
• Sleeping next to someone for 8 hours 2 mrem
• Medical Devices
• Xrays, Flouroscopy, PET, Nuclear Medicines
• Smoking (up to 16,000 mrem)?
• Po-210, radon
• The average cumulative dose for a human on earth
  is 360 mrem/year

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Natural Sources of Radioactivity

• Natural Sources in the U.S. (mSv/yr)
• Radon 2.0
• Internal to Body 0.39
• Terrestrial 0.28
• Cosmic 0.27
• C-14 , H-3 0.01
• TOTAL 3.0
• Person-made Sources
• Diagnostic Xrays 0.39
• Nuclear Medicine 0.14
• Consumer products 0.10
• TOTAL 0.6

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What are the properties of nuclear materials?

• Four types of Radioactive Decay
• Alpha emission
• Beta particle, electrons expelled by excited
  nucleus
• Gamma radiation
• Neutron
• Nuclear Reactions
• Fission
• Capture
• Scatter
• Half life

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Radioactive Decay

• Beta Decay (e-)?

• Alpha Decay

(2.45 Mev)?

• Gamma Decay

• Neutron Decay

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Radioactive Decay Equations

• dN/dT -N ? where N of Radioactive
  nuclei, t is time,
• and ? is the
  decay constant- time
• N ? is the activity of a sample in
  distintegrations/time
• Activity is measured in Curies (1 Ci 3.7 E10
  d/s)
• or Becquerels (bq) 2.707 E -11 Ci or 1 d/s
• NN

e
N Number radioactive nuclides surviving

• For N

N / 2
T
0.693/ ?
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Photon Interactions with Matter

• Photons (?s and x-rays) interact with matter in
  three ways
• - Photoelectric effect ? Z4 / E3
• - Compton Scattering elastic scattering of
  photon w/ free electron scattered photon
  has less energy than incident photon as E(?)
  increases, energy loss to incident photon
  increases
• - Pair Production at E(?) gt 1.02 MEV (1 MeV
  1.602 -13 Joules), the photon, in the
  presence of a nucleus, can spontaneously convert
  to an electron (e-) and positron (e) pair
• Pair production dominates over 5 Mev Compton
  Scattering dominates in the 0.5 to 5 Mev range

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Photoelectric Effect
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Compton Scattering
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Pair Production
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Total Gamma Ray Absorption
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Gamma Ray Absorption is Energy and Z Dependent
Values of the mass attenuation coefficient, ?/?
and the mass energy-absorption coefficient, ?en/?
as a function of photon energy.
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Gamma Ray Absorption is Energy and Z Dependent
Values of the mass attenuation coefficient, ?/?
and the mass energy-absorption coefficient ,
?en/? as a function of photon energy.
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Gamma Ray Absorption is Energy and Z Dependent
Values of the mass attenuation coefficient, ?/?
and the mass energy-absorption coefficient ,
?en/? as a function of photon energy.
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Mass Attenuation Coefficient (?/? cm2/g) of Al,
Fe, W, and U at 1.0, 3.0, and 8.0 MeV
Shield design must also take into account
buildup, inelastic neutron scatter, gammas from
neutron capture, geometry, thermal management,
radiation damage, and other factors.
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Comparison of Fission and Radioisotope Power
Radioisotope Decay
Fission
Heat Energy 0.024 MRV/nucleon (0.558 W/g
PU-238 Natural decay rate (87.7-year half-life)?
Heat Energy 0.851 MRV/nucleon Controllable
reaction rate (variable power levels)?

• Long history of use on Apollo and space science
  missions 44 RTGs and numerous RHUs launched by
  U.S. during past 40 years
• Heat produced from natural alpha (? ) particle
  decay of Plutonium (Pu-238)?
• Small portion of heat energy (5 - 20) converted
  to electricity via passive or dynamic processes.

• Many U.S. technology programs over last 50 years
  only one unit (SNAP-10A) was flown in 1965.
  Former U.S.S.R. flew over 30.
• Heat produced from neutron-induced splitting
  (fission) of Uranium (U-235). At steady-state, 1
  or the 2 to 3 neutrons from reaction cases a
  subsequent fission in a chain reaction process.
• Heat converted to electricity, or used directly
  to heat a propellant.

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What does fission produce?
Fission fragment 1

• Energy
• Radiation
• Neutrons

neutron
energy
235U
neutron
Fission fragment 2
neutron
235U
radiation
neutron
radiation
neutron
Fission fragment 1
energy
Fission fragment 2

• Chain reaction

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Nuclear Fission Process
Fission
Maximum Stability
Fusion
180 MeV prompt useful energy (plus 10 MeV
neutrinos) - additional energy released in form
of fission product beta particles, gamma rays,
neutron capture gammas (200 MeV total useful)?

• Neutron absorbed by heavy nucleus, which splits
  to form products with higher binding energy per
  nucleon. Difference between initial and final
  masses prompt energy released (190 MeV).
• Fissile isotopes (U-233, U-235 and Pu-239)
  fission at any neutron energy
• Other actinides (U-238) fission at only high
  neutron energies
• Fission fragment kinetic energy (168 MeV),
  instantaneous gamma energy (7 MeV), fission
  neutron kinetic energy (5 MeV), Beta particles
  from fission products (7 MeV), Gamma rays from
  fission products (6 MeV), Gamma rays from neutron
  capture (7 MeV).
• For steady power production, 1 of the 2 to 3
  neutrons from each reaction must cause a
  subsequent fission in a chain reaction process.

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Nuclear Fission Process
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Fission Products

• Fission events yield bimodal distribution of
  product elements.
• These products are generally neutron-rich
  isotopes and emit beta and gamma particles in
  radioactive decay chains.
• Most products rapidly decay to stable forms a
  few, however, decay at slow rates or decay to
  daughter products which have long decay times.
• Example fission products of concern
• Strontium-90 (28.8-year half-life)?
• Cesium-137 (30.1-year half-life)?
• Isotope amounts decrease by factor of 1,000 after
  10 half-lives and 1,000,000 after 20 half-lives.
• Decay power 6.2 at t0 (plus fission from
  delayed neutrons), 1.3 at 1 hour, 0.1 at 2
  months (following 5 years operation).

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Prompt vs Delayed Neutrons

• gt99 of total fission neutrons are prompt
  neutrons.
• Prompt neutrons released within 1.0 x 10-14 s
  of the instant of fission.
• Most prompt neutrons have energies between 1
  and 2 MeV, some gt 10 MeV

Prompt Fission Neutron Energy Spectrum for
Thermal Fission of Uranium-235
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Prompt vs Delayed Neutrons

• Fraction of delayed neutrons varies with
  isotope and (slightly) with neutron spectrum.
• Average delayed neutron energy significantly
  less than average prompt neutron energy.
• Delayed neutron importance factor gt1 for
  compact HEU systems.

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Reactor Kinetics
Reactivity ? (k-1)/k, where k is the
effective multiplication factor Reactivity in
? / ? Prompt neutron lifetime l 1 / (v
?a (1 L2Bg2))? Where l prompt neutron
lifetime v average neutron speed ?a
macroscopic absorption cross section 1 L2Bg2
nonleakage probability Prompt neutron lifetime
is on order of 10-6 s for compact, fast-spectrum
system and 10-4 for thermal reactor
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Reactor Kinetics
If there were no delayed neutrons, reactor would
be difficult to control Reactor period Tp l/
? Example ? 0.0001, l 1 microsecond gt
Tp 0.01 s Delayed neutrons stabilize
operation leff ? l (?-?)/? where ?
properly weighted average decay constant for six
actual delayed neutron groups ? 0.08 s-1 Actual
stable reactor period would be Tpeff (?-?)/? ?
800 seconds in previous example
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Delayed Neutrons
Example of Delayed Neutron Precursor
Other important neutronic parameters Nu ?
average number of neutrons liberated per
fission Eta ? neutrons liberated per neutron
absorbed
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Reactor Basics
Secondary Shield
Primary Coolant Loop
Primary Shield
Secondary Coolant Loop
Fission energy transfers to primary coolant
Energy to turbine or engine for power production
Heat Exchanger
Energy transfers to secondary coolant
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Nuclear Reactor Basics
Radial neutron flux distribution in spherical core
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Reactor Design

• The goal is to determine the amount of fissile
  and control material needed in order to
• Maintain the self-sustaining chain reaction
• Operate the system at a prescribed power
• Operate for a prescribed amount of time
• Achieving some type of operating goal
• Accounting for reactivity deficits
• Secondary design considerations may include
  waste, useful fissile material and medical
  isotope production
• Varied Material and geometry combinations in
  order to design a system that meets the operating
  goals at the minimum cost
• Mechanism Equations/Process
• Coefficients Nuclear Data

Refueling at Davis-Besse Nuclear Power Station
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Deterministic Method
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The Monte Carlo Method

• The Monte Carlo method uses probability theory to
  model a system stochastically
• Random sampling of events

Probability that a neutron moves a distance dx
without any interaction
Probability that a neutron has its first
interaction in dx p(x)dx

• Probability density function (PDF)? A real-valued
  function whose integral over any set gives the
  probability that a random variable has values in
  the set

• Cumulative Distribution Function (CDF) ?The
  probability that the variable takes a value less
  than or equal to x

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Reaction Sampling

• If particle interacts in a cell volume the
  isotope with which the particle interacted must
  be determined

• Then the reaction type must be determined

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Neutron Cross Sections
Measure of the probability of a particular
neutron-nucleus interaction. Property of the
nucleus and the energy of the incident
neutron. Symbolized ?, common unit is barn
1.0 x 10-28 m2 Neutron Flux nv ? n
neutrons / m3 v neutron speed (m/s)? Reaction
rate ? N ? N nuclei / m3 ? neutron flux
(neutrons / m2-s)? ? cross section (m2)?
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Comparison of Hydrogen and Deuterium Cross
Sections
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Simplified Depletion Equation
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Boiling Water Reactor
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Pressurized Water Reactor
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CANDU
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LMFBR
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Generation IV Reactor Core Modeling

• As advanced reactor concepts challenge the
  accuracy of current modeling technologies, a
  higher fidelity more robust depletion tool is
  necessary in order to properly model
  time-dependant core characteristics

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Fission Power Systems
Fissioning 12 fl oz (341 ml) of Uranium yields 50
times the energy contained in a Space Shuttle
External Tank Energy Density 82 billion joules
per gram
50 x

• Fission overcomes limitations of other candidate
  power sources
• Chemical already near theoretical performance
  limits
• Radioisotopes versatile and long-lived, but low
  power density and limited Pu-238 supply
• Natural sources (e.g., solar, EM tethers) highly
  dependent on location w/respect to sun or planet
• Advanced concepts (e.g., beamed energy, fusion)
  too immature, may not work, and/or require
  substantial in-space infrastructure and
  investment
• Greatly extends capability, sophistication and
  reach of future science missions
• Enables use of high-performance electric
  propulsion beyond inner solar system
• Provides long-duration, power-rich environments
  for sophisticated scientific investigations,
  high-data rate communications and complex
  spacecraft operations
• Improves safety, capability and performance of
  future human planetary missions
• Power-rich spacecraft and surface operations
• Rapid transportation to reduce extended exposure
  to solar/cosmic radiation and zero-g

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Radioisotope Power Systems

• NASA uses two types of Radioisotope Power Systems
  (RPS), both of which use heat from decay of
  plutonium-238.

• Radioisotope Thermal Generator (RTG)?
• Heat transferred to thermal-to-electric power
  conversion system
• Flown on many NASA missions Pioneer, Voyager,
  Viking, Apollo, Galileo, Ulysses, Mars
  Pathfinder, Cassini
• Radioisotope Heater Unit (RHU)?
• Heat keeps spacecrafts instruments warm and
  within designed operating temperatures
• Latest deployment onboard NASAs Mars Exploration
  Rovers Spirit and Opportunity
• NASA Project Prometheus is supporting
• Pluto/New Horizons mission
• Stirling Radioisotope Generator (SRG) Mission
• Multi-Mission Radioisotope Thermal Generator
  (MMRTG)?

Pu-238 Fuel Pellet
RHU
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Why Fission?

• Fission is the only near-term technology that can
  provide reliable high power at low mass for a
  large variety of missions.
• Conventional chemical systems.
• Very near their theoretical performance limit,
  low energy density.
• Solar power.
• Degrades rapidly as distance from the sun
  increases.
• Dependent on orientation, radiation field,
  eclipses, debris, etc.
• Radioisotope power.
• Energy density is many orders of magnitude below
  fission.
• Future sources of Pu-238 or other radioisotopes
  unknown.
• Advanced concepts.
• Fusion, anti-matter, pulsed-nuclear, tethers,
  solar sails, etc.
• These are many decades from being used for
  practical exploration.
• All of the above power sources have a valuable
  place in space exploration, but only fission can
  truly enable ambitious exploration in the
  near-term.
• Recent technology advances in power conversion
  systems, radiators, spacecraft design,
  electronics, etc., put less burden on the
  reactor.
• Reactor does not have to be a Ferrari right out
  of the gate.

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Potential Benefits of Space Fission Systems

• Space Fission Power can enable or contribute to
  many national and global missions, including the
  Presidents new exploration vision.
• Ambitious space science and exploration.
• Mars/lunar surface power/propulsion (robotic and
  manned).
• Outer-planet missions Jovian Moons, Pluto
  orbiter, etc.
• Interstellar precursor or solar missions.
• Enhanced national and planetary defense.
• High power and enhanced mobility for defense
  applications.
• Potential use for comet/asteroid defense.
• Synergy with advanced terrestrial and airborne
  defense systems.
• Significant commercial value in space.
• Satellite power, mobility, maintenance,
  retrieval.
• Space tourism (orbital, lunar, ?).

• Revitalizing the nations nuclear power
  infrastructure and capabilities
• Space fission programs inspire students and young
  professionals to pursue nuclear engineering (gt100
  applicants for LANL student-internships this
  year)?
• Many technologies developed could be used for
  advanced terrestrial power reactors
• Furthermore, for new and enabling technologies
  the majority of applications are not thought of
  until the technology is proven and in hand.

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Summary of Radioisotope/Fission System Differences
55
What makes a space reactor unique?

• Fission reactors represent a well established
  terrestrial technology.
• Commercial power plants
• Research/production reactors
• Naval applications
• There are several major requirements that
  distinguish a space reactor from a terrestrial
  reactor (to different levels depending on the
  application (i.e. commercial or naval)
• Specific mass (alpha mass/power)?
• Reliability and lifetime
• System Integration
• Operations/Environment
• Safety
• Safeguards
• Subclasses of space reactors also exist, but most
  of these distinguishing factors apply to all of
  them.
• I.e. space power, surface power, process heat,
  thermal propulsion, etc.

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Differences Between Terrestrial and Space Reactors

• Specific mass (alpha mass/power)?
• Reliability
• System Integration
• Operations/Environment
• Safety
• Safeguards
• Testing
• It is not practical to test the flight-unit with
  nuclear power. Nuclear test-units cannot be
  tested in prototypic environment and have vastly
  different safety requirements. On the plus side,
  small space systems can be designed to allow
  prototypic nuclear criticality testing and
  non-nuclear-powered thermal-structural system
  testing.

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Nuclear Subsystem Functions

• Reactor core
• Typically consists of an array of fuel pins
• Produces heat by fissioning of 235U
• Primary heat transport
• Tranfers heat from reactor core to power
  conversion subsystem
• Gas or liquid metal (loop or Heatpipe)?
• Usually includes an intermediate heat exchanger
• Radiation shield
• Limits gamma and neutron doses to other
  subsystems and payload
• Instrumentation and control
• Controls fission rate in reactor core
• Controls startup, shutdown, and other transients
• Maintains subcriticality under accident
  conditions
• Structure
• Supports NSS components during all mission phases
• Provides mounting interface to power conversion
  subsystem and NEP vehicle

Brayton subsystem
Control drives
Heat transport ducting
Nuclear subsystem
Radiation shield
Neutron relflectors
Reactor core
(Gas-cooled direct Brayton example shown)?
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Principle Nuclear Reactor Elements Core

• Designed to maintain stable chain reaction
  process while delivering power at desired
  temperature distribution.
• Options for core cooling include heat pipes,
  pumped gas, and pumped liquid metal. Each has
  advantages and disadvantages depending on overall
  system design and application.
• Core designed for safe, stable operation.
  Materials and geometry chosen so that power
  increase results in reactivity decrease.
• Minimum required fissile material mass typically
  less for moderated reactors than for
  fast-spectrum reactors. Total system mass may be
  similar because of other factors.
• Hydrogenous moderators limited to peak operating
  temperature of 1000 K, potentially limiting power
  conversion efficiency.

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Principle Nuclear Reactor Elements Fuel
Nb-1Zr Cladding Cap

• Numerous fuel options available. Choices
  include
• Geometry (e.g. pin, particle, prismatic, foil)?
• Isotope (e.g. U-233, U-235, Pu-239, Am242m)?
• Compound (e.g. U-metal, UZrH, UO2, UN, UC, UC2,
  UCZrN, (U,Zr,Nb)C, UF4)?
• Factors to consider include the following
• Required fuel operating temperature
• Fuel burnup (fraction of uranium fissioned)?
• Fuel operating environment
• Fabricability / technical risk
• Desired/required uranium density
• Core power density
• Two leading fuel options are UO2 and UN
• UO2 flown in space (TOPAZ), used by commercial
  power industry and Navy, available commercially
  and from National Laboratories
• UN (current baseline) developed during SP-100 and
  previous programs. High uranium loading, high
  thermal conductivity, low swelling/fission gas
  release

BeO Reflector Pellet
UN Fuel Assembly
BeO Reflector Pellet
Heatpipe Module
Nb-1Zr Cladding Cap
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Principle Nuclear Reactor Elements
Moderator/Reflector/Control/Shield
Clad Be Control Drum

• Moderator
• Hydrogen best moderator material for space
  systems. UZrH or UZrH good to 1000 K.
• Reflector
• Beryllium (Be) and Beryllium Oxide (BeO) best for
  space systems
• Be flown in space, easier to fabricate
• Control
• Control by varying neutron capture in reflector
  region (rotating control drums) or by varying
  rate of neutron escape (sliding or pivoting
  reflector)?
• Rotating control drums flown in space. Sliding
  reflectors potential advantage for certain
  designs
• Shield
• High-Z material for gamma attenuation (e.g. W)?
• Hydrogenous material for slowing down neutrons
• Reduce capture-gamma generation (Li-6 or B-10 for
  absorbing neutrons)?
• Reduce gammas produced by inelastic scatter
  (layered shield)?

Clad B4C Reaction Control Panel
Clad Be Reflector Panel
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Reactor Control and Safety

• Design for inherent stability
• Negative temperature and power reactivity
  feedback coefficients
• Neutron escape dominates in small, fast-spectrum
  system
• Provide redundant and diverse shutdown mechanisms
• Reflector control (e.g. drums, sliders)?
• Reflector ejection
• In-core control rods
• Minimize mission-ending single point failures in
  control system
• System remains operational following single or
  multiple control system failures
• Prevent inadvertent system start
• Preclude inadvertent system start during all
  credible launch accidents.
• System essentially non-radioactive at launch

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Summary

• Fission systems are in productive use today
  around the globe for military and commercial
  purposes.
• Fission, cross-section, radioactive decay, etc
  have been presented in a most elementary manner
  (w/o calculus or quantum mechanics). The fun for
  we nuc pukes is to take these concepts and make
  practical devices from them.

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Summary / Observations

• The physics of fission is very attractive.
  Fission process is easy to sustain, fission has
  effectively infinite energy density.
• The challenge of space fission power and
  propulsion is engineering systems that meet
  mission requirements and can be built, flight
  qualified, and launched in a timely and
  affordable fashion.
• Space fission systems are essentially
  non-radioactive at launch, and have other
  desirable attributes.
• Development of advanced fission systems depends
  on a successful series of evolving flight
  missions.

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Trades Fast vs Thermal Neutron Spectrum

• Mass
• Moderated spectrum can allow lower mass reactor
  (higher fission cross section)?
• Neutron shield mass also slightly smaller for
  moderated system because of lower flux and less
  leakage
• Gamma shielding about the same because of less
  high-Z material in core
• Overall system mass depends on the core
  temperature and thermal performance
• Thermal performance
• If mass benefit is realized for moderated system,
  then it will have higher power densities, which
  will nullify the mass advantage if the system is
  heat transfer limited
• In a small reactor, only hydrogen can be an
  effective moderator. No established technology
  for hydrogenous material above 1100 K
• UZrH 900 K to 1000 K, Yttrium hydride 1100 K
• Cooling of moderator adds a level of system
  complexity - reliability, cost, schedule
• Nuclear performance
• Moderated system has longer neutron lifetime, not
  a significant factor
• Moderated system has larger reactivity feedback
  coefficients
• Can help reactor stability in steady state
• Good at preventing overheating in extreme
  scenarios a big advantage for terrestrial
  systems.
• Is a disadvantage in startup and shutdown,
  especially because more excess reactivity is
  required in the system to allow startup.
• Moderated system more complex because of more
  feedback mechanisms
• Compact-fast system is easier to model and
  simulate - point kinetics is generally valid.

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Trades Fast vs Thermal Neutron Spectrum

• Lifetime
• If system lifetime is limited by fast fluence,
  then thermal system has an advantage
• Fluence does not appear to be a life-limiting
  factor for potential JIMO concepts
• If system lifetime is limited by fuel burnup,
  then fast system has an advantage
• Burnup limits will play a role in JIMO reactor
  designs above 500 kWt.
• A conservative fuel burnup limit (2) limits the
  power of a low mass reactor to 12 years at 500
  kWt
• A more risky fuel burnup limit.(6) limits the
  power of a low mass reactor to 12 years at 2
  MWt
• Moderator lifetime could be a big issue for
  moderated system
• Complexity
• Moderated system is more complex on about every
  level - design, development, fab, analysis,
  testing.
• Safety
• Fast spectrum system can take full advantage of
  spectral shift to remain subcritical in water,
  because poisons hardly affect nominal operation
• If a thermal system could be designed to be fully
  or over moderated, it would have an advantage,
  but this is probably impossible for this size
  reactor.

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Trades Fast vs Thermal Neutron Spectrum

• Safeguards
• A practical moderated system of this size will
  probably have about 1/2 the fuel loading of the
  fast system
• An advantage, but would have the same safeguards
  issues
• Moderated system might be able to use lower
  enriched fuel (corresponding mass penalty)?
• Cost
• The only significant cost advantage of the
  moderated system is that there is less fuel, most
  of the other factors mentioned favor the fast
  system as being lower cost.

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ISSUES Specific Mass

• Mass is not a significant concern for most
  terrestrial applications
• A space reactor must be light-enough to be
  launched as part of a spacecraft.
• A surface reactor must be light enough to be
  landed on surface.
• An NEP reactor must have high enough alpha
  (power/mass) to provide ample spacecraft
  acceleration.
• There is a tight relationship between alpha and
  technical risk.

68
ISSUES Reliability and Lifetime

• Terrestrial systems can utilize planned and
  unplanned maintenance, and very large suite of
  diagnostic tools
• Space reactor must start-up and operate for its
  entire lifetime without any hands-on assistance
  or maintenance.
• Remote maintenance and control will be
  significantly limited by the limited diagnostics
  that the system can accommodate, and by the
  communication time delay.
• Long-life at high-temperature with no maintenance
  is unique (and extremely difficult).

69
ISSUES Spacecraft Integration

• Advantages as compared to terrestrial
  applications
• No need for containment, emergency cooling, and
  many other systems required for terrestrial
  systems
• Challenges
• Versatility
• Space reactor development begins at the same time
  as other spacecraft components, must be flexible
  to meet possible changes in interface
  requirements as other systems are developed.
• It is desirable to have a space reactor design
  that can accommodate various mission performance
  and environmental requirements.
• Packaging
• System must fit with integrated spacecraft
  design, and accommodate launch shroud
  superstructure and center-of-mass issues
• Shielding
• Terrestrial systems can utilize very heavy and
  bulky shielding and large separation in many
  cases, space reactors must shield nearby
  components to allowable dose rates with as low as
  mass as possible.

70
ISSUES Operations/Environment

• Benefits as compared to terrestrial reactors
• Lack of atmosphere allows use of refractory
  metals
• Although ground testing at temperature still
  requires a vacuum
• Challenges as compared to terrestrial reactors
• Less efficient heat-sink
• Must radiate in-space, as opposed to using large
  volumes of water or cooling towers
• No real-time reactor operators
• Reactor must be designed for simple control, and
  control must be primarily autonomous
• Launch Loads
• These are in some ways similar to seismic
  evaluations for terrestrial reactors
• Very cold environment
• This can significantly complicate
  startup/shutdown
• Other
• Lack of gravity
• Affects different concepts in different ways,
  some positive and some negative.

71
ISSUES Safety

• Benefits as compared to terrestrial reactors
• Negligible radioactivity
• Reactor does not operate until after deployment -
  dose next to reactor is less than background.
• No significant safety impact of operational
  accidents
• Significantly simplifies design, operational
  safety is one of the key cost and risk factors
  for terrestrial reactor operation.
• If a ground test is performed, then the system
  will have to be designed and/or modified to meet
  ground safety requirements.
• Challenges as compared to terrestrial reactors
• Unique potential scenarios that could cause
  inadvertent criticality

72
ISSUES Safeguards

• Challenges
• Fuel is non-radioactive and highly enriched, need
  to adequately protect during storage, transport,
  and potential reentry after launch.
• Procedures exist for performing almost all of
  this function, but safeguards must be factored
  into the logistics of the program from the start.

73
Small-Steps Toward More Advanced Systems

• Recent technology advances in power conversion
  systems, radiators, spacecraft design,
  electronics, etc., can potentially put less
  burden on the reactor.
• Reactor does not have to be a Ferrari out of the
  gate (if reactor requirements can be kept
  simple)?
• The most evolvable early system is the one that
  successfully flies!
• Program continuity, experienced personnel, and
  infrastructure will enable development of
  progressively more advanced systems.

Nuclear fuels qualification In-core / near-core
materials Neutron reflector and control system
Reactor primary heat transport Radiation
shielding HPEP Vehicle Thermal management Power
conversion System integration, including
radiator System stowage and deployment Reactor
/ spacecraft separation booms Avionics Attitude
Control Automated Reactor / System Control
Thrusters Flight qualification Launch
approval Public opinion Program structure
Early robotic missions use established technology
Infrastructure, personnel, and experience from
early missions enable development of human-rated
systems
74
Obstacles to Keeping the First Step Small

• Mission studies will almost always suggest the
  use of a relatively high power reactor
• This reason is in the hands of mission designers,
  and there is no reason to expect this factor to
  disappear (they are just doing their job).
• It is usually felt that if a fission system is to
  be used, it must be far superior to any other
  power option for a specific mission (because of
  the perceived technical, programmatic, and safety
  risk). This drives a need for a higher-power,
  lower-mass system.
• This reason is in the hands of the top-level of
  the agency and policy makers
• There are usually plenty of paper (i.e.
  conceptual) reactors that can meet the
  higher-power, lower-mass requirements that are
  desired, plus at least one engineer to support
  the claim that this paper reactor can easily
  become a real reactor.
• This reason is in the hands of the space-reactor
  engineers, and unless the reigns are pulled in
  by the engineers, there is no reason not to
  specify a high-performance reactor.

75
Space Reactor Safety and Launch Approval

• 24 safe launches of RTGs since 1961. One
  reactor, SNAP-10A in April 1965.
• Since 1977 the approval for launch of nuclear
  material in the U.S. has been codified in
  Presidential Directive/ National Security Council
  Memorandum 25 (PD/NSC-25).
• Highlights of PD/NSC-25 are
• Presidential approval is required for the launch
  of a spacecraft with nuclear materials aboard. In
  certain cases, the head of the Office of Science
  and Technology Policy (OSTP) may grant approval.
• The agency head flying the mission must request
  launch approval through OSTP.
• For every mission an ad hoc review group is
  convened to assess the risks verses benefits of
  the mission - called the Interagency Safety
  Review Panel (INSRP).
• The INSRP consists of a representative from NASA,
  DOD, and DOE with supporting technical panels
  (e.g., meteorology, launch abort, etc). EPA and
  the NRC have advisory roles.
• United Nations agreements currently in place
• 1967 UN treaty on Outer Space
• Reactors can be used for peaceful purposes
  launching nation is internationally liable
  launching nation remains owner of the satellite
  Committee on Peaceful Uses of Outer Space
  (COPUOS) oversees space reactor activities
• UN Resolution 47/68
• Launching nation must perform a safety
  assessment.
• Principle 3 provides some guidelines and criteria
  for safe use.

76
Safety Engineering for Space Reactors

• Space reactor safety engineering is simplified by
  2 conditions
• The reactor does not present a significant
  nuclear safety risk prior to fission-powered
  operation.
• The reactor will not be powered-up until in a
  stable orbit is established with gt300 year decay
  time.
• Based on these 2 conditions, inadvertent
  criticality prevention is the only nuclear safety
  issue for a flight reactor.
• This greatly simplifies system design because
  engineers can focus only those features and
  events that impact reliability, and if
  functionality is lost, there is no need to
  evaluate or mitigate further damage or release.
• These 2 conditions also simplify safety testing
  requirements
• Zero-power criticals are required to verify
  nuclear safety
• Non-nuclear testing is needed to testing the
  control system, mechanical safety locks, etc.
• Nuclear testing is not significant to safety -
  only to reliability
• Program decision - does the increase in
  reliability justify the cost and increased safety
  risk
• Space reactors can be presented with unique
  scenarios that could cause criticality, primarily
  launch and transport accidents,
• Note it has not been shown that an accidental
  criticality event (burst or sustained) could
  significantly increase integrated mission risk to
  the public.
• Current systems are designed to prevent
  inadvertent criticality as a conservative measure.

77
Space Reactor Programs Historical Observations

• SNAP reactors (1960s to early 1970s)?
• UZrH fueled, liquid metal (NaK) cooled
  w/thermoelectrics or Rankine
• 500 We to 60 kWe (1 year life)?
• Several ground tests
• One (SNAP-10A) flown in Earth orbit
• Russian reactors
• U-Mo Alloy or UO2 fueled, liquid metal (NaK)
  cooled with thermoelectrics (gt30) or thermionics
  (2)?
• Low power (2-5 kWe), short life ( 1 year)?
• Over 30 reactors flown in Earth orbit
• Numerous other programs failed to lead to flight.
  
• In most cases this was because the reactor
  requirements and technology were too ambitious
  (too large of a first-step)?

0.5 kWe SNAP-10A thermoelectric
5 kWe TOPAZ thermionic
At least 14 major US programs in addition to SNAP
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80
Fission Reactor Operation

• System power controlled by neutron balance.
• Average 2.5 neutrons produced per fission
  (including delayed).
• Constant power if 1.0 of those neutrons goes on
  to cause another fission.
• Decreasing power if lt 1.0 neutron causes another
  fission, increasing if gt 1.0.
• System controlled by passively and actively
  controlling fraction of neutrons that escape or
  are captured.
• Burn 1 kg uranium per 1,000,000 kW-day

lt 1 ft
81
Space Reactor Engineering Historical
Observations

• Significant technology has been developed over
  the past five decades by programs aimed at
  fielding advanced space fission systems. With
  the exception of SNAP-10A (not listed), none of
  these programs resulted in flight.
• History shows we will need to be better
  (technically and programmatically) if we are to
  succeed with space fission system development and
  utilization.
• Technology with no flight heritage has little or
  no impact on space programs.

82
Space Reactor Engineering Advanced
SystemsEarly Flight Enables Development and
Utilization of more Advanced Systems

• Program continuity, experienced personnel, and
  infrastructure will enable development of
  progressively more advanced systems.
• The most evolvable early system is the one
  that successfully flies!

Nuclear fuels qualification In-core / near-core
materials Neutron reflector and control system
Reactor primary heat transport Radiation
shielding HPEP Vehicle Thermal management Power
conversion System integration, including
radiator System stowage and deployment Reactor
/ spacecraft separation booms Avionics Attitude
Control Automated Reactor / System Control
Thrusters Flight qualification Launch
approval Public opinion Program structure
Early robotic missions use established technology
Infrastructure, personnel, and experience from
early missions enable development of human-rated
systems
83
Two Aspects of Project Prometheus

• Radioisotope Power Systems
• Power via decay of radioactive atoms
• Well established space technology base

• Fission Power Systems
• Power via splitting of atoms, very high energy
  density
• Well established terrestrial technology base -
  limited space experience

Non-nuclear breadboard test of a nuclear-electric
propulsion system heat-pipe cooled reactor
coupled with Stirling engine powering an ion
thruster (in separate chamber).
84
Approach Emphasizes Fabrication and Testing

• Design to allow rapid hardware and test
  iterations.
• Design components and systems integrated into a
  test plan that maximizes the probability of
  program success.

• For JIMO, cost and schedule dictates that
  non-nuclear testing will be the work horse of
  the test program.
• Photos shown are non-nuclear testing of heat pipe
  cooled reactor

85
Non-Nuclear System Testing
86
JIMO Reactor Top-Level System Requirements

• All systems designed to provide power to a 100
  kWe power conversion system
• All systems designed to 12 year full-power
  lifetime
• Brayton systems designed to (parameters vary
  slightly for each design)
• Power 500 kWt
• Tin 890 K
• Tout 1150 K
• Gas 72He/28Xe
• Pressure 1.38 MPa
• Pressure drop allocation 2.5 dp/P
• Thermoelectric system designed to
• Power 2.36 MWt
• Tin 1265 K
• Tout 1350 K
• Neutron 5x1010 n/cm2 damage fluence at 25-m from
  the reactor core
• Gamma 25 kRad(Si) at 25-m from the reactor core

87
Space Reactors for JIMO

• The reactor power system for the Jupiter Icy
  Moons Orbiter (JIMO) mission is expected to
  provide 100 kWe for more than a decade.
• The current LANL space-reactor team has been
  investigating this class of reactor continuously
  for over 10 years.
• There are many potential reactor power system
  options for JIMO.
• Most of these options are rather similar, the
  major differences lie in primary reactor heat
  transport and the electrical power conversion
  system.

88
Three (of many) Potential Reactors are
Fast-Spectrum, U-235 Fueled, Be-Reflected,
Ex-Core Controlled
Gas-cooled concept
Heat pipe concept

• 400 kWt reactor
• UN/Nb-1Zr
• Na coolant (Heat Pipe)?

• 400 kWt reactor
• UN/Nb-1Zr
• He/Xe coolant

Principle difference is method of primary heat
transport and related design implications
89
JIMO Reactor Concepts
LANL has developed and evaluated several
potential reactor concepts for JIMO. The latest
4 concepts are shown here each provide thermal
power to a 100-kW electric system.
The reactor module mass generally ranges from
1500 kg to 3000 kg.
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91
This is an example of a small 1MWt liquid metal
(lithium) cooled Heat Pipe reactor
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95
Relative Reactor Sizes of 4 Potential JIMO
Reactors
HP-BR
GC-BR
50cm
LM-TE
LM-BR
96
In 1977, scientists exploring cracks in the ocean
floor deep beneath the sea found a big surprise.
Large, odd-looking animals were surviving without
sunlight. Strange plants and animals, never seen
on Earth before, were living deep on the ocean
floor. These plants and animals had no sunlight
to keep them warm. How could these new plants
live without the suns energy to live? How could
the animals live without the oxygen from plants?
97
Instead of warm energy from the sun, these plants
and animals get their warm energy from heat
produced in the center of the Earth. The heat is
released into the ocean from chimneys called
thermal vents, just like the fireplace in your
house releases heat up the chimney.
If youve ever been to Yellowstone National Park,
you may have seen Old Faithful, a thermal vent
called a geyser.
98
A spacecraft called Galileo traveled to Jupiter
in 2003 and discovered a frozen ocean on
Jupiters moon, Europa. The spacecraft also saw
things on Europa that looked like the cracks they
discovered on the ocean floor of Earth in
1977. Those cracks could let energy from Europa
heat its ocean water, just like the way thermal
vents heat the Earths ocean water.
99
Mars Surface Power

• Operation on Mars presents several design
  challenges
• Potential corrosion of reactor materials due to
  Martian atmosphere.
• Solution No refractory metals, Stainless-steel
  as the primary material.
• Although material changes due to carburization
  need to be considered
• Shielding issues caused by the Martian regolith
  and atmosphere.
• Solution Shield surrounds entire core,
  additional component shielding.
• Weather/environment - dust buildup, high wind,
  large temperature swings.
• Vertical radiator profile, thicker structural
  members, insulation/baffles.
• Operation on Mars also presents several potential
  advantages.
• Enhanced conductive heat transfer caused by CO2
  in gaps.
• Helps reduce temperatures in failed heatpipe
  condition
• Does not require He gap between fuel-pin and
  module
• This could greatly simplify handling at the Cape
• Enhanced radiative heat transfer - emissivity
  increased by oxidation/carburization.
• Also helps reduce temperatures in failed heatpipe
  condition
• Gravity assisted thermal-hydraulics.
• Can greatly simplify integration of core to power
  conversion system
• Mars lander aeroshell could potentially serve as
  Earth reentry aeroshell
• Eliminates significant design/mass concern if
  aeroshell is required for safeguards

100
FRINK is Part of an Integrated Space Reactor
Design Process

• MRPLOW Modular Reactor Powering Lunar Outpost
  Works
• MRPLOW is the workhorse design tool that drives
  the design process for lunar reactors.

• MONTEBURNS Automated MONTE Carlo BURNup tool
  that links MCNP and ORIGEN
• MONTEBURNS provides burnup reactivity, flux and
  fluence, decay heat, activity, etc.

• MOE MCNP Output Edits.
• This is the package of codes that reads MCNP
  output data, then normalized it and puts it in
  various forms to pass to different design tools.

• FRINK Fast Reactor Integrated Nuclear Kinetics
• FRINK calculates reactor transient response based
  on simplified assumptions of the power conversion
  system.

101
HOMER-15 and 3-kWe Stirling Power System
102
Dose Next to a JIMO-Class Reactor Before Operation

• Radiation transport calculations have shown that
  the radiation dose is below nominal background
  levels next to the SAFE-400 prior to operation.

12 hour hike up Colorado 14,000 ft peak 0.48
mR, Chest X-ray 40 mR, Average background 360
mR, Occupational dose limit 5 R. Dose rate data
obtained from EPA website www.epa.gov.

103
This class of reactor is radiologically benign
prior to operation.
Note The dominant dose contributor depends on
the reactor configuration and fuel form.
104
Radioactivity decays to very low levels after 300
years

• There radioactivity of a JIMO-class reactor will
  decay to benign levels of radioactivity

• If the reactor is used for an NEP spiral out of
  Earth orbit, the activity increases only as
  spacecraft moves out to a longer orbit.

105
Potential accident scenarios that could cause
inadvertent criticality

• Core flooding
• All void space in core filled with water or wet
  sand
• This includes flow channels, heatpipes,
  fuel-pins, or any potential voids within the
  core, even if they are individually hermetically
  sealed. The combination of forces to do this and
  keep the core intact may be impossible
• Core immersion
• All space surrounding core is filled with water
  or wet sand
• Wet sand adds more reactivity due to increased
  reflection
• Immersion must be analyzed both if system remains
  intact or if the core is stripped from the
  reflector and rest of system
• Core compaction
• Depending on core geometry, it may be possible to
  compact the core to a more reactive
  configuration. Reactors with solid packing
  have an advantage.
• Core disassembly on impact
• For some concepts, a rearrangement of the fuel
  pins can increase core reactivity, both on land
  or in water. Scenarios have to be evaluated on a
  concept specific probabilistic basis.
• Launch pad fires, transport accidents, etc.
• Analyzed as needed once the development and
  mission architecture is established, not usually
  a concern for fission systems that are not
  radioactive.
• Spurious/inadvertent control signals
• Can be handled with thorough redundancy,
  engineering, and testing.

106
Listing of Some Immersion Criticality Safety
Options

• Flooding/immersion scenarios are generally
  considered the most credible accident that could
  cause inadvertent criticality (although
  consequences are extremely low).
• Nominal shutdown margins are so large that only
  the increased moderation provided by water can
  bring the system close to critical.
• Potential options that can be considered to avoid
  inadvertent criticality (many options require
  unobstructed access to one end of the core)?
• Spectral - Thermal/resonance neutron absorbers
  are permanently placed in str