Transcript Lesson 13: Nuclear Propulsion Basics

Nuclear Propulsion
Basics
Dr. Andrew Ketsdever
Nuclear Propulsion Introduction
• Nuclear Thermal Propulsion (NTP)
– System that utilizes a nuclear fission reactor
– Energy released from controlled fission of material
is transferred to a propellant gas
– Fission
• Absorption of neutrons in a fuel material
• Excitation of nucleus causes fuel atoms to split
– Two new nulcei on average (Fission Fragments)
» High KE from release of nuclear binding energy
» Usually radioactive
– 1 to 3 free neutrons
» Necessary to keep reaction going
» Critical if each fission events leads to another
» Can be absorbed by reactor material or leak from reactor
Nuclear Propulsion
• ADVANTAGES
– High Isp (2-10x that of chemical
systems)
– Low Specific Mass (kg/kW)
– High Power Allows High Thrust
– High F/W
– Use of Any Propellant
– Safety
– Reduced Radiation for Some
Missions
A Nuclear/Chemical Comparison
• One gram of U-235 can
release enough energy during
fission to raise the
temperature of 66 million
gallons of water from 25oC to
100oC.
• By contrast, to accomplish the
same sort of feat by burning
pure octane, it would require
1.65 million gallons of the fuel
Nuclear Propulsion
• DISADVANTAGES:
– Political Issues
– Social Issues
– Low Technology Readiness Level (Maturity)
– Radiation issues (Shielding)
– High Inert Mass
Nuclear Propulsion Schematic
• Propellant Tank: Similar to
tanks discussed for liquid
propulsion systems. Tank
can also be used as a
radiation shield.
• Turbopump: Provides high
pressure propellants to the
heat exchange region of the
propulsion system. Warm
gas from regeneratively
cooled nozzle drives the
turbines.
• Radiation Shield: Protects
the payload from radiation
from the reactor by
absorbing or reflecting
neutrons and gamma rays.
Nuclear Reactor
Nuclear Reactor
Reactor Schematic
Nuclear Reactor
• Reflector
– Reflects neutrons produced in the reaction
back into the core
– Prevents neutron leakage
– Maintains reaction balance
– Can be used to reduce the size of the reactor
– Typically made of Beryllium
Nuclear Reactor
• Moderator
– Slows down neutrons in the reactor
– Typically made of low atomic mass material
• LiH, Graphite, D2O
• H2O absorbs neutrons (light water reactor)
• Slow (or Thermal) Reactor
– Uses moderator to slow down neutrons for efficient
fissioning of low activation energy fuels
• Fast Reactor
– No moderator. Uses high kinetic energy neutrons for
fissioning of high activation energy fuels
Nuclear Reactor
• Fuel Element
– Contains the fissile fuel
– Usually Uranium or Plutonium
– Contains the propellant flow channels
• High thrust requires high contact surface area for
the propellants
• Heat exchange in the flow channels critical in
determining efficiency and performance of the
system
Nuclear Reactor
• Control Rods
– Contains material that absorbs neutrons
• Decreases and controls neutron population
• Controls reaction rate
• When fully inserted, they can shut down the reactor
– Configuration and placement is driven by the engine
power level requirements
– Typically made of Boron
– Axial Rods
• Raised and lowered into place. Depth of rods in the reactor
controls the neutron population
– Drum Rods
• Rotated into place with reflecting and absorbing sides
NERVA
• Nuclear Engine for Rocket Vehicle Applications
–
–
–
–
Power: 300 – 200,000 MW
Thrust: 890 kN
Isp: 835 sec
Hydrogen propellant
• Reactor
– Uranium-Carbide fuel
– Graphite moderator
– 12 drum-type control rods
• Boron and Beryllium
PBR
• Particle Bed Reactor
– Core consists of a number of fuel particles packed in
a bed surrounded by moderator
• Maximizes the fuel’s surface area
• Increases the propellant temperature
• Propellant directly “cools” the fuel particles
– Advantages over the NERVA
• Higher Isp
• Higher F
• Higher F/W (~20 compared to ~4 for NERVA)
– Disadvantages over the NERVA
• Maturity
• Cost
CERMET
• Fast reactor uses high energy neutrons
(>1 MeV)
– No moderator
– Uranium-Dioxide fuel in tungsten matrix
• Advantages
– Long lifetime
– Ability to restart
– Fuel compatability with hydrogen propellant
Nuclear Propulsion
Table 1: Mars Mission Comparison - Round Trip
System
Chemical (H2/O2)
NTR - Solid Core
Payload Mass
100 tonnes
100 tonnes
Travel Time
1 year
1 year
Mission Delta-V
7.7 km/s
7.7 km/s
Isp
500 s
1000 s
Mass Ratio
4.806
2.192
Structural Mass
25 tonnes (e=0.05)
15 tonnes (e=0.10)
Propellant Mass
475 tonnes
137 tonnes
Total Initial Mass in LEO
600 tonnes
252 tonnes
Payload Fraction
0.167
0.397
Basic Atomic Structure
• Atoms are fundamental
particles of matter
– Composed of three types of
sub-atomic particles
• Protons
• Neutrons
• Electrons
– The nucleus contains protons
and neutrons
• Most of the atom’s mass
• Small part of the atom’s volume
– Electron Cloud
• Contains electrons
• Most of the atom’s volume
Basic Atomic Structure
• Atomic Number
– Number of protons in the nucleus
• Mass Number
– Number of protons AND neutrons in the
nucleus
• Mass of proton : 1.6726 x 10-27 kg
• Mass of neutron: 1.6749 x 10-27 kg
• Mass of electron: 0.00091x10-27 kg
Basic Atomic Structure
• Isotope
– Same element implies two atoms have the
same atomic number
– Isotopes of a given element have the same
atomic number but a different mass number
• Same number of protons in the nucleus
• Different number of neutrons in the nucleus
Hydrogen
Deuterium
Tritium
Basic Nuclear Physics
• An atom consists of a small, positively charged
nucleus surrounded by a negatively charged
cloud of electrons
• Nucleus
– Positive protons
– Neutral neutrons
– Bond together by the strong nuclear force
• Stronger than the electrostatic force binding electrons to the
nucleus or repelling protons from one another
• Limited in range to a few x 10-15 m
• Because neutrons are electrically neutral, they
are unaffected by Coloumbic or nuclear forces
until they reach within 10-15 m of an atomic
nucleus
– Best particles to use for FISSION
Fission
• Fission is a nuclear process in which a heavy
nucleus splits into two smaller nuclei
– The Fission Products (FP) can be in any combination
(with a given probability) so long as the number of
protons and neutrons in the products sum up to those
in the initial fissioning nucleus
– The free neutrons produced go on to continue the
fissioning cycle (chain reaction, criticality)
– A great amount of energy can be released in fission
because for heavy nuclei, the summed masses of the
lighter product nuclei is less than the mass of the
fissioning nucleus
Fission Reaction Energy
• The binding energy of the nucleus is directly
related to the amount of energy released in a
fission reaction
• The energy associated with the difference in mass
of the products and the fissioning atom is the
binding energy


  Z (m p  me )  ( A  Z )mn  M atom
E  c
2
Nuclear Binding Energy
Defect Mass and Energy
• Nuclear masses can change due to reactions because this "lost"
mass is converted into energy.
• For example, combining a proton (p) and a neutron (n) will produce
a deuteron (d). If we add up the masses of the proton and the
neutron, we get
– mp + mn = 1.00728u + 1.00867u = 2.01595u
– The mass of the deuteron is md = 2.01355u
– Therefore change in mass = (mp + mn) - md = (1.00728u + 1.00867u) (2.01355u) = 0.00240u
– An atomic mass unit (u) is equal to one-twelfth of the mass of a C-12
atom which is about 1.66 X 10-27 kg.
• So, using E=mc2 gives an energy/u = (1.66 X 10-27 kg)(3.00 X 108
m/s)2(1eV/1.6 X 10-19 J) which is about 931 MeV/u. So, our final
energy is 2.24 MeV.
• The quantity 2.24MeV is the binding energy of the deuteron.
Uranium 235 Energetics
•
•
•
•
•
Fission Products: 165 MeV
Primary Gamma Radiation: 7 MeV
Neutrons: 5 MeV
Beta and Gamma Decay of FP: 13 MeV
Neutrinos: 10 MeV
• TOTAL: 200 MeV
Radioactivity
• In 1899, Ernest Rutheford discovered
Uranium produced three different kinds of
radiation.
– Separated the radiation by penetrating ability
– Called them a, b, g
• a-Radiation stopped by paper (He nucleus, 24 He )
• b-Radiation stopped by 6mm of Aluminum
(Electrons produced in the nucleus)
• g-Radiation stopped by several mm of Lead
(Photons with wavelength shortward of 124 pm or
energies greater than 10 keV)
Half-Life
• The half life is the
amount of time
necessary for ½ of
a radioactive
material to decay
• Starting with 100g
of Bismuth
– Half life of 5 days
– 50 g of bismuth
after 5 days
– 50 g of thallium
a-Particle Decay
• The emission of an a particle, or 4He
nucleus, is a process called a decay
• Since a particles contain protons and
neutrons, they must come from the
nucleus of an atom
bParticle Decay
•
b particles are negatively charged electrons emitted by
the nucleus
– Since the mass of an electron is a small fraction of an atomic
mass unit, the mass of a nucleus that undergoes b decay is
changed by only a small amount.
– The mass number is unchanged.
• The nucleus contains no electrons. Rather, b decay
occurs when a neutron is changed into a proton within
the nucleus.
– An unseen neutrino, n, accompanies each b decay.
– The number of protons, and thus the atomic number, is
increased by one.
gRadiation Decay
• Gamma rays are a type of electromagnetic radiation that
results from a redistribution of electric charge within a
nucleus.
• A g ray is a high energy photon.
• For complex nuclei there are many different possible
ways in which the neutrons and protons can be arranged
within the nucleus.
– Gamma rays can be emitted when a nucleus undergoes a
transition from one quantum energy configuration to another.
– Neither the mass number nor the atomic number is changed
when a nucleus emits a g ray in the reaction
152Dy*  152Dy + g
Fission
Fission Probability
• When a neutron passes near to a heavy nucleus, for
example uranium-235 (U-235), the neutron may be
captured by the nucleus and this may or may not be
followed by fission.
• Capture involves the addition of the neutron to the
uranium nucleus to form a new compound nucleus.
– A simple example is U-238 + n  U-239, which represents
formation of the nucleus U-239.
– The new nucleus may decay into a different nuclide. In this
example, U-239 becomes Np-239 after emission of a beta
particle (electron).
• In certain cases the initial capture is rapidly followed by
the fission of the new nucleus.
• Whether fission takes place, and indeed whether capture
occurs at all, depends on the velocity of the passing
neutron and on the particular heavy nucleus involved.
Fission Probability
Pfission  n f ( En )dxdA
• The probability that fission or any another
neutron-induced reaction will occur is described
by the cross-section for that reaction.
– The cross-section may be imagined as an area
surrounding the target nucleus and within which the
incoming neutron must pass if the reaction is to take
place.
– The fission and other cross sections increase greatly
as the neutron velocity reduces for slow reaction
fuels.
– For fast reaction fuels, a large activation energy
requires high energy neutrons for fission
Fission Cross Sections
Fission Fragments
• Using U-235 in a thermal reactor as an example,
when a neutron is captured the total energy is
distributed amongst the 236 nucleons (protons &
neutrons) now present in the compound
nucleus.
• This nucleus is relatively unstable, and it is likely
to break into two fragments of around half the
mass.
– These fragments are nuclei found around the middle
of the Periodic Table and the probabilistic nature of
the break-up leads to several hundred possible
combinations.
Fission Fragments
Fission Fragments and the Chain
Reaction
Neutron Emission
• Creation of the fission
fragments is followed
almost
instantaneously by
emission of a number
of neutrons (typically
2 or 3, average 2.5),
which enable the
chain reaction to be
sustained
keff
neutrons _ produced

neutrons _ lost
keff = 1 implies critical mass
Want keff > 1
Fission Fragments
• About 85% of the energy released is initially the
kinetic energy of the fission fragments.
• However, in solid fuel they can only travel a
microscopic distance, so their energy becomes
converted into heat.
• The balance of the energy comes from gamma
rays emitted during or immediately following the
fission process and from the kinetic energy of
the neutrons.
– Some of the latter are immediate (so-called prompt
neutrons), but a small proportion (0.7% for U-235,
0.2% for Pu-239) is delayed, as these are associated
with the radioactive decay of certain fission products.
The longest delayed neutron group has a half-life of
about 56 seconds
Reactor Fuels
• U-235 is the only naturally occurring isotope which is thermally
fissile, and it is present in natural uranium at a concentration of 0.7
percent. U-238 is the main naturally-occurring fertile isotope
(99.3%).
• The most common types of commercial power reactor use water for
both moderator and coolant.
• Criticality may only be achieved with a water moderator if the fuel is
enriched.
• Enrichment increases the proportion of the fissile isotope U-235
about five- or six-fold from the 0.7% of U-235 found in natural
uranium.
– Enrichment is a physical process, usually relying on the small mass
difference between atoms of the two isotopes U-238 and U-235.
– The enrichment processes in commercial use today require the uranium
to be in a gaseous form and hence use the compound uranium
hexafluoride (UF6).
– The two main enrichment (or isotope separation) processes are
diffusion (gas diffusing under pressure through a membrane containing
microscopic pores) and centrifugation.
• In each case, a very small amount of isotope separation takes place in one
pass through the process.
• Repeated separations are undertaken in successive stages, arranged in a
cascade.
Uranium
Enrichment
Plutonium