Lesson 13: 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 TripSystem 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 (=0.05) 15 tonnes (=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

atomnep MmZAmmZ )()(

2cE

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

• -Radiation stopped by paper (He nucleus, )• -Radiation stopped by 6mm of Aluminum

(Electrons produced in the nucleus)• -Radiation stopped by several mm of Lead

(Photons with wavelength shortward of 124 pm or energies greater than 10 keV)

He42

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

-Particle Decay

• The emission of an particle, or 4He nucleus, is a process called decay

• Since a particles contain protons and neutrons, they must come from the nucleus of an atom

Particle Decay• 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 decay is changed by only a small amount.

– The mass number is unchanged.

• The nucleus contains no electrons. Rather, decay occurs when a neutron is changed into a proton within the nucleus. – An unseen neutrino, , accompanies each decay. – The number of protons, and thus the atomic number, is

increased by one.

Radiation Decay

• Gamma rays are a type of electromagnetic radiation that results from a redistribution of electric charge within a nucleus.

• A 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 ray in the reaction

152Dy* 152Dy +

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

dxdAEnP nffission )(• 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

lostneutrons

producedneutronskeff _

_

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