Nuclear Power and Nuclear

Propulsion in Space

Applications in a Microgravity

Environment

Dr. Ugur GUVEN

The Need for Power in Space

MissionsFor short times and low power levels, chemical or solar energy can be used to make

electricity. However, at high power levels for long periods of time nuclear energy is

the only way we know how to produce the necessary electrical power

The Demand for Energy in Space

Grows• Currently, levels of 100-500 watts are needed for small satellites while

human presence will require 10's of kWe and Lunar (or Mars) bases will

require 1-2 MWe of Energy.

• 1 kg of Nuclear Fuel contains 10,000,000 times the energy of 1 kg of

chemicals.

Ways to Create Electricity with

Nuclear Power in Space

Nuclear Power Race in Space

Thermal Radioisotope Generators• TRG have been used extensively in exploratory spacecraft both by the

Americans as well as the Russians.

• TRG uses a radioisotope such as Plutonium to create a constant heat

source.

• The prnciples of thermoelectricity are used to create a low voltage

differential that can be used for the onboard systems of the spacecraft.

Fission Reactors in Space

• As compared to RTG, 1 kg of Uranium in a

fission reactor can supply 500,000 times

the energy of a decay of Plutonium as a

Radioisotope Heat & Electricity Source.

Nuclear Electricity in Space

• The SP-100 nuclear reactor, designed in the U. S., would produce

100 kWe of continuous power for at least 7 years .

Nuclear Electric Propulsion

• The SP-100 can be used for nuclear electric propulsion

NERVA Achievements

Rocket Characteristics

• Sir Isaac Newton stated in his Third Law of Motion that "every action is

accompanied by an equal and opposite reaction." A rocket operates on this

principle. The continuous ejection of a stream of hot gases in one direction

causes a steady motion of the rocket in the opposite direction.

Specific Impulse

• Specific Impulse, Isp, is the most

important parameter in a spacecraft, as it

depicts the performance characteristics of

the spacecraft. Specific impulse will cause

the spacecraft to have more propulsion

capability, as well as more speed

capability.

g

VI e

sp

Specific Impulse and Nuclear

Power

• Specific Impulse can be significantly increased by

Nuclear Power

Delta V and Tsiolkovsky Rocket

Equation• Newton’s Second Law of Momentum states:

dt

dvmmaF

Thrust is defined as a function of the propellant mass flow rate and

the exhaust velocity Ve:

eVmF.

Combination of these equations will yield:

eVdt

dm

dt

dvm

Tsiolkovsky Rocket Equation is Achieved:

final

initialexhaust

M

MVV ln

Tsiolkovsky Equation• The implications of Tsiolkovsky equation are very clear. In order to

get a higher rocket speed performance (Delta V):

• a) The spacecraft needs to have either a very large Ve (exhaust gas

velocity)

• b) The spacecraft will need to have a very high proportion of m/mo

Advantages of Nuclear Propulsion

• As it can be seen, changing the Mass

Ratio is not a very efficient method. It is

essential to increase the exhaust velocity.

• The maximum Isp (specific impulse) which

can be achieved with chemical engines is

in the range of 400 to 500.

final

initialexhaust

M

MVV ln

Effects of Nozzle on Spacecraft

• By the proper geometrical design of the nozzle, the exhaust of the

propellant gases will be regulated in such a way that the maximum

effective spacecraft velocity can be reached.• The heated propellant is converged at the nozzle throat and then it

is propelled toward the divergent part of the nozzle. As the gas

expands under constant temperature, it will increase its stream

velocity to supersonic speeds.

Main Parameters of a Rocket• The parameters of interest are as shown, such as mass

flow rate, exit pressure Pe, exit velocity Ve, nozzle throat

exit area Ae, outside pressure Po, and the specific

impulse Isp.

Long Distances Require Nuclear

Propulsion• Having a high chamber temperature Tc can influence the exit

velocity Ve. This is extremely significant, as many long-range

missions will require having large specific impulses such as 5000

seconds or above in order to realize the scope of the mission.

Power of Fission

• Fission Reaction is used to generate energy that

is used in the spacecraft.

Nuclear Propulsion Techniques

• Fission Reaction produces energy in heat

form, then this energy is channeled with

thermodynamic means to heat the

propellant. Then the propellant is ejected

from the spacecraft.

Gas Core Nuclear Reactors

• Gas Core Nuclear Reactors allows the bypassing of

design limitations associated with solid core reactors.

The fission reaction and the heating of the propellant

takes place in gaseous form.

Essentials of Gas Core Reactors• Dilution Ratio (DR): Hydrogen Flow Rate

• Uranium Fission Rate

Burn-up Fraction (BF): The amount of uranium fissioned

• Total amount of uranium available

• The best possible uranium consumption rate is %1 of the hydrogen consumption rate

Design of a Gaseous Core Rocket

• The hydrogen fuel (propellant) is injected

in gaseous form, as well as the Uranium

Hexafluoride in gaseous form.

Parameters of Gas Core Reactor in

Spacecraft• -Type of Reactor: Gaseous Nuclear reactor

• -Type of Reaction: Simple Uranium Fission

• - Preferred Fuel: Uranium 235 (U235)

• - Nuclear Fuel Temperature: 25 C

• - The Form of the Fuel: Gaseous UF6

• - Preferred Propellant: Hydrogen (H2)

• - Propellant Temperature: -270 C

• - Form of Propellant: Gaseous

• - U235 Consumption Rate: %1

• - U235 Flow Rate: 100:1

• - Standard U235 Pressure: 68 atm = 70.3 kg force / sq cm- Required Core Diameter: Between 1 meter to 3 meters

(1m < D > 3m)

• - D2O Reflector Thickness: Between 35 cm to 50 cm

• - Required Uranium Velocity: 1/1000 of the hydrogen velocity

Temperature and Specific Impulse• The peak gas temperature in the nuclear reactor chamber along with

the resulting specific impulse can be calculated. As the temperature

goes up so does the exhaust velocity and the specific impulse.

Seperation Ratio

• In a gaseous core reactor in a spacecraft, some of the nuclear fuel will be

discharged from the exhaust without fissioning. This can cause serious

financial losses and it is shown by S:• Pc = Chamber pressure in the nuclear reactor of the spacecraft (atm)

• Isp = Specific impulse of spacecraft (sec)

• Itot = Total Impulse of the spacecraft (kg.sec)

• mf = The amount of fissionable fuel that must be used (kg)

• = gaseous fuel density required for nuclear criticality of the reactor (gm/cm^3)

• S = Separation Ratio (dimensionless)

f

ftot

sp

c

m

SI

I

P

3

2

How to Lower the Seperation Ratio

• Seperation ratio needs to be as low as possible

for the spacecraft to utilize as much nuclear fuel

as possible to minimize costs.

Heat Transfer in Nuclear

Propulsion

• Heat Transfer is important, since it is the mechanism by

which the heat from the nuclear reaction is passed on to

the propellant.

• Heat Transfer to the propellant is the net sum of:- Convection

- Conduction

- Thermal Radiation

- Ionization of Hydrogen

- Disassociation of Hydrogen

Flow Characteristics

• Very large Reynolds Numbers will be reached in the flow.

• Turbulent flow will be dominant due to high temperature and velocity

gradients.

• Viscosity will increase due to high temperature characteristics.

Selection of Gaseous Nuclear Fuel• You can use Uranium Hexafluoride as well as Uranium Tetrafluoride

in gaseous core reactors.

• Uranium Hexafluroide (UF6) is the better choice for gaseous core

reactors working in a microgravity environment. It also works better

with low pressure since American spacecraft have low pressure

since the Mercury & Gemini spaceship.

Geometry of UF6 Reactor

• The gaseous core reactor is surrunded by BeO reflector.

• The reactor has a right side up cylinder shape

• The pressure inside is kept between 70 to 200 atm or best results.

• The UF6 Gas Core Reactor design can be characterized as an

externally moderated, circulating fuel reactor with a thermal neutron

spectrum.

Criticality of a Gas Core Reactor• UF6 Gas Core Reactor has no fuel rods no control rods.

• The criticality of the reactor is maintained by:

- Controlling the amount of fissile material present

- Controlling the fuel density or enriching the fuel

- Controlling the pressure within the reactor core

- Rotating the BeO columns

Nuclear Shielding in Space

• The human crew aboard a nuclear spacecraft will be subjected to

both the cosmic radiation of space, as well as to the radiation that is

generated in the nuclear reactor.

• The shielding must protect against neutrons, electrons, alpha

beams, beta rays, photons, X rays, and Gamma Rays that emanate

from the fission reaction.

• In a nuclear system, neutrons are the hardest materials to stop,

since they dont interact electrically.

• The Effect of the Radiation flux = 2

1

d

Spacecraft Nuclear Shield Designs

• The designer is limited in choosing a

shield type due to budget and

technological constraints.

• Shadow shield is typically chosen as the

best shielding method.

Materials Technology

• The materials used in nuclear propulsion technology will have to

meet both nuclear requirements as well as spaceflight requirements.

• Special composite materials can be used in nuclear propulsion in

space.

• The materials need to be tested in microgravity conditions.

Sample Nuclear Spacecraft• The crew module is in the back of the spacecraft as the bulk of the

body acts as a shadow shield.

• Reactor is in the aft of the spacecraft and it is directly connected to a

nozzle.

• Propellant is placed near the crew module on the opposite side of

the spacecraft for storage safety purposes

• The overall shape is deemed to be cylindrical.

Sample Gas Core Reactor in a

Spacecraft• The gas core reactor will have two inlets. One inlet will have

gaseous or liquid H2, while the other will inject gaseous Uranium

Hexafluoride into the core. The slush mixture will heat and expand,

while it is discharged through a convergant-divergant nozzle at

supersonic speeds to create exhaust velocity in the spacecraft.

Design Parameters• The reactor will have a diameter of 3 m.

• It will be shielded by a neutron moderator shield with 0.50 cm in thickness

• The system reaction of the nuclear reactor in the spacecraft will be controlled by limiting the nuclear fuel and the propellant inlets

• For the purposes of simplification, it’s possible to assume a finite length and infinitely long cylinder in calculations

• Forced convection is assumed with heat transfer taking place as convection, conduction, thermal radiation, ionization and through disassociation.

• The Uranium is assumed to be flowing at a rate that is 1000 times slower than hydrogen

• For every 100 units of hydrogen, there is one unit of Uranium in the mass flow and in the nuclear reactor core

• The separation ratio of Uranium from the hydrogen during the exit phase is assumed to be less than 10-3

• The criticality of the nuclear reactor is controlled through pressure and by the amount of fissile material present in the system. No control rods are used.

• It is assumed that the hydrogen temperature is directly linked to reactor operating temperature and the stream exit velocity is proportional to temperature

Parameters of a Sample

Spacecraft• Projected Parameters

• Projected Specific Impulse for the Spacecraft :3000 sec

• Projected Thrust for the Spacecraft :11 MN

• Projected % of Payload : % 2

• Projected % of Structure of Spacecraft :% 18

• Projected % of Propellant & Nuclear Fuel :% 80

Operational Parameters

Tc = 8000 K

Pc = 45 MPa = 444 atm

Pe= 0 atm (Vacuum Pressure)

M = 4

ε = 77.5

ϒ = 1.4

R = 8.314 472 J K−1 mol−1

Propellant Fuel = H2 (Liquid Di-hydrogen)

Propellant Temperature Tpropellant = 14.01 K = - 259.14 C

Nuclear Parameters in the Sample

Spacecraft• Nuclear Parameters

• Nuclear Fuel = Uranium Hexafluoride (UF6)

• Nuclear Fuel Temperature Tfuel = 298 K = 25 C

• Mass Flow Rate of Nuclear Fuel = Mass flow of H2 / 1000

• UF6 Pressure = 68 atm = 70.3 kg force / cm2

Calculated Parameters for Sample

Spacecraft• Exhaust Velocity from Equation 4.1 : 29430 m/sec

• Calculated Mass Ratio (m/mo) from Table 4.1 : 5

• Delta V from Equation 4.6 : 47365.76 m/sec

• Subsonic H2 Stream Velocity from Equation 4.12: 341.18 m/sec

• Subsonic UF6 Stream Velocity : 0.341 m/sec

• Propellant H2 Mass Flow from Equation 4.3 : 373.77 kg/s

• Nuclear Fuel UF6 Mass Flow from Parameters: 0.37377 kg/s

NASA Nozzleflow Software Results

NASA Nozzleflow Software Results

Some Technological Problems

• The initialization of the nuclear rocket can be between 15 to 20 seconds, during which time two phase flow will exist in the propellant flow.

• The reactor will continue to be a heat source even after the mass flow is reduced to initiate a shutdown

• The corrosive properties of the propellant in the nuclear reactor system as well as the nozzle and the chamber flow system are formidable

• The temperatures that are needed for very high specific impulses (Isp > 9000 sec) requires materials that can withstand very high temperatures.

• Radiation can shorten the lifespan of many components in the spacecraft

• Nuclear fuel loss can be formidable as more than $10 million dollars of nuclear fuel can be wasted with a high seperation ratio.

Moon Mission Using Nuclear

Propulsion

• Mission to the Moon for colonization can

best be done with NTR (Nuclear Thermal

Rocket)

Mars Mission with Nuclear

Propulsion

• Mars Mission can be considered unfeasible without

nuclear propulsion due to 500 day duration of the

mission. Also, launch window can become infinite with

NTR.

Solar System Travel with Nuclear

Propulsion

• Even with relativistic speeds, travel within

the solar system requires time and energy.

Interstellar Propulsion

• For outer solar solar system exploration, more exotic means of

nuclear propulsion such as Fusion Drive or Antimatter Drive need to

be used.

Conclusion• Nuclear Propulsion is essential for space exploraiton due to high

power and high specific impulse requirements.

• Even countries without a space program have to be diligent in their

efforts for the future.

• With nuclear propulsion, mankind can reclaim his place in the stars.

THANK YOU

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