2. Space Reactors

(1) Characteristics of Space Sources

★ Why is the nuclear rocket needed in the space where the solar energy is

abundant ?

① Inefficiency of using the solar energy in deep-space

② Need of rechargeable battery due to the cyclical solar exposure

③ Limited sheer size of solar panels for increased electrical power

∴ It is necessary to use the nuclear power which is a reliable energy source

for long period

(2) Types

1) Nuclear Power Sources (NPS)

a) Radioisotope Thermoelectric Generator

; Conversion of the thermal energy emitted from radioisotopes into electrical	energy by using a thermocouple

- Fuel ; Pu238 (1 - 10Kg, 22Kg in Ulysses)

Recently, PuO2 (better HT and to meet enhanced safety criteria)

- Insertion of the pellet, sphere or disc shape of Pu fuel in the cylindrical	capsule

- The fuel capsule is surrounded by cladding to protect the entire assembly	from the accident

- No need of manual control, maintenance, and no rotational part in power	conversion unit

- Points to be considered

① Radiation shielding at the reentry (reentry burnup, marine environment

corrosion)

② Crash on the Earth

③ Safety of astronauts

b) Thermoelectric Nuclear Reactors

- 30 Kg of fully enriched U235 (∵ burnup at reentry)

- Nuclear Safe Orbit ; NSO

; to minimize the radioactivity outside the atmosphere with considering the	half-time of NPS (NSO has 300 years of orbit life)

- SNAP-10A is the unique experimental satellite (1965)

; Sodium-Potassium as a medium circulating between the nuclear reactor and

the thermoelectric instruments through the thermal-conductive pump

; Power conversion unit with no rotational component

2) Nuclear Propulsion

a) NEP(Nuclear Electric Propulsion)

-  Power electric thrust device using the electricity generated by the nuclear

reactor (～10MW)

b) NTP(Nuclear Thermal Propulsion)

- Cryogenic H2 is heated up to 2500～3500°K by nuclear power.

- Conventional rocket heats the liquid hydrogen by nuclear power (2400～	2700°K) instead of burning the O2 and H2

- Rapid expansion as exhaust out the rochet nozzle.

- Greater thrust and almost twice specific impulse than chemical rocket

- Nuclear reactor

; Graphite-moderated, [eqn: H_2]-cooled, 93.15% enriched [eqn: U^235]

∙UO particle

∙Uranriem - Carbide particle with carbide coating

∙U - Zr - Carbide composite particle with carbide coating

- high temperature and long - life fuel

(3) Propulsion principal of Rocket

; Momentum change of ejected mass

[eqn: F`=~V _{e} {dm} over {dt} `=~I _{sp} {dw} over {dt} `,~ rm where~I _{sp} = {V _{e}} over {g} `;``specific``impulse]      (1)

- Amount of propellant decreases as the specific impulse increases.

In the nongravitational and vacuum condition,

[eqn: F= M dV over dt]                                                      (2)

Since, in Eq. (1), the mass flow rate ejected from the rocket is same with

the mass reduction rate of the rocket,

[eqn: dm over dt ~=~ dM over dt]                                                (3)

Thus, the velocity increase is obtained by summing up and integration of

Eqs. (1), (2) and (3) as,

[eqn: triangle V = V_f ~ - ~ V_i = g CDOT Isp~ ln left (M_i over M_f right)]

- Velocity increment of mission

1) Orbital velocity on Earth surface : 7.9 Km/s

2) Escape velocity from the Earth: 11.2 Km/s

3) Escape velocity from the solar system: 16.7 Km/s

(4) Nuclear rocket

- Application on the propulsion

1) Direct momentum conversion method

2) Thermal propulsion method

3) Electrical propulsion method

- Nuclear engine has very high specific impulse but low weight ratio of gravity	to engine is lower than 1.

∴ Start from the Earth's orbit because the weight ratio of gravity to engine

should be more than 1before achieving orbit

- Solid core nuclear rocket

- Propellant : liquid H2

- Heating up over 2000 K in the core and 95 % of hydrogen is ejected from the	nozzle

- High enriched Uranium of Plutonium

★ Accidents

1) 1964. 4. 21

; Destruction of launch vehicle due to the guidance control malfunction

- fall Pu metal (17,000 Ci)

2) 1965. 5. 16

; SNAP - 10A (Experimental satellite)

; Voltage regulator failure inside reactor core

3) 1968. 5. 18

; Flight suspension after the launch due to the erratic rocket behavior

- SNAP - 19B2 RTG

- Pu-O2-Mb fuel was found in the ocean five month later, but the fuel capsule	and the graphite cladding were intact

4) 1970. 4.

; Flight suspension of Apollo-13 due to the explosion of O2 tank

(5) Development status

1) USA

◦ 1961. 6 - Using the nuclear power after the cruising satellite Transit 4A

◦ Rover plan (1955 ～ 1973)

- Completion of main design and experiments for nuclear propulsion rocket

- Propulsion by use of very high temperature hydrogen

- Design of “ NERVA engine” for high power and thrust, and “small engine”           for low power and thrust

- Interruption after Suspension of the human exploration of Mars

◦ SP - 100 Program - GE 300 KWe.

- Fuel pin using high-density uranium-Nitride (UN) fuel

- Refractory alloy liner and Niobium ～ 1% Zr alloy cladding

◦ Recently, USA performs experiments with Topaz II (5 kW) bought from

Russia without insertion of nuclear fuel

- Development of thermionic nuclear reactor

2) Russia

◦ 1964 - Use of RTG with Po210 fuel in Satellites

◦ After COSMOS-1402 accident in 1983, Russian spaceship adopts the nuclear	reactor with automatic raising system in the accident cases such as loss of	altitude control, depressurized accident of nuclear reactor or malfunction of	electric system.

◦ Development of Topaz II (Advanced space nuclear power system)

- Sale to USA

- 5 KW (3.9m height, 1.4m diameter of bottom surface, 1,000Kg weight)

- Core : 37.5 cm long, 26cm in diameter

- 96% enriched [eqn: rm C/O_2]