Generation: Alternative Sources: Nuclear Fusion

WEEK 03: GENERATION: ALTERNATIVE SOURCES: NUCLEAR FUSION

Sections: Definition | Theories | Systems | Breeder

Nuclear Fusion. Nuclei (ions) of elements of low atomic number are brought together in a plasma, at such temperature, i.e., velocity, as to give the nuclei sufficient speed to overcome repulsive columbric barrier between them (fusing) thereby transforming a part of their mass into kinetic energy.

Thermonuclear Reaction involves two phases of reactions between deuterium (D), tritium (T) and Lithium (Li), called the D-T-Li cycle.

a) D + T = ⁴He + n
Reactions between D and T produces an alpha particle, Helium-4 nuclei, about 3.5 MeV and a high energy neutron, n, about 14.1 MeV.

b) ⁶Li + n = T + ⁴He
⁷Li + n = T + ⁴He + n
Reactions between Li and a neutron produces from 15keV to 100keV.

Classes of Fuel
1. Deuterium-based, two types of fuel are categorized under this class: a) D - D and b) D - ³He, where ³He must be bred either via a D - D reaction or a decay of Tritium (half-life is about 12 years) or from prospective lunar mining.

2. Proton-based, at experimental stage, where hydrogen proton (P)is made to react to Boron 11 (¹¹B) to produce three alpha particles, thus: P + ¹¹B ----> 3a .

Advantages of Fusion¹

1. No atmospheric pollution -- the fusion reaction produces helium which is an inert gas
2. No long-term storage of radioactive waste
3. Inherently safe system -- it rapidly shuts itself down in under a minute.
4. Small-scale fuel -- a release of tritium would have no consequence beyond the power station site since the amount used is so small.
5. Fuel abundance -- fuels are plentiful and easily obtained.

Sections: Definition | Theories | Systems | Breeder

Theories

Power Production In a D-T-Li cycle, 80% of energy produced by the high-energy neutrons, which are chargeless thus no electric or magnetic fields. This element is converted to heat by extracting them through heat transfer systems. 20% are charged alpha particles, ⁴He often abstracted by thermal conversion or directly by causing it to interact with electrostaic or electromagnetic fields.

Energy Break-even Power supplied to start and sustain reaction must be equal to the power derived. Lawson criterion states that to breakeven, density confinement-time product, nt = 10¹⁴ions/cm³-s, where n is the plasma density in ions/cm³and t is the time of confinement in 1/s. For magnetic confinement systems, energy breakeven is achieved when t = 1/10 to several seconds, and n = 10¹⁴to 10¹⁶ions/cm³-s. In inertial systems, breakeven is acheived when t = 1/1000 microseconds, and n = 10²⁶ions/cm³-s. Plasma temperatures ranges from 10 to 15keV.

Power Balance Total input power should be greater than fusion-released power. Input power minus reaction power is equal to the useful output. Therefore, power-amplifiers are used.

Sections: Definition | Theories | Systems | Breeder

Systems

Confinement Technologies Fusion systems are often classified by the type of confinement used in the reactor. Plasma confinement is the use of material vessel to confine the plasma and contain the high temperature and pressures of the thermonuclear reaction, there are two types of confinement: magnetic confinement and inertial confinement.

Magnetic Confinement A technology that uses intense magnetic fields are generated in the reactor orientated with resepect to the reaction sphere such that the plasma ions and electrons experience an inward pressure of the plasma (ampules). The t = 1/10 of a second to several seconds. The n = 10¹⁴ to 10¹⁶.

Magnetic Confinement Classifications

1. Tokamak

2. Mirror

a) Simple Mirror
b) Field Reversed Mirror
c) Tandem Mirror

3. Toroidal

a) Steady State
b) Long pulsed
c) Pulsed

4. Compact Toroid

a) Stationary
b) Translating

5. Linear

a) Steady State
b) Pulsed

6. Very Dense

a) Fast Exploding Reactor
b) Dense Plasma Reactor
c) Wall-confined shock-heated reactor
d) Dense Z-pinch
e) Passive Linear

Inertial Confinement A technology that employs D-T fueled targets are bombarded by laser. Each laser beam is pulsed, and rapidly heats the surface of the target. Each pulse lasts about 10^-9 second, or 1 nanosecond and the pulse rate is about 1 per second. The symmetrical heating of the pellet causes it to ablate. This creates a strong inward force such that the target's interior is compressed. Fusion then occurs at the center and the reaction burns outward. Total energy generated is about 5MJ. The t = 1/1000 of a microsecond. The n = 10²⁶ ions per cm ³. This total is due to the addition of forces from the initial reaction of 10²² ions per cm³ and 10⁴ ions per cm³ from the laser beam.

Inertial Confinement Classifications

1. HYLIFE System

2. OFE Inertial Fusion Reactor

3. NOVA Upgrade ICF

4. GEKKO

5. Ion Beam Driver (IBD)

Sections: Definition | Theories | Systems | Breeder

Breeder Types

Breeder types are based in neutron multiplication. These types are as follows:

1) Fast-fission blanket The D-T fusion source is surrounded by a blanket of fertile materials, usually ²³⁸U or ²³²Th and a lithium compound for Tritium breeding.

Fast fissions in the fertile material multiply both the fusion energy (3 to 10 times) and the neutrons (approximately 2 to 4 neutrons per fusion neutrons). On of the neutrons is needed to breed Tritium from Lithium and the remainder are available for breeding fissile fuel.
2) Suppressed-fission blanket A non-fissioning, neutron-multiplying material such as Berrylium is used as a fertile fuel in the blanket.
3) Hybrid A type of experimental breeder which aims to combine both the functions of nuclear fission and fusion.

Sections: Definition | Theories | Systems | Breeder | Performance

Performance

For magnetic confinement, there are two factors involved in its performance: the plasma energy gain, Q, which is the ratio of the fusion energy produced to the input energy required to heat and sustain the plasma; and blanket power density, t., the first-wall fusion neutron wall loading, usually in MW/m². To produce electricity, a magnetic confinement reactor must have at least a Q of greater than 20, and a t of greater tham 3MW/m². For a Light Water Reactor, the Q is about 2 to 6, and a t is greater tham 1MW/m². For Fast-Fission Q is about 2 to 4, while for Suppresed-Fission, Q should be greater than 6.

For inertial confinement, the driver-efficiency target gain product, h_DG is considered. To produce electricity, the reactor must have a h_DG of greater than 20, but for ICF-hybrid, h_DG could be 6.

1. Rob Moss. Nuclear: Fusion Future, Asian Electricity Vol. 16 No. 08; Reed Business Publication, UK, October 1998, pp. 10-11.

Nuclear Fusion Reactors