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Lecture 17

Nuclear Power

• Energy associated with nuclear bonds (Fig. 1)

• Nuclear Fission

– Thermal reactors – in use; limited potential

– Breeder reactors – not available, large potential

– Thorium reactors – not available, large potential

• Nuclear Fusion – very large potential, not ready

– Magnetic confinement

– Inertial confinement

Fission

• Binding energy/nucleon of heavy elements is smaller than that for elements close to equilibrium peak à fission of heavy elements releases energy

• Uranium and Thorium are the only naturally occurring elements which can be fissioned

Uranium

• Uranium has two long-lived isotopes

– ²³⁵U – 0.7 % of natural U

– ²³⁸U – 99.3 % of natural U

• Fissile nucleus: can undergo fission during capture of neutron: ²³⁵U is the only naturally occurring fissile isotope

• Fertile nucleus: can be converted to form fissile isotope: ²³⁸U and ²³²Th are fertile isotopes

Production of uranium fuel

• Uranium mining à Yellow cake

• Enrichment à UF₆ is vaporized and sent through a stack of centrifuges à separation of ²³⁵U and ²³⁸U

• Enriched fuel: for thermal reactors need enrichment to ~ 3.5 % ²³⁵U

Fission of ²³⁵U : n + ²³⁵U à ²³⁶U à ⁹²Kr + ¹⁴¹Ba + 3n + Q

⁹²Kr and ¹⁴¹Ba are examples of fission fragments

The released neutrons can start a chain reaction (Fig. 2)

A competing process is the neutron capture by ²³⁸U and subsequent conversion into ²³⁹Pu (Fig. 3)

Neutron characteristics

Fast neutrons have an energy greater than 1 eV

Slow neutrons have an energy less than 1 eV.

Epithermal neutrons have an energy from 0.025 to 1 eV.

Hot neutrons have an average energy of about .2 eV.

Thermal neutrons have an average energy of about 0.025 eV.

²³⁵U works best with thermal neutrons, but fission releases mostly fast neutrons à need for moderator (²³⁸U reacts better with fast neutrons)

Moderators

Moderator: Material to slow down fast neutrons to thermal velocities à moderation occurs due to elastic collisions of neutrons with masses of similar size: H, He, C à most thermal reactors use water as coolant and moderator; other designs use gas (He) or graphite (C).

Cooling and heat transfer

• Nuclear reactions release heat à process needs cooling and transfer of heat

• Most thermal reactors use regular water; some use gases (He); CANDU design uses heavy water (enriched in D)

Basic design of commercial reactors (Fig. 4)

• All reactors in commercial use are thermal reactors

• The two most common designs are boiling water reactor (BWR) and pressurized water reactor (PWR); they use regular water à light water reactors

• A special design is the CANDU reactor, developed in Canada, a PWR using natural U (unenriched) and heavy water as coolant/moderator

• In the USSR, reactors using graphite as moderators were designed (Chernobyl) and are still in use

Efficiency of Reactors

• The energy capacity of a nuclear power plant can be given in two ways

– Thermal power (i.e. the energy released from the nuclear reactions) MW (th)

– Electric power (i.e. the electricity generated for the grid) MW (e)

– The two are connected by the efficiency of the system, typically between 35 and 45 %, similar to coal-fired power plants

– In most cases, the second definition is used, i.e. the electricity output is listed (without the (e))

Breeder Reactors (Fig. 5)

• Breeder reactors convert 238U (fertile) into 239Pu (fissile)

• Cannot use water as a coolant because it would moderate fast neutrons à liquid metals used (Na)

• No commercial breeder reactor in use, but several experimental designs under study

• USA decided against investment into breeder technology (during Carter administration)

Nuclear power in the world (2003)

Country	# of reactors	MW (e)	% of electricity
USA (Fig. 6)	104	96,298	19.86
Germany	18	20,643	28.10
France	59	63363	77.68
Russia	30 (+3)	20,793	16.54
Japan	53 (+3)	44,139	25.01
S. Korea	19 (+1)	15,850	40.01
China	8 (+3)	5,977	2.18
India	14 (+8)	2,550	3.30
World	439 (+31)	361,094	16