Lecture XV

Physics 367

Energy from Nuclear Reactions

Nuclear Energy

Nuclear energy may be released in two ways:

fusion

fission

All operating nuclear power plants use fission processes. The fission process looks like this:

Nuclear Power Plants

A nuclear power plant works in many ways as a coal-fired plant does. Both have a source of thermal energy, both turn water into steam, both have a steam turbine, and so on.

The big difference is the means used to produce the thermal energy.

The Boiling Water Reactor (BWR):

The Pressurized Water Reactor (PWR):

As of 2001 there were 35 BWR's and 69 PWR's operating in the U.S.

Chain Reactions

To make a sustained chain reaction that operates continuously, one neutron per fission must be used to cause another fission.

If the number of neutrons causing fissions were higher than 1, there is a possibility of a runaway chain reaction [an explosion]

control rods are used to absorb neutrons.

The neutron must also be slow in order to cause the Uranium-235 nucleus to break up water [and other materials such as carbon compounds] are used to moderate i.e. slow down, the neutrons enough to allow the nuclear reactions to occur.

Fission fragments carry about 80% of the energy released which is deposited in the moderator and/or coolant.

Controlling the Reaction

Recall that on average there are 2.4 neutrons produced in a Uranium-235 fission reaction.

The proportion of Uranium-235 used in reactors is adjusted to be 4%. This concentration controls the difficulty of a neutron from a fission to encounter a Uranium-235 nucleus.

Other Uranium-235 proportions to note are 90% in an atomic bomb and 0.7% in naturally occuring Uranium.

Another measure adopted to reduce the chance that a neutron hits a Uranium-235 nucleus is the distribution of the fuel rods in the reactor. They are separated by large gaps of water that circulates. These large spaces allow some neutrons to escape entirely from the region around the reactor.

Finally control rods are inserted into the reactor that can absorb neutrons. By moving the control rods in and out, the rate of generation of neutron can be controlled.

Conventional Reactor Types

Lack of international and business good sense proliferated designs of nuclear reactors too Many Designs for Nuclear Safety!

Light Water used as moderator
- Pressurized water (PWR, US:69).
- Boiling water (BWR, US:35). Steam connects reactor core to turbine; no PWR isolation.
Graphite-moderated, often CO₂ coolant.
- RBMK, no container vessel: Chernobyl!
- Advanced gas-cooled, UK, better thermal eff. than PWR.
- Magnox, UK, with natural uranium fuel.
Candu, Canadian PWR. Natural uranium and other advantages undermined by heavy water cost.
Breeder produces more fuel than it consumes; unmoderated natural uranium, fast neutrons sustain chain reaction. Few running: Russian: since 1981; Japan:14O MW. Others: Japan, India, China, UK, France, Germany, USA.

Concentration of Nuclear Power Plants

A single design

In 2000 the US Nuclear Regulatory Commission settled on a standard design AP600 that incorporates the best foreign/US practices and ideas. But none have been ordered!

Nuclear Power in the U.S

The average time to construct a new plant was 6.3 years in the U.S. in 1971. In 1991 the time was:

14 years in the U.S.

7 years in Japan

5 years in France

In the U.S. no plant ordered between 1974 and 1978 is still under under construction.

In the U.S. no new plants have been ordered since 1978.

Reactor construction in the U.S. has stopped. Reactor construction worldwide has slowed. Why? This is the topic of the next lecture.

Nuclear Production of Electricity

The active nuclear power plants (103) produced 754 billion kWh of electricity in 2000, 789 billion kWh in 2004 (the record), and 782 billion kWh in 2005. A comparison with other energy sources and predictions are shown below: