12-12-2012, 05:11 PM
Nuclear Reactor Materials and Fuels
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Power Reactors
The engineering designs of nuclear reactors are largely
governed by materials properties. The choices of nuclear
fuels and designs are limited by the characteristics of the
reactor cores, namely, the fuel enrichment, the nature of
the moderators and coolants selected, the operating temperatures
and pressures in the core, the fuel burnup and
exposure time, and the average neutron energy and fluence.
The most important nuclear power reactor concepts
at present include the following.
1. Light-water moderated and cooled reactors (LWR).
These may be either pressurized-water (PWR) operating
at about 14 MPa pressure and 300◦C or boiling-water
(BWR) operating at about 7 MPa and 300◦C. In the PWR,
the heat is transferred from the core to steam generators
via intermediate heat exchangers, whereas in the BWR,
the coolant water boils at the top of the core and furnishes
steam directly to the turbines. The fuel consists of slightly
enriched UO2.
2. Heavy-water moderated reactors (CANDU). In these
reactors, the heavy-water moderator is contained in a calandria,
through which insulated pressure tubes containing
the fuel elements circulate the pressurized light-water
coolant at 15 MPa and 300◦C to transfer the heat from the
fuel elements to steam generators. The fuel is natural UO2.
3. Carbon-dioxide gas-cooled graphite moderated reactors.
The first generation of these reactors (Magnox) are
cooled by circulating CO2 gas. The fuel elements consist
of natural-uranium metallic fuel rods clad with a magnesium
alloy. The second-generation advanced gas-cooled
(AGR) reactors use stainless steel clad slightly enriched
UO2 fuel rods, which permit steam generation at higher
temperatures.
NUCLEAR FUEL CYCLE
Fissionable Materials and Fission Products
The fissionable isotopes used in nuclear reactors include
233U, 235U, 239Pu, and 241Pu. The fertile isotopes are 238U
and 232Th. The fertile isotopes are converted into fissionable
isotopes by neutron absorption (238U into plutonium
isotopes and 232Th into 233U). Natural uranium contains
0.71%235U, 99.28%238U, and0.006%234U. Fuel enriched
in 235U, 233U, or plutonium is used to provide greater latitude
in selecting materials for use in the reactor system
and to achieve higher burnup. Since 233U and plutonium
must be produced from thorium and 238U, respectively,
by neutron capture, the neutrons are provided initially by
fission of 235U (Fig. 1).
The isotope 239Pu is present in minute quantities (1 part
in 100 billion) in uranium ores. It is produced by neutron irradiationof
238Ubythereactions showninFig. 1. Short periods
of irradiation produce mostly 239Pu, and longer irradiations
result in progressively more of the higher isotopes
of plutonium, up to 246Pu. The odd-number isotopes of Pu
are fissionable, whereas the even-number isotopes have
high neutron-absorption cross sections.
Reprocessing of Nuclear Fuel
The reprocessing of LWR fuel assemblies would reduce
the uranium needs and enrichment requirements by approximately
35%. The recycling of the plutonium for
LWRs has been studied extensively and can now be used
commercially.However, the institutional barriers to reprocessing
in the United States have, in effect, eliminated this
option for the time being in this country. Several other
nations are proceeding to use reprocessed fuels in their
LWRs. It should be pointed out that a typical core in a
LWR derives about 50% of its power from the fissioning
of bred-in plutonium isotopes near the end of an equilibrium
cycle. The performance of the mixed-oxide recycle
fuels (containing 3–6 wt. % PuO2) has been very impressive
and generally superior to that of the uranium dioxide
fuel. Other conservation measures include extended
burnup of fuel and optimization of plant availability or
capacity factor.