Page 227 - Managing Global Warming

186 Managing Global Warming

percentage of fissile nuclides compared to thermal-neutron-spectrum reactors. The
majority of the current nuclear power reactors have been designed as thermal-
neutron-spectrum reactors (PWRs, BWRs, PHWRs, AGRs, and LGRs), and only
two reactors—BN-600 and BN-800 in Russia are fast reactors cooled with sodium
(so-called SFRs).
In a nuclear fuel, the fission chain reaction is maintained by fission of fissile ele-
ments, which are capable of sustaining the fission reaction with neutrons of all ener-
gies. As such, fissile nuclides are used in the fuel of both thermal-neutron-spectrum
and fast-neutron-spectrum reactors. The fissile nuclides of importance for nuclear
reactors are U 233 ,U 235 ,Pu 239 , and Pu 241 . Among these fissile nuclides, only U 235
is a naturally occurring nuclide, while others are produced by neutron capture of other
nuclides during operation of a nuclear reactor. For instance, Pu 239 is produced by
neutron capture of U 238 ;Pu 241 —by neutron capture of Pu 240 ; and U 233 —by neutron
capture of Th 232 . The current nuclear reactors rely on U 235 as the primary fissile ele-
ment, often in an enriched proportion, but also use the Pu 239 produced during irradi-
ation by neutron capture in naturally occurring U 238 . In fact, the fuel composition of
most nuclear power reactors consists primarily of U 238 , which undergoes fission only
with very high-energy neutrons. In modern reactors, the degree of U 235 enrichment is
limited to <20% on the grounds of nonproliferation concerns, and the separation and
recycling of fissile Pu 239 from once-used fuel is subject to the international IAEA
safeguards. As shown in Table 4.17, fast-neutron-spectrum reactors require a higher
percentage of fissile elements than thermal-neutron-spectrum reactors as the probabil-
235 239
ity of fission reaction of fissile elements such as U and Pu with fast neutrons is
lower.
Even though the majority of nuclear power reactors are thermal-neutron-spectrum
reactors, there has been a continuous scientific effort for design and operation of fast-
neutron-spectrum reactors since the inception of the nuclear technology. There is a
renewed interest in fast-neutron-spectrum reactors to be included as part of the overall
nuclear fuel cycle, because of the advantages that these reactors offer. Currently, an
international collaboration has focused on the development of six concepts of the Gen-
eration IV nuclear reactors. There are several fast-neutron-spectrum reactors among
the selected designs (see Section 4.2).
Nuclear reactors can be designed on a basis of their fuel cycle, such that they
breed more fissile nuclides than what they use. Breeder reactors can utilize uranium,
thorium, and plutonium resources more efficiently. There are two types of breeder
reactors: (1) fast-neutron-spectrum-breeder and (2) thermal-neutron-spectrum-
breeder reactors, which are designed based on U 238 (99.2% natural abundance)
and Th 232 (100% natural abundance), respectively. Fertile nuclides U 238 and
Th 232 capture neutrons and transform, respectively, to fissile nuclides Pu 239 and
U 233 . Through this process, which is known as breeding, the reactor produces more
fissile nuclides than what it consumes. Fast-breeder reactors (FBRs) can also be used
in order to transmute the long-lived minor actinides in the spent fuel to radionuclides
with shorter half-lives. Thermal breeder reactors, on the other hand, produce less
232 233
minor actinides in the spent fuel. Th -U breeding cycle can be utilized in both
fast and thermal reactors. Even though both fast and thermal breeder reactors have