Nuclear-Fueled Power Plants - Energy and the Environment - page 133

The efficiency of fuel utilization in a breeder reactor is expressed as the ratio of the number of
fissile nuclei formed to the number destroyed. This ratio is called breeding ratio (BR):

Number of fissile nuclei produced
Number of fissile nuclei destroyed

BR = (6.11)

When BR is greater than 1, breeding occurs. Note that both in the numerator and denominator of
equation (6.11) fissile nuclei include not only 235 U, but 233 U and initially present 239 Pu.
Breeder reactors could also use thorium as a fuel. 232 Th is a fertile nucleus that can be converted
into fissile 233 U by the following reaction sequence:

232 Th 233 Th 233 Pa 233 U

+ n + γ → + β → + β → (6.12)

Here 233 Pa is an isotope of element 91, protactinium. Because worldwide thorium ores have about
an equal abundance as uranium ores, the use of thorium would extend the nuclear fuel resources
by about a factor of two. So far, thorium-based breeder reactors are only in the planning and
development phase.
A major problem with breeder reactors is the need to use liquid sodium as a coolant. (A
helium-cooled breeder reactor was designed by Gulf General Atomic Corporation in the 1960s,
but was not constructed.) In addition to the appropriate moderating characteristics of sodium, it
also has an excellent heat transfer capacity. Sodium melts at 90 ◦ C and boils at 882 ◦ C. This allows
the reactor to run hotter, and consequently a higher thermal efficiency is obtained. But sodium is
a nasty chemical. It burns spontaneously in air, and it reacts violently with water. Furthermore,
23 Na (the natural isotope) can absorb a neutron to convert first into 24 Na, and then into the stable
24 Mg. The intermediary

24 Na emits very energetic

radiation. Therefore, more than one
sodium loop is required, a separate loop for the sodium coolant circulation and another loop for
the water/steam cycle. A schematic of a LMFBR with two sodium loops is presented in Figure 6.5.
The reactor core, sodium loops, heat exchangers, and steam loops are all located in a contain-
ment vessel; the steam turbine, condenser, and the rest of the generating plant are outside of the
vessel.
Several fast breeder reactors were operated in the United States, United Kingdom, France,
Germany, India, Japan, and Russia. The largest (400-MW) breeder reactor in the United States
operated from 1980 to 1994 at Clinch River, Tennessee, but was terminated because of concern
about nuclear proliferation. Breeder reactors still in operation are the 250-MW Phenix in France
(the 1240-MW Superphenix was shut down in 1998), the 40-MW FBTR in India, the 100-MW
Joyo in Japan, the 135-MW BN 350 formerly in USSR but now in Kazakhstan, and the 10-MW
BR 10, the 12-MW BOR 60, and the 600-MW BN 600 in Russia.
Most breeder reactors, with the exception of those in Japan, were operated for the pri-
mary purpose of producing weapons-grade plutonium. This is certainly one of the drawbacks of
breeder-type power plants. While they produce more fissile fuel than they consume in the process of
power production, the fissile fuel, plutonium, can be fairly easily extracted to make atomic bombs.
Strict international supervision will be necessary if, in the future, breeder-type power plants will
commonly be used for electric power production.

β and γ