Page 243 - Dynamics and Control of Nuclear Reactors

244 APPENDIX B Advanced reactors

This Appendix emphasizes the features of advanced reactors that affect dynam-
ics, control, and safety.

B.2 Design possibilities

There are many possibilities for the constituents of a power reactor and for different
operational requirements and capabilities: The main choices include the following:

• Fuel: U-235, U-233, Pu-239
• Form of fuel: metal, metal oxide, metal carbide, fluid
• Fertile material: U-238, Th-232
• Moderator: water, heavy water, graphite, none (in fast reactors)
• Reactor coolant: liquid water, boiling water, liquid heavy water, helium, carbon
dioxide, molten salt, molten sodium, molten lead, molten lead-bismuth
• Secondary coolant: none (boiling water reactors), saturated steam, superheated
steam, helium, carbon dioxide
• Electricity production: Rankine cycle (steam turbine) Brayton cycle (gas
turbine)

Designers have chosen various combinations of these constituents and operational
features for potential advanced reactors. Designs for domestic use and export have
been prepared in several countries including the United States, Canada, France,
Japan, China, South Korea, India and Russia. Some of the various designs differ only
slightly from designs from other countries. But the number of different possibilities
is large and too extensive for inclusion of descriptions of every potential advanced
reactor in this book.

B.3 A note about reactors that use thorium
Some of the new reactor designs use Th-232 as a fertile material. They produce
U-233 by the following reactions:
Th-232 + n ! Th-233 ! Pa-233 + β ! U-233 + β (B.1)

Th-233 has a half-life of 22min and a very large capture cross section (about 2.7
times the U-233 fission cross section). Neutron losses to Th-233 would be major
if it had a longer half-life. The relatively short half-life causes the residence time
of Th-233 to be short and of rather small consequence.
The Pa-233 is a different story. It has a half-life of 27days and a significant appe-
tite for neutrons (absorption cross section for thermal neutrons is around 7.5% as
large as the U-233 cross section). Therefore Th-232/U-233 reactors experience a sig-
nificant loss of bred U-233 if the Pa-233 stays in the reactor. Reducing this problem
requires removal and sequestration of the Pa-233 in a low neutron flux region or