Page 248 - Dynamics and Control of Nuclear Reactors

APPENDIX B Advanced reactors 249

Both reactor designs use TRISO (Tristructural Isotropic) small fuel particles
(0.5mm) that can withstand high temperatures and contain fission products.
A TRISO coated fuel particle consists of an outer pyrolytic carbon layer
(0.92mm outer diameter), followed by a silicon carbide and an inner pyrolytic car-
bon layers, and an inner porous carbon buffer. The fuel kernel is in the center of the
particle. The silicon carbide’s high melting temperature (1600°C) protects against
particle failure.
Regardless of the specific reactor design, all advanced gas-cooled reactors have
the following features:
• Chemically inert Helium coolant
• Single phase Helium coolant (no issues due to boiling)
• Negligible neutron absorptions in Helium coolant (zero coolant temperature
coefficient of reactivity
• High thermal conductivity in graphite (avoids hot spots)
• Slow response to perturbations due to large heat capacity
• Dominant negative fuel temperature coefficient of reactivity due to the Doppler
effect
• Power stabilizes after a perturbation with no control action.

All of these features combine to enhance gas-cooled reactor safety. Both large and
small versions of gas-cooled reactors are candidates for implementation.

B.6.2 Liquid metal fast breeder reactors
The potential for breeding in fast reactors has been realized since the early days of
nuclear power and a number of prototype reactors have been operated (Refs. 18–20).
Two large fast reactors (BN 600 and BN 800) are operating in in Russia as this book
is being written (2018) and fast reactors are under construction in China [21] and
India [22].
Sodium fast reactors (SFRs) are a class of advanced reactor design that uses
sodium to remove heat from the reactor core, and transfer to heat exchangers and
steam generators. SFRs typically fall into one of two design categories: pool-type
and loop-type. Pool-type SFRs feature primary coolant system components such
as pumps and intermediate heat exchangers positioned inside the reactor vessel.
In loop-type reactors, these components are exterior to the reactor vessel. Pool-type
configurations reduce the possibility of many accident scenarios present in light
water reactors.
Since sodium (Na-23) atoms are heavier than hydrogen and oxygen atoms, neu-
trons lose less energy in collisions with sodium atoms than hydrogen and oxygen in
LWRs, thus enabling fast fission reactions. Sodium has a large range of temperatures
(371–1156K) in the liquid phase enabling the absorption of significant heat in the
liquid phase. A large sodium reservoir provides excellent thermal inertia against
overheating. Because of the high boiling point of sodium (compared to operating
temperatures), it is not necessary to pressurize the primary loop, thus enabling the