PLENTIFUL ENERGY

transferred in a heat exchanger inside the tank to a secondary cooling circuit. Only
non-radioactive sodium from the secondary cooling circuit is brought out of the
vessel. This piping may develop a leak, but there can be no spread of radioactivity
from it. Radioactivity from sodium leaks is a non-existent problem in the pool
reactor configuration.
The pool configuration is a conscious choice, just as the fuel and coolant
materials choices are. The reactor tank is sized large enough to accommodate all the
primary system components. The core itself, the primary piping, and the primary
heat exchanger (where the heat is transferred from the radioactive primary sodium)
are submerged in the pool of primary sodium. The tank boundary has no
penetrations; it is a smooth walled tank, and it in turn sits in another larger diameter
tank. This guard vessel provides double assurance that there will be no leaks to the
room. Unpressurized, a leak of sodium from the primary vessel would go into the
space between the two vessels. That space is ―inerted‖ with argon gas, and
instrumentation is provided to monitor the space for any leaks into it. (There were
none in the thirty-year lifetime of EBR-II.)
It should be noted that of the two possible reactor configurations, pool or loop,
each is suited to one particular coolant type. The water-cooled reactor, because of
its high pressures, needs a small-diameter reactor vessel and the loop design is
almost mandatory. The sodium-cooled reactor, because of its low pressure coolant
can have any sized vessel. The primary coolant is radioactive, so it‘s best to have
primary components, piping, and connections inside the primary tank. The pool is a
natural choice, and it was the choice of Argonne‘s designers of EBR-II in the late
1950s. The loop design, of course, is possible, and in fact it became the choice for
the U.S. breeder development in the late sixties and seventies, and several of the
breeder reactors built around the world were given this configuration, but for a
number of reasons it is not the natural choice for sodium cooling.
As will be seen in the chapter on safety, sizing the pool to provide enough bulk
sodium to absorb the heat of accident conditions adds some remarkable extra safety
properties to the system. It allows safe regulation of the reactor power even under
conditions where an accident has disabled the control and safety systems. In such an
accident the massive pool of sodium provides ballast—heat can be absorbed until
the natural reactivity feedbacks of a metallic-fueled core come in strongly enough
to reduce the reactor power to harmless levels.
These ―natural reactivity feedbacks‖ reduce reactivity as the core expands from
the increased temperatures of an accident. Neutron leakage is much more important
to reactivity in a fast reactor than a thermal reactor. In a fast reactor, neutron crosssections
are small and neutrons typically travel tens of centimeters before being
absorbed, compared to distances of fractions of a centimeter in thermal reactors.
The core dimensions are small too, so a large fraction of the neutrons are born close
110