Breeder Reactors

The fast breeder reactor (FBR) is so named because more fissile material is formed than consumed, a feat that can be accomplished if the fragment nuclei are also fis­sile. The breeding ratio is the ratio of fissile material produced to the fissile material consumed; any reactor that achieves a breeding ratio of >1 is characterized as a breeder reactor. In a conventional light water reactor, Pu-239 is formed by random neutron capture, but not at a rate high enough to sustain the chain reaction (hence the fuel must be enriched in U-235). By using a uranium-plutonium oxide fuel mixture (usually «20% Pu-239) bombarded with fast neutrons, Pu-239 can undergo fission to produce more second-generation neutrons than any other fissile isotope. At the same time, U-238 present in the fuel can undergo neutron capture to transmute to more Pu-239 (recall Equation 9.7). Furthermore, the reactor core is surrounded by a “blanket” of natural or depleted uranium to capture any stray neutrons and create even more Pu-239.

That a breeder reactor requires fast neutrons means that no moderator is needed, although (obviously) a coolant still is. By using a coolant with a high mass number, thermalization is minimized. The coolant of choice in FBRs is liquid sodium, the use of which clearly presents safety issues of its own as sodium reacts violently with both air and water (Equations 9.9a through 9.9c).

Na° + H2O ^ NaOH + H2

(9.9a)

Na° + O2 ^ ^Na2O

(9.9b)

4Na° + O2 ^ 2Na2O

(9.9c)

The BN-600 600 MW FBR in Russia had 27 sodium leaks between 1980 and 1997, resulting in 14 sodium fires (Cochran et al. 2010). Another serious safety issue is associated with the potential loss of coolant. In a fast-neutron reactor, the Pu-239 con­centration is high enough that the chain reaction will continue (or may even increase) if the coolant is lost, possibly leading to a small nuclear explosion. There are several other reasons that the commercialization of FBRs has failed. A key reason for its development in the first place was the scarcity of uranium fuel—but this has not proven to be an issue. Breeder reactors are both very expensive to build and to operate and have a reputation for being unreliable, needing extended periods off-line for main­tenance and repairs (primarily a result of using the liquid sodium coolant). Another major concern associated with breeder reactors is the proliferation of Pu-239 that can be used in nuclear weapons. Several FBRs have been built around the globe, but breeder reactors are, for all practical purposes, a dying breed (Cochran et al. 2010).

Updated: September 25, 2015 — 11:02 am