09 February 2011

Fast breeder reactor

      India’s first 40 MWt Fast Breeder Test Reactor (FBTR) attained criticality on 18 October 1985. India has the capability to use thorium cycle based processes to extract nuclear fuel. This is of special significance to the Indian nuclear power generation strategy as India has large reserves of thorium — about 360,000 tonnes — that can fuel nuclear projects for an estimated 2,500 years. The higher construction expense of the Fast Breeder Reactor in comparison with the Pressurised Heavy Water Reactors (PHWR) in use is one of the main reasons why India is looking at the cheaper option - uranium fuel. 


     The fast breeder or fast breeder reactor (FBR) is a fast neutron reactor designed to breed fuel by producing more fissile material than it consumes. The FBR is one possible type of breeder reactor.

      
 
       All large-scale FBR power stations have been liquid metal fast breeder reactors (LMFBR) cooled by liquid sodium. These have been of one of two designs:
Loop type, in which the primary coolant is circulated through primary heat exchangers external to the reactor tank (but within the biological shield owing to the presence of radioactive sodium-24 in the primary coolant).
Pool type, in which the primary heat exchangers and circulators are immersed in the reactor tank.

     In PWRs and BWRs, a vast majority of the fission reactions occur in U-235, which makes up for only 0.7 % of natural uranium and during fuel fabrication for these reactors it is enriched to a few percent. Accordingly, in the already mentioned reactor types (sometimes referred to as thermal reactors) U-238 is hardly applied as fissionable material. However, upon capturing a neutron, the nucleus of U-328 can transform into Pu-239 (via radioactive decay), which is a fissile material. For Pu-239, fission can also be induced using fast neutrons. The fast breeder reactors use both processes.

     The core of a fast breeder reactor consists of two parts. The fuel rods, which contain a mixture of uranium dioxide and plutonium dioxide, are found in the inner part. Here fission reactions dominate, while in the outer part the predominant process is conversion of U-238 to Pu-239. This part contains depleted uranium (i.e. uranium in which the U-235 content is even lower than the natural 0.7%). In such a reactor one can achieve a situation where more fissile plutonium nuclei are produced in a unit time than the number of fissile nuclei which undergo fission (hence the name "breeder"). On the other hand, neutrons are not thermalized, since fast neutrons are needed for the above described processes.

      Obviously, in a fast reactor there must not be any moderator, which implies that water is not at all suitable as coolant. Instead some liquid metal, usually sodium is applied. The boiling point of sodium is very high even at comparatively low pressures (at 10 bars about 900oC), there is no need to maintain a high pressure in the primary circuit and thus the construction and manufacturing of the reactor vessel is easier. 

     The heat of primary sodium is transferred to the secondary sodium in an intermediate heat exchanger, while the third heat exchanger is the steam generator. Application of three loops is necessitated by safety considerations (liquid sodium is very dangerous: the primary sodium is highly radioactive because of neutrons activation, which results in Na-24; the second sodium loop prevents radioactive sodium from accidental contact with water.) 

     The ratio between the Pu239 (or U235) fission cross-section and the U238 absorption cross-section is much higher in a thermal spectrum than in a fast spectrum. Therefore a higher enrichment of the fuel is needed in a fast reactor in order to reach a self-sustaining nuclear chain reaction. This same feature of fast neutrons also increases the ratio of breeding to fission.

      Since a fast reactor uses a fast spectrum, no moderator is required to thermalize the fast neutrons.