TECHNOLOGY

Generation II

FBR ( Fast Breeder Reactor)

Figure 18. Schematic diagram of FBR construction
Source: Own development based on "Power Reactors - Characteristics. 2010 WNA Pocket Guide", World Nuclear Association, July 2010 [25]

All thermal reactors described above have two main disadvantages. First is about non effective fuel usage - only 1-3 % of uranium, and second concerns very low steam parameters in turbine circuit caused by using water as coolant in primary circuit. Moreover need of moderator usage, lead to large constructions with rather low power density.

FBR reactors presents design different from the others. It has no moderator. Fast neutrons produced in fission reaction are not slowed down to lower energy intentionally. They slow down on construction elements and on fuel fractions directly but only to energy of 50 - 100 keV. Furthermore a part of fast neutrons cause conversion from non fissile by thermal neutrons U-238 to fissile Pu-239. Additionally fission reaction of Pu-239 produces more neutrons than reaction of U-238, so these neutrons can be used to U-238 to Pu-239 conversion. This process is named as fuel breeding. Fuel usage in FBR reactors is over 100 times more effective than in thermal reactors and there is no poisoning effect.

FBR reactors operates without moderator so the core dimensions are significantly smaller. Large amount of fuel in the core and small volume of the core cause that power density reaches level of several hundred MW/m3 (with highly enriched fuel). Such power density and proper coolant allow for obtain steam at temperature of 487 ºC under pressure of 17,7 MPa. This leads to efficiency of the FBR reactor at level of 40%. As coolant sodium is used. Only this does not act as moderator, and cools well. But necessity of sodium usage brings serious problems. One is producing radioactive sodium isotope what constrains triple circuit reactor. Another is sodium melting point - 98 ºC. To avoid its solidification sodium have to be heated even reactors is not operating. Special fuel UO2+PuO2 with 17,7% enrichment is also very expensive. All difficulties raise cost of FBR reactors by 50%, and makes them economically unprofitable, even though advantages are indisputable. [16],[21],[24], [29],[32]

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