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BWR Core Design and Core and Fuel Management
[1] Features of BWR core [3]
BWRs operate in a direct cycle in which the steam formed in the core goes directly to turbines to generate electricity. A big difference from other type reactor cores is that there is a distribution of moderator density within the BWR core because the neutron moderator, as coolant, is boiling as it flows through the core which leads to two-phase flow.
Figure 3.3 shows a BWR core cross-sectional view and structures. There are some distinctive features to BWR core structures. One is the set of a cruciform control rod and four surrounding assemblies; the sets are regularly arranged in the core. Another feature is that the control rod and fuel assemblies are mutually separated by channel boxes wrapping each fuel assembly, in which fuel rods and water rods are tied in an array of, for example, 8 x 8 or 9 x 9, by spacers. A third feature is that the coolant flow path in the core is divided by the channel boxes and each fuel assembly makes a coolant flow path. There is a water gap region, in which water does not boil, between adjacent fuel assemblies. The coolant flowing in from the core bottom comes to boiling and then a two-phase flow of water and steam is produced due to heat from the fuel rods within the fuel assemblies isolated by the channel boxes, but coolant is maintained in the non-boiling state outside the assemblies.
Those features of BWR core structures make it possible to improve or modify the intra-assembly design such as fuel rod size and array without changing specifications concerning the whole core construction such as control rod arrangement and fuel assembly shape.
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Table 3.2 History of BWR Specifications [1, 2]
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Table 3.3 BORAX and early test BWRs
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Fig. 3.3 BWR core cross-sectional view and structures