| 摘要 |
<p>A method for reducing the proportion of fast neutrons leaking out of a reactor core in a boiling water reactor. The reactor core comprises a plurality of fuel assemblies and is surrounded by a reactor vessel. Water flows upwards through the fuel assemblies and part of the water is thereby transformed into steam. A fuel assembly comprises a number of vertical channel-formed parts, so-called sub-channels (9g, 9h), to accommodate fuel bundles (15g). A fuel bundle comprises a plurality of fuel rods arranged between a bottom tie plate and a top tie plate. Openings (26a, 26b) are provided between adjacent sub-channels such that water and steam may flow between the sub-channels. The method comprises removing, in a plurality of those fuel assemblies which are located furthest out in the core, closest to the reactor vessel, one or more fuel bundles from sub-channels which are facing the reactor vessel, and replacing them with one or more devices (16h) for improving the neutron moderation in the upper portion of the fuel assembly. Such a device comprises means (17) for restricting the water flow in the upper portion of a sub-channel, and means (27) for blocking, downstream of the means for restriction, the openings (26b) between adjacent sub-channels.</p> |
