{"id":211,"date":"2015-11-07T08:33:20","date_gmt":"2015-11-07T08:33:20","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=211"},"modified":"2022-10-12T06:03:45","modified_gmt":"2022-10-12T06:03:45","slug":"control-rods","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-power-plant\/control-rods\/","title":{"rendered":"Control Rods"},"content":{"rendered":"
Control rods<\/strong> are rods, plates, or tubes containing a neutron absorbing material<\/strong> (material with high absorption cross-section<\/a> for thermal neutron) such as boron<\/a>, hafnium, cadmium<\/a>, etc., used to control the power of a nuclear reactor. A control rod<\/strong> is removed from or inserted into the reactor\u00a0core<\/a> to increase or decrease the reactor’s reactivity<\/b> (increase or decrease the neutron flux). This, in turn, affects the reactor’s thermal power, the amount of steam produced, and hence the electricity generated.<\/div><\/div>\n

By absorbing neutrons, a control rod prevents the neutrons<\/a> from causing further fissions. Control rods<\/strong> are an important safety system<\/strong> for nuclear reactors. Their prompt action and prompt response to the reactor are indispensable. Control rods are used for maintaining the desired state of fission reactions<\/a> within a nuclear reactor<\/a> (i.e., subcritical state, critical state, power changes). They constitute a key component of an emergency shutdown system (SCRAM)<\/strong>.<\/p>\n

\"Control<\/a>
Control rod assembly for VVER reactor. Absorber – boron carbide
Source: www.gidropress.podolsk.ru\/files\/proceedings
\/mntk2011\/documents\/mntk2011-108.pdf<\/figcaption><\/figure>\n

Control rods usually constitute cluster control rod assemblies<\/strong> (PWR<\/a>) inserted into guide thimbles within a nuclear fuel assembly<\/a>. The cladding protects the absorbing material (e.g.,, pellets of Boron Carbide), usually made of stainless steel. They are grouped into groups (banks), and the movement usually occurs by the groups (banks). The typical total number of clusters is 70. This number is limited, especially by the number of penetrations of the reactor pressure vessel head.<\/p>\n

A control rod<\/strong> is removed from or inserted into the reactor\u00a0core<\/a> to increase or decrease the reactor’s reactivity<\/b>\u00a0(increase or decrease the neutron flux). By the changes of the reactivity, the changes of neutron power are performed. This, in turn, affects the reactor’s thermal power, the amount of steam produced, and hence the electricity generated.<\/p>\n

In PWRs, they are inserted from above, with the control rod drive mechanisms being mounted on the reactor pressure vessel head. Due to the necessity of a steam dryer above the core of a boiling water reactor<\/a>, this design requires the insertion of the control rods from underneath the core.<\/p>\n

<\/span>Operability of control and shutdown rods<\/div>
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In PWRs, the shutdown and control rods’ operability (i.e., trip ability) is an initial assumption in all safety analyses that assume rod insertion upon reactor trip.<\/p>\n

Operability of control rods<\/strong><\/p>\n

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  1. ARI-1 condition<\/strong>. In all the safety analyses, it is usually assumed that control rods are fully inserted except for the single control rod of highest reactivity worth, which is assumed to be fully withdrawn (i.e., stuck rod).<\/li>\n
  2. Inoperability of the control rod<\/strong>. When one or more rods are inoperable (i.e., untrippable) during MODE 1 (Power Operation), there is a possibility that the required SDM may be adversely affected. Under these conditions, it is important to determine and ensure the SDM. The plant must be brought to a MODE or condition in which the LCO requirements are not applicable if the inoperable rod(s) cannot be restored to operable status. The unit must be brought to at least MODE 3 (Hot Standby) within (for example, 6 hours) to achieve this status.<\/li>\n<\/ol>\n<\/div><\/div>\n

    Control Rods usage<\/h2>\n