{"id":18541,"date":"2018-09-03T10:52:02","date_gmt":"2018-09-03T10:52:02","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=18541"},"modified":"2023-02-06T15:14:02","modified_gmt":"2023-02-06T15:14:02","slug":"excess-reactivity","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-power\/reactor-physics\/reactor-operation\/fuel-burnup\/excess-reactivity\/","title":{"rendered":"Excess Reactivity"},"content":{"rendered":"
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\"boron<\/a>
boron concentration vs. cycle burnup – PWR. Initial core reactivity requires about 9 g\/kg of boric acid to compensate it.<\/figcaption><\/figure>\n

The amount of fissile material contained in the core constantly decreases during the reactor\u2019s operation. If the reactor operates for a long period, fuel in excess<\/strong> (or excess reactivity<\/strong>) of that needed for exact criticality must be added when the reactor is refueled. For example, a typical 18-month fuel cycle requires an excess reactivity for 500 EFPDs. It means the core must be refueled after 500 days at 100% rated power or after 1000 days at 50% rated power.<\/p>\n

The excess reactivity<\/strong>, \u03c1ex<\/sub><\/strong>, is defined as the reactivity of a core if all control poisons (control rods<\/a>, burnable absorbers<\/a>, boric acid<\/a>) were instantaneously eliminated from the core. Large excess reactivities are undesirable because they require large amounts of neutron poisons<\/a> to be present in the core to compensate for them. Moreover, large excess reactivities require long fuel cycles (e.g., cycles longer than 24 months). These cycles are not economical due to the very high accumulation of fission products (see: Reactor Slagging<\/a>).<\/p>\n

The excess reactivity must be able to compensate:<\/p>\n