Nuclear reactivity Pressurized Water Reactor

The different reactivity control systems in a nuclear power plant allow keeping at any time the control of the nuclear fission reactions in the core power steering, safe reactor shutdown, wear compensation of the fuel. They are also part of the neutron protection of the out-of-core components. These systems can take various forms gas (such as helium 3 in some experimental reactors), liquid (soluble boron in pressurized water reactor (PWR) coolant to balance the reactivity evolution of the reactor), and most of the time solid (Table 15.1). In a reactor, they are most often combined [e.g., in PWR with Ag-In-Cd (AIC) plus boron carbide control rods and with boron present both as soluble boron and as boron carbide]. In all cases those materials incorporate neutron-absorbing nuclides, unlike the fuel which is a medium generally multiplier... 

The composition of boron carbide is approximately 80 atomic percent boron. The material is often considered as a source of boron, without the high reactivity of the latter. Like boron, B4C has a high neutron capture cross-section for thermal neutrons and a low secondary gamma radiation. As such, it provides an excellent neutron absorber and is used extensively to control the neutron flux in nuclear fission reactors, such as the boiling water, pressurized water, and fast breeding reactors. It is also used for the compact storage of spent fuel rods.l l... 

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