Why is californium-252 used to start up and calibrate nuclear reactors?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — transuranium element Modern version of the periodic table of the elements (printable).

Californium-252 is used to start up and calibrate nuclear reactors primarily because it serves as an extremely intense source of neutrons.12

Approximately 3 percent of californium-252 decay occurs through spontaneous fission, a process that releases a massive number of neutrons—specifically, one gram of the isotope emits about $2.3 \times 10^{12}$ neutrons per second. This high neutron output makes it an ideal tool for initiating the chain reaction required to start a nuclear reactor and for providing the necessary neutron flux to calibrate reactor instrumentation accurately.123