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Boron 10 Depletion

Natural boron consists primarily of two stable isotopes, 11B (80.1%) and  10B (19.9%). In nuclear industry boron is commonly used as a neutron absorber due to the high neutron cross-section of isotope  10B. Its (n,alpha) reaction cross-section for thermal neutrons is about 3840 barns (for 0.025 eV neutron). Isotope  11B has absorption cross-section for thermal neutrons about 0.005 barns (for 0.025 eV neutron). Most of (n,alpha) reactions of thermal neutrons are 10B(n,alpha)7Li reactions accompanied by 0.48 MeV gamma emission.

(n,alpha) reactions of 10B

Since the isotope 10B has a significantly higher neutron cross-section, the 10B depletes much faster than 11B. Without adding fresh boron (19,9% of 10B) into the primary coolant system, the enrichment of 10B in boric acid continuously decreases. As a result, the enrichment of 10B at the end of the fuel cycle can be, for example, below 18% of 10B. This phenomenon must be considered in all the criticality calculations (e.g.,, shutdown margin calculations, estimated critical conditions, or general core depletion calculations).

Boron 10. Comparison of total cross-section and cross-section for (n,alpha) reactions.
Source: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data Library
Boron letdown curve (chemical shim) and boron 10 depletion
Boron letdown curve (chemical shim) and boron 10 depletion during a 12-month fuel cycle. At the beginning of the specific fuel cycle concentration of boric acid is highest. At the end of this cycle, the concentration of boric acid is almost zero. A reactor must be refueled because there is no positive reactivity that can be inserted to compensate the negative reactivity of fuel burnup (increase in reactor slagging).

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Boric Acid – Chemical Shim

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Boron 10

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H3BO3 Converter