Double Contingency Principle

Spent fuel pool
Spent fuel pool. Source: wikipedia.org License: Public Domain

“The double contingency approach requires a demonstration that unintended criticality cannot occur unless at least two unlikely, independent, concurrent changes in the conditions originally specified as essential to criticality safety have occurred.”

Source: Nuclear Safety Technical Assessment Guide. NS-TAST-GD-041 Revision 5. ONR, 2016.

The double contingency principle discussed in ANSI/ANS-8.1 allows credit for soluble boron under other abnormal or accident conditions, since only a single accident need be considered at one time. For example, the most severe accident scenario is associated with the movement of fuel within spent fuel pool, and accidental misloading of a fuel assembly in the Region 2 of the spent fuel pool. This could potentially increase the criticality of Region 2. To mitigate these postulated criticality related accidents, boron is dissolved in the pool water.

References:
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  4. E. E. Lewis, W. F. Miller, Computational Methods of Neutron Transport, American Nuclear Society, 1993, ISBN: 0-894-48452-4.

See above:

Spent Fuel Pool