Double Contingency Principle

Spent fuel pool
Spent fuel pool. Source: 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.

Nuclear and Reactor Physics:
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  2. K. O. Ott, R. J. Neuhold, Introductory Nuclear Reactor Dynamics, American Nuclear Society, 1985, ISBN: 0-894-48029-4.
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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