ML21265A547

Criticality Safety Analysis for MNRC Spent Fuel Pits

ML21265A547
Person / Time
Site:	University of California-Davis
Issue date:	09/23/2021
From:	McClellan Nuclear Research Center
To:	Office of Nuclear Reactor Regulation
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ML21265A540	List: ML21265A541 ML21265A542 ML21265A543 ML21265A544 ML21265A545 ML21265A546 ML21265A547 ML21265A548 ML21265A549 ML21265A550 ML21265A551 ML21265A552 ML21265A553
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Text

The analysis below is given to demonstrate that the MNRC spent fuel storage pits will remain subcritical by an acceptable margin (keff<0.90) under any condition or fuel loading. This analysis will need to be incorporated into section 9.1.3 of the SAR.

Criticality Safety Analysis for MNRCs Fuel Storage Pit:

An MCNP model is used to simulate a single fuel storage pit based on the known geometry and composition of the MNRC fuel pits. The analysis simulates a fuel storage pit having 2 tiers of 19 fuel elements each in a hexagonal arrangement, for a total of 38 fuel elements. The inner diameter of the fuel storage pit is 10. There is a 6 distance from the bottom of lower-rack fuel elements to the bottom of the fuel storage pit. The distance between the bottom of higher-rack fuel elements and the top of lower-rack fuel elements is 3.3. The concrete surrounding the fuel pits was simulated as well because the hydrogen in the concrete will likely act as a reflector.

In order to be conservative for this analysis only fresh 8.5/20, 20/20, and 30/20 TRIGA fuel elements are modeled. The keff factors under each condition are summarized in the following table. In each case, the MCNP models were run long enough for the relative statistical error of keff to be less than 1%.

keff Factor 38 8.5/20 Fresh FEs 38 20/20 Fresh FEs 38 30/20 Fresh FEs Dry Condition 0.493 0.559 0.565 Flooded Condition 0.747 0.824 0.821 The worst-case scenario is that 38 fresh 20/20 TRIGA fuel elements are placed in a water-flooded fuel storage pit, resulting in a keff equal to 0.824. This result was not statistically different than the flooded condition with 38 fresh 30/20 fuel elements. The results in all cases are significantly lower than a keff of 0.900. A more realistic scenario is that 8.5, 20, and 30 wt% elements of different (non-zero) burnups will be placed in the fuel storage pits. As fuel burnup increases the keff will decrease slightly, thus increasing the safety margin. It should be noted once again MNRC has no plans to flood any of the MNRC spent fuel storage pits.