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XYLENE POWER LTD.

FNR CRITICALITY

By Charles Rhodes, P.Eng., Ph.D.

REACTOR CRITICALITY OVERVIEW:
A FNR has three extreme criticality conditions all of which must be met. 1) When the control portions of all of the active fuel bundles are fully inserted but the core fuel is depleted (12.5% Pu) the core must still be critical. This is the depleted fuel condition

2) When the control portions of all of the active fuel bundles are 1.2 m withdrawn and the core fuel is new (20% Pu) the reactor must be reliably sub-critical. This is the cold shutdown condition. In this condition the active fuel bundle control portion core fuel rods are all at one level and the surround portion core fuel rods are all at another level. The two levels are separated by 0.8 m of blanket rod material. This condition potentially slightly constrains the core fuel alloy mix.

3) When the fuel is new (20% Pu) and the control portions of every second full bundle are fully inserted while the control portions of the remaining active fuel bundles are 1.2 m withdrawn chequerboard style the reactor must be subcritical. This is the safety shutdown condition. This condition is analogous to the cold shutdown condition but half of the fuel bundles are in their maximum possible thermal power configurations. This condition is the most difficult shutdown condition to reliably meet and more heavily constrains the core fuel alloy mix.

The safety shutdown condition enables two fully independent reactor shutdown mechanisms and ensures that an isolated active fuel bundle with new fuel will not go critical if its control portion is accidentally fully inserted into its surround portion. In all fuel conditions fuel bundle criticality cannot occur if the control portions of the four nearest neighbour active fuel bundles are fully withdrawn.

Achieving condition #3 while allowing for the desired range of Pu-239 concentration decay in the core may require a 1.2 m control portion withdrawal, which in turn may require a blanket thickness of 1.6 m.

The relative fuel bundle insertion positions are shown below for these three extreme cases.

________FIX

Note that the maximum permitted vertical travel of an active fuel bundle control portion is 1.2 m which corresponds to the transition between the active fuel bundle control portion fully withdrawn and active fuel bundle control portion fully inserted states.
 

This web page last updated December 18, 2018

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