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XYLENE POWER LTD.

FNR CRITICALITY

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

A FNR has several criticality conditions all of which must be met.

FNR BASIC CRITICALITY REQUIREMENTS:
1) When the control portions of all of the active fuel bundles are fully inserted but the core fuel is nearly fully 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2) When the control portions of all of the active fuel bundles are 1.2 m withdrawn and the core fuel is new (20% Pu) the reactor must be reliably sub-critical.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 with new core fuel. In this condition the active fuel bundle control portion core fuel rods form a lower elevation layer and the surround portion core fuel rods form a higher elevation layer. The two layers are separated by 0.8 m of blanket rod material. Hence both the upper core fuel layer (the surround portion core fuel) and the lower core fuel layer (the control portion core fuel) must be individually subcritical.

This requirement for individual layer subcriticality limits the maximum Pu-239 concentration in the core fuel alloy.
 

TWO INDEPENDENT SHUTDOWN MECHANISMS:
If all of the fuel bundles are initially at their normal operating state and then half of the active fuel bundle control portions are withdrawn checkerboard style the reactor will reliably shut down. This safety shutdown condition enables two fully independent reactor shutdown mechanisms. In this shutdown state the core fuel is in three layers. This shutdown condition is easily met.
 

PROTECTION AGAINST A DEFECTIVE CONTROL SYSTEM:
In the event that due to multiple simultaneous control system failures a new fuel bundle is erroneously driven into its fully inserted state it is essential for the adjacent fuel bundles, which are separately controlled, to be able to shut down the defective fuel bundle.

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every second full bundle are fully inserted while the control portions of the remaining active fuel bundles are fully withdrawn chequerboard style the reactor must be subcritical. This condition ensures a cold shutdown condition with half of the fuel bundles jammed in their maximum possible thermal power states. This condition is the most difficult shutdown condition to reliably meet and heavily constrains the core fuel alloy mix.

This condition 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 because the other independent shutdown system will force a reactor shutdown. Hence, in all fuel conditions fuel bundle criticality cannot occur if the control portions of the four nearest neighbour active fuel bundle control portions are fully withdrawn.

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

FUEL BUNDLE STATE DIAGRAM:
The active fuel bundle control portion insertion positions are shown below for the extreme cases of reactor off and reactor at maximum power. In the reactor off state the core fuel forms two widely separated lower core fuel concentration layers. In the normal reactor on state these two layers are closer together. As the fuel ages the amount of active fuel bundle control portion withdrawl is gradually reduced making the two layers gradually merge. When the active fuel bundle control portion withdrawl reaches zero the two core fuel layers are fully merged at maximum rated reactor power and the fuel bundle should be reprocessed.
 

Note that the maximum permitted vertical travel of an active fuel bundle control portion is 1.2 m which causes the transition between the active fuel bundle control portion fully withdrawn (reactor off) and active fuel bundle control portion fully inserted (reactor maximum power) states.
 

This web page last updated January 21, 2019

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