Why is there a flux depression in CANDU fuel and how is it addressed in design?

[sandon]

let say there is an bundle of fuel with 4 rings. Where ring 1 is center most ring and ring 4 is outer most ring.

1st ring contains: 1 fuel element
2nd ring contains : 6 fuel elements
3rd ring contains: 12 fuel element
4th ring contains : 18 fuel elements

The general distribution of flux is that of a sinc function.

So why would 1 element in the outer most ring generate more power than the element in the inner most ring.

See attachment for (fuel element power) / (average fuel element power)

 

[Astronuc]

That sounds like a CANDU element/assembly/bundle.

The inner rod may contain a lower enrichment or burnable poison, or conversely, the outer ring has a slightly greater enrichment.

During irradiation, the fissile inventory (e.g., U-235) is depleted, so enrichment and power decreases with burnup. In addition, if there is a burnable poison, e.g., Gd-155, 157, it will deplete during irradiation, and the power will gradually increase with burnup.

Furthermore, with irradiation, some of the U-238 is converted to Pu-239 through n-capture in U-238 followed by two successive beta decays, and Pu-239 is fissile.

 

[QuantumPion]

My guess would be the outer ring has more moderator volume. Similar to higher power pins next to guide tubes in PWR rectangular lattice fuel.

 

[Stephan_doc]

1st ring contains: 1 fuel element
2nd ring contains : 6 fuel elements
3rd ring contains: 12 fuel element
4th ring contains : 18 fuel elements

This is CANDU 37-element fuel bundle with UO2 natural Uranium (0.71% U235). All elements in bundle has the same dimensions and composition. The CANFLEX bundle has 43 fuel elements, with two element sizes.The element in center contain poison (Dy).
Sure, the outer ring has more moderator volume (see CANDU lattice cell), i.e generate more power than the element in the inner most ring.

 

[Astronuc]

My guess would be the outer ring has more moderator volume. Similar to higher power pins next to guide tubes in PWR rectangular lattice fuel.

The outer ring could have slightly more moderation, depending on the clearance with the pressure tube.

I was also thinking that it could be surrounded by higher power bundles with slightly more enrichment.

But I suspect that the outer row might be more slightly enriched.

Newer bundle designs have more (43) rods.

 

[Hologram0110]

In Candu fuel design this is a well known phenomena usually just called "flux depression" (a fairly generic term). Just think about the neutron current. The fuel produces large amounts of fast neutrons. Most of these leave the fuel channel, and are thermalized in the moderator. Most of the thermal neutrons eventually diffuse into the fuel channels to be absorbed in the fuel (some of which induce fission). As you move towards the center of the bundle, more of the neutrons have been absorbed by the outer layers and the thermal flux is depressed. Then there is a positive feedback effect, higher flux on the outer ring leads to faster accumulation of fission products and Pu-239. These isotopes absorb even more of the thermal neutrons increasing the depth of the flux depression.

@Astronuc,
Designs for bundles with absorbers or enrichment gradients usually try to increase the absorption at the centre of the fuel element (but not always). This can be to either decrease flux depression and/or to reduce the coolant void coefficient (which is slightly positive for CANDUs). A positive void coefficient is commonly brought up as a safety concern for CANDU designs, however as hypothetical accidents progress the negative temperature coefficient actually takes over. As far as I know, all CANDU fuel designs currently being used commercially use uniform enrichment.