Factors to Consider in Long Term Reactivity Control for PWRs

[libertad]

One of my tasks these days is reseaching about long term reactivity control on a typical PWR. As I know in the long term reactivity control we must consider to the role of burnable absorber and boric acid concentration in reactor core during its operation cycles.
So I want to do some calculation to analysis of these two factors.
If you think which I must do some other tasks or consider to other criteria please let me know.
Thanks.

 

[Astronuc]

Reactivity control is accomplished by radial and axial enrichment design, burnable poisons in the fuel, burnable poison clusters, boric acid in the coolant (which is then buffered by LiOH to maintain a pH above 6.9 at operating conditions) and in some plants by 'grey' rods for axial power shaping.

The U-235 enrichments can be tailored to reduce the local peaking and flatten the flux by flattening the power profile. Different enrichments are used in the blankets and in different rods within symmetric assemblies and in different assemblies.

In the fuel, one will find two main types - IFBA and Gadolinia. There was third type which used Erbia, but this has largely been replaced by IFBA. The IFBA fuel contains UO₂ coated with ZrB₂ with the boron enriched in B-10. The coating is fractions of a mil depending on design.

Gadolinia fuel involves the mixture of Gd₂O₃ (gadolinia) and UO₂ in the same ceramic pellet. Disadvantages of Gd are the residual remaining toward end of the first cycle and the lower thermal conductivity. Enriching the Gd in isotopes 155 and 157 can allow one to reduce the gadolinia concentration thus reducing the penalties.

Solid burnable poisons include BPRAs (burnable poison rod assemblies) and wet annular burnable absorbers (WABAs) which look like control rod clusters. The burnable poison material (B₄ in an alumina matrix or a borosilicate pyrex) are encapsulated in long stainless steel tubes and placed in the guide tubes of selected assemblies (which are obviously not locations under control rods).

Boric acid is a chemical shim. The concentration begins at high levels at BOC and gradually decreases throughout a cycle. The pH is controlled with LiOH. Ideally the boric acid concentration is close to zero.

In a few plants in the US and several in France, 'grey' rods containing Inconel are used to locally suppress the flux (and thus power). The grey rods are less absorbing than burnable poisons or control rods.