BWR RIA during nuclear heating

[rmattila]

It seems that the recent developments in BWR fuels (increased enrichment, added uranium mass at the bottom, part-length fuel rods especially in the corners) have changed the control rod reactivity values in such a way that the potential reactivity insertion caused by the postulated rod drop accident is potentially quite high at certain points during nuclear heating. More specifically, at the stage when CR:s adjacent tho those already withdrawn are being taken out of the core. This may result into quite large reactivity insertions and it is not trivial that the RIA limits of fuel are fulfilled at every point (up until the spectral effect finally cuts the CR reactivity values), if the situation has not been considered thoroughly at some stage during the gradual fuel development.

I was wondering if someone else has come up with this issue, and if there are some findings that might be of interest? Especially the behaviour of Doppler and other feedback mechanisms during the heating phase would be interesting. I am planning to do some generic calculations on my own, but it would be nice to know if someone has already done something in this direction.

 

[Astronuc]

This is perhaps the most comprehensive report on the subject.
http://www.oecd-nea.org/nsd/reports/2010/nea6847-behaviour-RIA.pdf

In the US, many BWRs are now on 24-month cycles, while many, if not most, European BWRs (and LWRs) are still on annual cycles. Many BWR lattices have enrichments up to 4.9% in a many fuel rods.

 

[rmattila]

Yes, that is a very good report concerning the consequences of the postulated CR drop. However, even there on page 143 it is suggested that analyses are performed either at CZP (where the enthalpy rise has traditionally been limiting) or at HZP (which has traditionally been close to the maximum reactivity insertion point). What I'm mainly concerned is the validity of using only these two points, as it seems that the most limiting point might actually lie somewhere in between, or perhaps at a few % power, depending on the reloading pattern and the withdrawal sequence.

There are two competing effects: the CR worth, which tends to be the largest at a certain point rather late in the in the withdrawal sequence, and the negative feedbacks, which are more effective at increased temperature. Using just the CZP and HZP points in the analyses might fail to catch the most limiting transient.