What Factors Limit the Burn-Up of RBMK Reactor Fuel?

[vifteovn]

Hi

I'm wondering what puts the limits to the burn-up of RBMK fuel. Is it the build up of fission products or the fuel simply running out of sufficient u-235 to sustain a chain reaction?

The average burn-up of RBMK is between 20 - 30 GWd/tU, http://www.neimagazine.com/journals/Power/NEI/September_2004/attachments/NEISept04p26-35.pdf .

 

[Astronuc]

Hi

I'm wondering what puts the limits to the burn-up of RBMK fuel. Is it the build up of fission products or the fuel simply running out of sufficient u-235 to sustain a chain reaction?

The average burn-up of RBMK is between 20 - 30 GWd/tU, http://www.neimagazine.com/journals/Power/NEI/September_2004/attachments/NEISept04p26-35.pdf .

It is a matter of policy or fuel cycle management/strategy.

http://www.elemash.ru/en/production/Products/NFCP/RBMK/

Burnups could be increased to the range of LWR fuel by simply increasing the enrichment and increasing the residence time of the fuel. However, there are technical issues with respect to the dimensional stability and corrosion behavior that may play a role.

The lower burnups mean that there is less buildup of TU radionuclides beyond Pu, which is an issue for recycle of Pu from spent fuel.

Perhaps the Chernobyl accident influenced the policy regarding maximum burnup of RBMK fuel.

 

[vifteovn]

As far as I've read, the burnup was reduced after the Chernobyl accident, but after the addition of 0.4 % of erbium to the fuel the burnup was bumped up again.

Yeah, I was interested in the technical issues with increased burnup. If an increased burnup would not be possible without modifications to the fuel assembly, i.e. make the cladding thicker or something.

 

[Astronuc]

As far as I've read, the burnup was reduced after the Chernobyl accident, but after the addition of 0.4 % of erbium to the fuel the burnup was bumped up again.

Yeah, I was interested in the technical issues with increased burnup. If an increased burnup would not be possible without modifications to the fuel assembly, i.e. make the cladding thicker or something.

I'm not familiar with specific performance issues with RBMK fuel.

Erbium has a lower residual poison effect than gadolinium, and one can usually use less Er than Gd in fuel, so it's penalty on thermal conductivity of the fuel is less.

With regard to cladding material, if one thickens the cladding, this would be at the expense of the fuel material. One could compensate to some extent by increasing the density of the fuel.

Back in the earlier decades of LWR fuel, the cycles were annual and the discharge burnups were in the low 30 GWd/tHM. Reprocessing of LWR was part of the plan. However, fuel cycle strategies changed because reprocessing in the US was suspended indefinitely, and the industry was faced with no where to send the spent fuel which began accumulating in spent fuel pools. Cycle lengths were increased to improve capacity factor and eliminate refueling outages, which saved millions $. Also, the discharge burnup was increased to the high 50 or low 60 GWd/tU.

RBMK fuel cycle management is different than LWR fuel cycle management, and LWR fuel cycle management varies according to national policy and utility.