CANDU reactors are very novel in that refuelling can occur during normal operation. In fact, on-power refuelling is considered to be CANDU’s primary method of reactivity control. Using a conventional fuelling pattern, refuelling occurs four times per day, and serves to keep certain CANDU reactors functioning within a legislated operating envelope.

As CANDU reactors age, operational margins have begun to narrow. Power ripples that result from the insertion of fresh fuel bundles during on-power refuelling constantly challenge the limit of these boundaries. Therefore, reactivity control devices such as Liquid Zone Controllers (LZCs) are employed to prevent both the violation of these operational parameters and the initiation of a reactor trip.

To address the negative consequences of aging reactors, some research efforts have been directed towards improving CANDU operating margins. Significant gains have been realized from the addition of burnable neutron absorbers (BNAs), specifically 150 mg of Gd2O3 and 35 mg of B2O3 to a CANDU fuel bundle (24 × 106 mg)[3].

This study furthered the work by altering the conventional fuelling scheme in an iterative fashion with the addition of 8 bundle shift rings, in an attempt to reduce the fuelling frequency, and thus the demand on temperamental fuelling machines. Both NU and NU laced with BNAs were considered for this research study.

Results indicated that both the absolute maximum channel power and LZC fill fractions were reduced when 150 mg of Gd2O3 was added to NU fuel; these findings were unanimous for the various fuelling schemes considered, with a fixed fuelling frequency of four times per day. The mean discharge burnup of irradiated fuel containing the BNA also increased slightly for all core following studies. This is reassurance that gadolinium oxide reduces both channel power and LZC levels regardless of the chosen fuelling scheme. Further studies will re-examine the fuelling schemes proposed in this study with a fuelling rate of two times per day with the expectation that gadolinium oxide will successfully facilitate the use of more frequent eight bundle shifts in a CANDU reactor, and reduce fuelling machine usage without imposing a penalty to either burnup or LZC usage.

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