The Fuel String Relocation Effect - Why the Bruce Reactors Were Derated
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Abstract
In the CANDU Safety Analysis process, a series of design basis accidents arc chosen and analyzed to confirm safety system effectiveness. Of all the postulated accidents, the Large Break Loss of Coolant Accident (LBLOCA) - a postulated break in the Heat Transport System piping near a component that services a large number of fuel channels - sets the most demanding requirements on the speed and reactivity depth of the shutdown system devices - shutoff rods and liquid poison injection. In such an accident, coolant discharges out of the fuel channels, leading to a reactivity increase and power increase. turned around by shutdown system action within seconds. While the event is extremely improbable, it is reanalyzed periodically and its consequences examined to ensure continued shutdown
system effectiveness. In March 1993, an additional effect was identified: if the break occurred in U1e piping on the inlet side of the core this would cause sudden movement of the fuel bundles (so-called fuel string relocation) in a large number of channels. In Ontario Hydro's Bruce NGS A. Bruce NGS Band Darlington reactors, each channel is fuelled against the now. In this situation. the relocation or the fuel string results in a sudden positive reactivity increase. (By comparison, Ontario Hydro's Pickering A and Pickering B reactors, and the CANDU-6 reactors at Pt. Lepreau,
Gentilly and overseas arc all fuelled with the flow rather than against the flow, so that the fuel string relocation results in a negative reactivity pulse). This reactivity increase is in addition to the reactivity due to the core coolant voiding. The combined reactivity effect could lead to power pulses much higher than those that would arise due to coolant voiding alone. To maintain safety margins in the event of such a postulated accident, the eight Bruce NGS A and Bruce NGS B units were initially derated to 60 percent power within 2 days of the identification and confirmation of this effect. This paper: (1) describes the fuel string relocation phenomenon in detail: (2) explains why the consequences differ at the various Ontario Hydro reactors; (3) outlines the actions taken with respect to each of the Ontario Hydro reactors in the months following March 1993; (4) describes U1c design solutions implemented to mitigate the problem and return the Bruce reactors to higher powers.
system effectiveness. In March 1993, an additional effect was identified: if the break occurred in U1e piping on the inlet side of the core this would cause sudden movement of the fuel bundles (so-called fuel string relocation) in a large number of channels. In Ontario Hydro's Bruce NGS A. Bruce NGS Band Darlington reactors, each channel is fuelled against the now. In this situation. the relocation or the fuel string results in a sudden positive reactivity increase. (By comparison, Ontario Hydro's Pickering A and Pickering B reactors, and the CANDU-6 reactors at Pt. Lepreau,
Gentilly and overseas arc all fuelled with the flow rather than against the flow, so that the fuel string relocation results in a negative reactivity pulse). This reactivity increase is in addition to the reactivity due to the core coolant voiding. The combined reactivity effect could lead to power pulses much higher than those that would arise due to coolant voiding alone. To maintain safety margins in the event of such a postulated accident, the eight Bruce NGS A and Bruce NGS B units were initially derated to 60 percent power within 2 days of the identification and confirmation of this effect. This paper: (1) describes the fuel string relocation phenomenon in detail: (2) explains why the consequences differ at the various Ontario Hydro reactors; (3) outlines the actions taken with respect to each of the Ontario Hydro reactors in the months following March 1993; (4) describes U1c design solutions implemented to mitigate the problem and return the Bruce reactors to higher powers.
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