Searching for the best time shift for the fastest reactor recovery to nominal power after reactor trip

Automation of technological processes and production control


Zagrebayev A. M.*, Ramazanov R. N.**

National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), 31, Kashirskoe shosse, Moscow, 115409, Russia



A force majeure resulting in the necessity for temporary reactor trip may occur at the nuclear power plant (NPP) while normal operating mode. The paper envisages one of the operation c aspects of nuclear power plant operation in force majeure conditions. The station is assumed to consist of several power units being at various stages of the campaign. Let there is a necessity to NPP trip due to a force majeure. The threat termination can occur at an arbitrary point of time either. It is quite clear that not all power units in general will be able to raise the power up to nominal value immediately. This difference can be stipulated by, for instance, the presence of different operating reactivity margin for xenon poisoning compensation, or, in general, by different maneuver effectiveness.

The authors consider the system, consisting of two reactors. With brief shifts between reloads after unscheduled trips of both reactors, a period exists when neither can restore nominal power, since both stay in iodine well. For the rest of time both reactors can restore nominal power in no time. On the contrary, with large time shifts one of the reactors at a certain point would possess reactivity margin to compensate the iodine well, when the other still stays in the iodine well.

If the reactor stays at such a campaign stage, that it is impossible or undesirable to handle the control rods, to get out of the iodine well one should alter boric acid concentration in the active zone.

Reactivity margin due to boric acid presence in the coolant is proportional to its concentration. Let the boric acid concentration time dependence within single campaign be approximated with enough accuracy by linear function.

Since the moment of trip is random, we can speak about average flow time for starting one reactor at the power level depending on the shift. The paper offers setting and solution of the optimization problem for selecting the best time shift between the off-load refueling of nuclear reactors, allowing quick start of at least one unit after the reactor trip. This work demonstrates that the time shift between off-load refueling allows quick launch of one reactor, but does not allow the start of both. In the general, for NPP consisting of N reactors the optimum shift is 1/N of the reactor campaign.


nuclear power plant, nuclear reactor VVER, force majeure, optimization problem, iodine pit, compensation xenon poisoning


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