The transient processes induced in a water-moderated and -cooled reactor by prolonged (several seconds) insertion of absorbing rods into the reactor are analyzed using experimental and computational data. The change in the neutron flux density in core regions, forming the detector signal, during the motion of the rods, and the subsequent change in the distribution of delayed-neutron sources are examined. The spatial effect of the reactivity is determined from the coefficient of nonuniformity of the neutron flux density. The conditions under which regression analysis can be used to determine the efficiency of the rods are discussed.

A method of measuring the subcriticality of a reactor, according to which the reactor is transferred from a critical into a subcritical state twice by moving control rods with different velocities V 1 and V 2 , is substantiated. A correction to the indications of a reactivity meter for the spatial effect of reactivity is calculated according to detector signals which are recorded with each movement. The correction calculation does not require neutron-physical calculations. As an example this method is applied to water-moderated and -cooled reactors.In accordance with methods of measuring the effectiveness of regulation organs (absorption rods for emergency protection, control, and compensation), by moving a control organ, whose effectiveness is to be determined, from the position H ini into the position H fi n a nuclear reactor is transferred from a close to critical state into a subcritical state [1-3]. The difference of the reactor reactivity in the initial (before the rods are moved) and fi nal (after the rods are moved) states is taken as the effectiveness of the control organ. The reactivity is calculated in the course of the measurement by solving the inverse equation of point kinetics:(1)where ρ is the reactivity; t is the running time; Λ is the generation time; λ i , β, α i , and C i0 are generally accepted notations for the characteristics of the delayed neutrons; and I is the signal from a neutron detector.In practice, methodological errors complicate the use of Eq. (1). A signifi cant error arises as a result of changes in the spatio-energy distribution of the neutrons in the core during the measurements. A spatial effect of reactivity is observed: the reactivity calculated from a detector signal depends on the mutual arrangement of the detector and the moved working organ as well as on the velocity of the detector and the time when motion ceases. The problem of the spatial effect of reactivity has been a subject of discussion for many years [4][5][6].When the control organ stops moving under conditions where feedback can be neglected, as the neutron distribution corresponding to the change in the state of the core is established, the error owing to the spatial effect asymptotically goes to zero [7]. However, the possibility of measuring a detector signal and therefore calculating the reactivity is lost in a subcritical state of the reactor because as a rule the integral neutron fl ux decreases long before a new stationary distribution is established. To determine the effi ciency of a control organ the reactivity calculated from Eq. (1) at the end of the time interval when the detector signal can still be measured is used as the reactivity in the fi nal state of the reactor.The aim of the present work is to examine on the basis point kinetics the possibility of determining the effi ciency of a control organ as it moves from the initial to the fi nal position.

A regime of extension of an operation period is a stage of reactor operation that follows the moment when the reactivity excess is exhausted due to burnup. Extension of an operation period run of water-moderated water-cooled reactors is ordinarily accomplished (see, for example, [1][2][3]) by negative power and temperature effects of reactivity. Decreasing the first-loop parameters relative to their nominal values makes it possible to release additional reactivity.A regime of extension of an operation period is conducted in a fundamentally different manner in the case of cyclical variation of reactor power and suppression of 135Xe poisoning [4], when each cycle contains successive time intervals during which the power is correspondingly decreased, maintained at a lower level, increased, and maintained at the higher level. During the first interval, as a result of the power effect, the reactivity excess is released and is partially expended during the second interval in the presence of poisoning of the reactor. By the end of the second interval the reactivity excess should be sufficient to increase power. Excess xenon is burned up as the power increases during the third interval; this makes it possible to raise the reactor power to a high level (specifically, the nominal level) and maintain this level during the last interval. Further operation of the reactor requires repeating the cycle.Cyclical extension of an operation period has certain advantages over a regime in which the coolant parameters are decreased: the power generating equipment operates in standard regimes and there is no large decrease in the efficiency of a power-generating unit. The regime does not require special preparation of equipment (which could be especially important, if the need for extending an operation period arises externally as a result of a deficit of electricity in the power grid), and in the case of a 24-hour cycle the regime could correspond to a load-following regime, which facilitates regulation of the grid load in the power grid.The economic efficiency of a cyclical regime requires that the maximum possible power production be accomlished in each cycle. Therefore one can pose the problem of finding an optimal control which gives the maximum energy production in the regime. This problem is a xenon optimization problem. The formulation and solution of the problem, which is the goal of the present paper, are effected using the formalism of the principle of the maximum [5] in a point model of the kinetics similarly to [6], where this class of problems is studied in detail.Formulation and Solution of the Problem. We shall assume that the change in the reactivity excess is determined by oscillations of the reactor power (di --power coefficient of reactivity), burnup and slagging of the fuel (e --rate of change of the reactivity excess as a result of these processes), change in the xenon poisoning (t.t --coefficient of proportionality between poisoning and the concentration of xenon). The quantities 6, e, and ~ can be assumed to be c...

The generally accepted definition of an isothermal temperature effect of reactivity is a change of reactivity ρ caused by a change of temperature of all materials in the reactor core from T 1 to T 2 . The temperature coefficient of reactivity is determined as the increment to the reactivity accompanying a change of temperature by 1°C [1-4].The temperature coefficient α T [T, H cr (T)] is measured during heating or cooling of the reactor in the range T 1 ≤ T ≤ T 2 , maintaining the reactor in a near-critical state with the position of the rods H = H cr (T). The temperature effect is estimated by the quantity