RU2165109C2 - Method for experimental detection of local dips of multiplication constant in active zone of subcritical pile - Google Patents

Abstract

FIELD: monitoring of process overload of absorbing elements of the active zone of the pile being in the shut-down state and having an isotropic structure, for enhancing of safety during the whole overload period. SUBSTANCE: method consists in successive equivalent motion of rods-absorbents and determination of the response of the neutron field to these motions by signals of the neutron detectors, for determination of the fact of a local dip use is made of the static criterion in conformity with the interval average values of neutron rate measurement. According to the regressive dependence, setting the measured current values of relative variation of the interval rates of the neutron detector, determined is the unknown coordinate of local dip. The number of neutron detectors is selected proceeding from the necessity of partial overlap of the zones of minimum sensitivity of the neutron detectors to a local dip. EFFECT: facilitated procedure. 2 cl, 1 dwg

Description

 The invention relates to the nuclear industry, and in particular to methods of controlling the technological operation of overloading the absorbing elements of the core (pel assemblies) of a nuclear reactor in a muffled state and having an isotropic structure.

где k - коэффициент размножения до возмущения;
Δk - изменение коэффициента размножения после внесения возмущения, вызванное перемещением стержня регулирования;

- реактивность;
r - пространственная координата.The known method / 1 /, the closest in technical essence to the claimed invention, taken by the authors as a prototype, experimental determination of the distribution of neutron density N ₀ (r) over the radius of the reactor, which is in a stationary state, in which by sequentially equal extraction and immersion of the control rod in the active zone from the signals of the neutron sensors (DN) determines the response of the neutron field to this movement and measures the efficiency of the rod (for example, using a reactimeter - a device that performs which solves the solution of the inverted kinetics equation of the reactor in the point approximation and has an output signal equivalent to the reactivity introduced by the rod):

where k is the multiplication factor before the disturbance;
Δk is the change in the multiplication coefficient after introducing the disturbance caused by the movement of the control rod;

- reactivity;
r is the spatial coordinate.

 For example, the physical control system for the distribution of energy release SFKRE (p) along the radius of the reactor RBMK-1000/2 / provides control of the relative distribution of energy release in the range of reactor power from 5 to 120%, and the control of exceeding the maximum permissible levels of local energy release in the range of power from 10 to 120 % Therefore, after each subsequent overload on a muffled apparatus to verify compliance with nuclear safety conditions, the reactor is brought to a minimally controlled power level to determine the extrema of the energy release field.

The disadvantages that limit its practical significance and prevent the achievement of the following technical result when using the known method include the fact that in the known method:
- for measurements, it is necessary to transfer the reactor to a nuclear-dangerous critical state, interrupting all scheduled work with the CPS system;
- determination of the effectiveness of the absorber rod is made with a non-zero methodological error;
- the resulting flux density distribution will be instantaneous, i.e. will not correspond to the actual distribution after the first subsequent overload of the pel assembly.

 The objective of the invention is to increase the practical significance of the prototype method and increase the reliability of control by operational personnel over the state of subcriticality when overloading elements of the active zone of a nuclear reactor in order to avoid the appearance of uncontrolled local disturbances of the multiplication factor.

The task of controlling the distribution of propagating properties in the entire core for a heterogeneous physically large reactor as a control object with spatially distributed physical characteristics is especially important and most acute when loading assemblies of pels in a drowned reactor, when control of neutron power is difficult due to the low level of thermal flux density neutrons (PPTN) and large γ-background. When removing the assembly of pels in such a reactor, the formation of a peak of a neutron flux localized on a finite group of working cassettes surrounding it is characteristic. If as a result of an error of the technological personnel near such a local inhomogeneity of the neutron field (neutron "trap") extraction of another assembly (the formation of an interfering double neutron "trap") is started, then under certain unfavorable circumstances a zone with abnormally high propagating properties can form in the reactor ( high energy release) - the local "pot" with a neutron multiplication factor of K _∞> 1. it should be noted that the error can be "hidden" from the reactor control personnel , T. E. When the cell was charged core, without knowing it, absorbing elements assembled with absorbing properties different for any reason, the properties specified in the download cartogram.

 The technical result consists in determining the presence-absence of a local region of the active zone with an anomalous value of the reproduction coefficient, its detection and providing the possibility of continuous monitoring of its "movement" throughout the volume of the reactor core, i.e. in improving nuclear safety during the entire period of overloading a stopped apparatus.

 The specified technical result is achieved by the fact that in the known method, sequential, equal movement is carried out in the active zone of the absorber rods (pel assemblies, other sources of disturbance of propagating properties) and the neutron field response to these displacements is determined by the signals of the beam. The peculiarity is that the effectiveness of the absorber rod is not measured. It is understood that the system of neutron sensors (“grating” of radiation paths) is located in one plane perpendicular to the longitudinal axis of the core, and can be either intraband or extra-seasonal. In the latter case, the degree of removal of the DN placement plane should be such that the fact of the presence of a local disturbance at any point in the core is detected by at least two sensors. In addition, the number of sensors (determined by the ratio of the area of the active zone to the area of the minimum sensitivity zone of the neutron sensor to a local disturbance) is selected based on the need to partially overlap the zones of minimum sensitivity of the radiation path to the local disturbance, determined experimentally. The zone of sensitivity of the MD to a local disturbance (determined experimentally) is a circle with a radius equal to the distance from the DN to the center of the local disturbance, at which the relative change in the MD readings is still significant. As a criterion of significance and to determine the fact of the presence or absence of local heterogeneity of PPTN - local disturbance of the multiplication coefficient in the core, a statistical criterion is used as applied to the interval average values of neutron counting rates (CCH) of DNs. To reduce the variance of the analyzed DN signal, the last value is not the current value of the count rate, but the average value over the m-point interval — the interval value of the count rate. To determine the most probable coordinate of the local disturbance (extremum of the neutron field), a previously constructed regression dependence of the relative change in the sensor readings on the distance to the center of the disturbance is used, experimentally obtained with equal movement of the absorbers, which are predetermined initially, are identical in their absorbing properties in the direction of decreasing subcriticality. Given the measured current value of the relative change in the readings of the MD, the desired coordinate of the local disturbance is determined.

 The analysis of the prior art by the applicant, including a search by patent and scientific and technical sources of information, and the identification of sources containing information about analogues of the claimed invention, allowed to establish that the applicant did not find a source characterized by features identical to those of the claimed invention. The definition of the prototype, as the closest in the totality of the features of the analogue, made it possible to establish a set of essential distinguishing features in relation to the applicant’s perceived technical result in the claimed method set forth in the claims.

 Therefore, the claimed invention meets the condition of "novelty."

 To verify the compliance of the claimed invention with the condition of "inventive" level, the applicant conducted an additional search for known solutions to identify signs that match the distinctive features of the prototype of the claimed method. The search results showed that the claimed invention does not follow explicitly from the prior art for the specialist, since the influence of the transformations provided for by the essential features of the claimed invention is not revealed from the prior art determined by the applicant to achieve a technical result. The described invention is not based on a change in a quantitative characteristic, the presentation of such signs in relationship or a change in its appearance.

 Therefore, the claimed invention meets the condition of "inventive step".

 A distinctive feature of the proposed technical solution is that in the proposed method it is not necessary to transfer the reactor to a nuclear-hazardous critical state and calculate the effectiveness of the absorbers (both single and their groups) within the framework of accepted conditionally acting physical models. In addition, the control of the local perturbation of the multiplication coefficient over the entire core volume is carried out exclusively according to primary information — individual changes in the CCH of neutron sensors averaged over the same time interval. The named parameter can be controlled manually or using a computer continuously and in real time during the entire period of shutdown of the reactor.

регрессионная кривая;

- среднее по интервалу за время Δt значение CCH датчика нейтронов в состоянии с локальным возмущением; N_б - усредненные интервальные значения CCH; σ_б среднее квадратическое отклонение текущих интервальных значений N_t буфера; σ - среднее квадратическое отклонение полученных значений

определяемое гетерогенностью загрузки активной зоны реактора; r^* - наиболее вероятный радиус окружности с центром в точке размещения датчика нейтронов, которая (окружность) содержит ЛВ; Δr^* интервал неопределенности координаты r^*, обусловленный гетерогенностью загрузки активной зоны реактора.The illustrative method is illustrated in the drawing, where the drug is a local disturbance; DN - neutron sensor; R is the pitch of the grating of the pattern (in this case triangular) such that

regression curve;

- the average over the time interval Δt value of the CCH of the neutron sensor in a state with a local disturbance; N _b - averaged interval values of CCH; σ _{b is} the standard deviation of the current interval values N _{t of the} buffer; σ is the mean square deviation of the obtained values

determined by the heterogeneity of the loading of the reactor core; r ^* is the most probable radius of the circle centered at the point of placement of the neutron sensor, which (the circle) contains the drug; Δr ^* the uncertainty interval of the r ^* coordinate due to the heterogeneity of the loading of the reactor core.

где N_б= {N_i; N₂;...N_n} σ_б = {σ₁;σ₂;...σ_n} -буферы интервальных значений скоростей счета и средних квадратических отклонений соответственно для каждого нейтронного датчика;
Δt = const - половина минимального времени нахождения "ловушки" в активной зоне реактора, определяемого как интервал среднего времени от начала извлечения сборки пэлов из зоны до их возвращения в зону ("времени жизни" локального возмущения);
n=const - число интервалов (емкость буфера).The achievement of the technical result is carried out in the following sequence:
1. In the initial stable subcritical state (CPS rods moving, no pel assemblies, no drugs in the active zone), interval (averaged over the Δt interval) count rates (N _i ) and their mean square deviations (σ _i ) and their average values are calculated (N _b , σ _b ) for n-intervals (rest time t _p = n · Δt) of each neutron sensor:

where N _b = {N _i ; N ₂ ; ... N _n } σ _b = {σ ₁ ; σ ₂ ; ... σ _n } -buffers of interval values of counting rates and mean square deviations, respectively, for each neutron sensor;
Δt = const - half of the minimum time spent by the “trap” in the reactor core, defined as the interval of the average time from the beginning of the extraction of the pel assembly from the zone to their return to the zone (“lifetime” of the local disturbance);
n = const is the number of intervals (buffer capacity).

где ρ_n - минимальное значение "нормальной" подкритичности;
ρ_min - минимальное значение подкритичности, считающееся безопасным;
V $\genfrac{}{}{0ex}{}{\text{m}}{\text{ρ}}$ ^ax - максимальная скорость уменьшения подкритичности при возмущении-извлечении поглотителя.The choice of the value of the averaging interval Δt is based on the following considerations:
- to reduce the dispersion of the analyzed signal - interval CCH, it is necessary to have a maximum number of samples m of the current count rate in the interval Δt;
- the restriction from above is imposed by requirements determined by nuclear safety - the period for determining subcriticality (T _ρ = Δt) during overload should not exceed Δt _max

where ρ _n is the minimum value of "normal"subcriticality;
ρ _min - the minimum value of subcriticality, considered safe;
V $\genfrac{}{}{0ex}{}{\text{m}}{\text{ρ}}$ ^ax is the maximum rate of decrease in subcriticality during disturbance-extraction of the absorber.

где N_↑ - среднее значение CCH датчика нейтронов за время Δt в состоянии с локальным возмущением.2. Consecutive movement (extraction with returning to the starting position or other equally moving in the direction of decreasing subcriticality) of the same type (absorbing properties) sources of disturbance of propagating properties (CPS rods, pel assemblies) in pre-selected core cells and according to the results of the values obtained is performed the relative change in the signals of the neutron sensors is built regression dependence on the distance to the source of disturbance (see drawing):

where N _↑ is the average value of the CCH of the neutron sensor for the time Δt in a state with local disturbance.

3. When carrying out an overload, the fact of the presence of a local disturbance in the reactor core is determined by the change in the averaged (interval) values of the count rates of the DN in accordance with the following criteria:
N _gr = t _gr σ _b , (5)
where N _gr - the boundary value of the count rate;
t _gr - the boundary value of the quantile factor of the significance boundary for the distribution of interval counting rates, depending on the type of distribution and on the sample size of the analyzed values / 3 /;
according to which, when the current interval value of the count rate N _{t is} exceeded for any neutron channel of the average buffer value of the same channel N _b by t _g · σ _b (i.e., N _t -N _b ≥N _g ), a positive conclusion is made about the significance of the disturbance , i.e. about the presence of local disturbances in the reactor core. If the disturbance is considered insignificant, then the current interval value of the counting speed N _{t is} entered into the buffer with a shift to the left and removal of the first element of the buffer - the interval value CCH, according to the principle of "last logged in - last logged out", which ensures constant updating of the buffer. Then, the correction of the drift of the values of N _b , σ _b according to the formula (2). The described operation is necessary in order to take into account the variation of the neutron flux due to the drift of the physical properties of the core.

(см. чертеж). По регрессионной зависимости определяется наиболее вероятный радиус удаления r^* локального возмущения от датчика нейтронов. На чертеже зона наиболее вероятного местонахождения локального возмущения изображена в виде кольца, ограниченного радиусами r_min ^*, r_max ^*. Если число датчиков нейтронов, определивших текущее изменение скорости счета как значимое, больше или равно двум, то при использовании ЭВМ зона вероятного местонахождения ЛВ может быть обозначена на отображаемой дисплеем видеограмме активной зоны реактора как область пересечения нескольких окружностей (на чертеже выделено цветом). Кроме того, на чертеже указан уровень ″3σ″ (для нормального распределения интервальных скоростей счета t_гр = 3/4/) и соответствующее ему расстояние от ДН до ЛВ (радиус окружности с центром в точке размещения), на котором датчик еще может зарегистрировать возмущение как значимое.4. The local perturbation coordinate is determined by the regression dependence, given the measured current value of the relative change in the readings of the neutron sensor

(see drawing). Using the regression dependence, the most probable radius r ^{* of the} local disturbance removal from the neutron sensor is determined. In the drawing, the zone of the most probable location of a local disturbance is depicted in the form of a ring bounded by the radii r _min ^* , r _max ^* . If the number of neutron sensors, which determined the current change in the counting rate as significant, is greater than or equal to two, then using a computer the zone of the likely location of the drug can be indicated on the displayed videogram of the reactor core as the intersection of several circles (highlighted in color in the drawing). In addition, the drawing shows the level of ″ 3σ ″ (for the normal distribution of the interval counting speeds t _g = 3/4 /) and the corresponding distance from the MD to the drug (the radius of the circle centered at the location), at which the sensor can still detect disturbance as significant.

 The application of the invention allows to increase the reliability of control and to ensure its continuity over changes in propagating properties in real time in the entire physical volume of the active zone of a muffled reactor during overloads of absorbers, using the final optimal number of neutron sensors.

Sources of information
1. Galanin A. D. Theory of nuclear reactors with thermal neutrons. M: Atomizdat, 1957, p. 151.

 2. Dollezhal H.A., Emelyanov I.Ya. Channel nuclear power reactor. M .: Atomizdat, 1980, p. 109.

 3. Novitsky P.V., Zograf I.A. Error estimation of measurement results. - 2nd ed., Revised. and add. - L .: Energoatomizdat, 1991, p. 156-160.

 4. Urig R. Statistical methods in the physics of nuclear reactors. M .: Atomizdat, 1974, p. 20-25.

Claims (2)

1. A method for the experimental detection of local disturbances of the multiplication coefficient in the active zone of a subcritical nuclear reactor, which includes sequential, equal displacement in the active zone of the absorber rods and determination of the neutron field response to these displacements by signals of neutron sensors (ND), characterized in that for determining the presence of local disturbance use statistical criterion
N _t - N _b ≥ N _gr ,
where N _t is the current interval value of the neutron count rate (CCH) of the beam;

the average value of the buffer N _b = {N ₁ ; N ₂ ; ... N _n } of the interval values of the SS of the NAM;
N _gr = t _gr · σ _b - the boundary value of the CCH;
t _gr - the boundary value of the quantile factor of the significance border for the distribution of interval CCHs, depending on the type of distribution and on the sample size of the analyzed values;

the average value of the buffer σ _b = {σ ₁ ; σ ₂ ... σ _n } the mean square deviations of the interval values of the SSN of the NAM; as applied to the interval average values of CCH, and to determine the most probable coordinate of the local disturbance, the regression dependence Δ _b (r) of the relative change in the interval CCH of sensors from the distance to the center of the disturbance of propagating properties caused by the equal movement of the absorbent rods of the same type in the direction of decreasing subcriticality is preliminarily constructed

where N _↑ is the average value of the SSH of the LT for the time Δt in a state with local disturbance;
according to which, setting the measured current value of the relative change in the interval count rates of the neutron sensor, and determine the desired coordinate of the local disturbance.  2. The method according to claim 1, characterized in that the number of neutron sensors is selected based on the need to partially overlap the zones of minimum sensitivity of the neutron sensor to local disturbance.