RU2302676C1 - Method for evaluating effective intencity of neutron source in shutdown nuclear reactor - Google Patents

Abstract

FIELD: experimental evaluation of nuclear reactor physical characteristics and estimating its effective multiplication factor and reactivity.

SUBSTANCE: proposed method includes displacement of absorption rods or other disturbance source in reactor core, determination of neutron field response to these displacements by means of neutron detector, and gamma-ray intensity measurements. Effectiveness of neutron source intensity is found from equation Q_ef(t)=k_λλ_d(t), where λ_d(t), is gamma-ray intensity; k_λ is factor found in calibration experiment. This procedure needs no periodic repetition of displacing control rods affording reliable shutdown of reactor. Fission chambers are used as neutron and gamma-ray detectors. Gamma-ray intensity is measured by changing over fission chamber using integral current recording circuit.

EFFECT: enhanced reliability of measurement results, reduced amount of work involving handling of radioactive materials.

3 cl

Description

The invention relates to methods for experimental determination of the physical characteristics of nuclear reactors and can be used to evaluate such important parameters of a subcritical nuclear reactor as the effective multiplication factor, reactivity.

Reactivity measurements have been and remain the main measurements performed both at critical assemblies and at power reactors. This is due to the fact that the change in the neutron power of a reactor over time is determined by its reactivity. Therefore, to ensure nuclear safety, continuous monitoring of the reactivity of a nuclear reactor at stops is necessary, including during fuel refueling and during routine repair work.

In cases where the variation in the reactivity of the reactor does not lead to changes in the specified limits: 1) the effective intensity of the neutron sources and 2) the efficiency of the detectors, the relationship between the effective multiplication coefficient of the subcritical reactor and the count rate of the neutron detector has the form (Patent RU 2231145 C2, 20.06. 2004):

where k _eff is the effective multiplication factor of a nuclear reactor;

N _d is the count rate of neutron detectors in a nuclear reactor;

Q _eff is the effective intensity of the neutron source of a nuclear reactor.

Thus, having determined the effective intensity of the neutron source as a result of the experiment, in the future the task of measuring the reactivity of a nuclear reactor is reduced to the determination of _ef using the measured value of Q _eff .

There is a method of simultaneous and independent determination of the magnitude of the neutron source and reactivity [Mogilner A. I., Fokin G.N., Chaika Yu.B., Kuznetsov F.M. The use of small computers for measuring reactivity. - Atomic energy, 1974, vol. 36, issue 5, p. 358], which consists in the fact that the effective intensity of the neutron source of the reactor is determined by comparing the counting speed of the neutron detector of a critical or slightly supercritical (period greater than 200 s) reactor with a speed account of the same reactor into which the absorber is introduced at time t = 0. However, the application of this method is technically difficult, since for its implementation it is necessary to bring the nuclear reactor to a critical state.

A known method for determining the effective intensity of the neutron source of a nuclear installation, adopted as a prototype (Patent RU 2231145 C2, 20.06.2004). Q _eff by this method is calculated on the basis of the neutron field response measured by neutron detectors to enter negative reactivity into the core of a nuclear reactor. In this case, the input time of negative reactivity should not exceed 5 s, the total efficiency of the introduced rods should be more than 1%, the total time for measuring the count rate of the neutron detector should be at least 300 seconds, and the measurement time before the reset of the control rods was 10 ÷ 20 seconds. In the initial stationary state, before the rods are discharged, the effective multiplication coefficient (k _eff ) should be in the following range 1> k _eff > 0.95.

The method taken as a prototype, allows you to determine the effective intensity of the neutron source of a muffled nuclear reactor, however, its implementation is associated with certain technological and technical problems:

- the need to extract and then enter into the active zone a significant part of the control rods in order to create a sufficient perturbation of the neutron flux;

- it is assumed that Q _eff does not change over time. However, due to a decrease in the intensity of photoneutron generation within a few days after the nuclear reactor is shut down, the effective intensity of the neutron source varies significantly (more than 1.5 times), therefore, in order to increase the reliability of assessing the state of a nuclear reactor, and therefore safety, an experiment to determine Q _{eff is} necessary periodically repeat, which is associated with an increase in the volume of nuclear hazardous work.

The objective of the present invention is to increase the safety of a nuclear reactor and reduce the amount of nuclear hazardous work associated with the extraction during experiments to determine Q _{eff a} significant part of the control rods, providing reliable silencing of the nuclear reactor.

The problem is solved in that in a method for determining the effective intensity of a neutron source of a drowned nuclear reactor containing irradiated nuclear fuel, including moving in the active zone of the absorber rods or other local sources of disturbance, determining the response to these movements using neutron detectors, they additionally measure the gamma radiation, and the calculation of the effective intensity of the neutron source is performed by the ratio

where γ _d (t) is the intensity of gamma radiation;

k _γ is the coefficient determined as a result of the calibration experiment.

As detectors of neutrons and gamma radiation using fission cameras.

The intensity of gamma radiation is measured by switching the division cameras according to the integrated current registration scheme.

An example of the application of this method for measuring the reactivity of an industrial uranium-graphite nuclear reactor during a long shutdown of ~ 14 days.

One day after reactor shutdown calibration experiment conducted in which the reset control rods operate, is determined by dividing the pulse chambers CST-31 response to this neutron field perturbation and more calculated values and Q _{eff eff.} In the calibration experiment, Q _eff and k _eff were 22.5 pulses / s and 0.955, respectively.

Next, we measured the intensity of gamma radiation using fission cameras KNT-31, switched according to the registration scheme of the integrated current. The dose rate of gamma radiation was γ _d ≈50 P / s.

Based on the data obtained, the coefficient k _γ is determined:

For example, on the 8th day after the reactor shutdown, the count rate of the KNT-31 fission chambers was measured. By this time, it had fallen to 450 impulses / s. The KNT-31 fission chambers were switched according to the integrated current recording scheme and measured γ _d (8) ≈26 P / s. The value of the effective intensity of the neutron source, calculated by the relation (2), was

Q _eff (8) ≈k _γ · γ _d (8) = 0.45 · 26 = 11.7 pulse / s,

The value of the effective breeding coefficient is calculated by the formula (1):

The reliability of the obtained values of Q _eff and k _{eff was} confirmed in a repeated calibration experiment conducted on the 8th day after the reactor shutdown.

Thus, the use of the proposed method allows to obtain reliable results of the magnitude of the effective intensity of the neutron source without periodically repeating operations to move the control rods, ensuring reliable silencing of the nuclear reactor, and thereby reduces the amount of nuclear hazardous work.

Claims (3)

1. A method for determining the effective intensity of a neutron source of a muffled nuclear reactor containing irradiated nuclear fuel, including moving in the active zone of the absorber rods or other local sources of disturbance, determining with neutron detectors the response of the neutron field to these movements and calculating the desired value, characterized in that additionally measure the intensity of gamma radiation, and the calculation of the effective intensity of the neutron source is performed by the ratio Q _eff (t) ≈k _γ · γ _d (t), where γ _d (t) is the intensity of gamma radiation; k _γ is the coefficient determined as a result of the calibration experiment. 2. The method according to claim 1, characterized in that the fission cameras are used as neutron and gamma radiation detectors. 3. The method according to claim 2, characterized in that the gamma radiation intensity is measured by switching the division cameras according to the integrated current registration circuit.