RU2231145C2 - Method for evaluating effective intensity of nuclear power plant neutron source - Google Patents

Abstract

FIELD: nuclear power engineering.

SUBSTANCE: method includes measurement of count rate which is made when nuclear power plant is charged with fissionable material. Count rate is measured at digital time intervals of 1 s before, during, and after introducing control rods in plant. Control rod introduction time does not exceed 5 s. Total effectiveness of rods is over 1%. With total count-rate measuring time of minimum 300 s time for measuring count rate before introduction of control rods is chosen between 10 and 20 s.

EFFECT: enhanced safety of nuclear power plant operation.

2 cl

Description

The invention relates to the physics of nuclear reactors, namely, to ensure nuclear safety of nuclear reactors and critical assemblies, hereinafter referred to as nuclear facilities (nuclear installations), and can be used in the future when evaluating such basic parameters of a subcritical reactor as the effective multiplication factor, reactivity. Under nuclear safety at work associated with changing the composition or geometry YAU, in this case, is providing operator experimental evaluation of the values for _Aeff (effective multiplication factor) NF at each stage nuclear hazardous work, including during routine work on the stopped reactors NUCLEAR POWER STATION.

The determination of the effective intensity of the neutron source of a nuclear installation Q _{eff is} regulated by various standards: General Provisions for Ensuring the Safety of Nuclear Plants OPB-88/97, Nuclear Safety Rules for Critical Stations ПБЯ-КС-98, Nuclear Safety Rules for Research Reactors ПБЯ-ИР-98. All of them provide for the determination of Q _eff in the absence of fissile material in the nuclear substances. In the future, Q _{eff is} used, for example, to determine the reactivity of a reactor (RF patent No. 2088983, IPC G 21 C 17/00, 1997).

There is a method for determining the effective intensity of a neutron source neutron source (Nuclear Safety Rules for Critical Stations PBYA-KS-98 - prototype), which consists in measuring the counting speed of a neutron detector in a stationary state in the absence of fissile material in nuclear neutrons and determining the effective intensity of a neutron source Q _eff from the formula

v (k _eff ) is the count rate of the experimental neutron detector in the stationary state of the nuclear reactor.

When measuring the count rate of the neutron detector v (0) in a stationary state in the absence of fissile material in KU, K _eff = 0, and therefore Q _eff = v (0).

Once measured in this calibration experiment, the value of Q _{eff is} taken as an invariable parameter and is used further for estimating the _eff by the formula (1) according to the results of measurements of v ( _eff ) for all changes in the composition and geometry of the nuclear reactor. However, the value of Q _eff can change by several orders of magnitude due to changes in the composition and geometry of nuclear facilities, which reduces the reliability of the assessment of the state, and therefore, the security of nuclear weapons.

The objective of the invention is to increase the safety of nuclear facilities by increasing the reliability of measuring the value of Q _eff .

To this end, a method is proposed for determining the effective intensity of a neutron source in a nuclear installation, which consists in measuring the counting rate from a nuclear reactor with a neutron detector and calculating the value of the effective intensity of a neutron source in a nuclear installation Q _eff , the measurement being carried out if there is fissile material in the nuclear reactor with a discrete interval of time of at least 1 second before, during and after the introduction of the control rods into the nuclear reactor, the time of the introduction of the rods does not exceed 5 seconds, and the total efficiency of the introduced rods is pain more than 1%.

In this case, the total measurement time of the count rate of the neutron detector v (t) is at least 300 seconds, and the measurement time until the start of the reset of the control rods is selected equal to 10-20 seconds.

We show the possibility of determining Q _{eff in} this way.

The proposed method for determining Q _{eff is} based on the kinetics equations of the reactor in the following initial form:

i = 1-6

Initial conditions:

{dn (t) / dt = 0, dC _i (t) / dt = 0} for -∞ <t≤0, n (0) = n ₀ . Q _eff = Const

The notation in equations (2-8) is generally accepted (see J.R. Kipin. Physical fundamentals of the kinetics of nuclear reactors. Translation from English. Atomizdat, 1967).

The function n (t) / l is measured in relative units as the count rate of the neutron detector v (t).

Without introducing significant errors in the measurement of Q _eff, assume that (dn (t) / dt) ≈0 after reset rods. We introduce the following notation: S (t) = ∑λ _i C _i (t). By measuring the values of the function n (t) / l before, during and after the reset of the control rods, from equations (2-8), taking into account the initial conditions, we can determine the values of the function S (t). If in equation (2) substitute the values of the functions n (t) and S (t) for 2 times after resetting the control rods, as a result of the transformations, we obtain the following formula for calculating Q _eff :

where t ₁ and t ₂ are any two points in time after the reset of the rods.

The method is as follows.

A neutron detector is installed in a fissile material with a fissionable substance to measure the counting rate, which corresponds to measuring the function n (t) / l in relative units.

Sources of neutrons of nuclear weapons are spontaneous fission of the nuclei of fissile substances in the volume of nuclear weapons; In a stopped reactor, some of the isotopes produced during the campaign and photoneutrons are also sources of neutrons. In cases where the intensity of these neutron sources is insufficient, an additional neutron source should be installed to measure the count rate of the neutron detector with an acceptable error inside the nuclear reactor or at its surface. The proposed method will measure the effective total intensity of the sources emitting neutrons in the volume of nuclear explosives.

The main element of the technical support for measuring Q _eff is a PC computer connected to electronic equipment. PC is used to set up, set up and verify experimental equipment, to collect over time, process and display experimental information. In real conditions, to ensure the necessary measurement accuracy, it is desirable to use several neutron detectors operating in a pulsed mode, which also determines other electronic equipment: amplifiers, discriminators, pulse counters from the amplifiers output. The composition of the electronic equipment and the requirements for its characteristics are similar to the composition and requirements for the equipment used in the implementation of measurements of Q _{eff by} the prototype method.

After preparing all the equipment for operation, the PC is started for measuring v (t) by electronic equipment for 300 seconds with a discrete interval Δt = 1 second.

In order to minimize the random error in measuring the values of the function n (t) / l, one should take measurements at the maximum possible counting speed of neutron detectors, and also use a system of several detectors in the experiment. The measurement time before the discharge of the rods starts is at least 10 seconds, determined by the need to measure the values of the function n (t) / l with an acceptable error in the stationary state of the nuclear reactor. After that, the control rods are reset with a total efficiency of more than 1%, the time of the discharge of the rods should not exceed 5 seconds, in the initial stationary state, before the discharge of the rods, the effective multiplication coefficient should be in the following range: 1> _ef > 0.95. These practical recommendations are related to the requirements of measuring the difference [v (t ₁ ) -v (t ₂ )] in the formula for calculating Q _eff with an acceptable error.

The total measurement time should be at least 300 seconds, which is determined by the time the nuclear facility reaches the stationary mode.

After that, according to the above algorithm, Q _eff and the corresponding random measurement error Q _eff are calculated.

Thus, this method will determine the effective intensity of the neutron source of a nuclear installation in the presence of nuclear material in it, which in turn will make it possible in each state of nuclear research to know such basic parameters of nuclear research as the effective multiplication factor and reactivity with a greater degree of reliability than in the method prototype.

Claims (2)

1. A method for determining the intensity of a neutron source in a nuclear installation, which consists in measuring the counting speed from a nuclear installation by a neutron detector and calculating the effective intensity of a neutron source, characterized in that the counting speed is measured if there is fissile material in the nuclear installation, and the counting speed is measured in time with a discrete interval of at least 1 s before, during and after the introduction of the control rods into the installation, while the time of introduction of the rods does not exceed 5 s, the total effect ivnost rods administered more than 1%. 2. The method according to claim 1, characterized in that the total time for measuring the count rate is at least 300 s, and the time for measuring the count rate before the introduction of the rods is selected in the range of 10-20 s.