RU1766196C - Method of determination of nuclear-physical parameters of assembly containing fissionable substance - Google Patents

Abstract

FIELD: nuclear equipment. SUBSTANCE: assembly is subjected to primary radiation with pulse flux. Fission process is induced as a result of it. Pulse of primary radiation lasts 5-50 ns. Temporary distribution of momentary and delaying gamma-quanta formed in assembly is measured in the course of several hundred nanoseconds. Change of intensity of gamma-radiation in time relative to primary pulse characterizes physical processes going on in assembly. Precision of determination of parameters of assembly increases thanks to exclusion of influence of spread of times of flight of base by gamma-quanta and thanks to reduction of ratio of background- effect (owing to reduced measurement time) under conditions of remoteness of detector from assembly. EFFECT: increased precision of determination of nuclear-physical parameters of assembly containing fissionable substance. 3 cl, 1 dwg

Description

 The invention relates mainly to the field of research of subcritical assemblies, as well as for the study of any other structures, for example, fuel elements (fuel elements) containing fissile material. The invention can be used to determine reactivity, neutron lifetime, and other assembly parameters.

 A known method of measuring the energy spectra of fission neutrons from subcritical assemblies with low reactivity. According to the method, the assembly is irradiated by neutrons from a pulsed source with a pulse duration of several microseconds, and the fission neutron velocity, the time spectrum of which lasts several hundred microseconds, is measured by the time of flight of a given base.

 The disadvantage of this method is the relatively large background, since the detector that detects neutrons from the assembly is removed from the assembly several tens of meters (the geometric registration efficiency is low), and the measurement time is relatively long, since neutrons of different energies have a large spread in the time of flight of the base.

 Closest to the proposed one is a method for determining the nuclear physical parameters of an assembly, by which a radiation pulse of several microseconds is generated that initiates fission in the assembly, neutrons formed in the assembly are recorded, the temporal distribution of neutrons relative to the radiation pulse is measured, and the nuclear physical assembly parameters such as reactivity and lifetime of instant neutrons in the assembly. The neutron detector is placed in the immediate vicinity of the assembly.

 The aim of the invention is to increase the accuracy of determining the parameters of the assembly in conditions of remoteness of the detector from the assembly by eliminating the influence of the dispersion of the time of flight of the base particles and reducing the background-effect ratio.

 This goal is achieved by the fact that in the method for determining the nuclear physical parameters of an assembly containing fissile material, which includes creating a radiation pulse that causes the fission process in the assembly, registering particles formed in the assembly, measuring the temporal distribution of the recorded particles relative to the radiation pulse, determining nuclear -physical parameters of the assembly in the form of a temporal distribution, gamma rays that are separated from fission neutrons by time are used as registered particles the passage of a given database.

 The goal is also achieved by the fact that a radiation pulse is created with a duration of 5-50 ns, and the time distribution is measured for 200-2000 ns.

 In addition, in the form of a temporary distribution, the intensities of the instantaneous and delayed gamma radiation are jointly analyzed.

The drawing conventionally shows the temporal distribution of gamma rays for various assemblies, i.e. the dependence of the intensity of gamma rays in the detector (on a logarithmic scale in the ordinate) on the measurement time, and the leading edge of the accelerator pulse, the particles of which are sent to the assembly with fissile material, is taken as a zero time reference. The accelerator pulse is conventionally represented with infinitely steep leading and trailing edges, its duration being chosen significantly less than the measurement time after the accelerator pulse. Curves 1, 2 relate to assemblies with neutron multiplication in fissile material for two different reactivities; curves 3, 4 relate to assemblies with no neutron multiplication containing different amounts of fissile material. The indicated levels n ₁ -n _{4 are} determined by the irradiation pulse power and the quantities of fissile and structural materials in the assembly. The levels of 0.01n ₃ and 0.01n ₄ approximately correspond to the onset of manifestation of delayed gamma radiation from fission fragments.

 The method is as follows. The pulses of accelerator particles, say, protons or deuterons with energies of 10-100 MeV and a duration of 5-50 ns, are sent to the assembly under study. These particles, when interacting with the shell or external structural elements of the assembly, form neutrons and gamma rays with known angular distributions and energy spectra at every angle relative to the direction of the beam. Secondary particles cause a fission process in fissile material, as well as inelastic interactions in structural elements and fissile material.

Thus, during the accelerator pulse action _USH total number N of gamma rays on the detector is defined as inelastic interactions in different assembly _Sat material N, and the fission process in the fissile material N _cases where _{the Start} N N + N _{sb cases.}

The ratio between N _sat and N _affairs depends on the geometric location of the detector and the accelerator beam, as well as on the assembly design, the quantity and type of materials in it. It can be either N _sat <N _div , or N _sat > N _div .

A gamma-ray detector (which, in principle, can also detect neutrons) is located at a certain distance from the assembly (base). When this base passes, the neutrons are delayed relative to gamma rays (for example, at 10 or more than 200 ns) and, therefore, are easily separated by recording equipment. The clock from the accelerator starts for 200-2000 ns a time continuous analyzer. Each gamma-quantum is fed to the “Stop” input or the counting input of a temporary analyzer, which records the moment of arrival (registration) of gamma-quanta. The corresponding analyzer code is transferred to the computer memory. To obtain the necessary statistics, the accelerator pulses can be repeated as many times as necessary, for example 10 ³ -10 ⁵ , while for each accelerator pulse only one or several gamma-quanta can be recorded.

 After the accelerator pulse ends, the nature of the change in the intensity of gamma rays will have the form of an exponential or close to it decay curve due to the finite neutron lifetime or delayed gamma radiation of fission fragments. From the above time distributions, it is possible to determine the decay time constant α ≈ 30–4000 ns (for a measurement time of 200–2000 ns) the number of gamma quanta during and after the accelerator pulse and after.

For two assemblies with different reactivities, i.e. for curves 1, 2, the indicated time constants correspond to small reactivity values (ρ <-0.4, ρ ₁ > ρ ₂ ). The formulas for calculating ρ coincide with the formulas for the case of neutron registration in the prototype.

 For curves 3, 4, i.e. assemblies without neutron multiplication, after the end of the accelerator pulse, the intensity will remain, due only to the delayed gamma-ray fission quanta. In this case, delayed gamma-quanta means gamma-quanta from isomeric states of fission products with half-lives of 100-1000 ns, the proportion of which k of the number of instantaneous gamma-quanta is 5-10% i.e. k 0.05-0.1.

 The assembly design is considered known, i.e. it is possible to carry out numerical calculations to simulate time distributions. In addition, assemblies with known amounts of fissile material and without it are available for calibration measurements.

Calculation of the relationship k ˙N _cases / (N + N _{sb cases)} eliminates the beam intensity, the ratio can be calibrated for different amounts of fissile material (calibration curves can be obtained for the case of the curves 1, 2).

 The method of calibration curves is the simplest for the practical implementation of the method. Specific analytical formulas will be cumbersome and the more complicated the greater the required measurement accuracy, since they will include many calculated or experimentally measured correction factors.

 Curves 1, 2, as well as 3, 4, have a conditional illustrative character, i.e. the scales between curves 1, 2 and 3, 4 are not observed both along the ordinate and the abscissa. In addition, after the accelerator pulse, the curves may not have an exact exponential shape, but on curves 1, 2 there may be a “step” (derivative discontinuity) at the moment of the trailing edge of the accelerator.

 A positive effect was obtained by reducing the background-effect ratio, which is due to the reduction of the measurement time. To illustrate this effect, let us assume that the measurement time is 1000 ns and the span base is 10 m. The time spectrum of gamma rays is independent of the span base, and when neutrons are detected according to the prototype, the measurement interval 1000 ns is stretched by about 3 times due to the wide energy spectrum of fission neutrons. In this case, the background-effect ratio also increases by 3 times. In addition, the neutron decay curve of the assembly is substantially transformed.

и δN

при одинаковом времени измерения фона и эффекта + фон. Если N/N_ф ≈ 1, то при регистрации гамма-квантов ошибка уменьшается в

/

≈ 1,5 раза. Примерно во столько же раз вырастет точность определения параметра
Требование длительности импульса излучения 5-50 нс определяет конструктивные особенности ускорителя, которые, однако, можно реализовать. Применяют линейный резонансный ускоритель ионов водорода, работающий в пакетно-импульсном режиме: длительность ВЧ-импульсов в резонаторах 100-300 мкс, частота повторения 1-10 Гц. Необходимая длительность коротких импульсов (5-50 нс) выходящего пучка создается путем модуляции пучка в источнике в пределах каждого ВЧ-импульса, при этом короткие импульсы пучка повторяются с интервалом 1-2 мкс. Такой режим позволяет создать, например, за 1 с 10³ импульсов пучка. Если принять, что ток в импульсе пучка порядка 10 мА, а для модуляции пучка необходимо напряжение 1-10 кВ, то импульсная мощность модулятора пучка составляет не более 100 Вт при средней мощности 100 мВт.The true number of gamma rays after the accelerator pulse N is found as the difference of the effect + background and background N _f , i.e. N (N + N _f ) N _f . In the case of neutrons, N (N + 3 N _f ) 3N _f , while the number of neutrons and fission gamma rays is approximately assumed to be the same. The corresponding statistical errors are δN

and δN

at the same time for measuring the background and effect + background. If N / N _f ≈ 1, then when registering gamma rays, the error decreases in

/

≈ 1.5 times. The accuracy of parameter determination will increase by approximately the same amount
The requirement of a radiation pulse duration of 5-50 ns determines the design features of the accelerator, which, however, can be implemented. Apply a linear resonant accelerator of hydrogen ions operating in a burst-pulse mode: the duration of the RF pulses in the resonators is 100-300 μs, the repetition frequency is 1-10 Hz. The necessary duration of short pulses (5-50 ns) of the output beam is created by modulating the beam in the source within each RF pulse, while short beam pulses are repeated with an interval of 1-2 μs. This mode allows you to create, for example, for 1 with 10 ³ pulses of the beam. If we assume that the current in the beam pulse is of the order of 10 mA, and a voltage of 1-10 kV is required for modulating the beam, then the pulse power of the beam modulator is not more than 100 W with an average power of 100 mW.

The characteristic quantization time for the time analyzer is 3-15 ns, the allowed access time to the computer memory is 5˙10 ² -10 ³ ns, which is relatively easy to implement on the elements of modern electronics.

 The gamma-ray detector is, for example, a block of scintillating plastic, viewed by a temporary PMT, the pulse duration of the order of 1-3 ns.

 Thus, the proposed technical solution provides an increase in the accuracy of the measured parameters by at least 1.5 times.

Claims (3)

 1. METHOD FOR DETERMINING THE NUCLEAR PHYSICAL PARAMETERS OF THE ASSEMBLY CONTAINING THE FISSIBLE SUBSTANCE, including pulsed irradiation of the assembly, registration of particles formed in the assembly as a result of fission, measurement of the temporal distribution of the recorded particles relative to the primary radiation momentum, determination of the nuclear-physical parameters of the assembly by the shape of the temporal distribution, characterized in that, in order to improve the accuracy of determining the parameters of the assembly in conditions of remoteness of the detector from the assembly by eliminating the influence of scatter Remen span base particles and reduce background effect relationship, as particles are detected gamma rays, which are separated from the fission neutrons by passage of a predetermined time base.  2. The method according to p. 1, characterized in that the primary radiation pulse has a duration of 5-50 ns, and the temporal distribution is measured for 200-2000 ns.  3. The method according to PP. 1 and 2, characterized in that in the temporal distribution jointly analyze the intensity of instantaneous and delayed gamma radiation.

Images