RU2502087C2 - Method of detecting nuclear facilities by radio probing - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method of detecting nuclear facilities by radio probing, which involves detecting radiation, measuring the exceeding of the background by the detected radiation and outputting a signal on the presence of a facility, also involves radio-frequency scanning of the vicinity of the investigated facility, using equipment to detect presence of a reflected signal at the scanning frequency, measuring the value of said signal, determining the maximum value from frequency and, if said value exceeds the background, the investigated facility is a nuclear facility.

EFFECT: longer range of detecting nuclear facilities, high stealthiness and independence of using detection means, shorter search time.

Description

The invention relates to technical means for recording ionizing radiation of nuclear technology objects (SNF) and can be used to search for stationary and mobile SNF.

SNF is traditionally monitored by instruments and methods of radiation reconnaissance using gamma-neutron radiation of fissile materials as the main sign of detection. Radiation is recorded at small distances from the object, for example, for a mobile object, detection is carried out at distances of several tens of meters.

The well-known "Method for the remote detection of nuclear charges" [Patent RU 2068571 C1, IPC 6 G01T 1/29]. The method allows you to control the presence of nuclear charges at distances of several meters and the monitoring time, measured in hours. Detection is carried out, by comparing the doses of gamma radiation, in the ranges from 1.5 to 2 MeV and from 9.9 to 11.8 MeV. If the set value is exceeded, they conclude that nuclear charges are in the examination area.

The disadvantages of the method are the short detection range and the long exposure time, which is acceptable for a very small range of tasks for detecting stationary objects, and is unacceptable for mobile objects.

Closest to the technical nature of the claimed invention is the "Method for the detection of ionizing radiation sources (options)" [Patent RU 02230339 C2, G01T 1/167], which consists in radar surface layer of the atmosphere. The location is carried out by electromagnetic signals at the frequencies of radiation generated by free hydrogen atoms and OH hydroxyl molecules at a relative humidity of the surface air layer of at least 60%. At the same time, the emission line width for atomic hydrogen is not more than 150 kHz, for OH hydroxyl it is not more than 900 kHz. Registration of the presence of an ionizing radiation source is carried out by the magnitude of the signal absorption. Hydrogen atoms and OH hydroxyl groups absorb quanta of probe electromagnetic radiation. The signal reflected from the ionized layer of the atmosphere above the object, for example, nuclear energy, will be absent. Then, motionless dark spots appear on the screen of the circular view indicator of the radar station in the locations of the sources of ionizing radiation.

The disadvantage of the prototype is the presence of restrictions on the physical state of the atmosphere, which will not always be performed. In addition, radar at the indicated fixed frequencies precludes detection when conditions are realized under which the existing electron concentration in the vicinity of the object provides full reflection.

The aim of the invention is to increase the detection range of SNF due to the use as a sign of detection of radio emission reflected from the ionized vicinity of SNF. At the same time, the secrecy and independence of the use of detection tools is increased, the complexity of the search and detection of spent nuclear fuel is reduced, and the search time is reduced, which is especially important in the event of threats of nuclear terrorism.

This goal is achieved by the fact that in the known method, which includes registering the reflected radiation of a fixed frequency, measuring the excess of the detected radiation over the background and issuing a signal about the presence of an object, according to the invention, a radio frequency scan of the vicinity of the observed object is performed, the presence of the reflected signal at the scanning frequency is detected by technical means, measured its value, determine the maximum value of the frequency and, when it is higher than the background, make a decision on ownership n the observed object to the objects of nuclear technology.

The novelty of the present invention consists in the introduction of new operations using, as the main sign of detection, reflected from the ionized vicinity of spent nuclear fuel radio radiation, measured by means of detection, as a new sign of detection, significantly increasing the detection range and detection capabilities.

Thus, the claimed method meets the criteria of the invention of "novelty."

The analysis of known technical solutions in the studied area and related areas allows us to conclude that individually the components of the method under consideration are known. However, their use in the proposed method for solving the problem of detecting objects belonging to SNF gives this method new properties as a whole. The considered method also provides increased secrecy and independence of the use of detection tools, reduces the complexity of the search and detection of spent nuclear fuel, reduces the search time, which is especially important in the event of threats of nuclear terrorism.

Thus, an invention has an “inventive step”, since it does not explicitly follow from the level of development of science and technology for a specialist.

The invention can be used in the field of industry related to the creation of technical means for detecting spent nuclear fuel, in security and alarm systems for non-intrusive remote monitoring of the presence of radioactive materials, for remote monitoring of radioactive emissions, contamination and tracking their distribution, as well as in the transportation of new and spent fuel for nuclear reactors.

Thus, the invention meets the criterion of "industrial applicability".

A method for detecting spent nuclear fuel by radio sounding includes the following operations:

- radio frequency scanning of the neighborhood of the observed object;

- fixing by technical means the presence of a reflected signal at the scanning frequency;

- measurement of the magnitude of the reflected signal;

- determination of the maximum value of the reflected signal from the frequency;

- determination of the excess of the reflected signal over the background;

- making a decision on the belonging of the observed object to the objects of nuclear technology.

A method for detecting spent nuclear fuel by radio sounding works as follows.

Radioactive radiation of fissile materials, along with the traditional signs of detection due to the registration of gamma and neutron fluxes, create other fields that can be detected by existing or, if necessary, specially created by technical means. An unconventional informative sign of the presence of a source of radioactivity in the control zone may be an increased degree of air ionization in the vicinity of the object.

Registration of the presence of such an increase or its change in time will be an informative sign indicating the presence of a radioactive source inside the controlled object. Moreover, the detection sign under consideration can be unambiguously interpreted as the presence of SNF.

The transport or placement of open samples of fissile materials, with the exception of radioactive contamination, is hardly possible. Transportation of radioactive sources is carried out in containers, and the product itself is in a structure close to sealed, made of metal or other dense structural materials. This arrangement protects against the exit of materials from the product, moisture penetration into the structure, corrosion and, at the same time, absorbs radioactive α and β radiation. Gamma-neutron radiation, which is of interest for assessing the ionization picture in the vicinity of spent nuclear fuel, extends beyond the product or container in which it is located.

Gamma-neutron radiation has great penetrating power and propagates from a source at distances exceeding hundreds of meters. Due to this feature, the formation of ion pairs will have a distribution having a maximum value at a distance R _max from the source with a subsequent decrease. The value of R _max and the maximum concentration of ion pairs or ionization n _e will depend both on the power of the source of ionizing radiation and on the value of the initial energy of the emitted γ-quanta and neutrons.

The problem of neutron propagation, both in homogeneous and inhomogeneous media, has been dealt with for a long time. It has been studied quite well and presented, for example, for a homogeneous medium in the work [V. Klimanov. et al. Propagation of ionizing radiation in air. M., 1973].

The spatial distribution of the neutron flux density Ф is often approximated by a dependence of the form [V. Klimanov et al. Propagation of ionizing radiation in air. M., 1973]

where C (E), λ (E) are constants for a given spectral distribution of the source;

E is the initial neutron energy;

R is the distance from the source.

вместо кажущегося

, соответствующего обычному пространственному расхождению, оказывается более приемлемым, особенно для небольших R.Coefficient

instead of seeming

corresponding to the usual spatial divergence is more acceptable, especially for small R.

, где V - скорость нейтрона, то поглощение нейтронов происходит, в основном, при малых «тепловых» скоростях.At the mean free path, neutrons undergo scattering and absorption, and the macroscopic scattering cross section is much larger than the absorption. Therefore, the mean free path is determined mainly by the scattering length. Since the absorption of neutrons for many media, including air, is described by the law

, where V is the neutron velocity, then neutron absorption occurs mainly at small "thermal" speeds.

Absorbed by the nuclei of the medium, neutrons provide the output of ionizing gamma radiation. An analytical description of neutron cross sections and mean free paths in air is given, for example, in [Yavorsky B.M. and Detlaf A.A. Handbook of Physics. M., Science, 1977].

The spatial distribution of neutrons of a point source near the separation of two media differs significantly from the distribution in an infinite medium. This is mainly due to the difference in the density of the media (~ 1000). Consequently, the mean free path of neutrons will be very different at close distances, since an increase in neutron density is observed due to reflection. However, with increasing distance and falling velocity of neutrons, they are absorbed and the flux density decreases. Similarly, when the detector is positioned above the ground, its influence decreases and the distribution approaches the distribution in an infinite air environment. Thus, at distances exceeding the scattering length, the neutron distribution density will be close, and, therefore, the radiation absorption and ionization patterns will be close.

The mean free path L of the neutron in air can be described by the expression [V. Klimanov. et al. Propagation of ionizing radiation in air. M., 1973.]

where E is the initial neutron energy in electron volts;

L is the neutron mean free path in meters.

In the work [Klimanov V.A. et al. Propagation of ionizing radiation in air. M., 1973.] it is shown that the contributions of captures of intermediate and thermal neutrons are comparable up to distances of ~ 400 m. At large distances, the main process is the capture of thermal neutrons.

The occurring neutron capture is accompanied by the formation of gamma radiation, which, in turn, causes various secondary effects (ionization, air luminescence, current of Compton electrons generating electromagnetic fields, etc.).

, образующихся в единицу времени в окрестности точки

, где

- радиус вектор с началом в источнике нейтронного излучения, в результате радиационного захвата для воздушной среды будет определяться выражением [Климанов В.А. и др. Распространение ионизирующих излучений в воздухе. М., 1973.]The number of gamma rays

generated per unit time in a neighborhood of a point

where

- the radius of the vector with the beginning in the source of neutron radiation, as a result of radiation capture for the air medium will be determined by the expression [V. Klimanov et al. Propagation of ionizing radiation in air. M., 1973.]

where n _γ is the average output of gamma quanta per radiation capture;

- сечение радиационного захвата при скорости нейтрона V₀;

- cross section of radiation capture at a neutron speed of V ₀ ;

- полное макросечение взаимодействия нейтрона при скорости v₀;

- full macro section of the neutron interaction at a speed v ₀ ;

τ is the neutron age.

МэВ [Климанов В.А. и др. Распространение ионизирующих излучений в воздухе. М., 1973].The average energy of gamma radiation generated during the capture of neutrons with an energy E <4 MeV is equal to

MeV [Klimanov V.A. et al. Propagation of ionizing radiation in air. M., 1973].

каждый нейтрон, вышедший из источника, за счет гамма-излучения образует более 10⁴ электронов.So in the distance

each neutron emerging from the source, due to gamma radiation forms more than 10 ⁴ electrons.

The second factor affecting the ionization of air is the instantaneous and delayed gamma radiation accompanying the decay of fissile materials. The main process of interaction of gamma radiation with matter is scattering, in which electrons are knocked out of atoms and ionization occurs.

As a result of multiple scattering, gamma rays are accumulated in the low-energy region. For gamma rays with an energy of the order of 200 keV, the radiation transfer approaches the diffuse [V. Klimanov et al. Propagation of ionizing radiation in air. M., 1973].

Typically, when conducting calculations, the gamma-ray propagation region is divided into three components.

1) µR = 0.5 ÷ 1, weak accumulation of scattered quanta; spatial distribution ~ 1 / R;

2) µR = 1 ÷ 8, the spatial distribution has a complex dependence on R and the initial energy of the source;

3) µR> 8, characterized by a large accumulation of scattered quanta,

where µ is the linear attenuation coefficient of radiation.

Since for air µ <2.0 · 10 ^-4 cm ^-1 [Klimanov V.A. et al. Propagation of ionizing radiation in air. Moscow, 1973.], the first part is in the range from 25 to 50 m, and the third is more than 400 m. Estimates are made for the low-energy range of energies of the order of 100 keV. With an increase in energy, µ decreases, leading to an increase in the upper boundary of these components.

The attenuation of the intensities of gamma rays in a substance, provided that the beam is very narrow in energy, can be estimated from the expression [V. Klimanov. et al. Propagation of ionizing radiation in air. M., 1973.]:

where I ₀ is the initial value of the intensity of gamma radiation at the boundary of the constituent parts 1, 2, 3.

Μ is the sum of the attenuation coefficients

where µ _f , µ _c , µ _p are the attenuation coefficients, respectively, due to the photoelectric effect (energies 10 ^-2 ÷ 10 ^-1 MeV),

μ _c is the Compton effect (energy 1 ÷ 10 MeV) and μ _p is the formation of electron-positron pairs (energy more than 10 MeV).

On each of these ranges, the intensity of gamma rays is calculated taking into account the corresponding attenuation coefficient, which are available in the corresponding reference books, for example, in [Yavorsky B.M. and Detlaf A.A. Handbook of Physics. M., Science, 1977.].

The calculation of the attenuation of the intensity of gamma rays in the corresponding range makes it possible to determine the number of electrons formed due to ionizing gamma radiation, since the energy required is about 33 eV to form one pair of ions [BM Yavorsky and Detlaf A.A. Handbook of Physics. M., Science, 1977.].

The above dependences can be used to carry out evaluative results to determine the degree of ionization of the air under the combined influence of gamma-neutron radiation.

More accurate methods and corresponding calculation programs exist in large numbers to obtain more subtle results that explain the impact on electronic equipment and the control system.

Obtaining the average value of the electron concentration of the medium can be carried out by assessing the boundaries of the distribution of gamma-neutron radiation and the total amount of energy released by the source going to ionization.

It should be noted that the neutron component of ionization can have a maximum, and for the component from gamma radiation, according to (6), there will be three such mild maxima, according to the number of terms in expression (5).

However, these maxima will be strictly determined for monochromatic radiation, and in the case of wide beams of gamma rays characteristic of fissile materials, the maxima will blur along the coordinate.

Sounding by radio waves of the vicinity of the location of sources of ionizing radiation makes it possible to detect a change in the electron concentration and determine its value from the reflected signals from the arising inhomogeneities. In this case, the wavelength of the probe radiation should be close to critical, for which full reflection is realized, similar to reflection from ionospheric layers. The electron concentration can be calculated in the presence of initial data corresponding to the radiation of real sources, or obtained experimentally by conducting measurements on real objects.

When probing the vicinity of a source of radioactive radiation, the reflected signal returns back after a time ∆t _g , and the place of return is the value of R is not determined. However, the return point is the condition that the dielectric constant ε vanishes, i.e.

The value of the effective value of the radiation return range R _g is determined by the expression [Ginzburg V.L. Propagation of electromagnetic waves in a plasma. Fizmatgiz, M., 1960.]

;

;

where N (r) is the electron concentration at the point r;

ω is the cyclic frequency of the signal;

r is the radius vector;

m ₁ , e is the mass and charge of the electron;

R _u (ω) is the true distance to the return point.

Equation (7) can be solved with respect to R _u (ω). Knowing the true value of R _u (ω) allows us to find the electron concentration from the second formula of expression (7), since ε (R _u ) = 0 according to (6).

When the radio wave obliquely falls on the ionized region, reflection occurs at frequencies higher than normal incidence.

The critical reflection frequency with an inclined incidence f _{to, n} is determined by the expression

where f _n is the critical reflection frequency during normal incidence;

θ ₀ is the angle between the normal to the probed layer and the direction

signal propagation.

It can be seen from (8) that f _{k, n} can be quite different, that is, at low electron concentrations in the vicinity of the control object, high-frequency sounding equipment can be used. In this case, f _{k, n} , according to (8) and (7), can be higher by an order of magnitude or more, since the critical electron concentration in the region of the object is proportional to the square of the frequency.

The total reflection of the signal from the layer, as shown above, depends on the electron concentration, the shape and thickness of the layer, and the angle of incidence of radiation on the layer. The same parameters determine the partial reflection coefficient R _neg .

Reflection from a thick layer, the condition of which is determined by the expression

where Z _m is the thickness of the layer;

λ _k is the critical wavelength. The transmittance D _pr and reflection are related by the relation [Ginzburg V.L. Propagation of electromagnetic waves in a plasma. Fizmatgiz, M., 1960].

where ∆f = f _to -f and f is the frequency of the incident radiation;

c is the speed of light.

To fulfill (10), there must be ∆f / f _to << 1.

For control zones with a radius of the order of several hundred meters, condition (9) is satisfied, and the reflection coefficients and absorption can be calculated by formula (10).

For other conditions, when Δf / f _to > 1 or close to unity, the calculation formulas are given in [Ginsburg V.L. Propagation of electromagnetic waves in a plasma. Fizmatgiz, M., 1960]. In this case, the parameter taking into account the electron concentration is decisive, and the formulas are suitable for engineering calculations.

The magnitude of the electron concentration at which the complete reflection of the probed radio signal occurs can be achieved for relatively powerful sources of radioactive radiation.

In particular, for the lower boundary of the VHF range (30 MHz), the electron concentration according to expressions (7) should be of the order of 10 ⁷ el / cm.

For a signal with a wavelength of 100 m (3 MHz), the corresponding electron concentration should be of the order of 10 ⁵ el / cm ³ , and for 1000 m (30 kHz) - 10 ³ el / cm ³ . These values can be realized for conventional SNF. Moreover, the containers, as well as the walls of the wagons, are not able to significantly delay the penetrating radiation and affect the electronic concentration of the vicinity of spent nuclear fuel. With oblique sounding, as was shown above, the critical reflection frequency will be higher, which allows us to make an assumption about the possibility of detection even by means of the WMS range.

When implementing the invention, the operator selects the control object, turns on the transceiver, measures the background value of the radio signals in the operating frequency range, performs a radio frequency scan of the vicinity of the observed object, fixes by technical means the presence of the reflected signal at the scanning frequency, measures its value, determines the maximum value of the frequency and when it exceeds the background, it makes a decision on whether the observed object belongs to nuclear technology objects.

The positive effect of using the invention is:

- increasing the range of SNF detection due to the use of radio emission reflected from the ionized cloud of the vicinity of SNF as the main feature;

- increasing the secrecy of the use of technical means;

- ensuring the prompt search and detection of spent nuclear fuel;

- reducing the threat of nuclear terrorism.

The proposed method for detecting nuclear technology objects by radio sounding can be implemented using existing or specially developed domestic portable radar systems such as radar SBR 1L111, radar 1L120. It should be noted that determining the maximum of the reflected signal from the frequency allows us to calculate the level of electron concentration, which provides a potential opportunity to estimate the amount of fissile materials in the observation area.

Claims (1)

A method for detecting objects of nuclear technology by radio sounding, including registration of radiation, measuring the excess of the detected radiation over the background and issuing a signal about the presence of an object, characterized in that a radio frequency scan of the vicinity of the observed object is performed, the presence of a reflected signal at the scanning frequency is recorded by technical means, its magnitude is measured, its value is determined, determined the maximum value of the frequency and, when it exceeds the background, decide on whether the observed object belongs to The object of nuclear technology.