RU2554618C1 - Method for aerial radiation survey of terrain - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: method for aerial radiation survey of a terrain in the disaster area of a nuclear reactor with depressurisation of the reactor core includes measuring, at a flight altitude, gamma-radiation dose rate and reducing the obtained values to a height of 1 m over the surface of the earth, wherein radiation survey is carried out on-board an aircraft using a portable dose-rate meter with measurement time of not more than 2 s, the flight altitude is kept below 150 m, the flight speed is set at not more than 200 km/h; when measuring dose rate, altimeter readings are taken, the ratio of attenuation of gamma-radiation by the air layer is calculated using the formula K=2.019+0.027h^-1+1.128×10^-6h^-3, readings of the measured dose rate are multiplied by the coefficient K.

EFFECT: faster detection of the radiation environment at the initial phase of a disaster.

Description

The invention relates to the field of detecting radiation conditions at nuclear facilities after an accidental release of radioactive substances into the atmosphere.

Emergency situations associated with the depressurization of the reactor core cause the formation of pollution contaminated areas. The need to fully identify the situation to determine the parameters of pollution and, first of all, the configuration of hazardous areas for living requires a significant number of specialized complexes of ground and air radiation reconnaissance.

A known method of accounting for the attenuation factor K (h) of gamma radiation by an air layer equal to the flight height of the aircraft, implemented in a specialized GO-21 aerial radiation reconnaissance complex, by manually setting the sub-band switch to fixed positions [1]. This method is notable for its low reliability, since information on the parameters of the emergency release, including the radionuclide composition and the gamma-ray spectrum, as well as meteorological data, is not taken into account when measuring the dose rate.

There is an aviation dose rate meter IMD-31 with automatic reduction of radiation levels to a height of 1 m above the surface of the earth [2]. However, this device requires stationary installation on a helicopter, which after that can be used exclusively for radiation reconnaissance purposes. Currently, due to the high cost of such a solution, there are only a very limited number of such technical complexes.

One way to reduce the time to identify the situation in conditions of a lack of specialized systems may be to conduct radiation reconnaissance with the help of portable dose rate meters, for example, type MKS-01R [3], placed on aircraft. Naturally, the total measurement error will increase. However, such an approach will make it possible to quickly determine the general location of the hazardous areas of radioactive contamination of the area, which will ensure at the subsequent stage more efficient use of specialized reconnaissance equipment, as well as timely adoption of measures to protect the population, including temporary and permanent resettlement.

When conducting aerial radiation reconnaissance, knowledge of K (h) is required:

where P (1), P (h) is the gamma radiation dose rate at a height of 1 m and h m above the surface of the earth, respectively, R / h.

The values of P (h) can be determined based on the expression for the dose rate from a plane isotropic gamma-ray source in an infinite homogeneous air-equivalent medium [4], converted for the case of complex radionuclide contamination:

.Where $$r=\sqrt{R^2+h^2}$$

.

To _p - constant, dependent on the choice of units;

A _i is the activity of the i-th isotope in the reactor core, Bq;

K _vi is the output of the ith isotope from the reactor core during a first-class accident, rel.

To _γij is the differential quantum yield per decay for the jth line of the i-th isotope;

E _ij is the energy of quanta of the jth line of the i-th isotope, MeV;

σ _aij is the linear coefficient of energy absorption, m-1;

S is the area of radioactive contamination, m ² ;

h - reconnaissance height, m;

µ _ij - linear attenuation coefficient of radiation, m-1;

r is the distance from the elementary radiation source to the point of location of the radiation detector, m;

B _d - dose accumulation factor;

R is the distance from the projection point of the radiation detector to the surface to the elementary radiation source, m

Calculation of K (h) using formula (2) requires taking into account the dose accumulation factor for air in the energy range of 0.1-3 MeV. In addition, if you set a maximum exploration height of 300 m, the accumulation factor can be determined for a distance not exceeding 600 m, since the radiation of radionuclides from a surface bounded by a circle of radius 2h in a contaminated area determines the specific contribution to the total dose rate in excess of 0 95 [4].

The accumulation factor was calculated according to the formula that meets the advanced requirements, having the following form:

where A (E) = - 0.131 + 0.028E ^0.8 + 0.011E ^2.2 ;

B (E) = 1.792-1.92E + 1.225E ^1.2 ;

C (E) = - 0.315 + 0.297E-0.185E ^1.2 .

The formula was obtained by approximating the known experimental values of dose accumulation factors for wide gamma-ray beams [4, 5].

The emission parameters were modeled on the basis of reference data on the specific activity of fission products [6] and data on the release of radionuclides from the reactor core during a first-class accident [7, 8].

The rate of attenuation by a layer of air of gamma radiation of radioactive contamination of the area on the emergency release track was calculated for a reactor campaign from 10 to 720 days and a fuel holding time from 0 to 1800 days. The results of the analysis carried out to determine the range of possible values of the ratio of attenuation of gamma radiation [minK (h), maxK (h)] at a given altitude of radiation reconnaissance of the area are shown in Table 1

Table 1 Range K (h) for different intelligence heights Height, m min K (h), rel. units max K (h), rel. units fifty 3.26 3.36 one hundred 5.72 6.34 150 9.20 11.16 200 14.27 19.17 250 21.66 32,53 300 32,43 54.71

The ratio of min K (h) and max K (h) indicates that at low altitudes the attenuation coefficient of gamma radiation has a weak dependence on the campaign of the reactor t and the fuel holding time T. This makes it possible to use averaged K (h) in practical calculations by t and T. The average K (h) can be determined on the basis of the equality of the absolute values of the maximum positive and maximum negative error of bringing the dose rate to a height of 1 m above the ground:

where K _s (h) is the average g-radiation attenuation factor for height h;

maxK (h), minK (h) - the maximum and minimum for the height h, the multiplicity of attenuation of gamma radiation.

The advanced condition provides the minimum marginal error of conversion to a height of 1 m above the ground surface of the dose rate measured from the aircraft in the absence of accurate information on the radionuclide composition of the area.

Converting this relation allows us to obtain an expression for W:

The calculation results according to the formula (5) are presented in table 2.

The dependence K _s (h) for the reconnaissance altitude in the range from 50 to 300 m was approximated by a cubic polynomial:

where a = 2,019; b = 0.027 m ^-1 ; c = 1.28 · 10 ^-6 m ^-3 - approximation coefficients.

table 2 Values of Ks (h) and δK (h) for different intelligence heights Height, m K _s (h), rel. units δK (h), rel. units fifty 3.31 ± 0.02 one hundred 6.01 ± 0.05 150 10.09 ± 0.10 200 16.36 ± 0.15 250 26.00 ± 0.20 300 40.72 ± 0.26

A significant effect on the reliability of the measurement results in aerial reconnaissance is exerted by the value of the so-called dynamic error δР _days , which shows the effect of the integration of the radiation flux during the measurement in inhomogeneous radiation fields on the accuracy of the result:

where L is a line describing the route of movement when conducting reconnaissance;

A, B - the initial and final points of the segment of the route of movement along which the measurement was carried out;

| AB | = τV;

τ is the time of one measurement, s;

V - reconnaissance speed, m / s.

The greatest errors can occur when conducting reconnaissance in the cross section of a radioactive trace, where the gradient of the gamma radiation field is highest [4, 10]. Since the distribution of radiation levels in the trace section is described by the normal law [4, 10], the expression for the error takes the form:

- дисперсия уровней радиации в сечении следа на расстоянии х от точки выброса радиоактивных продуктов в атмосферу.Where $${\sigma }_y^2\left(x\right)$$

- the dispersion of radiation levels in the trace section at a distance x from the point of release of radioactive products into the atmosphere.

During the liquidation of the accident at the Chernobyl nuclear power plant, taking into account the radionuclide composition of the pollution, which determines the rate of decline in radiation levels, a territory with a gamma-radiation exposure dose rate of over 5 mR / h was taken as a resettlement zone. An analysis of the map of the radioactive contamination of the area in the Chernobyl region showed that the minimum track width with a boundary value of a radiation level of 5 mR / h is about 10 km, and the width of a track limited by a radiation level of 20 mR / h is about 5 km [11]. From these data it follows that the minimum value of σ _y , corresponding to the condition of the greatest gradient of radiation levels on the reconnaissance route, is approximately 2.6 km.

Taking into account the established value of σ _у , the error values δР _days were determined, which are presented in Table 3. During the calculations, it was assumed that the exploration was carried out by an instrument of the МКС-01Р type with an inertia (measurement time) τ equal to 2 s [3].

Table 3 The error values | δР _day | for different rates of radiation reconnaissance, rel. units Dose rate, mR / h Speed of reconnaissance, km / h 150 200 250 300 32,0 0.00 0.00 0.00 0.00 29.7 0.01 0.02 0.02 0,03 23.8 0,03 0,03 0.04 0.05 16,4 0.04 0.05 0.06 0,07 9.9 0.05 0.06 0.08 0.10 5,0 0.06 0.08 0.10 0.12 2.2 0,07 0.09 0.12 0.14

Let us determine the permissible errors δK and δР _days and, therefore, the acceptable modes of radiation reconnaissance using a portable dose rate meter. To do this, we first determine the other partial components of the measurement error.

The main measurement error δ _os taken by the MKS-01R type instrument does not exceed ± 30% with a confidence level of 0.95. The error in the readings δ _ez due to the energy sensitivity of the detector of the device does not exceed ± 30% [3].

The margin of permissible additional error when exposed to elevated temperature (upper value of the working ambient temperature + 40 ° C) and lower temperature (lower value of the working temperature -10 ° C) is 10% for every 10 ° temperature change. Therefore, for the most anticipated conditions (temperature change in the range from -10 ° С to + 20 ° С), the additional error δ _tm , due to the influence of the ambient temperature on the instrument readings, will not exceed ± 20%.

The change in the readings of the device depending on the direction of the flow of ionizing radiation does not exceed ± 20% at an energy of 1.25 MeV, ± 30% at an energy of 0.66 MeV, ± 60% at an energy of 0.04 MeV. The error due to the anisotropy of the detector sensitivity δ _an is systematic and, in contrast to the main error, is the same for all devices of the same type. Uncertainty in this case is introduced only by the lack of knowledge of the radiation energy. However, because the emission spectra of the fission products is observed, usually significant contributions of radiation with energies near the boundaries of a range of values, the limit values _an error δ can be regarded as ± 30%.

The listed errors by physical nature are systematic. However, if we consider the totality of situations, then the realized values of the errors will be random. In particular, the basic error is systematic, but has its own values for various devices. Therefore, if measurements are carried out by anonymous instruments, then it should be assumed that δ _oc has a random value from the range of possible values. Similarly, if the spectrum of gamma radiation at each point is unknown, and the ambient temperature is not controlled during measurements, then the values of δ _ez and δ _tm should be considered random.

In that case, if the practically limit value of the error δ _{i is} known, then its standard deviation (RMS) σ _i can be estimated as σ _i = δ _i / 3 [12]. The boundaries of the basic error δ _oc are set for a confidence probability of 0.95, then its standard deviation σ _oc is equal to δ _oc / 1.96. Therefore, the standard deviation of the total measurement error at the location of the device will be equal to

Errors δK and δР _days must be considered as systematic errors. Indeed, reconnaissance in the area of infection will be carried out by various instruments, then the measurement error will have a different value in each case, but the errors δK and δР _days , if the reconnaissance is carried out in accordance with the methodology at a specific height and at a given speed, will have a constant value . Only if we consider two cases of reconnaissance, significantly spaced in time, then δK and δР _days will differ due to some change in the radionuclide composition of the pollution. Therefore, the error in determining radiation levels at a height of 1 m above the earth's surface must be determined by the formula

where t _α is the coefficient specified by the selected confidence value.

As the minimum critical value of the reliability of information, a value in excess of 0.8 is usually selected.

In [13], the probabilities of the correct identification of dose intervals from the standard deviation of the measurement error are given. From the presented data it follows that the range of radiation doses at which the maximum possible use of protective equipment (shelters) is to be considered and the question of evacuating the population is considered, can be identified with a probability higher than 0.8 if the _standard deviation of the measurement error σ _{crt is} not more than 0.28 . Therefore, the methodological measurement error, which in this case is determined by the two partial components δK and δР _days , should not exceed

Here, the coefficient t _{α is} chosen equal to 3, which corresponds to the almost limit value of confidence probability [12]. This is due to the fact that for the errors δK and δР _days the limiting values that can be realized are determined.

When analyzing the data of Tables 2 and 3, taking into account the need to reliably determine the boundary of the hazardous infection zone (in this case, 5 mR / h), it can be noted that it is impossible to provide a methodological error below the critical value if the flight altitude exceeds 150 m. In the event that reconnaissance carried out at an altitude of 150 m, the flight speed should be limited to 200 km / h.

Thus, taking into account the above generalizations, a method for conducting airborne radiation reconnaissance in the area of an accident at a nuclear reactor with depressurization of the active zone is proposed, which consists in measuring gamma radiation dose rate values at a flight altitude and bringing the values obtained to a height of 1 m above the earth’s surface, which differs the fact that radiation reconnaissance is carried out from the aircraft by a portable dose rate meter with a measurement time of not more than 2 s, the flight altitude is maintained up to 150 metro , Flight speed is set not more than 200 km / h, when the dose rate measurements are taken altimeter, a calculation multiplicity of gamma radiation attenuation air layer according to the formula ^{K = 2,019 + 0,027h -1 + 1,128} × 10 ^-6 h ^-3, readings measured dose rates are multiplied by K.

BIBLIOGRAPHY

1. Technical description and operating instructions for the GO-21 device. - 87 p.

2. Meter dose rate IMD-31. Technical description and instruction manual. - 132 p.

3. Radiometer-dosimeter MKS-01R. Technical description and instruction manual. - 116 p.

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6. Gusev N.G., Mashkovich V.P., Obvintsev G.V. Gamma radiation of radioactive isotopes and fission products. Theory and Tables [Text]. - M .: State. Publishing House of Phys.-Math. Lit., 1958. - 208 p.

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8. Bespalov S.E., Karatov A.V., Mikhailov L.V. and others. Features of calculating the yield of fission products from the destroyed active zone in severe accidents [Text]. - Atomic energy, 1995, vol. 79, issue 2, 134-138 s.

9. Radiation safety standards (NRB-99). M., Center San.-epidemic. regulation, hygienic certification and examination of the Ministry of Health of Russia, 1999. 116 p.

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11. Chernobyl. Five difficult years. The collection of materials on the work on liquidation of the consequences of the Chernobyl accident in 1986-1990. [Text]. - M .: Izdat., 1992. - 186 p.

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Claims (1)

The method of conducting airborne radiation reconnaissance in the area of an accident at a nuclear reactor with depressurization of the active zone, which consists in measuring gamma radiation dose rate values at a height of flight and bringing the values obtained to a height of 1 m above the earth’s surface, characterized in that radiation reconnaissance is carried out from onboard the aircraft with a portable dose rate meter with a measurement time of not more than 2 s, the flight altitude is maintained up to 150 m, the flight speed is set to not more than 200 km / h, when Eren dose rate altimeter readings are taken, a calculation multiplicity of gamma radiation attenuation air layer according to the formula ^{K = 2,019 + 0,027h -1 + 1,128} × 10 ^-6 h ^-3, measured power readings dose multiplied by coefficient K.