JPH0521515B2 - - Google Patents

Description

Detailed Description of the Invention (Industrial Application Field) The present invention scans and measures gamma rays emitted from contaminated radioactivity inside a pipe from outside the pipe using a gamma ray detection system having a collimator window. This method relates to a method for determining the radioactivity distribution non-destructively by quantifying the deposited radioactivity that is distributed inside a pipe and the radioactivity of a substance that has a radioactivity distribution only in the depth direction inside the pipe.

(Prior art) Conventionally, in order to measure the distribution of deposited radioactivity inside a pipe, a part of the pipe at the measurement point was cut out and used as a measurement sample, and this sample was cut at appropriate intervals and measured in an analysis room, etc. , measurement methods that determine the distribution of radioactivity have been mainly used. This method involves cutting out the measurement sample and unsealing the radioactivity, so there are problems in that the measurement locations are limited and it takes time to obtain measurement results.

In addition, as a non-destructive method for measuring the radioactivity inside the pipe, as shown in Fig. 1, a method is used in which the entire pipe 3 is viewed through the collimator window 2 of the gamma ray detection system 1 and the deposited radioactivity 4 on the inner surface of the pipe is determined. It's here. With this method, if the radioactivity inside the pipe exists in a deposited state and in a liquid or gaseous state,
There was a problem in that it was not possible to quantify each of these, let alone measure their radioactivity distribution.

(Means for Solving the Problems) The present invention non-destructively analyzes radioactivity distribution for each state of non-uniform deposited and liquid radioactivity inside contaminated piping of a nuclear reactor primary cooling system. It is a method devised for the purpose of seeking.

A principle diagram showing the measurement method of the present invention is shown in FIG. Gamma rays emitted from a pipe 5 whose interior is radioactively contaminated are measured from the outside of the pipe using a gamma ray detection system 9 having a structure in which a detector 8 is covered with a shield 7 having a rectangular collimator window 6. It's a method. As shown in Figure 2, the radioactivity inside the piping as seen from the collimator window of the gamma ray detection system at any three points 11, 12, 13 on the line 10 connecting the center of the detector and the center of the collimator, or at three or more points. Gamma rays emitted from nuclides are measured and the number of gamma rays at each measurement position is calculated.

On the other hand, in determining the radioactivity contamination inside the pipe, the deposited radioactivity 1
4, 15, and the radioactivity 16 of the substance filled inside the pipe sandwiched between these is assumed to be uniformly distributed within the local range desired by the collimator window of the gamma ray detection system. The count of gamma rays in each state reaching the gamma ray detection system at each measurement position is determined by the distance 18 from the pipe axis 17 to the detector, the inner radius 19 of the pipe, the outer radius 20 of the pipe, the radiation absorption coefficient of the pipe, It can be calculated and determined using the radiation absorption coefficient of the material filled inside the pipe, the thickness 21 of the collimator, and the vertical width 22 of the window. The number of gamma rays measured at each measurement position is the sum of the number of gamma rays emitted from deposited radioactivity and the radioactivity of substances filled inside the pipe counted by the gamma ray detection system. It corresponds to the sum of the products of the count calculated for the case where the radioactivity is a unit of radioactivity and the radioactivity of each state, which is an unknown quantity.

The point of view of the present invention is that when the gamma ray detection system is moved on a line connecting the center of the detector and the center of the collimator and the geometrical conditions with respect to the piping are changed to measure the radioactivity inside the piping, the deposited The reason is that the ratio of the radioactivity and gamma ray count of the radioactivity of the substance filled inside the pipe differs depending on the measurement position. If the number of measurement positions at this time is n, then n-dimensional simultaneous linear equations are independently established. By solving these simultaneous equations, it is possible to separate and quantify the deposited radioactivity at the two locations inside the pipe and the radioactivity of the substance filled in the pipe sandwiched between them, for each state. can.

(Example) An example of the present invention is shown in FIG. 2 inside the piping
It is assumed that the deposited radioactivity 23, 24 at the location and the liquid radioactivity 25 filling the inside of the pipe are uniformly distributed in the local range desired by the collimator window of the gamma ray detection system, and the center 26 of the detector On the straight line 28 connecting and the center 27 of the collimator
When the gamma ray detection system 29 is moved,
Calculate the number of gamma rays in each state in which the radioactivity inside the pipe reaches the detector.

In the calculation, each parameter used is defined as follows.

Radius inside the pipe 30: R1cm Radius outside the pipe 31: R2cm Length from the detector to the pipe axis 32: Lcm Vertical width of the collimator 33: Wcm Thickness of the collimator 34: Hcm Detector position (y axis) 35: ycm Radiation absorption coefficient of piping: Ap Radiation absorption coefficient of water: Aw Radiation absorption coefficient of gas: Ag Unit radioactivity in deposited state: σp Unit radioactivity in liquid state: ρw. The origin 36 of the coordinates is the center of the front surface of the gamma ray detector, and the coordinates of the intersection of the pipe and the uppermost line 37 of the range where the collimator desires the pipe are (x1, y1) 38 and ( x2, y2) 39. From the relationship of similarity of triangles, (y1-y):W/2=x1:H holds true, and when we find the relationship between x1 and y1, we get y1=y+W×x1/(2×H). On the other hand, from the Pythagorean theorem, R1 ² = (L−x1) ² + (y+W×x1/(2×H)) ²
holds true. Coordinates (x1, y1) from the above two formulas
Find x1=(2×L-W×y/H)−√(2×L-W×y/
H) ² /2×(1+((W/2)/H) ² −4×(1+((W/2)/H) ² ×(L ² +y ² −R1 ²
)y1=y+((W/2)/H)×x1. Similarly, when calculating the coordinates (x2, y2), x2=(2×L-W×y/H)+√(2×L-W×y/
H) ² /2×(1+(1+((W/2)/H ² −4×(1+((W/2)/H) ² ×(L ² +y ² −R1 ²
)y2=y+((W/2)/H)×x2. Next, as shown in FIG. 3, the coordinates of the intersection of the lowermost line 40 of the range where the collimator desires the piping and the piping are (x3, y3) 41 and (x4, y4) 42.
Then, in the same way as above, x3=(2×L+W×y/H)−√(2×L+W×y/
H) ² /2×(1+((W/2)/H) ² −4×(1+(W/2)/H) ² ×L ² +y ² −R1 ² )y3
=y+((W/2)/H)×x3 x4=(2×L+W×y/H)+√(2×L+W×y/
H) ² /2×(1+((W/2)/H) ² −4×(1+((W/2/H) ² ×(L ² +y ² −R1 ² )
y4=y+((W/2)/H)×x4 is found. Also, as shown in Figure 3, the coordinates of the intersection of the straight line connecting the center of the detector and the center of the collimator with the pipe are coordinates (x5, y5) 43 and coordinates (x6, y6).
44 and coordinates (x7, y7) 45, do the same as above: x5=L−√1 ² − ² y5=y x6=L+√1 ² − ² y6=y x7=L−√2 ² − ² y7 =y is found.

Using these coordinates, we can calculate that the deposited radioactivity (μCi/cm ² ) at two locations inside the pipe and the liquid radioactivity (μCi/cm ³ ) sandwiched between these locations are uniformly distributed locally. , calculate the number of gamma rays in each state that reach the gamma ray detection system.

First, determine the deposited radioactivity inside the pipe. The area 46 of the deposited radioactivity sandwiched between coordinates (x1, y1) and coordinates (x3, y3) is S1cm ² , and the area 46 of the deposited radioactivity sandwiched between coordinates (x2, y2) and coordinates (x4, y4) is If the radioactive area 47 is S2cm ² , then S1 = R1 x (sin ^-1 (y1/R1) - sin ^-1 (y3/R1)) S2 = R1 x (sin ^-1 (y2/R1) - sin ^-1 (y4/R1)).

Furthermore, if it is approximated that these radioactivities reach the detection system through the center line of the collimator, the distance 48T at which the radiation is absorbed by the piping is determined by T=x5-x7.

Therefore, if the count of gamma rays due to the deposited radioactivity of area S1 measured when gas is in the pipe is Np1, and the count of gamma rays due to the deposited radioactivity of area S2 is Np2, then according to the above, Using the values and coordinates ^obtained by
x5)/(4×π×x6 ² ).

Under the above conditions, if there is water in the pipe, Np2 only needs to take water absorption into account, so Np2 = σp x S2 x ε - Ap x T x ε - Aw x (x6 -
x5)/(4×π×x6 ² ).

Next, the liquid radioactivity filled inside the pipe is transferred to a fourth
Find it based on the diagram. The desired radioactivity is the area surrounded by the upper line and lower line where the collimator wants the piping, i.e. coordinates (x1, y1) 49, coordinates (x2, y2)
50, coordinates (x3, y3) 51 and coordinates (x4, y4)
This is the radioactivity in the area surrounded by 52. In order to perform calculations, the range of integration is approximated to a trapezoid 53 as shown in Figure 4, and the range of integration is set at coordinates (x5, y5).
54 to the coordinates (x6, y6) 55.

In Figure 4, the coordinates (x, y) of an arbitrary point 5
6, and if the desired length 57 of the collimator in the y direction at that time is u, then from the relationship of similarity of triangles, u:d=x:H holds, and u becomes u=(d×x)/H. .

Therefore, the minute volume dv58 is dv=u×H×dx/x, and the liquid radioactivity Nw is calculated for this minute volume dv, taking into account the self-absorption of radiation by water.
Integrate from x5 to x6, Nw=∫ ^x6 _x5 [W×ε−Aw×(x−x5)×ε−Ap×
It can be obtained as T/(4×π×H×x)]dx.

As an example of the above calculation formula, the outer diameter of the pipe is 20 cm,
Co-60 as a radionuclide inside the 1cm thick pipe
exists in a deposited state and a liquid state mixed with water,
Based on this measurement method, gamma rays with an energy of 1.33 MeV were measured. Each parameter: Inside radius of the pipe; R1 = 9cm Outside radius of the pipe; R2 = 10cm Collimator thickness: H = 5cm Collimator length: W = 0.5cm Position of the detection system perpendicular to the pipe axis:
y = 5 cm Radiation absorption coefficient of piping: Ap = 0.414 (cm ^-1 ) Radiation absorption coefficient of water: Aw = 0.0204 (cm ^-1 ) Radiation absorption coefficient of gas:
Ag=0.0000242 (cm ^-1 ) Unit radioactivity of deposited state:
σp=37000 (1μCi/cm ² ) Unit radioactivity in liquid state:
Set ρw = 37000 (1μCi/cm ³ ), substitute it into the above formula, and change the length L between the pipe axis and the detector (x-axis direction) from 15cm to 40cm at 5cm intervals.
Calculations were made for the case where the distance was changed to cm. Figure 5 shows calculation results 59, 60, and 61 regarding deposited radioactivity and liquid radioactivity at two locations inside the pipe.
These results show that the ratio of counts due to the deposited radioactivity at two locations and the liquid radioactivity sandwiched between the two locations differs when measured while changing the distance L between the piping and the gamma ray detection system.

For this reason, the deposited radioactivity at two locations in the pipe to be determined is defined as Cp1 (μCi/cm ² ) and Cp2 (μCi/cm ² ),
If liquid radioactivity is Cw (μCi/cm ³ ), these radioactivities can be determined as follows. When n measurements are performed while changing the distance L between the piping and the gamma ray detection system, the measured gamma ray count rate for each of the n measurement points is M(1) to M(n),
Further, if the counts due to the deposited radioactivity obtained by the above calculation are Np(1) to Np(n), and the counts due to n liquid radioactivity are Nw(1) to Nw(n), for each measurement point, Cp1×Np(1)+Cp2×Np(1)+Cw×Nw(1)=M(1) Cp1×Np(2)+Cp2×Np(2)+Cw×Nw(2)=M(2) … Cp1×Np (n)+Cp2×Np(n)+Cw×Nw(n)
=M(n) The n-element simultaneous linear equations are independently established. This simultaneous equation has three unknowns, Cp1, Cp2, and Cw, so it can be solved if there are three or more equations. That is, by measuring at three or more locations, the deposited radioactivity Cp1, Cp2 and liquid radioactivity Cw at two locations within the pipe can be determined.

Furthermore, as shown in FIG.
The radioactivity inside the pipe is measured at three or more locations using the measurement method described above at each measurement location. At each measurement location, the detector detects the deposition conditions at two localized locations desired by the detector. Radioactivity 6
4,65 and the liquid radioactivity 66 sandwiched between them.
By quantifying each, the distribution of deposited radioactivity and the distribution of liquid radioactivity throughout the pipe can be measured in detail.

(Effects of the Invention) As described above, by using the measurement method of the present invention, it is possible to non-destructively measure the distribution of each radioactivity in the non-uniform deposition state inside the pipe and in the liquid state, which was not possible with the conventional technology. Since it can be easily measured, it becomes possible to obtain detailed information on the radioactivity inside the piping in advance when cutting the reactor primary system piping. Further, it becomes possible to easily perform detailed radioactivity monitoring work and maintenance work in the piping during nuclear reactor operation.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a case where deposited radioactivity in a pipe is measured using a conventional non-destructive measurement method. Figure 2 shows
FIG. 2 is a diagram showing the principle of the measurement method of the present invention. FIG. 3 is an explanatory diagram for measuring deposited radioactivity in piping by the measuring method of the present invention. FIG. 4 is an explanatory diagram for measuring liquid radioactivity in piping by the measuring method of the present invention. FIG. 5 is a distribution diagram showing the results of measurement and calculation of deposited radioactivity and liquid radioactivity in piping using the measuring method of the present invention. (Assuming that the deposited radioactivity (1 μCi/cm ² ) and the liquid radioactivity concentration (1 μCi/cm ³ ) in the two locations inside the pipe are uniformly distributed, the energy is 1.33 MeV.
For gamma rays, calculations were performed for the case where the distance L between the pipe axis and the detector was changed from 15 cm to 40 cm at 5 cm intervals. FIG. 6 is a diagram showing the principle of scanning the radioactivity distribution in a pipe in a direction perpendicular to the axis of the pipe by the measuring method of the present invention. 1... Gamma ray measurement system, 2... Collimator window, 3... Piping, 4... Deposited radioactivity on the inner surface of the pipe, 5... Piping with radioactive contamination inside, 6... Rectangular collimator window, 7... ...shielding body, 8... gamma ray detector, 9... gamma ray detection system, 10
...The line connecting the center of the detector and the center of the collimator,
11...Measurement position 1, 12...Measurement position 2, 13
...Measurement positions 3, 14...Deposited radioactivity inside the pipe (perigee), 15...Deposited radioactivity inside the pipe (apogee), 16...Radioactivity of the substance filled inside the pipe, 17... ...Axis of the pipe, 18...Distance from the axis of the pipe to the detector, 19...Inner radius of the pipe,
20...Radius of the outside of the pipe, 21...Thickness of the collimator, 22...Length of the collimator, 23...Deposited radioactivity inside the pipe (perigee), 24...Deposited radioactivity inside the pipe (apogee) ), 25... liquid radioactivity, 26... center of detector, 27... center of collimator, 28... line connecting the center of detector and the center of collimator, 29... gamma ray detection system,
30... Radius inside the pipe, 31... Radius outside the pipe, 32... Distance from the detector to the pipe axis, 33
... Collimator length, 34 ... Collimator thickness, 35 ... Detector position (y axis), 36 ... Coordinate origin, 37 ... Uppermost line of the range where the collimator desires piping, 38 ...Coordinates (x1, y1), 39
...Coordinates (x2, y2), 40...The lowest line of the range where the collimator desires piping, 41...Coordinates (x3,
y3), 42...coordinates (x4, y4), 43...coordinates (x5, y5), 44...coordinates (x6, y6), 45...coordinates (x7, y7), 46...of deposited radioactivity Area (perigee), 47... Area of deposited radioactivity (apogee), 48
...Distance at which radiation is absorbed by piping, 49
...Coordinates (x1, y1), 50...Coordinates (x2, y2),
51... Coordinates (x3, y3), 52... Coordinates (x4,
y4), 53... Integral range approximated to a trapezoid, 54...
...Coordinates (x5, y5), 55...Coordinates (x6, y6), 5
6...Coordinates of an arbitrary point, 57...Length of the collimator in the y direction, 58...Minimum volume, 59...Example of calculation of deposited radioactivity at perigee when there is water in the pipe, 60... ...Example of calculating the deposited radioactivity at the apogee when there is water in the pipe, 61...Example of calculating the liquid radioactivity in the pipe, 62...Gamma ray detection system, 63... Direction perpendicular to both the straight line connecting the center of the detector and the center of the collimator, 64...Deposited radioactivity at perigee, 65...
...Deposited radioactivity at the apogee, 66...Liquid radioactivity in the pipe.

Claims (1)

[Claims] 1. Deposited radioactivity distributed on the inner surface of the pipe, and radionuclides released from contaminated pipes in a system in which the inside of the pipe is filled with a substance with radioactivity distribution only in the depth direction. A method for determining the radioactivity inside a pipe by measuring the spectrum of gamma rays with a specific energy, using a gamma ray detection system in which the detector is covered with a shielding body with a rectangular collimator window from the side of the pipe. , emitted from radioactive nuclides inside the piping viewed from the collimator window of the gamma ray detection system at any three or more locations on a straight line connecting the center of the detector of the gamma ray detection system and the center of the collimator. Depending on the measurement position, the gamma ray detection system detects the radioactivity emitted from the deposited radioactivity in two localized areas of the pipe and the radioactivity of the substance filled inside the pipe sandwiched between these two locations. Using the fact that the ratio of gamma ray count values is different, we can calculate the measured gamma ray count value, the inner radius and outer radius of the pipe, the distance from the pipe axis to the detector, the radiation absorption coefficient of the pipe, and the inside of the pipe. Using the radiation absorption coefficient of the filled material and the dimensions and thickness of the collimator window, the gamma ray detection system can calculate the amount of deposited radioactivity in two localized areas of the piping and the inside of the piping sandwiched between these. An internal radioactivity distribution measurement method that quantifies the radioactivity of each substance. 2 In the measurement method of 1 above, the gamma ray detection system is moved parallel to the vertical direction of the axis of the pipe to detect deposited radioactivity distributed inside the pipe and substances distributed only in the depth direction inside the pipe. A method for measuring radioactivity distribution inside a pipe, which is characterized by detailed measurement of radioactivity.