JPH0513591B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention scans and measures gamma rays emitted from contaminated radioactivity inside a pipe from outside the pipe using a gamma ray detector having a collimator window. The present invention relates to a measurement method for separating and nondestructively quantifying deposited radioactivity, aqueous radioactivity, or gaseous radioactivity inside a pipe.

(Conventional technology) Conventionally, in order to measure radioactivity inside a pipe, a part of the pipe at the measuring point is cut out, or an aqueous solution or gas inside is extracted as a measurement sample, and this sample is measured in an analysis room, etc. The measurement method that determines the radioactivity in each state has been mainly used. This method involves cutting out the measurement sample and unsealing the radioactivity.
There were problems in that the measurement locations were limited and it took time to obtain measurement results.

In addition, as a non-destructive method, as shown in FIG.
A method has been used to find in this way,
If the radioactivity inside the pipe exists in the form of a deposit, an aqueous solution, or a gas, there is a problem in that it is not possible to separate and measure the radioactivity.

(Means for Solving the Problems) The present invention aims to non-destructively quantify radioactivity in the form of deposits, aqueous solution, or gas inside the contaminated pipes of the reactor primary cooling system in each state. This is a method devised for this purpose.

A principle diagram showing the measurement method of the present invention is shown in FIG. A detector 7 is constructed by a shield 6 having a collimator window 5 whose length in the vertical direction of the rectangular window can be changed.
A gamma ray detection system 8 is installed to face the central axis 10 of the pipe 9 to be measured, and the vertical length of the collimator window is changed to detect gamma rays emitted from the pipe whose inside is contaminated with radioactivity. Measure gamma rays. Next, the gamma ray count distribution is determined by sequentially subtracting the measurement results when the length of the collimator window in the vertical direction is long from the measurement results when the collimator window is one length shorter.

On the other hand, when the length of the collimator window 22 in the vertical direction is changed, the number of gamma rays reaching the detector is uniform depending on whether the deposited radioactivity 11, aqueous radioactivity 12, or gaseous radioactivity 13 inside the pipe is the same. Assuming that there is unit radioactivity distributed in The absorption coefficient can be calculated and determined using the vertical width 17 and thickness 18 of the collimator. Next, in the same way as in the above measurement, the calculation results when the length of the collimator window in the vertical direction is longer are sequentially subtracted from the calculation results when the length is shorter by one, to obtain the counts for each state. The measurement result in the longitudinal width of each collimator is the sum of the number of gamma rays emitted from deposited, aqueous or gaseous radioactivity and counted by the detector, so the radioactivity in each state is expressed as unit radioactivity. The number corresponds to the sum of the products of the counts calculated for the case where the radiation is generated and the radioactivity of each state, which is an unknown quantity. The point of view of the present invention is that the ratio of gamma ray counts of deposited and aqueous or gaseous radioactivity obtained by the above measurement method differs depending on the longitudinal length of the collimator. If n is the number of measured values obtained by measuring different lengths and sequentially taking the differences, then an n-dimensional simultaneous linear equation holds true independently. By solving this simultaneous equation, the precipitated radioactivity and aqueous radioactivity inside the pipe, or the precipitated radioactivity and gaseous radioactivity, can be quantified for each state.

(Example) An example of the present invention is shown in FIG. Assuming that the deposited radioactivity 19, aqueous radioactivity 20, or gaseous radioactivity concentration 21 inside the pipe is uniformly distributed and has unit radioactivity, and when the vertical length of the collimator window is changed, The number of gamma rays in each state reaching the gamma ray detection system 23 is determined.

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

Radius inside the pipe 24: R1cm Radius inside the pipe 25: R2cm Distance from detector to pipe axis 26: Lcm Collimator width 27: Wcm Collimator thickness 2
8: Radiation absorption coefficient of Hcm pipe: Ap Radiation absorption coefficient of water: Aw Radiation absorption coefficient of gas: Ag Unit radioactivity in deposited state: σp Unit radioactivity in aqueous solution state: ρw Unit radioactivity in gas state: ρg . The origin 29 of the coordinates is the center of the front surface of the gamma ray detector,
The uppermost line 30 of the range where the collimator wants the piping
As shown in Figure 3, the coordinates of the intersection between
(x1, x2) 31, (x2, y2) 32 and (x3, y3)
33. From the relationship of similarity of triangles, x1 and y1
When finding the relationship, y1:w/2=x1:H holds, and y1=W×x1/(2×H). On the other hand, from the Pythagorean theorem, R1 ² =(L−x1) ² +y1 ² holds true. Coordinates (x1, y1) from the above two formulas
Solving, x1=2×L−√(2×L) ² −4×(1+((w/2)
/H) ² )×( ^L2 − ^R12 )/2×(1+((w/2)/H
) ² ) y1=(W/2×H)×x1. Similarly, coordinates (x2, y2) and coordinates (x3,
y3), x2=2×L−√(2×L) ² −4×(1+((w/2)
/H) ² )×( ^L2 − ^R12 )/2×(1+((w/2)/H
) ² ) y2=(W/2×H)×x2 x3=2×L−√(2×L) ² −4×(1+((w/2)
/H) ² )×( ^L2 − ^R12 )/2×(1+((w/2)/H
) ² ) y3=(W/2×H)×x3. Next, if the vertical width W and dcm of the collimator are widened to make W + d34, the coordinates of the intersection of the outermost line 35 and the pipe are determined as shown in the figure: coordinates (x4, y4) 36, coordinates (x5, y5) 37 and coordinates (x6, y6) 38 are x4=2×L−√(2×L) ² −4×(1+((W+d/
(2×H) ² )×(L ² −R1 ² )/2×(1+((w+d/
(2×H) ² ) y4=(W/2×H)×x4 x5=2×L+√(2×L) ² −4×(1+((w+d(
2×H) ² )×(L ² −R1 ² )/2×(1+((w+d(2
×H) ² ) y5=(W/2×H)×x5 x6=2×L−√(2×L) ² −4×(1+((w+d(
2×H) ² )×(L ² −R1 ² )/2×(1+((w+d(2
×H) ² ) y6=(W/2×H)×x6.

Using these coordinates, the vertical width of the collimator is W
The number of gamma rays in each state that reach the gamma ray detector through the collimator when expanded from W+d to W+d is calculated.

First, determine the deposited radioactivity inside the pipe. The area 39 of the deposited radioactivity sandwiched between coordinates (x1, y1) and coordinates (x4, y4) is S1cm ² , and the area 39 of the deposited radioactivity sandwiched between coordinates (x2, y2) and coordinates (x5, y5) is If the area 40 of is S2cm ² , S1=R1×(tan ^-1 (y4/(L−x4))− ^tan
(y1/(L-x1))) S2=R1×(tan ^-1 (y5/(L-x5))-tan ^-1
It is found as (y2/(L-x2))). The distance that the radioactivity of S1 and the radioactivity of S2 reach the gamma ray detector, respectively.
If Q1 and Q2 are becomes.

In addition, when these radioactivity passes through the piping, it is absorbed by the piping, so the coordinates (x1,
T1 is the length from coordinates (y1) to coordinates (x3, y3), and T2 is the length from coordinates (x4, y4) to coordinates (x6, y6).
When calculated as, T1=√(1−3) ² +(1−3) ² T2=√(4−6) ² +(4−6) ² . The average length T that gamma rays pass through is calculated as follows: T=(T1+T2)/2.

If the gamma ray detector's count due to the deposited radioactivity of S1 measured when the vertical width W of the collimator is widened by d is Np1, and the gamma ray detector's count due to the deposited radioactivity of S2 is Np2, calculate as above. Using these values, it can be determined as Np1=σp×S1/4×π×Q1 ² Np2=σp×S2/4×π×Q2 ² .

Therefore, when there is gas in the pipe, the count Np due to the deposited radioactivity of the part of the pipe where the vertical width of the collimator is expanded from W to W + d is calculated by correcting the radiation absorption of the gas for S2. , can be asked.

Under the above conditions, if there is water in the pipe, Np will be corrected for the radioactivity absorption of water with respect to S2. can be asked.

Next, the aqueous radioactivity and gaseous radioactivity inside the pipe are determined based on FIG. The desired radioactivity is obtained by expanding the vertical width of the collimator from W to W+d and obtaining the desired part of the piping, and using the coordinates (x1, y1) 41, (x2, y2) 42, (x4, y4) 4
Find the part surrounded by 3 and (x5, y5)44.
For calculation purposes, the integral range 45 is approximated as shown in FIG. The range of the x-axis to be integrated is the coordinates (x7, y7)46
From (x8, y8)47, and finding this as the average of the above four coordinates, we get x7 = (x1 + x4) / 2 y7 = (y1 + y4) / 2 x8 = (x2 + x5) / 2 y8 = (y2 + y5) / 2 .

In FIG. 3, the coordinates 48 of an arbitrary point are (x,
y), and if the desired length 49 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 is expressed as u=(d×x)/H. Become.

On the other hand, y is based on the similarity of triangles, W+d/2:y=H:x holds true, and y is y=(W+(d/2))×x/H becomes.

Therefore, the minute volume dv50 is dv=u×H×dx/x becomes.

Aqueous radioactivity Nw is calculated from x7 to x8 while correcting radiation self-absorption due to water.
Integrate up to It can be found by

In addition, gaseous radioactivity Ng can be obtained by integrating this microvolume from x7 to x8 while performing self-absorption of radiation by gas in the same way as above. It can be found by

As an example of calculating the above calculation formula, the outer diameter of the pipe is 20cm,
Co-60 as a radionuclide inside the 1cm thick pipe
exists in the form of a deposit and an aqueous solution, or in the form of a deposit and a gas, and the energy is calculated based on this measurement method.
This was done for the case where gamma rays of 1.33 MeV were measured. Radius inside the pipe: R1 = 9 cm Radius inside the pipe: R2 = 10 cm Distance from detector to pipe axis: L = 20 cm Collimator thickness: H = 5 cm Radiation absorption coefficient of pipe: 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 of radioactivity in deposited state: σp = 37000 (1 μCi/cm ² Unit of aqueous solution state) Radioactivity: ρw = 37000 (1μCi/cm ³ ) Unit radioactivity in gas state: ρg = 37000 (1μCi/cm ³ ) Substitute into the above formula, and calculate the vertical width W of the collimator.
Calculations were made for the case where the openings were opened from 0 cm to 6 cm at 0.5 cm intervals. Calculation results 51 and 52 for the case where deposited radioactivity and aqueous radioactivity exist inside the pipe are shown in FIG. Further, calculation results 53 and 54 for the case where deposited radioactivity and gaseous radioactivity exist inside the pipe are shown in FIG. The above calculation results show that the count distribution of deposited radioactivity and aqueous radioactivity, or of deposited radioactivity and gaseous radioactivity, differs depending on the length of the collimator's longitudinal width.

For this reason, the deposited radioactivity in the piping that should be determined is
Cp (μCi/cm ² ), aqueous radioactivity Cw (μCi/cm ³ )
It is calculated as follows. The measured gamma ray count rate M(l) to M(1) for each of the n measurement points measured by changing the length of the collimator's vertical width, and the count due to the deposited radioactivity obtained by the above calculation. of
Np(l)～Np(1), count by aqueous radioactivity is Nw
(l) ~ Nw(1), for each measurement point, Cp×Np(1)+Cw×Nw(1)=M(1) Cp×Np(2)+Cw×Nw(2)=M(2) ... An n-element simultaneous linear equation of Cp×Np(n)+Cw×Nw(n)=M(n) is established. These n equations hold true independently because the ratio of counts due to radioactivity in each state is different, as shown by the above calculation. Therefore, in this simultaneous equation, the unknowns are Cp and
Since there are two Cw, it can be solved if two or more are satisfied, and deposited radioactivity and aqueous radioactivity can be separated and quantified.

In addition, the deposited radioactivity in the pipe to be determined is Cp
(μCi/cm ² ) and when gaseous radioactivity is Cw (μCi/cm ³ ), it can be determined in the same manner as above.

Note that the measurement method of the present invention allows scanning measurement of gamma rays emitted from radioactivity inside the pipe by simply changing the vertical length of the rectangular window of the collimator in front of the detector. Therefore, the mechanism of the scanning device that needs to measure the entire piping can be made simple and compact.

In addition, since the gamma ray count rate is determined by sequentially subtracting the measurement results when the collimator's vertical width is one length shorter than the measurement results when the collimator's vertical width is long, Background gamma rays from areas other than the area to be measured are automatically subtracted, making it possible to perform measurements without the influence of the background.

(Effects of the Invention) As described above, by using the measurement method of the present invention, it is possible to non-destructively quantify each type of radioactivity in the form of deposits, aqueous solution, or gas inside a pipe, which was not possible with conventional techniques. This makes it possible to easily measure the radioactivity inside the piping, including its existing state, when cutting the reactor primary system piping. In addition, it has become easier to monitor radioactivity in piping and perform maintenance work during reactor operation.

[Brief explanation of drawings]

FIG. 1 is a diagram in which deposited radioactivity inside a pipe is measured using a conventional non-destructive measurement method. 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 inside a pipe by the measuring method of the present invention. FIG. 4 is an explanatory diagram for measuring aqueous radioactivity and gaseous radioactivity inside a pipe by the measuring method of the present invention. FIG. 5 is a distribution diagram of the results of measurement and calculation of deposited radioactivity and aqueous radioactivity inside a pipe using the measuring method of the present invention. (Deposited radioactivity inside the pipe (1μCi/cm ² )
Based on the condition that the and aqueous radioactivity (1 μCi/cm ³ ) are uniformly distributed, the vertical width W of the collimator of the gamma ray detection system is set to 0 at intervals of 0.5 cm for gamma rays with an energy of 1.33 MeV.
Figure 6 is a distribution diagram of the results of measurement and calculation of deposited radioactivity and gaseous radioactivity inside the pipe using the measurement method of the present invention. . (Deposited radioactivity inside the pipe (1μCi/cm ² ) and gaseous radioactivity (1μCi/cm ³ )
For gamma rays with an energy of 1.33 MeV, under the condition that they are uniformly distributed,
The vertical width W of the collimator of the gamma ray detection system is
Calculations were made for the case where the openings were opened from 0 cm to 6 cm at 0.5 cm intervals) 1... Gamma ray measurement system, 2... Collimator window, 3... Piping, 4... Deposited radioactivity on the inner surface of the pipe, 5... Rectangular collimator window, 6... Shielding body, 7... Gamma ray detector, 8... Gamma ray detection system, 9... Piping with radioactive contamination inside, 1
0... Central axis of the pipe, 11... Deposited radioactivity inside the pipe, 12... Aqueous radioactivity, 13... Gaseous radioactivity, 14... Distance from the pipe axis to the detector, 15... ... Radius inside the pipe, 16... Radius outside the pipe, 17... Vertical width of the collimator, 18... Thickness of the collimator, 19... Deposited radioactivity inside the pipe, 20... Radioactivity in aqueous solution, 21 ... Gaseous radioactivity, 22 ... Collimator window, 23 ... Gamma ray detection system, 24 ... Inner radius of pipe, 25 ...
... Radius of the outside of the pipe, 26 ... Distance from the detector to the pipe axis, 27 ... Vertical width of the collimator, 28 ...
Collimator thickness, 29... Origin of coordinates, 30...
...the uppermost line of the range where the collimator wants the piping to go,
31... Coordinates (x1, y1), 32... Coordinates (x2,
y2), 33... Coordinates (x3, y3), 34... Vertical width of the collimator W + dcm, 35... In the case of 34, the outermost line where the collimator wants the piping, 36... Coordinates (x4, y4), 37... ... Coordinates (x5, y5), 38... Coordinates (x6, y6), 39... Area of deposited radioactivity (perigee), 40... Area of deposited radioactivity (apogee),
41... Coordinates (x1, y1), 42... Coordinates (x2,
y2), 43... Coordinates (x3, y3), 44... Coordinates (x4, y4), 45... Integral range approximated to a trapezoid,
46... Coordinates (x5, y5), 47... Coordinates (x7,
y7), 48... Coordinates of an arbitrary point, 49... Length of the collimator in the y direction desired, 50... Minute volume, 5
1... Calculation result of deposited radioactivity when there is water in the pipe, 52... Calculation result of aqueous radioactivity in the pipe, 53... Calculation of deposited radioactivity when there is gas in the pipe Result, 54... Calculation result of gaseous radioactivity in the pipe.

Claims (1)

[Claims] 1 Radioactivity almost uniformly deposited on the inside of the pipe and aqueous radioactivity almost uniformly distributed inside the pipe, or radioactivity almost uniformly deposited on the inside of the pipe and gas almost uniformly distributed inside the pipe Gamma rays with a specific energy emitted from radioactive nuclides in pipes whose interior is contaminated with radioactivity are absorbed by the gamma ray detector. In the method of determining the radioactivity inside a pipe by measuring the spectrum using a gamma ray detection system with a vine structure, the radioactivity inside the pipe is viewed from the collimator window of the gamma ray detection system placed at the center next to the pipe and facing the central axis of the pipe. Gamma rays emitted from nuclides are measured using the vertical length of three or more types of collimator windows. Utilizing the fact that the ratio of gamma ray count values emitted from gaseous radioactivity and gaseous radioactivity is different, the detector can be detected from the measured gamma ray count value, the inner radius and outer radius of the pipe, and the axis of the pipe. By using the distance to A pipe internal radioactivity measurement method characterized by quantifying both radioactivity and gaseous radioactivity.