JPH0222918B2 - - Google Patents

Description

Detailed Description of the Invention [Technical Field of the Invention] The present invention relates to fuel assemblies, particularly irradiated fuel assemblies used in nuclear reactors and subjected to neutron irradiation, the composition of which is no longer accurately known. Pertains to neutron multiplication factor measurement method.

[Technical background of the invention and its problems] After the irradiated fuel assembly is removed from the nuclear reactor, it is stored in a fuel storage rack in a fuel storage pool. Subcriticality during fuel exchange and storage racks is extremely important for ensuring safety.
Subcriticality must be achieved with sufficient margin.

However, the fuel composition of irradiated fuel assemblies is significantly different from that of unused fuel assemblies, and it is unclear what kind of composition and nuclear properties they actually have. Although theoretical values of nuclear properties are given through nuclear design calculations, it is unclear to what extent these correspond to actual nuclear properties.

Therefore, in the past, excessive subcriticality was secured compared to the subcriticality based on the theoretical value of nuclear properties. Therefore, the number of fuels that can be stored in a fuel storage rack has been set to be much smaller than the number that can be stored theoretically.

Because of the above conditions, it is desirable to be able to measure the neutron multiplication factor of the irradiated fuel assembly and thereby evaluate the reliability of the design code.

However, although various methods have been proposed for measuring the neutron multiplication factor of irradiated fuel assemblies, there is no prospect of practical use of these methods due to poor workability and low measurement accuracy.

Conventional knowledge suggests that when an artificial neutron source is placed from the outside on one side of a fuel assembly placed underwater and a neutron detector is placed on the other side to measure the neutron flux _φs , the value is It was assumed that it is expressed by equation (1), where S is the source strength and β is the constant.

φ _s = βS / 1−k _eff ...(1) The inventor of the present application also disclosed the previously disclosed Japanese Patent Application Laid-open No. 159586 (1983) (formulas (2a) and (2b)) and
Equation (1) was used in Publication No. 26417 (Equations (1) and (2)). Equation (1) originally represents the neutron multiplication system as 1
This is correct when viewed as a point (or an infinitely wide system of uniform composition), but in reality the size of the system is finite, and the neutron multiplication region with a finite size (for example, a fuel assembly placed in water) is correct. There has been little consideration as to how accurate the approximation is when a neutron source is placed locally near the body.

As a simple example, suppose that a neutron source is placed locally on the side of a fuel assembly underwater, and the neutron flux is measured near the neutron source. In this case, it is clear that the neutron count rate obtained in the measurement is determined approximately by the intensity of the neutron source, and the contribution of the neutron multiplication effect of the fuel assembly is only small.
Nevertheless, it is unreasonable to use the above equation (1) in general.

In order to find a simple formula that can be expressed even in such cases, the present invention conducted various analytical calculations and experiments, and devised an extremely simple experimental formula. The result is simply adding a constant term to the right-hand side of equation (1). That is, φ _s = αS + βS/1−k _eff ……(2) This equation also becomes φ _s = βS/1−k _eff {1+α/β(1−k _eff )}……(2
a) It can also be written as

Comparing equation (2a) with equation (1), {1+(α/β)(1−k _eff )} on the right side of equation (2a) is a correction factor associated with the local placement of the neutron source, and (1−k It can also be thought of as adding a first-order perturbation correction term using _eff ). When we applied Equation (2) to various calculated and measured values, we found that φ _s and k _eff were extremely accurate over a wide range.
It turns out that it is possible to express the relationship between After examining many numerical calculation results in detail, we found that although (αS) and (βS) vary depending on the measurement conditions, the neutron source and neutron detector were placed between the fuel assembly placed in the water. When placed, the value of αS may be negative, but usually |αS|<
βS/(1−k _eff ), and under certain conditions |αS|
It was found that ≪βS/(1− _keff ). In other words, it has become clear that equation (1), which has been known from the past, generally has a worse approximation than equation (2).

[Object of the Invention] The present invention was made based on the above-mentioned circumstances, and an object of the present invention is to obtain a method for measuring the neutron multiplication factor of an irradiated fuel assembly that is easy to work with and can be measured with high accuracy.

[Summary of the Invention] In the present invention, a neutron source is placed on one side of a fuel assembly placed in water, and a neutron detector is placed on the other side opposite to the side to detect the neutron flux φ _s . and at least two types of calibration assemblies including at least one type of fuel assembly whose effective neutron multiplication factor k _eff is known, that is, k _eff
Two types of unirradiated fuel assemblies with significantly different values or unirradiated fuel assemblies with known k _eff and a dummy rod are housed together with water in a channel box (BWR
The same measurements as above are performed on calibration assemblies such as simulated irradiated fuel assemblies that simulate the neutron transfer area (fuel example), and the neutron measurement values (count values) for these calibration assemblies are calculated as (1 -k _eff ) as a variable of the hyperbolic function to determine the products αS and βS of the constants α and β determined by the measurement conditions and the neutron source strength S, and then calculate k _eff from equation (2) above. The above objectives have been achieved.

[Examples of the Invention] The present invention will be described below with reference to Examples. In Fig. 1, a neutron source S is located on one side of the fuel assembly F, passing through the center of the cross section of the irradiated fuel assembly F placed in the water and parallel to the side surface of the fuel assembly.
and a neutron detector D is placed on the other side. According to theoretical calculations, the distance from the side of the fuel assembly F can be set at 3 to 6 cm.

The neutron flux φ at the position of the neutron detector D is
The neutron flux φ ₀ formed by the internal neutron source inside the fuel assembly F in the absence of the neutron source S
and the neutron flux φ _s formed by the external neutron source S, φ=φ ₀ +φ _s (3).

Therefore, φ ₀ = γS ₀ /1−k _eff ... (4) where S ₀ ... internal neutron source strength (spontaneous neutron emission rate) k _eff ... (effective) multiplication factor γ ... proportionality coefficient or , φ _s =αS+βS/1− _keff ...(5) However, S...External neutron source strength α, β...Proportional coefficient.

Note that in equation (5), the first term on the right side is a correction term caused by the fact that the external neutron source is not dispersed within the fuel assembly F but is localized outside.
The second term is that neutrons from the external neutron source S enter the fuel assembly F and are doubled by the same behavior as neutrons from the internal neutron source, or leak out, or are absorbed and reduced in vain. This is the neutron flux formed as a result of Note that the value of α in equation (5) changes depending on the location, such as being negative at the position of the neutron detector D and positive at the position near the neutron source S.

It seems that the function of neutron flux and multiplication factor can be found from both equations (4) and (5), but in reality, in the case of irradiated fuel assemblies, even if the multiplication factor is the same value, the neutron flux φ Since the value of ₀ is not constant and changes over time, it is difficult to determine the relationship between the front and back from Equation (4), and it is preferable to find it from Equation (5).

Therefore, since α, β, and S in equation (5) are constants,
It can be seen that φ _s has a hyperbolic relationship with (1−k _eff ). Furthermore, as a matter of course, 1/φ _s also has a hyperbolic relationship. If |α|≪|β|, then 1/φ _s
is proportional to (1- _keff ). Also, when k _eff approaches 1, the first term on the right side of equation (5) can be ignored, so 1/φ _s can be considered to be approximately proportional to (1-k _eff ).

In the system shown in Figure 1, two-dimensional diffusion calculations were performed with various compositions of the fuel assembly.
FIG. 2 shows the results of determining the relationship between 1/φ _s and (1−k _eff ).

In this figure, a solid curve A indicates a neutron detector D, an external neutron source S, and a fuel assembly F as shown in Figure 1.
This is the case where the two-dot chain curve B
This is the case where the neutron detector D is placed near the external neutron source S. From this figure, it can be seen that for both curves A and B, 1/φ _s has a hyperbolic relationship passing through the origin with respect to (1-k _eff ). In addition, in curve A, the slope of curve A is larger than the slope of curve B in that range for the fuel assembly k _eff range of 0.4 to 0.7, indicating that the arrangement of curve A has better detection sensitivity. Recognize.

Furthermore, in the arrangement of the neutron source S, fuel assembly F, and neutron detector D shown in FIG. At the position, almost no influence of the composition or composition distribution within the fuel assembly F was observed on the measured value, but 1/φ _s was (1−k _eff )
shows the hyperbolic relationship passing through the origin. Moreover, when k _eff = 0, the extrapolated value of this hyperbola roughly matches the calculated value of a simulated fuel assembly constructed using iron or stainless steel, especially a system in which a fuel assembly is arranged that simulates the neutron transfer area. I found out what to do.

This means that the composition is known exactly and k _eff
Using two fuel assemblies that can be accurately calculated,
Show that the proportionality coefficients α and β can be determined.

Therefore, in the present invention, an unirradiated fuel assembly (the composition distribution may be almost arbitrary) is used as the first calibration assembly, and a neutron transfer area is simulated as the second calibration assembly. A simulated fuel assembly made of iron or stainless steel is used. If the neutron detector D is 5 cm away from the aggregate surface,
Thermal neutron transfer distances do not necessarily have to be the same.

Using the above two calibration assemblies, measure φ _s for each assembly, find 1/φ _s , determine proportional coefficients α and β from the hyperbolic relationship shown in Figure 2, and calculate 1/φ s. Find the correlation equation between _s and (1-k _eff ).

From the above correlation equation and 1/φ _s in the irradiated fuel assembly F, 1−k _eff and therefore k _eff can be determined.

FIG. 3 shows the second calibration assembly.
The channel box 1 is made of Zircaloy or aluminum, and inside the channel box 1 there are dummy rods 2 made of materials such as SUS, Al, and Fe-filled rods with the same diameter as the fuel rods and arranged in the same way as the fuel rods in a normal fuel assembly. It is accommodated. The axial length of this aggregate is approximately three times the length of one side of the cross section.
Channel box 1 is filled with water.

As is clear from the above, according to the present invention, by performing measurements using the first and second calibration assemblies,
Once α and β have been determined, the irradiated fuel assembly in the arrangement shown in Figure 1 can be measured, and from the measured values,
k _eff can be found.

Note that if unirradiated fuel assemblies with significantly different k _effs can be used, the fuel assemblies can be used for the first and second calibrations without preparing a second calibration assembly equipped with a dummy rod. The present invention can be implemented as an assembly for use.

[Effects of the Invention] According to the present invention, k _eff of an irradiated fuel assembly of unknown fuel composition can be determined by actual measurement.
This makes it possible to evaluate the reliability of the design code, and, for example, to ensure subcriticality in fuel storage racks, it is possible to reduce the excess margin that has been taken until now to ensure subcriticality. It is possible to store more fuel in the rack than before.

[Brief explanation of drawings]

Fig. 1 is a plan view of one embodiment of the present invention, and Fig. 2 is a plan view of one embodiment of the present invention.
A diagram showing the relationship between ~φ _s and (1-k _eff ), and FIG. 3 is a plan view of the calibration assembly. S...neutron source, F...irradiated fuel assembly, D...
...neutron detector.

Claims (1)

[Claims] 1. A neutron source is placed on one side of a fuel assembly placed in water, and a neutron detector is placed on the other side opposite to the side to determine the neutron flux φ _s . The same measurements as above are performed on at least two types of calibration assemblies including at least one type of fuel assembly whose effective neutron multiplication factor k _eff is known, and the neutron measurement values for these calibration assemblies are expressed as ( 1-k _eff ) as a variable of the hyperbolic function, and determine the products αS and βS of the constants α and β determined by the measurement conditions and the neutron source strength S, and from φ _s = αS + βS / 1-k _eff A method for measuring the multiplication factor of irradiated fuel, characterized by determining k _eff .