JPH0743437B2 - Method for determining subcriticality of nuclear fuel specimens - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is, for example, a solution fuel handled in a treatment process of a nuclear fuel cycle facility as a test object, and accurately obtains the subcriticality of this test body over a wide range with high accuracy. And a method for determining subcriticality of a nuclear fuel test body.

[Conventional technology]

At nuclear fuel cycle facilities that reprocess spent fuel in nuclear reactors, it is necessary to maintain the work of the reprocessing process below the critical level in terms of safety management. For this reason, in conventional nuclear fuel cycle facilities, a criticality safety design is performed using a safety concept based on the so-called double contingency principle so that no single contingency will occur in a critical state. The mass control, shape control, and concentration control are performed. This is because the conventional nuclear instrumentation technology cannot obtain accurate subcritical data in a wide range of subcritical areas, and the criticality safety design is performed using the design code verified with the critical data of one point. .

That is, as a neutron flux monitor, there is a monitor with high measurement accuracy used in the level region close to the critical point in the past.
A monitor has been developed, which has a wide range of high-precision sub-criticality in the sub-critical region where the neutron flux level is low, has high sensitivity and responsiveness that can measure in a short time, and accurately verifies the measurement data of the monitor. The current situation is that there are none. Especially in the subcritical low-level region (effective multiplication factor K _eff = 0.3 to 0.8)
When measuring the subcriticality with, because the number of neutrons is small, it takes a huge amount of time and cost to measure with a conventional monitor, and verification of the measurement data requires calibration based on critical data, Moreover, the data processing requires a complicated procedure using a computer.

[Problems to be Solved by the Invention]

As described above, the conventional nuclear measurement technology is a monitoring technology for measuring the subcriticality of nuclear fuel, in particular, the subcriticality in the deep subcriticality region with an effective multiplication factor K _{eff of} about 0.3 to 0.8 in a highly accurate and short time. Therefore, in the conventional nuclear fuel cycle facility, the criticality safety design of each treatment process is performed with an excessive margin. However, in such a reprocessing facility that has an excessive margin in terms of safety design, the plutonium content, which is important for criticality safety management, such as fuel reprocessing of fast reactors and nuclear fuel of light water reactors, which will increase in the future, is important. There is a problem that the nuclear fuel cycle facility becomes larger and larger when reprocessing a large amount of fuel, which has a lot of fuel.

Thus, for example, the critical mass for such a solution fuel to be handled by the processing steps of the fuel cycle facilities, and extensive and accurate, and monitoring techniques are established that can be determined in a short time of the subcriticality to about 1 to 0.3 in the effective multiplication factor K _eff If this is done, it is possible to operate near the safety limit by drastically cutting down the margin on the critical safety design in the nuclear fuel cycle facility described above based on a large number of data obtained in the subcritical region. , This will enable a large amount of nuclear fuel to be reprocessed with a small-scale facility.

The present invention has been made in view of the above points, and is intended for a nuclear fuel test specimen including a solution fuel or powder fuel, a fuel pin, etc. handled in the treatment process of the above-mentioned nuclear fuel cycle facility, and its subcriticality. It is an object of the present invention to provide a method for determining the subcriticality of a nuclear fuel test body, which enables accurate determination in a short time with high accuracy over a wide range.

[Means for Solving the Problems]

In order to solve the above-mentioned problems, the method of the present invention uses, as a test facility, a reactor whose reactor characteristics are known in which a core is defined as a driver region and a test body region is defined in the central portion of the reactor, and After the nuclear fuel test body is housed in the test body area of the above, a coupled core is constructed, and then the reactivity control system of the reactor is adjusted to bring the coupled core into a critical state. The subcriticality of the nuclear fuel test specimen alone is calculated based on the degree.

[Action]

In the above, the reactor used as the test facility is an existing heavy water moderator reactor whose reactor characteristics are well known, or a light water reactor that can make the reactor critical by adjusting the moderator, and In the center of the, there is a void defined as the test body area. In such a nuclear reactor, a reactivity control system, for example, by adjusting the water level of heavy water as a moderator to change the reactivity, with the driver fuel body loaded in the driver region of the core surrounding the test body region, The reactor goes from subcritical to critical, and the critical state of the reactor is measured by nuclear instrumentation installed in the reactor. Also, the reactor characteristics of the reactor used here as test equipment are well-known based on a wealth of experimental data obtained through operation results over many years. The subcriticality of the driver area alone has already been accurately determined by measurement and calculation over a wide range.

Then, by accommodating the nuclear fuel test body (for example, the nuclear fuel test body simulating each processing step of the nuclear fuel cycle facility) in the test body area defined in the core of the nuclear reactor, The combination will form a combined core. For this coupled core, the reactivity K in the whole core system is expressed as a functional expression K = f (K _D , K _T ) of the reactivity K _{D in the} driver region and the reactivity K _{T in the} test body region, It can be simply expressed as K = K _D + K _T (1).

Therefore, the nuclear fuel test body to be verified is accommodated in the test body region of the reactor core described above, and the reactivity control system of the reactor, for example, the water level of heavy water as a moderator is adjusted to control the entire system of the combined core. To the critical state (K = 1), and the reactivity K in the whole core system is measured at multiple points while changing the heavy water level near the critical point. Here, the measured value of each state in the above functional equation (1) is measured. K, and by solving the equation by substituting known values K _D corresponding thereto, specimen region, i.e. a single reaction of the nuclear fuel specimen accommodated in the specimen area K _T K _T = K −K _D
Can be asked as Furthermore, if the nuclear fuel test specimen is a solution fuel contained in a tank, the critical amount and subcriticality of the solution fuel in the tank are changed by changing the solution level in the tank, and the subcriticality is reduced to an effective multiplication factor of 1 to 1. 0.
Can make accurate and quick decisions over a wide range of up to about 3.

In addition, in this case, since the reactor is subjected to critical adjustment by controlling the moderator, stable operation can be performed without danger of runaway. This is because, for example, when the plutonium solution fuel is compared with heavy water, which is a moderator of the nuclear reactor, the latter has a reactivity addition rate per unit volume that is smaller by about one digit than the former.

Note that the nuclear fuel test body is not limited to the solution fuel contained in the tank, but powder fuel, pin fuel, or an interfering body for these may be used.
It is possible to determine the subcriticality of various types of test bodies of various combinations. Therefore, by preparing test specimens of various system patterns that simulate each processing step of the nuclear fuel cycle facility described above, and determining the non-temporary degree over a wide range, and accumulating a large number of data in the subcritical region, It is possible to operate the facility safely after significantly reducing the margin for the safety design of the cycle facility.

Further, by comparing the subcriticality obtained by the above method with the measurement data of the subcriticality measurement monitor currently under development, it is possible to accurately verify the measurement data of the monitor.

〔Example〕

1 and 2 are plan views showing a core structure of a nuclear reactor used as a subcriticality measurement demonstration test facility according to an embodiment of the present invention,
And an elevation view of the test body region containing the nuclear fuel test body in the core, FIGS. 3 (a), (b), 4 (a), and (b) are different from the nuclear fuel test body shown in FIG. FIG. 5 is a plan view and a configuration diagram of a test body region containing the test body, and FIG. 5 is an effective multiplication curve showing the subcriticality of the test body region and the driver region obtained by the method of the present invention.

First, in FIG. 1 and FIG. 2, reference numeral 1 is a core of, for example, a heavy water moderator reactor used as a test facility, and a test body region as a space for accommodating a nuclear fuel test body, which will be described later, is provided in a central portion of the core 1. 2 is defined, and the core region surrounding the test body region 2 is defined as the driver region 3, and the driver fuel body 4 is loaded therein. Further, 5 is heavy water as a moderator introduced into the driver area 3 of the core.

Although not shown, the reactor is equipped with a water level control system for heavy water 5 with respect to the core 1, and nuclear instrumentation for monitoring neutron flux. Also, this reactor are those furnace characteristics are familiar, as represented by the solid line K _D of FIG. 5, the critical amount alone about the drivers area 3, and the effective water level level X and subcritical regions of heavy water 5 The relationship with the multiplication factor K _eff is known in advance because it has been accurately obtained by measurement and calculation.

On the other hand, a nuclear fuel test body 6 to be verified whose subcriticality is unknown is housed in the test body region 2 of the nuclear reactor core 1 described above, and in this state, it is combined with the fuel body 4 loaded in the driver region 3 described above. Form a combined core. The test body 6 described above is, for example, a model of a treatment process of a nuclear fuel cycle facility, and the test body 6 illustrated in FIG. 1 is arranged around the tank 8 containing the solution fuel 7 and the tank 8. It is configured as a combination with the interference body 9.

In the above state, the reactivity of the heavy water 5 as the moderator introduced into the driver region 3 of the nuclear reactor is adjusted to change the reactivity, so that the entire system of the combined core reaches the critical state.
Furthermore, by measuring the neutron flux while changing the water level of heavy water around the critical point and monitoring the reactivity of the entire core system, the test body region in the coupled core 2,
That is, the individual critical amount and subcriticality of the nuclear fuel test body 6 are required.

That is, the reactivity measured in the entire system of the coupled core is K, the single reactivity of the driver region 3 corresponding to the state is K _D (known value), and the single reactivity for the test body region 2 is K _T ( Unknown value), and here, the known values K and K _D are the above-mentioned functional expressions K _T = K
By substituting into −K _D and calculating, the reactivity K _T of the test object alone can be immediately determined. Furthermore, the tank 8 of the test body 6
By repeating the same measurement operation while changing the level Y of the solution fuel 7 accommodated in No. 1, the subcriticality (effective multiplication factor K _eff of 1 to 0.
A range of about 3) can be determined. The subcriticality of the test specimen alone thus obtained is shown as an effective multiplication factor K _{eff in} FIG. 5 as a dotted line K _T. The subcriticality K shown
_T is the solution fuel 7 of 250gPu / l as the nuclear fuel test specimen
An example in which a 0 cm tank 8 is used and the solution level Y in the tank is variously changed will be shown.

Further, FIGS. 3 (a) and (b) and FIG. 4 (a),
(B) shows an example of a test body having a system pattern different from that of the nuclear fuel test body 6 shown in FIG. 1, and FIG. 3 shows the test body 6
Is divided into a fixed side portion 6a incorporating a solution fuel tank 8 and a movable side portion 6b incorporating an interfering body 9, and the position of the movable side portion 6b is moved up and down so as to be positioned relative to the fixed side portion 6a. Those in which the relationship can be changed, and FIG. 4 is a test body in which a powder fuel body 10 is incorporated in place of the solution fuel and interference body in FIG. It should be noted that these test bodies 6 shown in each drawing are produced by simulating the respective processing steps of the nuclear fuel cycle facility, and the subcriticality is similarly obtained for various test bodies other than the illustrated examples. You can

〔The invention's effect〕

As described above, the present invention uses, as a test facility, a reactor in which the reactor characteristics are known in which the core is defined as the driver region and the test body region is defined in the central portion thereof, and the test body region within the core is used for the reactor. After constructing the combined core by accommodating the nuclear fuel test body in the reactor, the whole system of the combined core is brought to the critical state by adjusting the reactivity control system of the reactor, and the known subcriticality of the driver region in this state The following effects are obtained by determining the subcriticality of the nuclear fuel test body alone based on the above. That is, (1) by combining the nuclear fuel test body with the driver region of the reactor core used as a test facility to form a combined core, and adding neutrons from the driver region to the test body to bring the combined core into a critical state. , The subcriticality measurement range, which was impossible with conventional nuclear instrumentation technology, is 1 to 0.3 in terms of effective multiplication factor K _eff .
The degree of subcriticality of the test piece can be accurately determined by expanding to a certain degree.

(2) In this case, by controlling the moderator on the reactor side, the criticality adjustment of the entire core system can be performed, and the reactor can be operated in a stable state. Moreover, the subcriticality of the test body alone can be immediately obtained without requiring complicated data processing.

(3) Further, the nuclear fuel test body is not limited to a specific system, but a solution fuel system, a powder fuel system, a fuel pin system, or an interference system in which these fuel systems and an interfering body are three-dimensionally combined. For example, the subcriticality can be obtained for test objects having various system patterns.

(4) Therefore, by preparing various test bodies that simulate the treatment process in the nuclear fuel cycle facility as the nuclear fuel test bodies, and by accumulating the data for a wide range of subcriticality for these test bodies, Based on this data, the subcritical region, which was impossible until now (effective multiplication factor K _eff = 0.
You can verify the design code of 3 to 1). This significantly cuts the margin in criticality safety design, and reprocessing of fast reactor fuel that will increase in the future in small-scale facilities, or large amount of reprocessing of fuel with high plutonium content such as fuel of high burnup light water reactor. There are economic advantages that can be addressed.

(5) Further, by using the subcriticality obtained by the method of the present invention as reference data and comparing it with the data measured by the subcriticality monitor currently under development, it is possible to determine whether or not the measured data of the monitor is correct. It can be verified accurately.

[Brief description of drawings]

1 and 2 are plan views showing a core structure of a nuclear reactor used as a subcriticality measurement demonstration test facility according to an embodiment of the present invention,
And an elevation view of the test body region containing the nuclear fuel test body in the core, FIGS. 3 (a), (b), 4 (a), and (b) are different from the nuclear fuel test body shown in FIG. FIG. 5 is a plan view and a configuration diagram of a test body region accommodating the test body, and FIG. 5 is a diagram showing an effective multiplication factor curve representing the subcriticality of the test body region and the driver region obtained by the method of the present invention. . In the figure, 1: reactor core, 2: test body region, 3: driver region, 4: driver fuel body, 5: heavy water (moderator), 6: nuclear fuel test body.

Claims (1)

[Claims] 1. A nuclear reactor having a known reactor characteristic in which a core is defined as a driver region and a test body region is defined in a central portion of the reactor is used as a test facility, and a nuclear fuel test body is provided in the test body region in the core for the reactor. After accommodating and constructing the coupled core, the reactivity control system installed in the reactor is adjusted to bring the entire system of the coupled core to a critical state, and based on the known subcriticality of the driver region in this state. A method for determining a subcriticality of a nuclear fuel test specimen, characterized by obtaining a subcriticality of a nuclear fuel test specimen alone.