JPH0458184A - System for uranium accountancy and method for uranium accountancy - Google Patents

Abstract

PURPOSE:To hermetically determine the total quantity of uranium in a raw material without sampling by providing a neutron detector on the outer side of a hermetic cell for sepn. and enrichment and further, providing a gamma ray measuring instrument around a flow passage which carries enriched uranium and waste uranium. CONSTITUTION:The total quantity of the uranium in metal uranium is measured by the neutron detector 7 on the outer side of the hermetic cell 3 for separating and enriching the uranium. The enrichment is determined inline together with the total quantity of the uranium measured by the neutron detector 7 if the presence quantity of <235>U is measured by the gamma ray measuring instrument 13 in the flow passages 10a, 10b which are corrected to the hermetic cell 3 and carrying the enriched uranium and waste uranium separated from the uranium raw material in accordance with the object of measurement. The uranium enrichment in the raw material is measured inline in this way while the contact with the atm. air is averted without sampling the raw material.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Field of Application) The present invention relates to a uranium accounting and management system and a uranium accounting and management method in a facility that handles metallic uranium, such as a uranium enrichment facility using an atomic laser method.

(Conventional technology) Nuclear fuel cycle facilities such as uranium enrichment plants, uranium conversion plants, fuel fabrication plants, and fuel reprocessing plants generally require process control, quality control, criticality control, safeguards (material balance of uranium throughout the enrichment plant). The amount of uranium present in factories and processes, the degree of enrichment (isotopic ratio) of uranium, Metrology control is required to measure the flow rate of molten uranium.

Conventionally, the main methods for enriching uranium have been gas diffusion methods and gas centrifugation methods. In these methods, the chemical form of uranium treated is uranium hexafluoride (UF, ). Uranium hexafluoride has a low vapor pressure at room temperature, so if the storage container (cylinder) is even slightly warmed, it can be transported as a gas through the process through piping connected to the cylinder. Furthermore, by cooling a container containing UF6 gas, the UF6 gas can be condensed and solidified or liquefied and collected.

In uranium enrichment facilities using gas centrifugation, uranium is measured as follows.

The raw material UF6 is stored in a large cylinder in solid or liquid form and transported to a nuclear fuel cycle facility. The weight of the UF6 in the cylinder is determined by first measuring the weight of the entire cylinder, and then subtracting the weight of the cylinder tare by 1. If the raw material UF6 is obtained from natural uranium, the enrichment level is not particularly measured, but if necessary, a small amount can be sampled after turning it into a gas and measured with a mass spectrometer, or it can be measured from outside the cylinder. Obtained by measuring the γ-ray spectrum.

On the other hand, when measuring non-destructively with UF5 gas entering the process piping system, it is determined from the ratio of the U gas concentration and the UF6 gas concentration. In this case, it is determined from the count rate of 186 keV γ-rays generated by the decay of . The flow rate of UF6 gas can be measured with a general gas flow meter (current meter).

In addition to the above-mentioned γ-ray spectrum measurement method, when measuring the concentration of UF6 filled in a cylinder non-destructively,
There is a method of measuring neutrons detected from U Fa. Neutrons emitted from U F 6 are classified into spontaneous fission neutrons and (α, n) reaction neutrons based on the emission process (nuclear reaction).

Spontaneous fission neutrons are mostly generated by spontaneous fission of 238U in low enriched uranium, in which U usually accounts for 95% or more. On the other hand, (α, n) reaction neutrons are the nuclear reaction between fluorine and α rays released by α decay of uranium19
F(α,n) Generated by 22Na. In enriched uranium produced by gas centrifugation from U F 6, which is made from natural uranium, 234U, which has a high α specific radioactivity, is enriched, so as the enrichment increases, the (α, n) reaction neutron generation rate increases. increase Therefore, the (α) of the cylinder.

n) Enrichment can be determined from the ratio of the measured neutron emission rate and the calculated spontaneous fission neutron emission rate based on the net U F 6 weight measurement.

(Problems to be Solved by the Invention) On the other hand, in enrichment facilities using the atomic laser method, the enrichment operation is carried out inside a normally vacuum airtight cell (separation cell) in which metallic uranium is melted and liquefied and further evaporated. The supply of raw materials and the storage of products and waste take place in the form of uranium metal (solid).

Because metallic uranium oxidizes in the atmosphere and sometimes has the risk of burning, it must be stored and handled under inert gas or vacuum. For this reason, for example, it is difficult to sample solid metal uranium in an airtight container without exposing it to the atmosphere.
It's significantly more difficult than F6. Furthermore, the molten metal uranium that flows through the channels leading to the separation cell is at a high temperature of over 1,100 degrees Celsius, making sampling difficult.

Metallic uranium emits spontaneous fission neutrons (caused by 238U), but fluorine and other (α) neutrons are emitted.

n) Since it does not contain any elements that cause a reaction, (α, n) reaction neutrons are not emitted. Therefore, even if neutrons are measured from outside a container containing metallic uranium, it contains almost no information about the enrichment level.

In other words, in the current situation where there is no suitable sampling method for detecting gamma rays and alpha rays, it is impossible to effectively measure the enrichment level of metallic uranium in solid or molten state, and it is impossible to perform quantitative control.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a uranium accounting and management system and a uranium accounting and management method in facilities that handle metallic uranium in a solid or molten state, such as uranium enrichment facilities using atomic laser methods.

[Structure of the invention]

(Means for Solving the Problems) In order to solve the above problems, the present invention provides a neutron detector on the outside of an airtight cell for separating and concentrating metallic uranium using an atomic laser method, and further includes a neutron detector inside the airtight cell. A gamma ray measurement device, an α-ray spectrum sample production device, an α-ray spectrum measurement device, a mass spectrometry sample production device and a mass spectrometer, and the above-mentioned A uranium measurement and management system is provided, which is equipped with at least one type of SF6 gas replacement device for replacing the atmosphere in an airtight cell with SF gas.

Further, the present invention provides uranium measurement management that includes an airtight container in which a concave measurement hole is provided in a casing and that accommodates metal uranium, and a gamma ray detector disposed outside the airtight container including the inside of the measurement hole. a system, an airtight container having a concave measurement hole in the casing and housing metal uranium, a neutron moderator and a neutron absorber surrounding the airtight container, and an inside of the neutron moderator and neutron absorber. Provided is a uranium metrology and management system that includes a neutron detector embedded in a neutron detector and a signal counting circuit that counts the multiplicity of neutron detection signals emitted from the neutron detector.

The present invention further includes (1) a step of adding a tracer of a uranium isotope to a metallic uranium raw material to be separated by an atomic laser method, and (2) a step of adding a uranium isotope tracer to a raw material of metallic uranium to be separated by an atomic laser method; (3) measuring the concentration of tracer contained in each of the above steps (
A uranium measurement management method is also provided that includes a step of calculating the total amount of uranium in product uranium and waste uranium based on the tracer concentration determined in 2).

(Function) The uranium measurement management system related to the airtight cell for uranium separation and enrichment of the present invention first measures the total amount of uranium in metallic uranium using a neutron detector outside the airtight cell. Then, depending on the purpose of measurement, two measurements are taken using a gamma ray measuring device in the flow path following the airtight cell.
By measuring the amount of 35U present, the degree of enrichment can be determined in-line by combining it with the total amount of uranium measured by a neutron detector. Furthermore, if an α-ray sample measurement device and an α-ray spectrum measurement device are used, α-decay can occur without stopping the flow of product uranium and waste uranium in the flow path or breaking the airtightness.

Isotope ratios such as 235U and 238U can be quickly determined. Similarly, mass spectrometry of uranium isotopes can be performed using a mass spectrometry sample preparation device and a mass spectrometer. Furthermore, after the separation and enrichment work of uranium is completed using an airtight cell, if the atmosphere inside the airtight cell is replaced with SF6 gas using an SF6 gas replacement device, the (α, n) reaction will occur based on the fluorine of SF6. By measuring neutrons with a neutron detector outside the hermetic cell, the amount of alpha contamination in the hermetic cell can be determined.

Further, according to the uranium measurement management system for an airtight container containing uranium metal of the present invention, the amount of 238U can be determined by using a neutron detector or a gamma ray detector placed outside the airtight container without sampling to break the airtightness. The degree of concentration can be determined, and by arranging these detectors in the measurement holes, it is possible to calibrate deviations in detected values due to shape factors of the airtight container.

Furthermore, according to the uranium measurement control method using the tracer of the present invention, the total amount of uranium mixed in the supplied uranium raw material can be determined without sampling the raw material by measuring the tracer concentration in the product uranium and the waste uranium. You can ask for it.

(Example) The present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a schematic cross-sectional view of a uranium measurement and management system 1 according to a first embodiment of the present invention.

Solid metallic uranium as a raw material for producing enriched uranium by the atomic laser method is sent from a raw material supply cell 2 to a raw material supply device 4 in an airtight cell 3 connected thereto, and then to an airtight container via a raw material supply path 5. 3 to an evaporation crucible 6.

Two (one or more may be used) neutron detectors 7 are arranged outside the airtight container 3.

In the evaporation crucible 6, the solid metallic uranium receives an electron beam whose irradiation direction is adjusted by a deflection magnetic field from an electron gun (not shown), and is given thermal energy and melted. When the molten metallic uranium is further heated by an electron beam, it evaporates.

A laser beam with a wavelength tuned to the absorption level of U is emitted from a laser beam oscillator (not shown) to the vaporized metallic uranium. Then, only 235U of metallic uranium absorbs excitation energy and becomes ionized. Therefore, the ionized 235U is collected on the product collection plate 8 which also serves as an electrode, and the non-ionized component, which is mainly composed of 238U, is collected on the waste product collection plate 9 without being affected by the electrode.

The uranium components collected in the product recovery plate 8 and the waste recovery plate 9 in a gaseous state are then liquefied due to a decrease in temperature, and pass through channels 10a and 10b, respectively, to a product recovery liquid reservoir 11 and a waste recovery liquid reservoir (not shown). leading to. Finally, the product uranium is removed from the product removal cell 12 connected to the hermetic cell 3.

In this embodiment, on the outside of the airtight cell 3 through which the flow paths 10a and 10b pass, a gamma ray measuring device 13, an alpha ray spectrum sample manufacturing device 14, and a mass spectrometer A sample manufacturing device 15 is installed.

An α-ray spectrum measuring device and a mass spectrometer (not shown) are connected to the α-ray spectrum sample manufacturing device 14 and the mass spectrometry sample manufacturing device 15, respectively, while maintaining airtightness.

Now, the neutron detector 7 constantly measures neutrons with high substance permeability emitted from the uranium in the airtight cell 3 during implementation of the atomic laser method. Since the neutrons emitted from inside the airtight cell 3 are mainly due to spontaneous nuclear fission of 238U,
The counting rate of neutrons detected by the neutron detector 7 is proportional to the amount of 238U in the airtight cell 3. and airtight cell 3
Since most of the uranium inside is 238U, the total amount of uranium inside the airtight cell 3 can be approximately measured from this neutron count rate.

FIG. 2 is an enlarged sectional view of the gamma ray measuring device 13 shown in FIG. 1.

An intermediate liquid reservoir 16 is provided in the flow path 10a for the product uranium housed in the airtight cell 3, and the inner wall surrounding the flow path 10a of the airtight cell 3 is provided with liquid that is released from the molten metallic uranium flowing through the flow path 10a. A heat insulating material 17 is attached to block heat.

Then, on the outside of the airtight cell 3 around the intermediate liquid reservoir 16, corrinters 18a and 18b are respectively housed.
A gamma ray source 19 such as Cs and a gamma ray detector 20 are arranged facing each other. In this embodiment, these collimators 18a,
The 18bs gamma ray source 19 and the gamma ray detector 20 constitute the gamma ray measuring device 13.

In this gamma ray measuring device 13, the gamma ray detector 20 detects gamma from the molten uranium product temporarily retained in the intermediate liquid reservoir 16.
The amount of 235U in the intermediate reservoir 16 is measured by measuring the radiation spectrum and counting the 1 86 keV gamma rays caused by 235U.

On the other hand, the counting rate of γ-rays that pass through the molten metal uranium in the intermediate reservoir 16 from the γ-ray source 19 and reach the γ-ray detector 20 (the γ-ray energy to be counted depends on the type of the γ-ray source 19) is measured. Then, the liquid volume and density of uranium can be measured. Therefore, when combined with the previously determined amount of 235U in the intermediate reservoir 16, the enrichment in the intermediate reservoir 16 can be obtained, so the uranium enrichment can be determined in-line without sampling and while avoiding contact with the atmosphere. can be weighed.

On the other hand, in the α-ray spectrum sample manufacturing device 14 shown in FIG. 1, the product uranium and waste uranium flowing through the channels 9a and 9b are irradiated with a laser beam or an electron beam to evaporate a small amount of uranium, and the evaporated uranium is deposited on a target plate. Form a vapor deposited film. The formed vapor-deposited film is moved to an α-ray spectrum measuring device while keeping it airtight, and the α-ray spectrum is measured while keeping it airtight. Therefore, according to this embodiment, it is possible to measure the isotope ratio of uranium that emits α rays such as U and US U without exposing the metallic uranium to the atmosphere.

Furthermore, in the mass spectrometry sample manufacturing apparatus 15 shown in FIG. 1, a wire is immersed in, for example, product uranium and waste uranium flowing through channels 9a and 9b, and a small amount of uranium is attached to the wire and then pulled out. The uranium sample is then sent to a mass spectrometer without breaking the airtight seal for mass analysis. In this way, the abundance ratio of each isotope of uranium can be measured quickly and with high precision without the risk of exposure to air, while saving labor.

Furthermore, when SF6 gas is irradiated with α rays, (α, n
) It has the property of causing a reaction and generating neutrons. Therefore, separation and
After the concentration is finished and the crucible 6, product recovery liquid reservoir 11, etc. are removed, the airtight cell 3 is filled with SF6 gas using an SF gas replacement device (not shown). After that, if the neutron detector 7 placed outside the airtight cell 3 counts the neutrons emitted from the airtight cell 3, the amount of uranium present in the airtight cell 3 due to adhesion to the inner wall, that is, the amount of uranium present in the airtight cell 3, The amount of α contamination can be determined.

FIG. 3 is a perspective view of an airtight container 21 containing uranium metal according to a second embodiment of the uranium measurement and management system 1 of the present invention. This airtight container 21 has a concave measurement hole 22 at the bottom.
is formed.

Therefore, the airtight container 21 is surrounded by a neutron moderator and a neutron absorber (not shown), and a neutron detector is embedded within the neutron moderator and neutron absorber. In this way, neutrons emitted from metallic uranium in the airtight container 21 are moderately decelerated in the surroundings, fast neutrons are absorbed, and thermal neutrons are detected by a neutron detector.

Therefore, the total amount of 238U in the airtight container 21 can be measured without contacting with the atmosphere.

At this time, a signal counting circuit is connected to the neutron detector,
Number of neutrons detected almost simultaneously within a certain period of time (
By measuring the signal multiplicity), U/
The ratio of U, that is, the degree of enrichment, can also be determined.

However, it is affected by the shape of the airtight container 21 obtained in this way, the packing density of uranium, etc. Therefore, the measurement hole 22
The detected values of the neutron detector placed in the same way,
The above-mentioned measured values can be corrected using detected values obtained by inserting a calibration neutron source such as 252Cf into the measurement hole 22.

By the way, the half-life of 234Th, which is a daughter nuclide of U (half-life 4.51×109 years), is 24.1 days. Therefore, after being stored in the airtight container 21 for 2 to 3 months,
Radiation equilibrium is reached, and gamma rays are emitted in proportion to the amount of U. Since γ-rays are easy to measure even outside the airtight container 21, the degree of enrichment (235U/238U ratio) can be analyzed by arranging a γ-ray detector around the airtight container 21 and measuring the γ-ray spectrum. Even in this case, the γ
By comparing the values of the ray detector and the gamma ray detector placed outside the measurement hole 22, deviations due to the shape of the airtight container 21, etc. can be corrected.

Finally, the separation and separation of metallic uranium by the atomic laser method of the present invention
An embodiment of the uranium measurement control method related to the enrichment process will be described.

First, in the first step, uranium isotopes (U, U, U.

At least one type of U) is added as a tracer to raw metal uranium for a certain period of time, and then supplied to the uranium separation process.

Then, in the second step, the product uranium and waste uranium that have undergone the separation and concentration process are transferred to, for example, the flow path 1 shown in FIG.
0a, 10b, the product recovery liquid reservoir 11, the waste product recovery liquid reservoir, etc., and continuously measure the concentration of the added tracer.

The tracer concentration then changes over time depending on the amount of uranium and the mixing ratio of the main uranium isotopes (238U and 235U) in the separation and enrichment process.

In this case, when using 232U as a tracer,
Since U has a high α-specific radioactivity and the energy of α-rays is larger than that of other uranium isotopes, it can be measured by adding a very small amount using the α-ray spectrum sample production device 14 and the α-ray spectrum measuring device shown in Figure 1. can do.

Then, in the third step, if an appropriate calculation formula is used, the characteristics of the separation/concentration process, such as the total amount of molten metal uranium during the separation/concentration process and the mixing speed of the tracer, can be determined from the measured tracer concentration to continue the concentration/separation process. You can check it while you are at it.

Figure 4 shows the product uranium produced when the process was operated while adding 232U as a tracer to the uranium raw material at a constant concentration ρ0 from time t to t2 (time T) after the separation/concentration process was operated until it became steady. Figure 3 shows the time change of 232U concentration in The same thing applies to scrap uranium.
The time variation of 32U concentration is obtained.

The U concentration ρ in the product uranium is approximately ρ(, (1-erp(-λt)) (λ is the decay constant) after an initial delay from the addition of U to the exit of the product uranium. The added concentration ρ0 is asymptotically approached according to a curve a expressed by ρ. More precisely, ρ represents the isotopic ratio of 232tr 7238U.

If the total amount of raw uranium supplied during the tracer addition period T and the total amount of uranium taken out as products and scrap are equal (the amount supplied and the amount taken out are balanced),
The area of the rectangular part of TY x ρ0 in Figure 4 is SEo for products and waste products, respectively.

S. , and the area surrounded by curve a (including al, which is an extrapolated curve of curve a), straight line ρ, and the vertical axis of 1=11 is SEl, S81, and period TY for products and scraps, respectively.
2 in the product uranium and waste uranium taken out
If the amount of 38U is MEY2MDY, the period Ty
The total amount M1 of molten uranium inside is calculated by the formula M1=REφMEY・
(SE1/5Eo)+RD-MDY・ (SDl/5D
o). Here, RE and R8 indicate the ratio of total uranium amount/238U amount in the product and waste product, respectively.

By the way, after the tracer addition ends (time t2), the curve a□ that would be predicted if the addition continues is not actually measured, but if T is set to be appropriately long, ρ becomes R0 at time t.
, and the error in the estimated value of the area surrounded by curve a including curve a1, straight line ρ, and vertical axis t1 is extremely small (
Become.

The total amount of molten uranium during the separation and enrichment process was 23
232U is detected if it is extremely large compared to 2U.
The concentration ρ rises slowly like a curve.

In addition, in the atomic laser method, 232U has a different particle number from 235U, which is resonantly excited and ionized by laser light, and behaves the same as U, so it can also be used as a tracer to investigate the behavior of U. can.

〔Effect of the invention〕

As explained above, according to the uranium measurement management system related to the airtight cell for uranium separation and enrichment of the present invention, the enrichment level, isotope ratio, You can know the amount of α contamination in an airtight cell. Further, according to the uranium measurement management system for an airtight container containing uranium metal of the present invention, the amount of 238U and enrichment can be determined without sampling to break the airtightness, and the detected value can be determined by the shape factor of the airtight container. The deviation can also be calibrated. Further, according to the uranium measurement control method using the tracer of the present invention, the total amount of uranium mixed in the supplied uranium raw material can be determined without sampling the raw material. Therefore, quantitative management of uranium becomes possible based on these measured values.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a uranium metrology and management system according to a first embodiment of the present invention, FIG. 2 is a cross-sectional view of a gamma ray measuring device in this uranium metrology and management system, and FIG. 3 is a second embodiment of the present invention. FIG. 4 is a perspective view of an airtight container used in the uranium measurement management system according to the present invention, and FIG. 3... Airtight cell, 7... Neutron detector, 10a...
・Product uranium flow path, 10b... Waste uranium flow path, 13
... γ-ray measurement device, 14 ... α-ray spectrum sample manufacturing device, 15 ... mass spectrometer, 21 ... airtight container,
22...Measurement hole.

Claims (1)

[Scope of Claims] 1. A neutron detector is provided on the outside of an airtight cell for separation and enrichment of metallic uranium using an atomic laser method, and a flow for transporting enriched uranium separated from uranium raw materials and waste uranium in the airtight cell. α, a gamma ray measuring device installed around roads
Ray spectrum sample production equipment and α-ray spectrum measurement equipment,
and a mass spectrometry sample production device, a mass spectrometer, and an SF in which the atmosphere in the airtight cell is replaced with SF_6 gas.
_6 A uranium measurement management system comprising at least one type of gas replacement device. 2. A uranium measurement and management system having a concave measurement hole in a casing, an airtight container containing uranium metal, and a gamma ray detector disposed outside the airtight container including the inside of the measurement hole. 3. An airtight container in which a concave measurement hole is provided in the casing and contains uranium metal, a neutron moderator and a neutron absorber surrounding the airtight container, and a neutron moderator and a neutron absorber that contain A uranium measurement and management system that includes a buried neutron detector and a signal counting circuit that counts the multiplicity of neutron detection signals emitted from the neutron detector. 4. (1) Adding a uranium isotope tracer to the metallic uranium raw material separated by the atomic laser method, and (2) adding a uranium isotope tracer to the product uranium and waste uranium separated from the metallic uranium containing the tracer by the atomic laser method. A uranium measurement management method comprising: measuring the concentration of tracer contained; and (3) calculating the total amount of uranium in product uranium and waste uranium based on the tracer concentration determined in step (2).