JPS60260876A - Method and device for measuring concentration of plutonium in process liquid flow - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the reprocessing of irradiated nuclear fuel to separate uranium, plutonium and fission products from the irradiated nuclear fuel. For accounting and technical purposes, it is necessary to ascertain the plutonium concentration in the various process streams of the reprocessing process. The present invention relates to a method and apparatus for measuring plutonium concentrations in such process streams.

It is possible to monitor plutonium in liquid streams by simple passive neutron counting. However, it is known that the total neutron emission from plutonium strongly depends on its isotopic composition. Therefore, the plutonium concentration cannot be determined unless the isotopic composition is known.

Although high-resolution gamma spectroscopy is widely used to determine isotopic composition, this method requires sophisticated equipment that is difficult to accommodate and expensive due to space limitations. This method also treats fission product contaminants and
Affected by the presence of A-levels.

There are many advantages to be gained by using the technique exclusively for neutron counting. In particular, neutrons can be easily measured through plant stream containment, and the detectors are relatively rugged and inexpensive. Therefore, it is an object of the present invention to provide a practical measuring device based on neutron counting and capable of producing all the information required for criticality control, plant control, and accounting.

According to the information, the method for measuring the concentration of plutonium in the process stream during the plutonium production stage in separating plutonium, uranium and fission products from irradiated nuclear fuel is to use mi-neutron coincidence counting to determine the "even" mass. A first step of measuring the intrinsic neutron emission due to spontaneous fission of plutonium isotopes, hence the plutonium concentration, i.e. the equivalent mass per unit volume of 240 pu, and the fission produced in fissile "odd" mass isotopes due to the presence of an external neutron source. The additional neutrons produced by the
The equivalent mass per unit volume of 39pu is measured, and tm9pu
Equivalent mass per unit volume / * 4 (l p u Equivalent mass per unit volume using the conversion factor determined from the ratio of equivalent mass per unit volume "C"'Pu equivalent mass per unit volume t39p,, equivalent mass per unit volume and a second step of converting the plutonium concentration into the total plutonium concentration.

Apparatus for carrying out this method comprises an annular liquid container used in series with a separation plant or used on a representative sample from the separation plant and having an inlet and an outlet for said process liquid flow, and around said annular liquid container. a plurality of neutron counter tables arranged in a front annular liquid container, a neutron source positioned at an axial position relative to the front annular liquid container and arranged to be extracted from the axial position, an amplifier, and a scaler;
It includes a coincidence unit and a data processor and has associated electronic circuitry for measuring and displaying the results of neutron coincidence counting.

The principle involved in this method is as follows.

As mentioned earlier, passive neutron emission from plutonium depends on its isotopic composition. Neutrons are the result of spontaneous fission of even mass isotopes or of light elements such as oxygen (
α, n) Neutrons are released from the reaction. Each nuclear fission flat,
An average of 2.2 neutrons are emitted simultaneously, while (α1.n
) Only one neutron is released per reaction. This large number of neutrons from fission is reduced by neutron coincidence (α,
n) allows to distinguish it from the reaction, thus making the measured value more clear. Coincidence involves analyzing the output from multiple neutron detectors to establish a "true" coincidence, ie, the number of coincidences due to simultaneous prompt neutrons from nuclear fission.

During the ``passive'' neutron coincidence phase of monitoring, the ``true'' coincidence rate corrected for bank ground, isotopes in solution undergoing spontaneous fission, ie! 38pu, t40Pu and t4Npu concentrations are represented. The concentration of these isotopes is z
It is expressed as the mass of alcohol per unit volume of aopu, that is, the mass of alcohol per unit volume of 240 Pu, where: 84 (mass of alcohol per unit volume of lpu = zao p u + l, 57 24"P u +2.
49''P ut46pu is the concentration of 210pu, t4Zpu is the concentration of !4!pu, and t311Pu is the concentration of 2311pu.

Factors 1.57 and 2.49 are t4Zpu and 2, respectively.
It exhibits a high spontaneous fission rate of 311 pu/gram.

Therefore, with proper calibration of the monitor, *40p by CP
The amount of alcohol per unit volume can be determined. However, this alone cannot tell you the entire plutonium concentration. That is, part of the total plutonium concentration, Rta. This is because the final 24+lpu changes considerably depending on the history of the fuel.

The "active" phase of monitoring uses an external neutron source to pose the solution with thermal neutrons, causing any fissile isotopes present to undergo fission. The simultaneous neutrons released during nuclear fission increase the "true" coincidence compared to the coincidence in passive measurements by an amount proportional to the amount of fissile material present, ie CA.

The fissile concentration is expressed as the equivalent mass per unit volume of 23''Pu, that is, the equivalent amount per unit volume of 2J9pu, and if there is no ffi3s U here, 23qpu
Mass of alcohol per unit volume = 239 p u + ( 241 F / !39 F) 2
41 p u1239pu is the concentration of 239pu, 241pu is the concentration of Z41pu, and ioF is the fission cross section of isotope i for thermal neutrons.

Again, with proper calibration of the monitor, by CA! 39pu
You can know the amount of alcohol per unit volume. ! 39p
Although the amount of alcohol per unit volume is a required parameter for criticality control, the criticality limit is usually written as the total plutonium concentration in the absence of information about the isotopic composition. However, just knowing the fissile concentration itself is not sufficient.
It is not possible to know the plutonium concentration as the equivalent mass per unit volume of 239 pu. This is because a fraction of the total plutonium concentration, namely R2, varies significantly over fuel history.

The amount of alcohol per unit volume of 239 pu measured and 24
Assuming an equal amount of plutonium per unit volume of 0 pu, in order to determine the total plutonium concentration Pu in the solution, R13, is R1. . must know.

These could be determined assuming detailed knowledge of the isotopic composition, but this is of course not available. 239p
It is known that the amount of alcohol per unit volume is a function of odd mass isotopes, and t40pu is a function of even mass isotopes. 239pu Equilibrium mass per unit volume / 2
4 0 p u The ratio of equal alcohol mass per unit volume is Rz3
w'O values can be indicated (both are a function of irradiation). R23, by plotting the ratio of 2311 pu mass per unit volume / t 40 p 11 isovolume mass per unit volume for rated, initial concentration or irradiation of 2000 MWE/t e or more. A well-defined relationship is obtained for all fuel types without any particular variation. Rta.

For Z19pu, equivalent mass per unit volume / 240
An equally well-defined relationship is known by blowing the equal volume of alcohol per unit volume of pu.

Therefore, the measured ratio Zff9. pu mass per unit volume / 2 411 pu Convert the fissionable concentration 239 pu mass per unit volume (or t411 pu 411 pu single mass) to total plutonium concentration . be able to.

If U-235 is present in significant amounts, the above method of determining the total Pu concentration is not valid.

Therefore, this method is employed for process streams involved in post-U separation operations.

Isotopic sources with long half-lives are the optimal means of generating interrogating neutron flux for active operation. Such isotope sources require no maintenance, are physically very small, and allow efficient interrogation of solutions. Sources that emit disordered neutrons also have advantages over fission sources such as Contributes to ground ratio.

The source neutrons must also be of low average energy to ensure rapid thermalization within the solution to be measured. The 241 Am/L i source appears to be the most suitable. It has a half-life of 433 years and emits disordered neutrons with an average energy of 500 KeV.

The optimal configuration for the question is obviously to have the source completely surrounded by solution. This is because every neutron released goes into solution.

In practice, the closest clue to this situation holds when the source is surrounded by an annular volume of plant liquid. By attaching the counter to the annular ring surrounding the solution container,
An example of an apparatus for applying the method of the invention is shown in FIGS. 1 and 2. Annular with 3 inlets and 3 outlets
A cylindrical solution container l is provided with its axis horizontal to facilitate access to the neutron source and detector through the side wall 2. The bottom of the container 1 is slightly sloped towards the inlet 3 to prevent solids from accumulating during operation. Interrogation neutron source 4
and the detector 5 are each accessible via a tube 6.7 passing through the wall 2. When the neutron source is in the retracted position, it is attached to a ram (not shown) with a relatively long stroke or to a teleflex cable 8 to ensure effective extraction from the vicinity of the solution container l. .

To minimize the influence of the neutron source on the bank ground of the passive counting width detector, the neutron source can be drawn into a small tl, 6-body 9. The danger of radiation sources associated with sources of constant strength is very small. Neutron counter is BF3
type or 3He type, the typical number is 10 symmetrically arranged around the container 1.
or 12 (not shown). ') When employing le counters, it is possible to obtain the same accuracy or sensitivity with fewer counters than when using B F s counters. Optimal sensitivity is obtained by completely surrounding the counter with a polyethylene reflector/shield (not shown).

Calibration of the device is performed using plutonium solutions of known isotopic composition and varying concentrations. This is most conveniently done off-line using a calibration vessel of design equal to that of the monitor.

The operation steps are as follows. Prior to initial operation, at regular intervals, vessel 1 is filled with clean (plutonium-free) acid to establish true background levels with and without interrogator R4. When calibrating a "passive" count rate, the unsourced bank ground, i.e., CPI
When using I to determine the net "active" count rate,
Use the background or C- with the source in place.

The source 4 is then retracted into the shield 9 and the passive count CP is taken. Determine the equivalent volume of alcohol per 240 pu unit volume using the net passive counts, (, , N and the established calibration data.

That is, CPN=Cp Cp8 ・' ・ ”' Equivalent amount of alcohol per unit volume of Pu Next, place the interrogation neutron source 4 at a predetermined position as shown in Fig. 1 and take the active count, CA.Net active count Determine the equivalent amount of alcohol per unit volume of 239 pu using CAN and calibration data.

That is, CAN=CA-C)-CPN...239Pu
Equivalent mass per unit volume Next, 239Pu Equivalent mass per unit volume / 2: Opu
R239 can be determined by calculating the amount of alcohol per unit volume. 239 pu In conjunction with the determination of the isodium mass per unit volume, R23 gives the total plutonium depth.

Under normal operating conditions, the isotopic composition of plutonium does not change rapidly and there is probably no need to alternate between passive and active measurements. Repeat the active counts to monitor the fissile concentration and use the previously determined passive counts to
2,l, and convert the measured equivalent mass per unit volume of 239 pu to a plutonium concentration. The device can be programmed to perform passive measurements on demand or at predetermined intervals in connection with batch-type dispensing.

The electronic circuitry for neutron coincidence counting, including amplifiers, scalers, coincidence units and data processors, is conventional and known to those skilled in the art.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a device for measuring plutonium concentration in a process liquid stream according to the present invention, and FIG. 2 is a front view thereof. 1... Annular liquid container, 4... Neutron source, 5... Neutron counter.

Claims (1)

[Scope of claims] The inherent neutron emission from spontaneous fission of an even-mass plutonium isotope, hence the effective concentration of this plutonium isotope, i.e. 240 pu
The first step is to measure the equivalent mass per unit volume and the additional neutrons resulting from the fission produced in the fissile "odd" mass plutonium isotope by the presence of an external neutron source, and hence the effective fissile concentration of this plutonium isotope. That is, 2
Measure the equivalent mass per unit volume of 39 pu, then 21
Using the conversion factor determined from the ratio of 9 pu equivalent mass per unit volume / Z 40 pu equivalent mass per unit volume, 240 pu equivalent mass per unit volume or 2:19
A method for measuring plutonium concentration, comprising a second step of converting the equivalent mass per unit volume of pu to total plutonium concentration. (2) an annular liquid container having an inlet and an outlet for liquid from the process liquid stream; a plurality of neutron counters arranged about the annular liquid container; and an axial position relative to the annular liquid container. a monotron source positioned and arranged to be extracted from the axial position, an amplifier, a scaler, a simultaneous unit and a data processing sensor;
Device for measuring plutonium concentration, consisting of associated electronic circuitry for measuring and displaying the results of neutron coincidence counting. (3) The plutonium according to claim (2), wherein the inlet and outlet are connected in series with a plutonium production stage of a plant for separating plutonium, uranium and fission products from irradiated nuclear fuel. Concentration measuring device. (4) The inlet and outlet are used for a representative sample from the plutonium production stage of a plant for separating plutonium, uranium and fission products from irradiated nuclear fuel. plutonium s- measuring device. (5) The plutonium concentration measuring device according to claim (2), wherein the neutron counter is of the BF3 type. (6) The plutonium concentration measuring device according to claim (2), wherein the neutron counter is of the Co-He type. (7) the counter is completely surrounded by a reflector/shield;
A plutonium concentration measuring device according to claim (5) or (6). (8) The plutonium concentration measuring device according to any one of claims (2) to (8), wherein the neutron source is an americium-241/lithium source.