JPH0519957B2 - - Google Patents

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 is also affected by the presence of fission product contaminants and variable 241 _An levels.

Many advantages can be obtained from the use of techniques based entirely on neutron counting. In particular, neutrons can be easily measured through plant stream containment, and the detectors are relatively rugged and inexpensive. Therefore, the object of the present invention is based on neutron counting,
Another object of the present invention is to provide a practical measuring device that can output all the information required for criticality control, plant control, and accounting.

According to information, the method for measuring the concentration of plutonium in the process stream during the plutonium production stage for the separation of plutonium, uranium, and fission products from irradiated nuclear fuel uses neutron coincidence counting to determine the "even" mass. The first step of measuring the intrinsic neutron emission due to spontaneous fission of the plutonium isotope, and hence the plutonium concentration, i.e. the equivalent mass per unit volume of ²⁴⁰ Pu, and the presence of an external neutron source leads to the fissile "odd" mass isotope. measuring the additional neutrons resulting from the resulting fission, hence the effective fissile concentration, i.e. the equivalent mass per unit volume of ²³⁹ Pu;
Using a conversion factor determined from the ratio of equivalent mass per unit volume of ²³⁹ Pu/equivalent mass per unit volume of ²⁴⁰ Pu, the equivalent mass per unit volume of ²⁴⁰ Pu or the equivalent mass per unit volume of ²³⁹ Pu is calculated as the overall plutonium concentration. and a second step of converting into.

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 counters arranged in the annular liquid container, a neutron source positioned at an axial position relative to the annular liquid container and arranged to be extracted from the axial position, an amplifier, and a scaler;
Contains simultaneous units and data processors,
and associated electronic circuitry for measuring and displaying neutron coincidence results.

The principle involved in this method is as follows.

As mentioned earlier, passive neutron emission from plutonium depends on its isotopic composition. Neutrons are released as a result of spontaneous fission of even mass isotopes or from (α,n) reactions with light elements such as oxygen. On average, two neutrons are released simultaneously per fission, while only one neutron is released per (α,n) reaction. This large number of neutrons from fission allows it to be distinguished from the (α,n) reaction by neutron coincidence, thus making the measurements more clear. Coincidence involves analyzing the output from multiple neutron detectors to obtain a "true" coincidence, i.e.
Establish the number of coincidences due to simultaneous prompt neutrons from nuclear fission.

During the “passive” neutron coincidence phase of monitoring, the “true” background-corrected coincidence rate (C _P ) is:
Isotopes in solution that undergo spontaneous fission, namely ²³⁸ Pu,
Represents the concentration of ²⁴⁰ Pu and ²⁴² Pu. The concentration of these isotopes is ²⁴⁰ Pu equivalent mass per unit volume, i.e.
It is expressed as the equivalent mass per unit volume of ²⁴⁰ Pu, where: Equivalent mass per unit volume of ²⁴⁰ Pu = ²⁴⁰ Pu + 1.57 ²⁴² Pu + 2.49 ²³⁸ Pu ²⁴⁰ Pu is the concentration of ²⁴⁰ Pu, ²⁴² Pu is the concentration of ²⁴² Pu, ²³⁸ Pu is the concentration of ²³⁸ Pu.

Factors 1.57 and 2.49 represent high spontaneous fission rates/gram of ²⁴² Pu and ²³⁸ Pu, respectively. Therefore, with appropriate calibration of the monitor, the equivalent mass per unit volume of ²⁴⁰ Pu can be determined by C _P. However, this alone does not allow us to know the entire plutonium concentration. This is because a fraction of the total plutonium concentration, ²⁴⁰ Pu as R ₂₄₀ , varies considerably over the history of the fuel.

The "active" stage of monitoring uses an external neutron source to interrogate 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 C _A .

The fissile concentration is expressed as the equivalent mass per unit volume of ²³⁹ Pu, that is, the equivalent mass per unit volume of 239 Pu ^. If there is no ²³⁵ U here, the equivalent mass per unit volume of ²³⁹ Pu = ²³⁹ Pu + ( ²⁴¹ F / ²³⁹ F) ²⁴¹ Pu, ²³⁹ Pu is the concentration of ²³⁹ Pu, ²⁴¹ Pu is the concentration of ²⁴¹ Pu, and ^io F is the fission cross section of isotope i for thermal neutrons.

Again, with proper calibration of the monitor, C _A
²³⁹ You can find the equivalent mass per unit volume of Pu. ²³⁹ The equivalent mass per unit volume of Pu is a required parameter for criticality control, which is usually written as the total plutonium concentration in the absence of information about isotopic composition. However, simply knowing the fissile concentration itself does not allow us to know the plutonium concentration as the equivalent mass per unit volume of ²³⁹ Pu. This is because a portion of the total plutonium concentration, namely R ₂₃₉ , varies significantly over the fuel's history.

Equivalent mass per unit volume of ²³⁹ Pu measured and
To determine the total plutonium concentration in the solution, Pu, assuming an equivalent mass per unit volume of ²⁴⁰ Pu,
_R239 must know _R240 . These could be determined assuming detailed knowledge of the isotopic composition, but this is of course not available. It is known that the equivalent mass per unit volume of ²³⁹ Pu is a function of odd mass isotopes, and ²⁴⁰ Pu is a function of even mass isotopes. ²³⁹ Pu equivalent mass per unit volume /
The ratio of equivalent mass per unit volume of ²⁴⁰ Pu can indicate the value of R ₂₃₉ (both are a function of irradiation): equivalent mass per unit volume of ²³⁹ Pu / equivalent mass per unit volume of ²⁴⁰ Pu to R ₂₃₉ By plotting the ratio of equivalent masses, the rating, initial concentration or
Variations in irradiation over 2000 MWE/te yield well-defined relationships for all fuel types without any particular differences. An equally well-defined relationship is known by plotting equivalent mass per unit volume of ²³⁹ Pu/equivalent mass per unit volume of ²⁴⁰ Pu against R ₂₄₀ .

Therefore, by the measured ratio ²³⁹ Pu equivalent mass per unit volume / ²⁴⁰ Pu equivalent mass per unit volume, the fissile concentration ²³⁹ Pu equivalent mass per unit volume (or
²⁴⁰ Pu equivalent mass per unit volume) can be converted to total plutonium concentration.

If a significant amount of U-235 is present, the total Pu
The above methods of determining concentration are 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 ²⁵² cf. I mean,
Neutron coincidence effectively distinguishes the source disordered neutrons, thus contributing to a high signal-to-background ratio. The source neutrons must also be of low average energy to ensure rapid thermalization within the solution to be measured. The 241Am/Li 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. Because,
This is because any neutrons released will enter the solution. In practice, the closest approach to this situation holds when the source is surrounded by an annular volume of plant liquid. By attaching the counter to an annular ring surrounding the solution container, the solution itself can be used to shield the counter from interrogating source neutrons.

An example of an apparatus for applying the method of the invention is shown in FIGS. 1 and 2. Inlet 3 and outlet 3'
An annular cylindrical solution container 1 with a side wall 2 is provided with its axis horizontal to facilitate access to the neutron source and detector. The bottom of the container 1 is slightly sloped towards the inlet 3 to prevent solids from accumulating during operation. The interrogating neutron source 4 and the detector 5 are accessible via tubes 6 and 7, respectively, which pass through the wall 2. When in the retracted position, the neutron source is mounted on a ram (not shown) with a relatively long stroke or on a teleflex cable 8 to ensure effective extraction from the vicinity of the solution container 1. There is. To minimize the influence of the neutron source on the background of the detector during passive counting, the neutron source can be drawn into a small shield 9. The danger of radiation sources associated with sources of constant strength is very small. The neutron counters are of the BF ₃ type or even of the ³ He type, with a typical number of 10 or 12 (not shown) arranged symmetrically around the vessel 1. be. When employing ³ He counters, it is possible to obtain the same accuracy or sensitivity with fewer counters than when using BF ₃ 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 by varying the concentration. This is most conveniently done off-line using a calibration vessel of equal design 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 interrogation source 4 present. When calibrating the "passive" count rate, use the background without a source, i.e., C _P ^B , and when determining the net "active" count rate, use the background with a source in place, i.e., C _a ^B. do.

Next, the source 4 is retracted into the shield 9 for passive counting.
Take C _P. Using the net passive counts, C _P ^N and established calibration data, determine the equivalent mass per unit volume of ²⁴⁰ Pu.

That is, C _P ^N = C _P - C _P ^B ... ²⁴⁰ Equivalent mass per unit volume of Pu Next, the interrogation neutron source 4 is placed at a predetermined position as shown in Figure 1, and the active count, C _A take. Determine the equivalent mass per unit volume of ²³⁹ Pu using the net active count C _A ^N and the calibration data.

That is, C _A ^N = C _A −C _a ^B −C _P ^N ... ²³⁹ Pu equivalent mass per unit volume Next, calculate the equivalent mass per ²³⁹ Pu unit volume / ²⁴⁰ Pu equivalent mass per unit volume. You can learn about R ₂₃₉ . In conjunction with the determination of the equivalent mass per unit volume of ²³⁹ Pu, R ₂₃₉ gives the total plutonium concentration.

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 calculate R ₂₃₉ and
²³⁹ Convert the measured value of equivalent mass per unit volume of Pu to plutonium concentration. The device can be programmed to take passive measurements on demand or at predetermined intervals in connection with batch feeding.

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 an apparatus 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)

[Claims] 1. A method for measuring the concentration of plutonium in a process stream during the plutonium production stage in the separation of plutonium, uranium and fission products from irradiated nuclear fuel, using neutron coincidence counting for "even" mass plutonium isotopes. The first step is to measure the intrinsic neutron emission from spontaneous fission and hence the effective concentration of this plutonium isotope, i.e. the equivalent mass per unit volume of ²⁴⁰ Pu, and the presence of an external neutron source reduces the fissionable "odd" mass. Additional neutrons resulting from the multiplication of splitting in the plutonium isotope,
Therefore, the effective multiplicity concentration of this plutonium isotope, i.e. the equivalent mass per unit volume of ²³⁹ Pu, is determined,
Next, using the conversion factor determined from the ratio of equivalent mass per unit volume of ²³⁹ Pu/equivalent mass per unit volume of ²⁴⁰ Pu, the equivalent mass per unit volume of ²⁴⁰ Pu or the equivalent mass per unit volume of ²³⁹ Pu is converted to the total plutonium. A method for measuring plutonium concentration, comprising a second step of converting into concentration.