JPH0453398B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a method for non-destructive measurement of spent fuel for evaluating the nuclear properties of spent fuel by non-destructive measurement.

[Technical background of the invention and its problems] Generally, spent fuel taken out from a nuclear reactor is stored in a fuel storage pool for a certain period of time, and after its radioactivity, which has a short half-life, decays, it is stored in a transport container. , transported to reprocessing plants or long-term storage facilities.

When such spent fuel is transported from a nuclear power plant, data such as initial enrichment Ei and burnup BU are handed over to the spent fuel receiving side, such as a reprocessing plant or long-term storage facility.

Shippers of spent fuel from nuclear power plants, etc., carry out the removal work to be as error-free as possible, but those who receive the spent fuel must ensure that subcriticality is maintained throughout the storage and processing processes. It is necessary to carry out independent measurements of the initial enrichment Ei, burnup BU, etc. of the completed fuel and reconfirm that there are no errors.

Gamma ray spectrum measurement, passive neutron measurement, active neutron measurement, and the like are conventionally known as non-destructive measurement methods for burnup, BU, etc. of spent fuel.

The conventionally known gamma ray spectrum measurement method measures the gamma rays emitted from the fission product FP accumulated in the spent fuel, and uses a certain correlation to determine the combustion rate based on the count rate or count rate ratio of the gamma ray of interest. This is a method to derive the degree BU, the total plutonium to total uranium concentration ratio Pu/U, and the cooling period Tc.

Examples of certain correlations include the following:

Correlation between ¹³⁷ Cs gamma ray count rate and burnup BU.

Correlation between ¹³⁴ Cs/ ¹³⁷ Cs gamma ray count ratio and burnup BU or Pu/U concentration ratio.

Correlation between ¹⁵⁴ Eu/ ¹³⁷ Cs gamma ray count ratio and burnup BU or Pu/U concentration ratio.

Correlation between ¹⁴⁴ Pr/ ¹³⁷ Cs gamma ray count ratio and cooling period Tc.

However, conventionally, the derivation of these quantities is
Each is performed almost independently, and there is a problem in that when deriving each, it is necessary to provide data in advance for some quantities as known values.

For example, deriving the burnup BU from the correlation of or requires a cooling period Tc, and deriving the burnup BU from the correlation of or requires the cooling period Tc, initial enrichment Ei, and irradiation history. be. Furthermore, to derive the Pu/U concentration ratio from the correlation of or, a cooling period Tc is required,
Information on the burnup BU is required to derive the cooling period Tc from the correlation.

Therefore, the conventional method for non-destructive measurement of spent fuel using gamma ray spectroscopy requires various data, and there is a problem in that non-destructive measurement of spent fuel cannot be easily performed.

[Object of the Invention] The present invention has been made in response to the above-mentioned conventional situation, and the measured value can be calculated only by non-destructive gamma ray measurement without requiring any special information about the spent fuel other than the fuel type in advance. burnup based on
The present invention aims to provide a non-destructive measurement method for spent fuel that can easily and quickly determine nuclear characteristic quantities such as BU, initial enrichment Ei, U concentration, Pu concentration, and fissile nuclide concentration Fiss.

[Summary of the Invention] That is, the present invention provides a method for measuring ¹³⁷ Cs, ¹³⁴ Cs or ¹⁵⁴ Eu, ¹⁴⁴ Pr by gamma ray spectroscopy, which measures the spectrum of gamma rays emitted from spent fuel.
Measure the gamma ray count rate of
This is a method for non-destructive measurement of spent fuel, which is characterized by determining the Pu/U concentration ratio, U concentration, Pu concentration, fissile nuclide concentration Fiss, and cooling period Tc.

[Embodiment of the Invention] In this embodiment, information regarding the spent fuel to be measured includes boiling water reactor or pressurized water reactor, number of fuel rods, fuel pellet diameter,
Assume that only the fuel type, such as the fuel rod pitch, is known.

FIG. 1 is a flowchart showing an embodiment of the method for non-destructive measurement of spent fuel according to the present invention.

As shown in the figure, in this example, gamma rays emitted from fissile nuclides ( ¹³⁷ Cs, ¹³⁴ Cs or ¹⁵⁴ Eu, ¹⁴⁴ Pr) accumulated in spent fuel are measured, and these photopeak count rates or The burnup BU, cooling period Tc, initial enrichment Ei, Pu/U concentration ratio, U concentration, Pu concentration, and fissile nuclide concentration Fiss are derived from the measured values of the count rate ratio.

This derivation procedure will be explained in detail below using FIG.

First, using the correlation between ^{the 137} Cs gamma ray count rate (at the end of irradiation) and the burnup BU shown in Figure 2, the burnup approximate value BU ₁ is determined from the ¹³⁷ Cs gamma ray count rate obtained by measurement. . Here, the correlation shown in Figure 2 has been calibrated in advance using destructive measurements of burnup BU, non-destructive measurements using other methods such as passive neutron measurement, or a method using a standard ¹³⁷ Cs gamma ray source. There is. The relationship shown in FIG. 2 depends on the fuel type, but is almost independent of the initial enrichment Ei, irradiation history, etc.

Using the correlation between ^{the 144} Pr/ ¹³⁷ Cs gamma ray count ratio and the cooling period Tc shown in FIG. 3, the cooling period Tc is determined from the gamma ray count ratio obtained by measurement and the burnup approximate value BU ₁ determined by. In addition,
The correlation shown in FIG. 3 is a relationship that depends on the burnup BU.

^{The 137} Cs gamma ray count rate at the end of irradiation is determined by taking into account the cooling period Tc determined in
BU is required.

¹³⁴ Cs/ ¹³⁷ Cs or ¹⁵⁴ Eu/ obtained by measurement
Cooling period determined from ¹³⁷ Cs gamma ray count ratio
Using Tc, the value at the end of this irradiation is determined.
After this, these count ratios and burnup shown in Figure 4 are calculated.
The initial enrichment Ei is determined by considering the dependence of the correlation with BU on the initial enrichment Ei. Note that here, the value determined as burnup BU is used. In addition, the correlation shown in Figure 4 is the initial enrichment level.
The dependence on Ei is very large.
From the gamma ray count ratio at the end of irradiation of ¹³⁴ Cs/ ¹³⁷ Cs or ¹⁵⁴ Eu/ ¹³⁷ Cs, the difference between these and
The Pu/U concentration ratio is determined using the correlation with the Pu/U concentration ratio.

In addition, the initial uranium amount determined by the fuel type
Using the burnup BU determined from _U0 , the amount of uranium after irradiation U is determined from the correlation between the amount of uranium after irradiation and the burnup BU shown in FIG. 6, and the amount of Pu is derived. Note that the correlation shown in FIG. 5 has relatively little dependence on the initial concentration Ei, post-irradiation history, etc.

Considering the dependence on the initial enrichment Ei in the correlation between the ¹³⁷ Cs gamma ray count rate and the fissile nuclide concentration Fiss shown in Figure 7, ^{the 137} Cs gamma ray count rate at the end of irradiation ( ¹³⁷ Cs) was determined in the process of The fissile nuclide concentration Fiss is determined using the value of the initial enrichment Ei determined with ₀ . Note that the correlation shown in FIG. 7 has a large dependence on the initial enrichment degree Ei.

As described above, nuclear characteristic quantities such as burnup BU, initial enrichment Ei, Pu concentration, U concentration, and fissile nuclide concentration Fiss can be measured by non-destructive gamma ray measurements without requiring special information other than fuel type in advance. can be determined easily and reliably.

Furthermore, by storing the correlations shown in FIGS. 2 to 7 in advance in the data processing device, it becomes possible to derive the results online at the same time as gamma ray spectrum measurement is completed.

Note that the present invention can be applied to fuels other than boiling water reactor fuels or pressurized water reactor fuels, and can also be applied to spent fuels other than fuel assemblies or fuel rods. can.

[Effects of the Invention] As described above, according to the method for non-destructive measurement of spent fuel of the present invention, if only the fuel type is known, gamma ray spectrum measurement can be performed without requiring any other special information data. Burnup only by law
Nuclear properties such as BU, initial enrichment Ei, Pu concentration, U concentration, and fissile nuclide concentration Fiss can be easily and reliably determined.

This makes it possible to derive nuclear properties of spent fuel online.

[Brief explanation of the drawing]

FIG. 1 is a flowchart showing one embodiment of the method for non-destructive measurement of spent fuel according to the present invention, and FIGS. 2 to 7 are graphs showing various correlations used in the present invention.

Claims (1)

[Scope of Claims] 1 (a) A gamma ray spectrometry method that measures the spectrum of gamma rays emitted from spent fuel can produce ¹³⁷ Cs, ¹³⁴ Cs (or ¹⁵⁴ Eu) and
Steps to measure each gamma ray count rate of ¹⁴⁴ Pr, and (b) ¹³⁷ Cs gamma ray count rate (at the end of irradiation) determined in advance from the measured value of ¹³⁷ Cs gamma ray count rate.
and (c) calculating the approximate burnup value Bui obtained in step (b) above using the correlation between
From the measured values of ¹⁴⁴ Pr and ¹³⁷ Cs gamma ray count rates, the ` <sup>`144</sup> Pr/ ¹³⁷ Cs gamma ray count ratio,
(d) From the cooling period Tc obtained in step (c) and the measured value of the gamma ray count rate of ¹³⁷ Cs, the irradiation at the end
The gamma ray count rate ^{of 137} Cs is determined, and from this ¹³⁷ Cs gamma ray count rate at the end of irradiation, the above-mentioned combustion (e) calculating the measured values of each gamma ray count rate of ¹³⁷ Cs, ¹³⁴ Cs (or ¹⁵⁴ Eu), and ¹³⁷ Cs and the cooling obtained in step (b) above; From the period Tc, calculate the gamma ray count rates of ¹³⁷ Cs, ¹³⁴ Cs (or ¹⁵⁴ Eu), and ¹³⁷ Cs at the end of irradiation, and combine these gamma ray count rates at the end of irradiation with the predetermined ¹³⁴ Cs/ ¹³⁷ Cs. or (f) obtaining the initial enrichment Ei using the initial enrichment Ei dependence in the correlation between each gamma ray count rate ratio of ¹⁵⁴ Eu/ ¹³⁷ Cs and the burnup BU; ¹³⁷ Cs at the end of irradiation,
From each gamma ray count rate of ¹³⁴ Cs ( ^{or 154 Eu} ) and ¹³⁷ Cs, the predetermined
¹⁵⁴ Eu/ ¹³⁷ Cs gamma ray count rate ratio and Pu/U concentration ratio. (g) Initial uranium amount U ₀ determined by fuel type and step ( (g) Step (d) of calculating the Pu concentration and U concentration from the burnup BU obtained in step (d) using the previously determined "correlation between the amount of uranium after irradiation and the burnup Bu"; From the gamma ray count rate of ¹³⁷ Cs obtained at the end of irradiation and the value of the initial enrichment Ei obtained in step (e), the "correlation between each gamma ray count rate of ¹³⁷ Cs and the fissile nuclide concentration Fiss" was determined in advance. A method for non-destructive measurement of spent fuel, comprising: a step of determining the fissile nuclide concentration Fiss using .