SU1681338A1 - A method of assessing fuel isotope composition in nuclear reactor core - Google Patents

Abstract

The invention relates to nuclear power engineering and can be used to determine the isotopic composition of the fuel in the cores of the nuclear reactor. The aim of the invention is to increase safety and efficiency, as well as to ensure the possibility of remote determination of the isotopic composition of the fuel during the operation of the reactor. The method consists in measuring the spectral energy characteristics of nuclear fission products and determining the burnout of 235U and accumulation of 239Pi. The energy spectrum of positrons from the antineutrino reverse / decay reaction on protons of a hydrogen-containing target in the energy range 1.2–7.2 MeV is measured, the positron energy spectra from the antineutrino reverse / decay reaction on protons from isotope fission of 23 U, 239 2323 are determined 241 Hz and the fractions of 235U, 239Pu divisions are calculated from them. 238U, 241Pi. 3 il., Table.

Description

The invention relates to nuclear power engineering and can be used to determine the isotopic composition of the fuel in the cores of nuclear reactors.

To determine the burnout of some components and the accumulation of others in nuclear reactors, 3 known methods are used.

The calculation method is based on modeling the neutron fields in the core of the nuclear reactor, taking into account the initial enrichment of fuel in each heat release assembly and the concentration of moderator in the core.

The disadvantages of the method are low accuracy and unreliability of estimates.

Known radiochemical methods for analyzing the composition of spent fuel.

Such an analysis is fairly accurate, but it is used only 2 to 3 years after the fuel has been removed from nuclear installations. It is not contactless — it requires the destruction of the fuel assembly, which is related to the radiation safety of the operating personnel.

The closest to the intended is a y-spectrometric method for analyzing spent fuel in fuel assemblies.

This method is non-destructive, however, it can be used only after 2-3 years after the end of the fuel cycle, laborious, requires special hot chambers and the corresponding equipment.

The γ-spectrometry method does not allow promptly, directly during the work, to measure the burnup U and

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239Pi accumulation. This method is associated with radiation protection and dosimetry monitoring.

The aim of the invention is to improve the safety and efficiency of determining the isotopic composition of the fuel in the core of a nuclear reactor and to enable remote determination of the isotopic composition during the operation of the reactor.

The goal is achieved by measuring the energy spectrum of positrons from the antineutrino reverse-decay reaction on protons of a hydrogen-containing target in the energy range of 1.2–7.2 MeV, determining the energy spectra of positrons from the reverse / 3-decay reaction of antineutrinos on trotons from dividing U isotopes

239 238 at 241 rts and nQ they calculate the shares

divisions 23eU, 239Pu, 238U, 241Pu.

In the course of operation of the nuclear reactor, the composition of the nuclear fuel (contributions in terms of the number of fissions due to the burning out of some U, U isotopes) and the accumulation of the other 239Pi changes. Consequently, the resulting, total antineutrino spectrum during the operation of the nuclear facility also changes.

Antineutrino radiation recording and energy spectrum analysis are performed using an antineutrino spectrometer.

The main element of the antineutrino spectrometer is a liquid or plastic organic scintillator. Hydrogen. part of the scintillator, is the target of the reverse / 3-decay reaction

ve + + e +, (1)

where Ve is the electron antineutrino;

n is the neutron;

p is a proton;

e + is a positron whose spectrum is measured in a spectrometer.

The ve identification is performed by registering the products of the reaction (1) - positron and neutron. The positron energy spectrum is measured.

Fig. 1 is a diagram illustrating an implementation of a neutrino method for measuring the isotopic composition of a nuclear plant fuel; figure 2 - the differential energy spectra of the positrons of the reaction (1), calculated by the energy spectra of antineutrinos from delo fragments / C O IQO TA

nor / j u, pu and u; on fig.Z - ratio

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spectra for U and Pu isotopes.

A nuclear installation that implements the proposed method contains an active zone 1 with biological protection 2, At some

The spectrometer 3 of an antineutrino with a hydrogen-containing target is located at a distance from the active zone. The antineutrino spectrometer contains analog electronic

circuit 4 with signal amplification paths and power supply units connected to the analyzing electronics 5, which includes a CAMAC system (amplitude and time sampling of information) and a mini-computer (information analysis and accumulation).

The method is implemented as follows. The energy spectrum of the positrons of reaction (1) is measured (the energy spectrum of positrons is obtained from the difference

energy emission spectra measured with the nuclear plant operating and stopped to eliminate the background).

The spectrum of positrons produced in reaction (1) in a hydrogen-containing target,

is uniquely associated with the spectrum of ve, since the kinetic energy of the positron Ee + (MeV) is determined by the energy of the incident antineutrino

E “(MeV) Eve (MeV) -1.8 MeV, (2)

where 1.8 MeV-threshold of reaction (1).

The energy spectrum of the antineutrino ve from the nuclear facility is formed by the J-decay of the fission fragments of four isotopes: 239Pu, 238U and 241Pi. Therefore, the total positron spectrum of reaction (1) in a hydrogen-containing target can be written as

five

Se + (Ee +) ff5S5 (Ee +) + a9Sg

(Ee +) + a8S8 (Ee +) + CЈlSl (Ee +)

(3)

where asagfcaa i are the shares of divisions, respectively, U. ®Ри. 238U 0 positron energy;

SsCEe 1 S9 (Јe +) S8 (Ee + Sl (Ee +)

the energy spectra of the positrons of the reaction (1) in a hydrogen-containing target from

fission fragments, respectively

five ,

235,

U,

235,

0

five

The division ratio of each isotope U, 239ri star and 24ipu is superimposed

normalization.

"5+" 9 + "8 +" 1 1 (4)

The differential energy spectra of positrons of reverse ft-decay (1) shown in Fig. 2, calculated from the energy spectra of antineutrinos from fission fragments 235U (1), 239Pu (2), and 238U (3) (the spectrum from 24tPu is not shown, since its contribution to the division process is insignificant), differ greatly from each other (2.0-2.5 times).

A standard is used for the analysis.

X2 method:

/

. S. (Ј. +) -Ј a, S, - (E. +) J.

 , t

, &,;

where S e (E e) is the value of the analyzed spectrum at the Jth point on the positron energy scale of reaction (1) in a hydrogen-containing target;

j 1st x of the considered points on the energy scale (arbitrary);

SP804 (Е в + J) - values of the spectra of positrons from the fission fragments of the 1st isotope at J-x points on the energy scale;

I - 5.9.8,1 - isotope designations, respectively 235U, 239Pu, 238U and 241Pi;

cn - contributions of the 1st isotope by the number of divisions;

Oj is the experimental error of the spectrum at the jth point.

The essence of the X2 method is to find by varying the parameters corresponding to the minimum value of Hnin2.

The found values of sc min are sought.

The energy range Eve (3-9) MeV was chosen from the need to provide high static accuracy and the highest effect / background ratio. Fig. 3 shows the spectral ratio for U and 239Pi isotopes, from which it can be seen that the greatest difference in spectra is achieved precisely for this energy range. At the same time, outside the given energy range, the count rate of positron events is small, since; at an energy of E ve below Z MeV, the accuracy of determining the spectrum decreases due to the large contribution. Yes, the background, which has to be subtracted, and at an energy of B ve greater than 9 MeV, the measurement accuracy is significantly reduced due to the rapid decay of the neutrino spectrum, i.e. a small number of neutron events above 9 MeV.

Measuring the energy spectrum of antineutrino, you can determine the amount

235 (23B} y and 239 rts found in the MO in the active zone, and measuring the initial load (the spectrum of the positrons of the reaction (1) at the beginning of the campaign), determine the amount of burnt and accumulated nuclear fuel at a given time. Antineutrino radiation due to a small cross section interaction with the substance freely penetrates the active zone and the biological protection of the nuclear installation and propagates isotropically

all directions. The antineutrino spectrometer can be located anywhere outside the biological defense at distances from a few meters to tens of meters from the active zone, i.e. The positron energy spectra of reaction (1) (antineutrino spectra) are measured remotely and non-contact.

Let us consider the possibility of using the antineutrino method using the example of a VVER reactor.

The table shows the typical composition of the nuclear fuel of the VBER-440 reactor.

The table shows that the total contribution of 235U and 239Pi is (85-90}%.

According to the table, the main changes for the campaign are 235UJero, the share decreases from 70% to 51% and 239Pi (its

the proportion increases from 20 to 35%), the contribution of U is almost unchanged, and the contribution of 241Pi is insignificant.

To implement the method, it is necessary to constantly (or periodically)

measuring the energy spectrum of the positrons of the reaction (1) using an antineutrino spectrometer, which can be located at a distance of 10-100 m from the active zone.

The proposed method, based on the measurement of the energy spectrum of the antineutrino, makes it possible to determine the burnup 235 @ h8} and and the accumulation of g Pu promptly, contactlessly, remotely during the campaign. The amount of burned out 235 (23E); j in the nuclear installation determines the duration of the fuel cycle. Direct measurement of the content in the active zone directly during the campaign makes it possible to predict its duration and, consequently, more efficient use of nuclear fuel, which improves the technical and economic performance of nuclear power plants. The method is operational, non-destructive and

safe, as it does not require contact with the core or with the fuel. Remote monitoring of 239Pi accumulations in nuclear installations is also important for monitoring the non-proliferation of fissionable nuclear materials in terms of the IAEA safeguards system.

Claims (1)

Claims The method of determining the isotopic composition of the fuel in the core of a nuclear reactor consists in measuring the spectral energy characteristics of nuclear fission products and determining burnout U and 239Pi accumulation, characterized in that, in order to increase safety and efficiency, as well as the provision of bo-energies of 1.2–7.2 MW, determine the energetic capacity of the remote determination of positron spectra from the reaction of the isotopic composition of the fuel during the process of f-decay of antineutrinos at reactor bots, measure the energy protons from the fission of 23SU, 23gPu isotopes, the spectrum of positrons from the inverse reaction 5 2 and from them calculate the deler-decay fractions of antineutrinos on water protons 235U, 239Pu, U, 241Pu. generic target in the range Typical isotopic composition of VVER-440 reactor - contributions by number of fissions U.Pu. Fig / V ) " about O4 34 Phie2 .. ..:at .. pftt ° Је ,, м.в. NfVW 2.0 t, 8 1.6- /, Vt r, o / 5 6 fig.Z 7 In E} etMs6