JPH0222355B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a reactor pressure vessel monitoring test piece for accurately measuring the amount of neutron irradiation in the reactor pressure vessel in order to monitor the health of the reactor pressure vessel during its service life. .

Conventionally, reactor pressure vessel monitoring test pieces have been installed inside the reactor pressure vessel in order to investigate the effects of fast neutron irradiation on the steel materials of the reactor pressure vessel, which undergo changes in their mechanical properties.

This test piece, which is used to examine changes in mechanical properties, is taken from the same steel material used during the manufacture of the reactor pressure vessel, exposed to neutron irradiation within the reactor pressure vessel for a certain period of time, and then removed from the reactor pressure vessel. Subjected to mechanical testing.

The amount of neutron irradiation received by the test piece is measured in order to find out the correlation between the mechanical influence of the test piece and the amount of neutron irradiation.The test piece used to measure the amount of neutron irradiation is what is called a monitor wire, High purity
It is a wire made of Ni, Fe, and Cu metals.

When these monitor wires are irradiated with neutrons in a nuclear reactor pressure vessel, radioactive isotopes are produced by a nuclear reaction between some isotopes in the metal and neutrons. After the reactor is shut down, the monitor wire is removed from the reactor pressure vessel and the intensity of gamma rays, which have energy specific to each element among these radioactive isotopes, is measured using a measuring device such as a Ge(Li) semiconductor detector. By determining the radioactivity intensity, it is possible to determine the amount of neutron irradiation that the monitoring test piece actually received inside the reactor pressure vessel, and from this it is possible to estimate the amount of neutron irradiation that the reactor pressure vessel received. .

In general, the influence of neutron irradiation on the mechanical properties of steel materials is that the neutron energy is 0.1~
It is said that fast neutrons in the 1 MeV range are responsible for the radiation, so when it comes to monitoring test pieces for measuring neutron irradiation, it is only necessary to consider radioactive substances produced only by nuclear reactions with fast neutrons, and the test The material of the piece must be one that makes it easy to measure radioactivity with a Ge(Li) semiconductor detector, etc.

Therefore, the aforementioned Ni, Fe, and Cu wires have traditionally been used for measuring neutron irradiation. In other words, as shown in the graph of the nuclear reaction cross section of the test piece in Figure 1, Ni wires mainly undergo ⁵⁸ Ni(n,p) reactions due to fast neutrons of 2 MeV or higher, while Fe wires mainly undergo 58 Ni(n,p) reactions.
⁵⁸ Co, 54 Mn and 60 are radioactive isotopes produced mainly by the ⁵⁴ Fe (n, p) reaction caused by fast neutrons of 2 MeV or more and ^{the 63} Cu (n, α) reaction caused by fast neutrons of ⁶ MeV or ^more in Cu wire. Coα radioactivity is used.

However, these reactions require the energy of the neutron to
It is limited to 2 MeV and 6 MeV or above, and there is an unmeasurable range l ₀ below 2 MeV, and the measured neutron irradiation amount is also limited to those above this energy, and it cannot be measured within the originally required range of 0.1 to 1 MeV. In order to estimate the fast neutron irradiation dose, it was necessary to correct the energy range using methods such as calculations.

In view of this, an object of the present invention is to provide a reactor pressure vessel monitoring test piece that can directly estimate the fast neutron irradiation dose in the range of 0.1 to 1 MeV without requiring correction of the energy range.

This objective is achieved by sealing the fissile material within a container.

The monitoring test piece according to the present invention includes, in addition to the conventional test piece for monitoring changes in mechanical properties and a monitor wire for measuring neutron irradiation, a fissile material that causes nuclear fission due to fast neutrons is housed in a sealed container. .

In measuring the fast neutron irradiation dose using such a monitoring test piece, the irradiation dose due to neutrons with an energy of several MeV or more can be obtained from the conventional radioactivity measurement of a monitor wire, and the irradiation dose due to neutrons with an energy of several MeV or more can be obtained from the radioactivity measurement of a conventional monitor wire, and the irradiation dose with a small energy, that is, 0.1~
The irradiation dose by neutrons with an energy of 1 MeV can be obtained from radioactivity measurements of fission products in fissile materials.

For example, fissile materials such as ²³⁸ U, ²³⁷ Np and
If ²³² Th is selected, as shown in Figure 2, ²³⁸ U and ²³⁷ Np will undergo nuclear fission due to a reaction with neutrons of approximately 2 MeV or higher, and ²³² Th will undergo nuclear fission through a reaction with neutrons of approximately 0.1 MeV or higher, so the resulting fission products will be Contains various radioactive isotopes, the amount of which is produced is uniquely determined by the number of nuclear fission. After the reactor is shut down, the monitoring test piece is removed from the reactor pressure vessel, and the intensity of gamma rays emitted by ¹³⁷ Cs, which is easy to measure, is measured using a Ge(Li) semiconductor detector, etc. to determine the radioactivity intensity. For example, it is possible to easily estimate the amount of neutron irradiation in the energy range of 0.1 MeV to 2 MeV received by the fissile material of the test piece.

As described above, when a fissile material using fast neutrons is added as a monitoring test piece, it is possible to measure the neutron irradiation amount with energy in the range of 0.1 to 2 MeV, and by combining this fissile material test piece with a conventional monitor wire test piece. This has the effect that it is possible to estimate the necessary range of energy distribution of neutrons received by a monitoring test piece for investigating mechanical properties.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between neutron energy and nuclear reaction cross section of a conventional test piece, and FIG. 2 is a graph showing the relationship between neutron energy and nuclear reaction cross section of a test piece of the present invention. l ₀ ...Unmeasurable range.

Claims (1)

[Claims] 1 A plurality of fissile materials that undergo nuclear fission at different neutron energies and a plurality of radionuclide generating materials that undergo nuclear reactions at different neutron energies are sealed in a storage container, and ²³⁸ U, ²³² Th, and ²³⁷ Np are used as the fissionable materials. A nuclear reactor pressure vessel monitoring test piece, characterized in that ⁵⁸ Ni, ⁵⁴ Fe, and ⁶³ Cu are used as the radionuclide generating substances.