HIGH FLUENCE NEUTRON DOSIMETRY METHOD

Abstract: A high fluence neutron dosimetry method, for measurement of the embrittlement of a nuclear reactor pressure-container owing to neutron fluence, is described, including the steps of: exposing a dosimeter containing an alpha-emitting target isotope to neutron irradiation to form an alpha-emitting product isotope; determining the alpha decay rates of the target nucleus and the product nucleus; and using known alpha decay constants for the target nucleus and the product nucleus and the determined alpha decay rates of the target nucleus and the product nucleus to determine the neutron capture rate of the product nucleus.

Description: BACKGROUND OF THE INVENTION This invention relates to a method for monitoring neutron fluence and, more particularly, to a highly accurate method for measuring reaction rates induced by neutron exposure. Federal government regulations require that nuclear reactor coolant pressure containers or boundaries should have sufficient margin to ensure that the boundary behaves in a non-brittle manner when stressed under operating, maintenance, testing and postulated accident conditions, and that the probability of rapidly propagating fracture is minimized. These requirements necessitate prediction of the amount of deterioration of the reactor vessel due to radiation throughout its service life, which in turn requires that the neutron exposure to the pressure vessel be monitored. Prior art methods currently used for such monitoring, known generally as high fluence neutron dosimetry, include helium accumulation fluence monitors, solid state track recorders, and radiometric monitors. In each of these methods, a neutron-induced reaction rate is measured. In the case of helium accumulation fluence monitors, integral (n, α) rates are measured by isotope dilution helium mass spectrometry. In the solid state track recorder method, neutron-induced integral fission rates are measured by counting fission tracks. In the case of radiometric monitors, neutron-induced reaction rates are measured by radiometric counting of activation products. When these high fluence neutron dosimetry methods are applied at a nuclear power reactor, high measurement accuracy is a goal with low uncertainty of 3-5% or better being a general requirement. These conventional high fluence neutron dosimetry methods, however, have inherent accuracy limitations due to uncertainties in dosimeter mass and detection of product isotopes. In light thereof, a simpler and more accurate high fluence neutron dosimetry method is needed for measuring neutron exposure within a nuclear power reactor. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a high fluence neutron dosimetry method which is simpler and is characterized by a much higher inherent accuracy than conventional methods, wherein the only practical accuracy limitation is the counting statistics of the product isotope. The invention in its broad form resides in a high fluence neutron dosimetry method to measure embrittlement of a nuclear reactor pressure container, comprising the steps of: (a) exposing a dosimeter containing an alpha-emitting target isotope to neutron irradiation to form an alpha-emitting resultant product isotope; characterized by (b) determining the alpha decay rates of the target nucleus and the product nucleus; and (c) using known alpha decay constants for the target nucleus and the product nucleus and the determined alpha decay rates of the target nucleus and the product nucleus to determine the neutron capture rate of the resultant product nucleus. As described hereinafter, there is provided a high fluence neutron dosimetry method comprising the steps of: exposing a dosimeter containing an alpha emitting target isotope to neutron irradiation to form an alpha emitting product isotope; determining the alpha decay rates of the target nucleus and the product nucleus; and using the known alpha decay constants of the target nucleus and the product nucleus and the determined alpha decay rates of the target nucleus and the product nucleus to determine the neutron capture rate of the product nucleus. The accuracy of the method according to the present invention is dependent only on the accuracy of counting statistics and decay constants. Errors usually associated with conventional methods, such as counting efficiency, geometry, dosimeter mass, self-absorption, scattering, etc. are completely eliminated. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments according to the present invention will now be described. The capture of a neutron to produce a target nucleus is represented by the following general formula: A + n → (A + 1) (1) where the target nucleus has a mass number A and the product nucleus has a mass number A+1. Assume that the target nucleus emits alpha radioactivity and that the product nucleus either emits alpha radioactivity directly or decays, to an alpha emitter. Then the neutron "capture rate" (indicated by "captures/atom") for the target nucleus A is represented by the following formula: where N A+1 is the number of atoms of the product nucleus formed in a neutron irradiation and N A is the number of atoms of the target nucleus. Since both are alpha active: λ A N A = alpha decay rate of A = R A (3) and λ A+1 N A+1 = alpha decay rate of A+1 = R A+1 (4) where the quantities λ are alpha decay constants. If a dosimeter containing atoms of target nucleus A is subjected to neutron irradiation, counted with an alpha spectrometer, and the alpha peaks for A and A+1 are separated by an alpha spectrometer, the following proportions necessarily obtain, with the same proportionality factor: R Aα λ A N A (5) and R A+1α λ A+1 N A+1 (6) These proportions accordingly can be relied on to determine the capture rate as follows: The only quantities required for calculating the capture rate are the alpha activities (R) and the decay constants (λ) of the target nucleus and the product nucleus. Decay constants for alpha emitters are generally known to better than 1%, as evidenced by the following Table 1. Decay Constants and Associated Uncertainties for Selected Alpha Emitters Nuclide t 1/2 (y) (sec⁻¹) Uncertainty (%) 230-Th (7.7±0.3)x10⁴ 2.9x10⁻¹³ 3.9 232-Th (1.405±0.006)x10¹⁰ 1.563x10⁻¹⁸ 0.43 233-U (1.592±0.020)x10⁵ 1.380x10⁻¹³ 1.3 235-U (7.038±0.005)x10⁸ 3.121x10⁻¹⁷ 0.071 238-U (4.468±0.003)x10⁹ 4.916x10⁻¹⁸ 0.067 237-Np (2.14±0.01)x10⁶ 1.026x10⁻¹⁴ 0.47 236-Pu 2.851±0.008 7.704x10⁻⁹ 0.28 238-Pu (8.774±0.004)x10¹ 2.503x10⁻¹⁰ 0.046 239-Pu (2.411±0.010)x10⁴ 9.110x10⁻¹³ 0.41 240-Pu (6.537±0.010)x10³ 3.360x10⁻¹² 0.15 242-Pu (3.763±0.020)x10⁵ 5.837x10⁻¹⁴ 0.53 Therefore, the limitation on accuracy of the capture rate measurement lies effectively only in the accuracy to which the two alpha decay rates (R) can be detected. Generally, when a nuclear reaction is induced, the product nucleus is less stable than the target nucleus. In the case of alpha emission, this results in the product nucleus having a higher decay rate per atom than the target nucleus and also a higher alpha particle energy as per the well known Geiger-Nuttall Rule. Thus, although only a small number of capture reactions may be induced, the decay rate per atom will be higher, and the alpha energy will result in a peak higher than the target nucleus peak, both allowing the product nucleus to be detected more easily. This effect has been observed in the recalibration of ²³⁷Np fission rate dosimeters used in SSTR fission rate measurements at a boiling water nuclear reactor. When subject to recalibrations, two ²³⁷Np dosimeters were found to have total alpha decay rates which were about 2% higher then before the neutron exposure. This increase was due to: The 2% increase in alpha activity was due to ingrowth of ²³⁸Pu. In the case of one of the dosimeters, analysis of alpha spectra resulted in the observation of 915 counts in the peak due to ²³⁸Pu(Eα=5.499 MeV) and 42,646 counts in the peak for ²³⁷Np(Eα=4.788 MeV). Since as shown in Table 1 the decay constants for ²³⁷Np and ²³⁸Pu are 1.026 x 10⁻¹⁴ sec⁻¹ (±0.47%) and 2.503 x 10⁻¹⁰ ±0.046%), respectively the capture rate is determined as follows: Since uncertainties in the decay constant are small, the overall uncertainty is determined mainly by the uncertainty of the number of counts of ²³⁸Pu (3.3%). Counting of the dosimeters for a longer time period resulting in the accumulation of more than 10,000 counts in the ²³⁸Pu peak will result in determination of the capture rate to an absolute uncertainty of better than 1%. Several other reaction rate possibilities exist for other target/product pairs, as summarized by the following Table 2. Capture Reactions For High Fluence Neutron Dosimetry Target Isotope Product Isotope Target α Half Life Product α Half Life Capture Cross Section (barns) ²³²Th ²³³U 1.40x10¹⁰y 1.59x10⁵y 74.85 ²³⁵U ²³⁶U 7.04x10⁸y 2.34x10⁷y 98 ²³¹Pa ²³²U 3.28x10⁴y 72y 210 ²³⁸U ²³⁹Pu 4.67x10⁴y 2.41x10⁴y 27 ²³⁷Np ²³⁸Pu 2.14x10⁶y 87.7y 170 ²³⁹Pu ²⁴⁰Pu 2.41x10⁴y 6.54x10³y 200 ²⁴⁰Pu ²⁴¹Pu 6.54x10³y 14.7y 290 ²²⁶Ra ²²⁷Ac 1600y 21.8y 110 ²⁴¹Am ²⁴²Cm 432y 163d 630 ²⁴⁸Cm ²⁴⁹Bk 3.39x10⁵y 320d 25 ²⁴⁴Pu ²⁴⁵Cm 8.3x10⁷y 8.5x10³y 17 ²⁴²Pu ²⁴³Am 3.76x10⁵y 7.38x10³y 190 ²⁴⁷Bk ²⁴⁸Cf 1.4x10³y 334d 100 ²⁵²Cf ²⁵³Es 2.64y 20.47y 20 As can be seen, many different half life combinations exist, allowing dosimetry sets to be tailored to specific dosimetry applications (e.g., long duration/low fluence vs. high fluence/short duration). Thermal and epithermal reaction rates can be measured using cadmium covers. Table 2 is by no means an exhaustive list. That is, other non-capture reactions are contemplated by the present invention, such as (n,n'), (n,2n), (n,p) and (n,α). One example would be: These reactions are generally threshold reactions opening up the possibility of providing neutron spectrally sensitive reaction rates. Production of thin alpha emitting deposits for these applications has already been accomplished (e.g., for solid state track recorder dosimetry) and alpha spectrometry methods are already routine. There are several advantages to this method over the prior art methods described above: 1. Higher accuracy--uncertainty determined mainly by the alpha decay rate of the product nucleus R A+1 . 2. The dosimeter mass need not be known. 3. The counter efficiency need not be known. 4. The sample-counter geometry factor need not be known. 5. The method is ideal for high neutron fluence. In addition, sample self-absorption or self-scattering may be rendered unimportant and dosimeter dose rates can be made small by using materials that have low neutron activation. These last two advantages can be realized by making a thin layer of target material (less than 100µ g/cm²) on a substrate such as aluminum which will not activate during the neutron bombardment. For small deposit areas, the method is insensitive to uniformity of the deposit. The foregoing is considered illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described. Accordingly, resort may be had to all suitable modifications and equivalents falling within the scope of the invention and the appended claims as would be understood within the art.

Claims: A high fluence neutron dosimetry method to meausre embrittlement of a nuclear reactor pressure container, comprising the steps of: (a) exposing a dosimeter containing an alpha-emitting target isotope having a mass number A to neutron irradiation to form an alpha-emitting resultant product isotope having a mass number A+1; characterized by (b) determining the alpha decay rates R A =λ A N A and R A+1 =λ A+1 N A+1 of the target nucleus and the product nucleus; and (c) using known alpha decay constants λ A and λ A+1 for the target nucleus and the product nucleus and the determined alpha decay rates of the target nucleus and the product nucleus to determine the neutron capture rate of the resultant product nucleus. The method as recited in claim 1, wherein step (b) comprises the following substeps: (i) subjecting the exposed dosimeter to a counting of the number of atoms N A of the target nucleus and the number of atoms N A+1 of the product nucleus using an alpha-spectrometer, and (ii) separating the respective alpha peaks representing the decay rates of the target nucleus and the product nucleus using the alpha-spectrometer. The method as recited in claim 1, wherein the target nucleus and the product nucleus are selected from the target/product isotope pairs ²³²Th-²³³U, ²³⁵U-²³⁶U. ²³¹Pa-²³²U, ²³⁸U-²³⁹Pu, ²³⁷Np-²³⁸Pu, ²³⁹Pu-²⁴⁰Pu, ²⁴⁰Pu-²⁴¹Pu, ²²⁶Ra-²²⁷Ac, ²⁴¹Am-²⁴²Cm, ²⁴⁸Cm-²⁴⁹Bk, ²⁴⁴Pu-²⁴⁵Cm, ²⁴²Pu-²⁴³Am, ²⁴⁷Bk-²⁴⁸Cf and ²⁵²Cf-²⁵³Es. The method as recited in claim 2, wherein the target nucleus and the product nucleus are selected from the target/product isotope pairs ²³²Th-²³³U, ²³⁵U-²³⁶U, ²³¹Pa-²³²U, ²³⁸U-²³⁹Pu, ²³⁷Np-²³⁸Pu, ²³⁹Pu-²⁴⁰Pu, ²⁴⁰Pu-²⁴¹Pu, ²²⁶Ra-²²⁷Ac, ²⁴¹Am-²⁴²Cm, ²⁴⁸Cm-²⁴⁹Bk, ²⁴⁴Pu-²⁴⁵Cm, ²⁴²Pu-²⁴³Am, ²⁴⁷Bk-²⁴⁸Cf and ²⁵²Cf-²⁵³Es.

Inventors: LIPPINCOTT, EZRA PARVIN, RUDDY, FRANCIS HENRY
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1987
Main classification: G21C17
IPC codes: G21C17, G01T3