Radioactivity evaluation method and decay heat evaluation method

Abstract

To allow for non-destructively assessing at least radioactivity of irradiated nuclear fuel such as spent nuclear fuel.SOLUTION: Bremsstrahlung rays radiated together with γ-rays from irradiated nuclear fuel are detected using a detector so as to assess radioactivity of 90Sr and of 90Y contained in the irradiated nuclear fuel.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a radioactivity evaluation method and a decay heat evaluation method. [Background technology]

[0002] As is well known, 90 Sr and 90 Y is produced by nuclear fission when nuclear fuel is used in a nuclear reactor, and is a representative nuclide responsible for the radioactivity, toxicity, and decay heat that accumulates in spent nuclear fuel. 90 Sr decays by beta decay with a half-life of 28.8 years. 90 Y. On the other hand, 90 Y has a half-life of 64.1 hours 90 It undergoes beta decay in a much shorter period of time than Sr. 90 Sr and 90 The Y's are in "permanent equilibrium" in which their radioactivities are equal unless either one moves.

[0003] Also, 90 Sr is easily deposited in bones, etc., and once deposited, 90 Sr and 90 Because Y irradiates tissues with β-rays for a long time, it is a highly toxic nuclide that can easily induce cancer, etc., and it is important to manage it from the perspective of internal exposure. For this reason, if nuclear fuel is damaged underwater, etc., it is important to understand its radioactivity. 90 A common method for determining the radioactivity of Sr is "milking and beta-counting" of the sample.

[0004] The sample containing Sr was left for about two weeks after removing Y and other elements by chemical separation. 90 The radioactivity of Y is allowed to reach a permanent equilibrium state. Here, only Y is separated and extracted by chemical manipulation. 90 The beta rays of Y are counted using a liquid scintillator or a 4π gas detector, and then divided by the detection efficiency. 90 Obtain the radioactivity of Y. Then, based on the conditions of permanent equilibrium 90 Radioactivity of Sr 90The radioactivity of Y is obtained as the same value. The operation of separating and extracting only Y results in almost no change in radioactivity over a period of several months. 90 From Sr, the same radioactivity 90 Since Y can be obtained repeatedly every two weeks, this is called "milking."

[0005] For example, in section 9.4 of the following non-patent document 1, an overview of the above-mentioned milking and beta ray counting is given. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] Ministry of Education, Culture, Sports, Science and Technology, Science and Technology Policy Bureau, Nuclear Safety Division, Disaster Prevention and Environmental Measures Office, 2003 Revised Radioactivity Measurement Method Series 2 "Radioactive Strontium Analysis Method" Summary of the Invention [Problem to be solved by the invention]

[0007] By the way, in the above background art (milking and beta-ray counting), 90 Coexistence with Sr 90 It is necessary to remove or separate / extract Y using chemical treatment. In other words, the background art (milking) is a destructive measurement method that destroys the measurement sample, and is practically impossible to apply to spent nuclear fuel actually generated in nuclear power generation.

[0008] For example, for the spent nuclear fuel generated in nuclear power generation, there is a demand for more accurate measurement of decay heat in order to ensure its sound management. However, the above-mentioned background art (milking and beta ray counting) is a destructive measurement method that requires the destruction of the spent nuclear fuel, and therefore, in order to ensure the sound management of the spent nuclear fuel, it is necessary to measure the decay heat, which is the main component of the spent nuclear fuel. 90 Sr, 90 It is virtually impossible to determine the radioactivity of Y.

[0009] The present invention has been made in consideration of the above-mentioned circumstances, and has an object to provide an evaluation method capable of non-destructively evaluating at least the radioactivity of irradiated nuclear fuel such as spent nuclear fuel. [Means for solving the problem]

[0010] In order to achieve the above object, the present invention provides a first solution relating to a method for evaluating radioactivity, which comprises detecting brake lines emitted from irradiated nuclear fuel together with gamma rays using a detector, thereby determining the amount of bremsstrahlung contained in the irradiated nuclear fuel. 90 Sr radioactivity and 90 The method adopted is to evaluate the radioactivity of Y.

[0011] The present invention employs, as a second solution relating to a radioactivity evaluation method, a solution in which, in the above-mentioned first solution, the radioactivity is evaluated based on the gamma rays and the bremsstrahlung rays having energies in the range of 1.6 to 1.7 MeV.

[0012] In the present invention, as a third solution relating to a radioactivity evaluation method, in the first or second solution, 106 Rh and 144 Calculated based on the absolute value of the gamma ray emission spectrum and the absolute value of the electron beam emission spectrum from the Pr counting rate. 106 Rh and 144 A correction is performed by subtracting the count rate of Pr, and the radioactivity is evaluated based on the result of the correction.

[0013] The present invention employs, as a fourth solution related to a radioactivity evaluation method, a solution in which, in any one of the above first to third solutions, the irradiated nuclear fuel is spent nuclear fuel stored in a spent fuel pool, and the brake wire is detected by immersing the detector in shielding water of the spent fuel pool.

[0014] The present invention employs, as a fifth solving means related to the radioactivity evaluation method according to the fourth solving means, a means in which the detector is housed in a watertight container and immersed in the shielding water.

[0015] The present invention employs, as a sixth solution related to a radioactivity evaluation method, a solution in any one of the first to third solutions, in which the irradiated nuclear fuel is spent nuclear fuel stored in a spent fuel pool, and the detector detects the brake line via a collimator embedded in a wall of the spent fuel pool.

[0016] In the present invention, as a solution relating to a decay heat evaluation method, a means is adopted in which the decay heat of the irradiated nuclear fuel is evaluated based on the radioactivity evaluated by the radioactivity evaluation method relating to any one of the above first to sixth solutions. Effect of the Invention

[0017] According to the present invention, it is possible to non-destructively evaluate at least the radioactivity of irradiated nuclear fuel such as spent nuclear fuel. [Brief description of the drawings]

[0018] [Figure 1] FIG. 2 is a block diagram showing the functional configuration of a measurement device used in one embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic diagram showing a setting state of an HP-Ge detector in the measuring device. [Diagram 3] 1 is a flowchart showing the steps of a radioactivity evaluation method according to one embodiment of the present invention. [Figure 4] FIG. 1 is a characteristic diagram showing the evaluation results of the pulse height spectra of gamma rays and braking lines and the proportion of 90Y braking lines in the count rate in one embodiment of the present invention. [Diagram 5] 1 is a flowchart showing the procedure of a decay heat evaluation method according to one embodiment of the present invention. [Figure 6] FIG. 11 is a characteristic diagram showing an evaluation result of decay heat in one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The method for evaluating radioactivity and decay heat according to the present embodiment uses a measuring device A shown in FIG. 90 Sr radioactivity and 90 In addition to evaluating Y radioactivity, etc., the decay heat of spent nuclear fuel X is also evaluated.

[0020] Here, the above-mentioned spent nuclear fuel X is an assembly of fuel rods, and is an irradiated nuclear fuel that radiates gamma rays (gamma rays) to the surroundings as a result of beta decay. The spent nuclear fuel X also contains high-speed electrons generated by beta decay, and these high-speed electrons are decelerated by the nuclide, emitting braking rays (a type of electromagnetic wave) to the surroundings. In other words, the spent nuclear fuel X emits gamma rays and braking rays to the surroundings as a result of beta decay.

[0021] Although details will be described later, the measurement device A in this embodiment simultaneously detects gamma rays and braking rays of such spent nuclear fuel X, and obtains the spectrum of the gamma rays and braking rays to measure the temperature of the spent nuclear fuel X. 90 Sr radioactivity and 90 Y radioactivity was evaluated and the relevant 90 Sr radioactivity and 90 Evaluate the decay heat of spent nuclear fuel X based on its radioactivity Y.

[0022] More specifically, the measurement device A includes an HP-Ge detector 1, a preamplifier 2, a linear amplifier 3, a pulse height analyzer 4, and a computer 5, as shown in the figure. The HP-Ge detector 1 is a high-purity germanium detector that converts the gamma rays and braking rays emitted from the spent nuclear fuel X into a current (signal current). The signal current is a pulse current through which a charge flows according to the energy imparted to the HP-Ge detector 1 by the gamma rays and braking rays.

[0023] The preamplifier 2 integrates this signal current and outputs a pulse current having an amplitude proportional to the integral value to the linear amplifier 3. The linear amplifier 3 linearly amplifies the pulse current input from the preamplifier 2, shapes the waveform, and outputs it to the pulse-height analyzer 4 as a proportional integral signal current. The amplification degree in this linear amplifier 3 is set so that gamma rays and braking rays with energies of about 0.5 MeV to 3.6 MeV can be measured. The pulse-height analyzer 4 outputs the amplitude of the proportional integral signal current input from the linear amplifier 3 to the computer 5 as a spectrum evaluation signal (digital signal).

[0024] The computer 5 is also equipped with an arithmetic unit, a storage device, an operation device, an output device, etc. The computer 5 performs a predetermined signal processing on the spectrum evaluation signal (digital signal) based on an analysis program previously stored in the storage device, thereby performing a spectrum analysis of gamma rays and brakes rays, and outputs the results of the spectrum analysis to an output device. The results of this spectrum analysis are temporarily stored in the storage device, and then displayed as an image on a display device (output device), and are also output to another output device according to the operation instructions of the administrator inputted to the operation device.

[0025] Here, the installation mode of the HP-Ge detector 1 relative to the spent nuclear fuel X will be described with reference to Figures 2(a) and (b). In this embodiment, the spent nuclear fuel X is contained in a spent fuel pool B as an assembly of fuel rods as shown in Figure 2. In contrast, the HP-Ge detector 1 is disposed near the spent nuclear fuel X while being contained in a watertight container 6, for example, as shown in Figure 2(a).

[0026] 2(a), the HP-Ge detector 1 is immersed in the shielding water C (cooling water) in the spent fuel pool B while being protected from the shielding water C by the watertight container 6. The watertight container 6 accommodates the HP-Ge detector 1 airtightly against the shielding water C, and is made of a material that transmits gamma rays and braking rays without attenuation. In this installation mode, the HP-Ge detector 1 detects gamma rays and braking rays emitted from the spent nuclear fuel X through the shielding water C and the watertight container 6.

[0027] On the other hand, the HP-Ge detector 1 detects gamma rays and braking rays via a collimator D embedded in the wall of the spent fuel pool B as shown in Fig. 2(b) instead of being immersed in the shielding water C described above. The collimator D is embedded so as to penetrate the wall of the spent fuel pool B, and causes the gamma rays and braking rays emitted from the spent nuclear fuel X to be incident on the sensitive surface of the HP-Ge detector 1.

[0028] 2(b), the HP-Ge detector 1 does not require a watertight container 6 because it is not immersed in the shielding water C. That is, the HP-Ge detector 1 in this installation mode detects gamma rays and braking rays emitted from the spent nuclear fuel X via the shielding water C and the collimator D.

[0029] Next, the detailed procedure of the radioactivity evaluation method according to this embodiment will be described with reference to the flowchart shown in FIG.

[0030] In the radioactivity evaluation method according to this embodiment, first, a measurement device A is used to measure the spectral distribution (counting rate distribution) of gamma rays in the entire energy range of gamma rays and braking rays emitted from nuclides contained in spent nuclear fuel X (step S0). As shown in FIG. 4(a), this spectral distribution indicates the counting rate of gamma rays over the energy range of 0 to 4 MeV. From this measured spectral distribution, 106 Rh and 144 The gamma rays emitted from Pr are counted (step S1). For example, 106The energy of gamma rays emitted from Rh (energy deposited on the detector) is 2.366MeV and 2.406MeV. 144 The energy of gamma rays emitted from Pr is typically 2.186 MeV.

[0031] In addition, 106 Rh and 144 Pr also emits gamma rays of energies (energy imparted to the detector) different from the above-mentioned 2.366 MeV, 2.406 MeV, and 2.186 MeV. In consideration of this, in step S1, the gamma rays of the different energies may be counted.

[0032] In this radioactivity evaluation method, the response function and detection efficiency in the measurement of β rays (beta rays) and γ rays are then calculated (step S2). 90 Y, 106 Rh and 144 The spatial distribution and energy spectrum of beta and gamma rays emitted from Pr are evaluated. For the energy spectrum, the well-known data from "ICRU-56" (Explanation of External Beta Dosimetry for Radiation Protection) and the well-known data from "Table of Isotopes" can be used.

[0033] The following evaluation method can be used for the spatial distribution: It is assumed that the space in the spent nuclear fuel X is uniform, that the spatial distribution in the spent nuclear fuel X can be expressed by an analytical function (for example, a distribution proportional to a cosine function in the vertical direction), and that the measured values ​​of the burnup distribution and gamma ray dose distribution are similar to the spatial distribution of radioactivity.

[0034] As yet another method, under these assumptions, the spatial distribution of the above-mentioned beta and gamma radiation generation is actually calculated. For this calculation, the assembly burn-up calculation function of the SCALE code system created by the Oak Ridge National Laboratory in the United States, or calculation codes such as SWAT and MVP-BURN developed by the Japan Atomic Energy Agency (JAEA) can be used. These calculations can be made by inputting the time change in thermal output of spent nuclear fuel X, which is an assembly of fuel rods, the type of assembly, and the density of the cooling water.

[0035] Response function for electrons R e (r,E e →h) and the response function R γ (r,E γ →h) represents the probability that an electron beam or gamma ray with energy E generated at position r is detected as energy h by the HP-Ge detector 1. The detection efficiency ε(h) is calculated by multiplying the absolute value of the gamma ray generation spectrum by S γ (r,E γ ), the absolute value of the electron beam emission spectrum is S e (r,E e ), it is expressed by the following equations (1) and (2).

[0036]

number

[0037] Before measurement, the absolute value S γ (r,E γ ), S e (r,E e ) is unknown, but if its relative value can be evaluated or assumed, the detection efficiency ε(h) can be calculated. To calculate this relative value, the MCNP code developed by the Los Alamos National Laboratory in the United States, the EGS code developed by the Japan Atomic Energy Agency and the High Energy Accelerator Research Organization, and the PHITS code developed by JAEA which incorporates the functions of the EGS code can be used. All of these codes input the positions, composition, and density of the nuclear fuel and HP-Ge detector to be measured, as well as the atmosphere, and calculate the above absolute value S γ (r,E γ ), Se (r,E e ) or absolute value S γ (r,E γ ), S e (r,E e ) the detection efficiency ε(h) can be calculated by inputting the relative value of

[0038] In this radiation assessment method, 106 Rh and 144 The number of gamma rays generated for Pr is calculated (step S3). In this step S3, the number of gamma rays generated for Pr obtained in step S1 is calculated. 106 Rh and 144 By dividing the peak count rate C(h) of Pr by the detection efficiency ε(h) obtained in step S2, 106 Rh and 144 Determine the number of gamma rays emitted at energies specific to Pr, then 106 Rh and 144 Absolute value of gamma-ray emission spectrum for Pr S γ (r,E γ ) is calculated.

[0039] Here, the energy h indicates the peak energy of gamma rays, for example, 106 For gamma rays emitted from Rh, h = 2.3660 MeV, 2.406 MeV, 144 For gamma rays emitted from Pr, h = 2.186 MeV. The count rate C(h) is given by the following equation (3), so 106 Rh and 144 The number of gamma rays generated by Pr is given by the following equation (4), obtained by dividing the count rate C(h) by the detection efficiency.

[0040]

number

[0041] In this radiation assessment method, 106 Rh and 144 Absolute value S of the electron emission spectrum for Pr e (r,E eIn step S4, the absolute value S of the gamma-ray generation spectrum obtained in step S3 is calculated from the data in the "Table of Isotopes" and "ICRU-56" (step S4). γ (r,E γ ) and the absolute value of the electron beam emission spectrum S e (r,E e ) is known, so the absolute value S of the electron beam emission spectrum can be calculated based on this relationship. e (r,E e ) is calculated.

[0042] In this method of evaluating radioactivity, the following is performed: 106 Rh and 144 The count rate of Pr is calculated (step S5). In calculating this count rate, the above-mentioned MCNP code, EGS code, and PHITS code are used to calculate the system information and the absolute value S of the gamma-ray generation spectrum obtained in step S3. γ (r,E γ ) and the absolute value S of the electron beam emission spectrum obtained in step S4 e (r,E e ) to calculate.

[0043] More specifically, the absolute value of the gamma-ray emission spectrum S γ (r,E γ ) and the absolute value S of the electron beam emission spectrum obtained in step S4 e (r,E e ) is input and the gamma ray detection efficiency ε γ (h) and electron beam detection efficiency ε e (h) is calculated based on the following formulas (5) and (6), and the absolute value of the gamma-ray emission spectrum S γ (r,E γ ) and the absolute value of the electron beam emission spectrum S e (r,E e ) are multiplied by each other and the results are added together. In equations (5) and (6), the range of energy h is 1.6 to 1.7 MeV.

[0044]

number

[0045] That is, by calculating the following formula (7) based on the above formulas (5) and (6), the energy h in the range of 1.6 to 1.7 MeV is 106 Rh and 144 The count rate of Pr is obtained. Here, the absolute value S of the gamma ray generation spectrum is γ (r,E γ ) and the absolute value of the electron beam emission spectrum S e (r,E e The sum of the integrals of 106 Rh and 144 This includes all of the gamma rays and braking rays emitted from Pr.

[0046]

number

[0047] In this radiation assessment method, 90 Calculate the count rate of Y (step S6). In this step S6, the count rate of Y is calculated in the range of 1.6 to 1.7 MeV for the energy h obtained in step S1. 106 Rh and 144 The count rate of Pr obtained in step S5 106 Rh and 144 By subtracting the count rate of Pr 90 Calculate the count rate of Y.

[0048] Here, in step S6 90 The calculation process of the count rate of Y is performed in the next step S7. 90 It includes a correction process to further improve the evaluation accuracy (calculation accuracy) of the radioactivity of Y. 90 If it is acceptable to sacrifice some of the accuracy of the evaluation of the radioactivity of Y, 106 Rh and 144 Pr counting rate, i.e. measured by measuring device A 106 Rh and 144The count rate of Pr obtained in step S5 106 Rh and 144 Measured by measurement device A without subtracting the Pr count rate 106 Rh and 144 Pr counting rate as is 90 It may also be the count rate of Y.

[0049] In this radiation assessment method, 90 The Y radioactivity is calculated (step S7). 90 By dividing the count rate of Y by the detection efficiency ε(h) evaluated in step S2, 90 Calculate the beta ray generation rate for Y, and then use the electron ray generation rate per decay listed in the "Table of Isotopes" mentioned above. 90 By dividing the beta radiation rate by Y 90 Calculate the Y radioactivity.

[0050] Furthermore, in this radioactivity evaluation method, 90 Sr radioactivity is calculated (step S8). In this step S8, the above-mentioned permanent equilibrium conditions are used to calculate the Sr radioactivity. 90 Sr radioactivity 90 Calculate the amount as equal to Y radioactivity.

[0051] The radioactivity evaluation method according to this embodiment is realized by a series of processes consisting of steps S1 to S8 described above. According to this embodiment, the radioactivity evaluation method according to this embodiment is realized by performing at least 90 Sr radioactivity and 90 It is possible to evaluate Y radioactivity non-destructively.

[0052] Figure 4(a) shows the calculation results of the pulse height spectrum of the brake lines due to gamma rays and beta decay for spent nuclear fuel X with a burnup of 52 GWd / t and a cooling period of 13 years. From this calculation result, it is clear that the majority of the count rate is in the range of 1.6 to 1.7 MeV of energy applied to the detector (HP-Ge detector 1). 90 It can be seen that most of the radiation is from Y.

[0053] Figure 4(b) shows the percentage of the count rate in the region of 1.6 to 1.7 MeV of energy deposited on the detector (HP-Ge detector 1). 90 The figure 3(b) shows the proportion of the Y braking line in the region of 1.6 to 1.7 MeV of deposited energy. 90 This shows that the ratio of the Y braking line is about 94% regardless of the burnup after 13 years of cooling. Therefore, for spent nuclear fuel X that has been cooled for this long, simply calculating the count with an imparted energy of 1.6 to 1.7 MeV and dividing it by the detection efficiency will give an error of within 6%. 90 Y radioactivity and 90 Sr radioactivity can be determined.

[0054] Therefore, the evaluation targets spent nuclear fuel X with a cooling period of 13 years or more. 90 Sr radioactivity and 90 If a measurement error of 6% regarding the Y radioactivity is allowed, the correction process in step S6 may be omitted. 106 Rh and 144 Without performing correction processing for the count rate of Pr, 106 Rh and 144 Based on Pr count rate alone 90 Y radioactivity may be calculated.

[0055] Furthermore, as a result of investigating gamma rays that cause interference in the 1.6 to 1.7 MeV imparted energy range (pulse height range), it was found that if the spent nuclear fuel X is 2 cycles = 26 GWd / t or more, 144 Ce→ 144 Pr and 106 Ru→ 106 It is due to Rh, and these two components 90 Sr → 90 It was found that the combined Y bremsstrahlung components accounted for 98% of the 1.6-1.7 MeV counts.

[0056] 144 Ce→ 144 Pr and 106 Ru→106 Rh emits gamma rays with a peak structure, so it is easy to measure and quantify this radioactivity. 144 Ce→ 144 Pr and 106 Ru→ 106 By subtracting the Rh component and then dividing by the detection efficiency, 90 Sr → 90 The radioactivity of Y can be measured and evaluated from a stage earlier than the cooling period of 13 years. For example, in Figure 4(b), 90 Sr → 90 This shows that the radioactivity of Y can be evaluated.

[0057] Next, the decay heat evaluation method of irradiated nuclear fuel according to this embodiment is based on the radioactivity evaluation method according to this embodiment described above. 90 Sr and 90 The decay heat of spent nuclear fuel X (irradiated nuclear fuel) is evaluated by using radioactivity such as Y. The procedure of the decay heat evaluation method according to this embodiment will be described below with reference to the flowchart of FIG.

[0058] In this decay heat evaluation method, first 106 Rh radioactivity and 144 The radioactivity of Pr is calculated (step S9). That is, in this step S9, 90 Sr radioactivity and 90 The radioactivity of Y was evaluated using the method 106 Rh and 144 By dividing the number of gamma rays and electron beams emitted by Pr by the number of gamma rays and electron beams emitted per decay given in the "Table of Isotopes", 106 Rh radioactivity and 144 Calculate the radioactivity of Pr.

[0059] In this decay heat evaluation method, 106 Ru and 144 The radioactivity of Ce is calculated (step S10). That is, in this step S10, 106Rh and 144 Pr each 106 Ru and 144 Since the condition of permanent equilibrium with Ce is satisfied, 106 Rh radioactivity and 144 As the same amount of Pr radioactivity 106 Ru and 144 Obtain the radioactivity of Ce.

[0060] In this decay heat evaluation method, in step S0, a measuring device A is used to measure the decay heat of spent nuclear fuel X (irradiated nuclear fuel). 134 Cs, 137m Ba and 154 The spectrum includes gamma rays emitted from Eu. 134 Cs, 137m Ba and 154 The gamma rays emitted from Eu are counted (step S11). The peak energy of the gamma rays is, for example, 134 0.605 MeV and 0.796 MeV for Cs, 137m For Ba, it is 0.662 MeV, and 154 For Eu it ​​is 1.274 MeV, but each decay produces multiple gamma rays. 134 Cs and 154 Eu is not limited to this. For this reason, in step S11, 134 Cs, 137m Ba and 154 The gamma ray peak spectrum for Eu is counted.

[0061] In this decay heat evaluation method, the response function and detection efficiency are then calculated (step S12). That is, in this step S12, the response function and detection efficiency in gamma ray measurement are obtained by using a method similar to that in step S2 described above.

[0062] In this decay heat evaluation method, 134 Cs, 137m Ba and 154The number of gamma rays emitted from Eu is calculated (step S13). That is, in this step S13, the peak count obtained in step S11 is divided by the detection efficiency obtained in step S12 to obtain the following: 134 Cs, 137m Ba and 154 Obtain the number of gamma rays emitted from Eu.

[0063] In this decay heat evaluation method, 134 Cs, 137m Ba and 154 The radioactivity of Eu is calculated (step S14). That is, in this step S14, the number of gamma rays generated in step S13 is divided by the number of gamma rays generated per one decay listed in the "Table of Isotopes" to obtain the following: 134 Cs, 137m Ba and 154 Obtain Eu radioactivity.

[0064] In this decay heat evaluation method, 137 The radioactivity of Cs is calculated (step S15). 137m Ba and 137 Since the conditions for permanent equilibrium are met for Cs, 137 The Cs radioactivity was obtained in step S14. 137m This is the same amount of radioactivity as Ba.

[0065] In this decay heat evaluation method, the decay heat of the spent nuclear fuel X (irradiated nuclear fuel) is finally calculated (step S16). From the data listed in "JENDL / DDF-2015", the calculated value Q of the sum of the beta ray energy per beta decay heat is β and the calculated sum of gamma ray energies Q γ Read out radioactivity × (Q β +Q γ )of 134 Cs, 137 Cs, 137m Ba, 154 EU, 90 Sr, 90 Y, 144 Ce, 144 Pr, 106 Ru and 106Add up the Rh. This total value indicates the amount of heat generated from nuclides that account for 75-90% or more of the decay heat of spent nuclear fuel X.

[0066] As described above, the decay heat evaluation method according to this embodiment is realized by a series of processes from steps S9 to S16. 134 Cs, 137 Cs, 137m Ba, 154 EU, 90 Sr, 90 Y, 144 Ce, 144 Pr, 106 Ru and 106 Since the decay heat of spent nuclear fuel X is calculated based on the sum of the energy obtained from each radioactivity of Rh, it is possible to obtain the data non-destructively and to significantly improve the reliability of decay heat evaluation by radiation measurement.

[0067] Here, the radioactivity of typical light water reactor fuel burned at burnups of 13 GWd / t, 26 GWd / t, 39 GWd / t and 52 GWd / t for 406 days, 2 x 406 days, 3 x 406 days and 4 x 406 days and then cooled (removed from the core and simply stored) was calculated using the ORIGEN-ARP module of the SCALE-6.1 code developed by Oakridge National Laboratory in the United States.

[0068] Figure 6(a) shows the percentage of total decay heat obtained based on this calculation. 90 Sr → 90 Y. The proportion of these two nuclides in the decay heat is the largest between 1 and 20 years after removal, as shown in Figure 6(a), which shows that radioactivity evaluation of these two nuclides is essential for decay heat evaluation.

[0069] Figure 6(b) shows the ratio of measurable nuclides to the total decay heat. In this figure, the ratio of measurable nuclides to the total decay heat measured by SKB in Sweden is 134 Cs, 137 Cs and 154The decay heat rate that can be evaluated from the Eu gamma ray count rate is shown by a line, and the decay heat rate that can be evaluated from the neutron measurement is shown by a line. 244 The radioactivity of Cm and the radioactivity evaluation method according to this embodiment 90 Sr → 90 Y, 106 Ru→ 106 Rh, 144 Ce→ 144 The decay heat that can be evaluated from the Pr radioactivity is shown by dots.

[0070] According to Fig. 6(b), for the three conventional nuclides, the proportion of decay heat that could be actually measured was around 40% even after five years of cooling, but after five years of cooling, it becomes around 90%. This shows that the proportion of decay heat that must be compensated for by numerical calculation is greatly reduced. Therefore, by evaluating decay heat using the radioactivity of each nuclide obtained by the radioactivity evaluation method according to this embodiment, the reliability of the decay heat evaluation can be greatly improved.

[0071] The present invention is not limited to the above-described embodiment, and the following modifications are possible. (1) In the above embodiment, the evaluation target is the spent nuclear fuel X (irradiated nuclear fuel) stored in the spent fuel pool B, but the present invention is not limited to this. That is, the evaluation target of the present invention is irradiated nuclear fuel represented by the spent nuclear fuel X.

[0072] (2) In the above embodiment, as shown in FIGS. 2(a) and 2(b), the HP-Ge detector 1 is immersed in the shielding water C of the spent fuel pool B, or the gamma rays and braking rays emitted from the spent nuclear fuel X are detected through the collimator D embedded in the wall of the spent fuel pool B. However, the present invention is not limited to this.

[0073] (3) In the above embodiment, the detailed configuration of the measuring device A is shown in Fig. 1, but the present invention is not limited to this. In other words, the configuration of the measuring device used in the radioactivity evaluation method and decay heat evaluation method according to the present invention is not limited to the configuration shown in Fig. 1. [Explanation of symbols]

[0074] A Measuring device X Spent nuclear fuel (irradiated nuclear fuel) 1 HP-Ge detector 2 Preamplifier 3. Linear Amplifier 4. Wave height analyzer 5. Computers

Claims

1. A first step of detecting gamma rays and bremsstrahlung rays emitted from an irradiated nuclear fuel with a detector and measuring a spectral distribution indicating a distribution of count rates of the gamma rays and the bremsstrahlung rays; a second step of counting the gamma rays emitted from 106 Rh and 144 Pr from the spectral distribution; A third step of calculating a response function and a detection efficiency in measuring the beta rays and the gamma rays emitted from 90 Y, 106 Rh, and 144 Pr contained in the irradiated nuclear fuel; Using the count rate of 90 Y and the value obtained from the detection efficiency calculated in the third step, 90 Sr radioactivity and 90 a fourth step of assessing the radioactivity of Y; A method for evaluating radioactivity, comprising the steps of:

2. The method for evaluating radioactivity as described in claim 1, characterized in that the fourth step evaluates the radioactivity based on the gamma rays and the braking rays having energies in the range of 1.6 to 1.7 MeV.

3. The method for radioactivity evaluation according to claim 1 or 2, characterized in that in the fourth step, the count rate of the gamma rays obtained in the second step is used as the count rate of 90 Y.

4. The fourth step comprises calculating an absolute value of a gamma ray generation spectrum and an absolute value of an electron beam generation spectrum using the gamma ray count rate obtained in the second step and the detection efficiency calculated in the third step; calculating count rates of 106 Rh and 144 Pr based on the absolute value of the gamma ray emission spectrum and the absolute value of the electron beam emission spectrum; calculating a count rate of 90 Y by subtracting the count rates of 106 Rh and 144 Pr from the count rate of the gamma rays obtained in the second step; The method for evaluating radioactivity according to claim 1 or 2, further comprising:

5. the irradiated nuclear fuel is spent nuclear fuel contained in a spent fuel pool; The method for evaluating radiation according to any one of claims 1 to 4, wherein the brake line is detected by immersing the detector in shielding water of the spent fuel pool.

6. The method for evaluating radioactivity according to claim 5, wherein the detector is housed in a watertight container and immersed in the shielding water.

7. the irradiated nuclear fuel is spent nuclear fuel contained in a spent fuel pool; 5. The method for evaluating radiation according to claim 1, wherein the detector detects the brake line through a collimator embedded in a wall of the spent fuel pool.

8. A decay heat evaluation method, characterized in that the decay heat of the irradiated nuclear fuel is evaluated based on the radioactivity evaluated by the radioactivity evaluation method described in any one of claims 1 to 7.

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