SE0950394A0 - A method for measuring the distribution of voids in the refrigerant of a model nuclear reactor - Google Patents

Description

10 15 20 25 30 35 The United States Patent 5,098,641 describes a method and apparatus for measuring sub-channel voids in a light water reactor test fuel assembly. The emulated fuel bundle has individually electrically heated hollow rods. A gamma-emitting source is placed on a probe and mounted for vertical excursion inside a selected emulated hollow fuel rod. A detector, typically a Geiger-Muller counter, is placed for corresponding vertical excursion inside another and preferably adjaceht fuel rod. The intensity of gamma radiation transmitted from the source to the detector through the walls of the emulated fuel rods and across the coolant volume is measured. By utilizing a relatively low energy gamma source, the attenuation of the gamma signal registered by the detector can be directly attributed to gamma absorption in the liquid moderator fraction between the emulated fuel rod containing the detector and the emulated fuel rod containing the source. The apparatus and method described will, however, only provide the void distribution in a limited part of the assembly volume. 10 15 20 25 30 35 The United States Patent 5,098,641 describes a method and apparatus for measuring sub-channel voids in a light water reactor test fuel assembly. The emulated fuel bundle has individually electrically heated hollow rods. A gamma-emitting source is placed on a probe and mounted for vertical excursion inside a selected emulated hollow fuel rod. A detector, typically a Geiger-Muller counter, is placed for corresponding vertical excursion inside another and preferably adjaceht fuel rod. The intensity of gamma radiation transmitted from the source to the detector through the walls of the emulated fuel rods and across the coolant volume is measured. By utilizing a relatively low energy gamma source, the attenuation of the gamma signal registered by the detector can be directly attributed to gamma absorption in the liquid moderator fraction between the emulated fuel rod containing the detector and the emulated fuel rod containing the source. The apparatus and method described will, however, only provide the void distribution in a limited part of the assembly volume.

Summarv of the invention An object of the present invention is to provide a method for determining the void distribution in a fuel assembly.Summarv of the invention An object of the present invention is to provide a method for determining the void distribution in a fuel assembly.

Another object of the present invention is to provide a method which enables the determination of the void distribution everywhere in the coolant volume of a fuel assembly, efficiently and accurately.Another object of the present invention is to provide a method which enables the determination of the void distribution everywhere in the coolant volume of a fuel assembly, efficiently and accurately.

At least one of these objects is fulfilled with a method according to the independent claim.At least one of these objects is fulfilled with a method according to the independent claim.

Further advantages are achieved with the features of the dependent claims.Further advantages are achieved with the features of the dependent claims.

A basic idea of the present invention is to provide a method for measuring the void distribution in the liquid coolant in a model 10 15 20 25 30 35 reactor tank by introducing a radioactive isotope into the coolant and producing an image of the distribution of the radioactive isotope using tomographic reconstruction.A basic idea of the present invention is to provide a method for measuring the void distribution in the liquid coolant in a model 10 15 20 25 30 35 reactor tank by introducing a radioactive isotope into the coolant and producing an image of the distribution of the radioactive isotope using tomographic reconstruction.

The radioactive isotope may be provided in the form of a chemical compound, called a radio-tracer, which comprises the radioactive isotope. This is the only practical way of providing some radioactive isotopes as they may not easily be chemically isolated.The radioactive isotope may be provided in the form of a chemical compound, called a radio-tracer, which comprises the radioactive isotopes. This is the only practical way of providing some radioactive isotopes as they may not easily be chemically isolated.

According to the invention a method of measuring the void distribution in the liquid coolant of a model nuclear reactor is provided. The model nuclear reactor comprises a reactor tank with a length axis, electrically heated rods arranged in the reactor tank, and a coolant arranged in the reactor tank surrounding the rods.According to the invention a method of measuring the void distribution in the liquid coolant of a model nuclear reactor is provided. The nuclear reactor model comprises a reactor tank with a length axis, electrically heated rods arranged in the reactor tank, and a coolant arranged in the reactor tank surrounding the rods.

The method is characterized in that it comprises the steps of providing a gamma emitting radioactive isotope into the coolant of the model nuclear reactor, providing an electrical current to the rods for heating of the rods, measuring the gamma radiation in a number of different positions around the length axis on the outside of the reactor tank, and determining an image of the distribution of the radioactive isotope, and thus also the void distribution, by using the measurements of the gamma radiation and tomographic reconstruction.The method is characterized in that it comprises the steps of providing a gamma emitting radioactive isotope into the coolant of the model nuclear reactor, providing an electrical current to the rods for heating of the rods, measuring the gamma radiation in a number of different positions around the length axis on the outside of the reactor tank, and determining an image of the distribution of the radioactive isotope, and thus also the void distribution, by using the measurements of the gamma radiation and tomographic reconstruction.

With a method according to the present invention the measurement is less distorted by noise and is geometrically more complete as compared with methods according to the prior art. This is due to the fact that methods according to the prior art are relying on gamma attenuation as the basic mechanism for characterizing the distribution of coolant and thus also the distribution of void.With a method according to the present invention the measurement is less distorted by noise and is geometrically more complete as compared with methods according to the prior art. This is due to the fact that methods according to the prior art are relying on gamma attenuation as the basic mechanism for characterizing the distribution of coolant and thus also the distribution of void.

Unfortunately, for such methods, the attenuation in structure material, such as the reactor tank and in the heated rods themsetves, is stronger than in the coolant. This is true for most coolants and in particular for water and deteriorates the signal-to- noise ratio. The method according to the present invention relies on gamma rays which are emitted from the actual coolant. This allows 10 150 20 25 30 35 the use of more accurate tomographic reconstruction techniques, which are less sensitive to attenuation effects.Unfortunately, for such methods, the attenuation in structure material, such as the reactor tank and in the heated rods themsetves, is stronger than in the coolant. This is true for most coolants and in particular for water and deteriorates the signal-to- noise ratio. The method according to the present invention relies on gamma rays which are emitted from the actual coolant. This allows 10 150 20 25 30 35 the use of more accurate tomographic reconstruction techniques, which are less sensitive to attenuation effects.

The coolant may flow along the rods and be re-introduced via an external loop, or it may be static.The coolant may flow along the rods and be re-introduced via an external loop, or it may be static.

Furthermore, a method according to the present invention also provides for the use of gamma photons with higher energy than in methods according to the prior art where the photon energy has to be chosen so that the gamma radiation is sufficiently attenuated in the coolant.Furthermore, a method according to the present invention also provides for the use of gamma photons with higher energy than in methods according to the prior art where the photon energy has to be chosen so that the gamma radiation is sufficiently attenuated in the coolant.

Tomographic reconstruction is a well known method and will therefore not be described in detail herein.Tomographic reconstruction is a well known method and will therefore not be described in detail herein.

An image of the distribution of the radioactive isotope may be determined for at least one plane being essentially perpendicular to the length axis. by using measurements from positions situated in the plane. ln this way an image of the distribution of radioactive isotopes, and thus also the distribution of void, in the plane will be provided. lt is of course also possible to provide an image of the distribution of the radioactive isotope from measurements at different heights along the length axis.An image of the distribution of the radioactive isotope may be determined for at least one plane being essentially perpendicular to the length axis. by using measurements from positions situated in the plane. ln this way an image of the distribution of radioactive isotopes, and thus also the distribution of void, in the plane will be provided. lt is of course also possible to provide an image of the distribution of the radioactive isotope from measurements at different heights along the length axis.

An image of the radiotracer distribution may be determined for a number of planes along the length axis. ln this way, a 3-dimensional image of the distribution of void may be provided.An image of the radiotracer distribution may be determined for a number of planes along the length axis. ln this way, a 3-dimensional image of the distribution of void may be provided.

Measurements from single positions may be used as input for the tomographic reconstruction, i.e. the gamma radiation measured in one point is used as one of a number of input data for the tomographic reconstruction.Measurements from single positions may be used as input for the tomographic reconstruction, i.e. the gamma radiation measured in one point is used as one of a number of input data for the tomographic reconstruction.

The radioactive isotope may be chosen so that it emits gamma photons having an energy of at least 500 keV, and preferably at 10 15 20 25 30 35 least 1 MeV. Such gamma photons are subject to low attenuation in the reactor walls, the rods and the coolant. This is advantageous as this provides for a strong signal at the detector and, thus, a low noise level in the measurements.The radioactive isotope may be chosen so that it emits gamma photons having an energy of at least 500 keV, and preferably at 10 15 20 25 30 35 at least 1 MeV. Such gamma photons are subject to low attenuation in the reactor walls, the rods and the coolant. This is advantageous as this provides for a strong signal at the detector and, thus, a low noise level in the measurements.

The desirable low attenuation of the gamma rays is an important difference from the methods according to the prior art according to which it is desirable to have a measurable attenuation in the coolant in order to be able to detect the voids in the coolant. However, photons having an energy making them subject to attenuation in the coolant are also subject to attenuation in the reactor walls and the rods. Thus, the signal in the methods according to the prior art is only a small ripple on a high background level. This makes the methods according to the prior art sensitive to noise.The desirable low attenuation of the gamma rays is an important difference from the methods according to the prior art according to which it is desirable to have a measurable attenuation in the coolant in order to be able to detect the voids in the coolant. However, photons having an energy making them subject to attenuation in the coolant are also subject to attenuation in the reactor walls and the rods. Thus, the signal in the methods according to the prior art is only a small ripple on a high background level. This makes the methods according to the prior art sensitive to noise.

The radioactive isotope is preferably chosen from the group comprising 24Na, WLa, 59Fe, ”Ca 46Sc, 5800, mi, G5Zn, 54Mn, 22Na, and 6°Co.The radioactive isotope is preferably chosen from the group comprising 24Na, WLa, 59Fe, ”Ca 46Sc, 5800, mi, G5Zn, 54Mn, 22Na, and 6 ° Co.

The measurement of the gamma radiation in each position may be performed as a detection of gamma photons which are coincident in time with gamma photons detected in an opposite position. This may be achieved by using a radioactive isotope which emits positrons. The positrons will annihilate on contact with electrons after travelling a short distance. ln water this distance is approximately equal to 1 mm. Each annihilation produces two 511 keV photons travelling in opposite directions. According to the second embodiment of the invention these photons are detected with pairs of opposite detectors such as sodium iodide scintillators or Geiger counters, An advantage of using an isotope emitting positrons is that collimation of the gamma photons travelling to the detectors is not needed as long as the opposing detectors are sufficiently smalt or are placed at sufficiently large distance from each other. Also, shielding against background radiation emitted from above and below the measurement plane is not needed as long as the rate of random Coincidences is kept sufficíently low. 10 15 20 25 30 The radioactive isotope may be wF. “F decays into stable 150 with a decay half-life of 110 minutes, which means that the model reactor does not have to be decontaminated after the measurement.The measurement of the gamma radiation in each position may be performed as a detection of gamma photons which are coincident in time with gamma photons detected in an opposite position. This may be achieved by using a radioactive isotope which emits positrons. The positrons will annihilate on contact with electrons after traveling a short distance. ln water this distance is approximately equal to 1 mm. Each annihilation produces two 511 keV photons traveling in opposite directions. According to the second embodiment of the invention these photons are detected with pairs of opposite detectors such as sodium iodide scintillators or Geiger counters, An advantage of using an isotope emitting positrons is that collimation of the gamma photons traveling to the detectors is not needed as long as the opposing detectors are sufficiently smalt or are placed at sufficiently large distance from each other. Also, shielding against background radiation emitted from above and below the measurement plane is not needed as long as the rate of random Coincidences is kept sufficíently low. 10 15 20 25 30 The radioactive isotope may be wF. “F decays into stable 150 with a decay half-life of 110 minutes, which means that the model reactor does not have to be decontaminated after the measurement.

The gamma radiation may be measured with a detector such as, e.g., a sodium iodide scintillator or another type of gamma spectrometer. These detectors provide information on the energy of the gamma photons . The energy information is generally needed in order to provide a high-quality reconstruction in case that gamma photons having different energies are emitted from the radioactive isotope. Furthermore, sodium iodide crystals provide a high efficiency, a sufficient energ-y resolution and are relatively inexpensive as well as easy to use as they do not require cooling.The gamma radiation may be measured with a detector such as, e.g., a sodium iodide scintillator or another type of gamma spectrometer. These detectors provide information on the energy of the gamma photons. The energy information is generally needed in order to provide a high-quality reconstruction in case that gamma photons having different energies are emitted from the radioactive isotopes. Furthermore, sodium iodide crystals provide a high efficiency, a sufficient energy-y resolution and are relatively inexpensive as well as easy to use as they do not require cooling.

The method may comprise the step of initiating a disturbance in the supplied electrical power, the coolant flow, the coolant temperature, or the reactor tank pressure. This provides the opportunity of determining what happens with the distribution of the coolant during, and immediately after the disturbance. ln the following preferred embodiments of the invention will be described with reference to the appended drawings.The method may comprise the step of initiating a disturbance in the supplied electrical power, the coolant flow, the coolant temperature, or the reactor tank pressure. This provides the opportunity of determining what happens with the distribution of the coolant during, and immediately after the disturbance. ln the following preferred embodiments of the invention will be described with reference to the appended drawings.

Short description of the drawinos Fig .1 shows in a side view a model model nuclear reactor on which a measurement of the coolant void distribution according to a first embodiment of the present invention is performed.Short description of the drawinos Fig. 1 shows in a side view a model nuclear reactor on which a measurement of the coolant void distribution according to a first embodiment of the present invention is performed.

Fig 2 shows in a top view the model nuclear reactor of Fig 1.Fig. 2 shows in a top view the model nuclear reactor of Fig. 1.

Fig 3 shows in a top view the model nuclear reactor of Fig 1 when a measurement of the void distribution according to a second embodiment of the present invention is performed. 10 15 20 25 30 35 Description of preferred embodiments ln the following description of preferred embodiments of the invention similar features will be denoted with the same reference numera! in the different figures.Fig 3 shows in a top view the model nuclear reactor of Fig 1 when a measurement of the void distribution according to a second embodiment of the present invention is performed. 10 15 20 25 30 35 Description of preferred embodiments ln the following description of preferred embodiments of the invention similar features will be denoted with the same reference nowadays! in the different figures.

Fig 1 shows in a side view a model model nuclear reactor 1, which comprises a reactor tank 2 with a length axis 3. Electrically heated rods 4 are arranged, essentially parallel to the length axis 3, in the reactor tank 2. A coolant, which in the described embodiment is water, is arranged in the reactor tank 2 surrounding the rods 4. Due to the heating of the rods 4 the coolant will be heated so that it vaporizes into voids 5 in the form of steam bubbles. The coolant is forced to flow upwards through the reactor tank 2 from the inlet tube 6 to the outlet tube 7. The coolant that leaves the outlet tube 7 is transferred to a heat exchanger (not shown) in order to cool off and is the returned into the reactor tank through the inlet tube 6. During the flow through the reactor tank 2 the coolant will heat and will be hotter in the top of the reactor tank. Thus, voids 5 are more common in the top of the reactor tank 2. A detector 8, such as a sodium iodide (Nal) scintillator, is connected to a control unit 9 with a wire 10. The control unit 9 may be an ordinary computer. The detector 8 may be positioned in any one of a number of different heights of which a few are shown by dotted lines.Fig 1 shows in a side view a model nuclear reactor 1, which comprises a reactor tank 2 with a length axis 3. Electrically heated rods 4 are arranged, essentially parallel to the length axis 3, in the reactor tank 2. A coolant, which in the described embodiment is water, is arranged in the reactor tank 2 surrounding the rods 4. Due to the heating of the rods 4 the coolant will be heated so that it vaporizes into voids 5 in the form of steam bubbles. The coolant is forced to flow upwards through the reactor tank 2 from the inlet tube 6 to the outlet tube 7. The coolant that leaves the outlet tube 7 is transferred to a heat exchanger (not shown) in order to cool off and is the returned into the reactor tank through the inlet tube 6. During the flow through the reactor tank 2 the coolant will heat and will be hotter in the top of the reactor tank. Thus, voids 5 are more common in the top of the reactor tank 2. A detector 8, such as a sodium iodide (Nal) scintillator, is connected to a control unit 9 with a wire 10. The control unit 9 may be an ordinary computer. The detector 8 may be positioned in any one of a number of different heights of which a few are shown by dotted lines.

Fig 2 shows in a top view the model nuclear reactor 1 of Fig 1 in a cross section along the line A-A. The detector 8 may be positioned in a number of positions for each height of which a few positions are shown by dotted lines in Fig 2. Fig 2 shows a plane which is perpendicular to the length axis 3 in which a measurement of the void distribution is to be performed. ln order to measure the coolant void distribution a radioactive isotope is introduced into the coolant, possibly in the form of a chemical compound, called a radio-tracer, which comprises the radioactive isotope. A detector 8 such as a sodium iodide (Nal) scintillator is arranged in a number of different positions, in the 10 15 20 25 30 35 same plane perpendicular to the length axis 3, on the outside of the reactor tank 2. The gamma counts are registered by the control unit for a predetermined time period in each position. After measurement of the gamma counts in all positions in one measurement plane, an image of the distribution of the radioactive isotope, and thus also the distribution of the radio-tracer, may be determined using the measurements of the gamma radiation and tomographic reconstruction. As the radioactive isotope is evenly distributed in the coolant, the image of the distribution of the radioactive isotope corresponds to the distribution of the coolant.Fig. 2 shows in a top view the model nuclear reactor 1 or Fig. 1 in a cross section along the line A-A. The detector 8 may be positioned in a number of positions for each height of which a few positions are shown by dotted lines in Fig 2. Fig 2 shows a plane which is perpendicular to the length axis 3 in which a measurement of the void distribution is to be performed. ln order to measure the coolant void distribution a radioactive isotope is introduced into the coolant, possibly in the form of a chemical compound, called a radio-tracer, which comprises the radioactive isotope. A detector 8 such as a sodium iodide (Nal) scintillator is arranged in a number of different positions, in the 10 15 20 25 30 35 same plane perpendicular to the length axis 3, on the outside of the reactor tank 2. The gamma counts are registered by the control unit for a predetermined time period in each position. After measurement of the gamma counts in all positions in one measurement plane, an image of the distribution of the radioactive isotope, and thus also the distribution of the radio-tracer, may be determined using the measurements of the gamma radiation and tomographic reconstruction. As the radioactive isotope is evenly distributed in the coolant, the image of the distribution of the radioactive isotope corresponds to the distribution of the coolant.

For the measurement of the gamma radiation in different directions it is possible to either use one or more detectors 8 which are moved to different positions or a large number of detectors 8 which are fixed in different positions.For the measurement of the gamma radiation in different directions it is possible to either use one or more detectors 8 which are moved to different positions or a large number of detectors 8 which are fixed in different positions.

The detector(s) 8 are preferably shielded by means of, e.g., lead I blocks (not shown) in order to restrict the detection of gamma rays to a small part of the object in a plane. lt ís possible to use isotopes emitting gamma photons with an energy below 500 keV. lt is, however, favourable to choose an isotope which emits gamma photons with an energy of at least 500 keV, and preferably at least 1 MeV. At such an energy the gamma radiation will be less attenuated by the wall, the rods, and the coolant as compared to the case of using an isotope producing photons having a lower energy.The detector (s) 8 are preferably shielded by means of, e.g., lead I blocks (not shown) in order to restrict the detection of gamma rays to a small part of the object in a plane. lt is possible to use isotopes emitting gamma photons with an energy below 500 keV. lt is, however, favorable to choose an isotope which emits gamma photons with an energy of at least 500 keV, and preferably at least 1 MeV. At such an energy the gamma radiation will be less attenuated by the wall, the rods, and the coolant as compared to the case of using an isotope producing photons having a lower energy.

Fig 3 shows in a top view the model nuclear reactor of Fig 1 when a measurement of the void distribution according to a second embodiment of the present inventíon is performed. According to this second embodiment of the inventíon, a positron emitting isotope is used. An example of a suitable radioactive isotope is 'BF which decays into stable 180.Fig 3 shows in a top view the model nuclear reactor of Fig 1 when a measurement of the void distribution according to a second embodiment of the present inventíon is performed. According to this second embodiment of the invention, a positron emitting isotope is used. An example of a suitable radioactive isotope is' BF which decays into stable 180.

The resulting positrons subsequently annihilate on contact with electrons after travelling a short distance. ln water this distance is approximately equal to 1 mm. Each annihilation produces two 10 15 20 25 30 35 511 keV photons travelling in opposite directions. According to the second embodiment of the invention these photons are detected with pairs of opposite detectors. ln Fig 3 a first detector 11 is arranged opposite to a second detector 12 along the detection line 13. The detectors are connected to the same control unit 9 by means of a first wire 14 and a second wire 15, respectively.The resulting positrons subsequently annihilate on contact with electrons after traveling a short distance. ln water this distance is approximately equal to 1 mm. Each annihilation produces two 10 15 20 25 30 35 511 keV photons traveling in opposite directions. According to the second embodiment of the invention these photons are detected with pairs of opposite detectors. Fig. 3 is a first detector 11 arranged opposite to a second detector 12 along the detection line 13. The detectors are connected to the same control unit 9 by means of a first wire 14 and a second wire 15, respectively.

The control unit 9 is arranged in such a way that only detection events occurring within a pre-determined time-window are regarded as being coincident and are counted. The detected coincidence events correspond to different lines 16 of projection throughthe reactor tank 2 (cf. Fig 3), and are used in tomographic reconstruction to create an image of the distribution of the radioactive isotope, which, according to the above, corresponds to the distribution of the coolant.The control unit 9 is arranged in such a way that only detection events occurring within a pre-determined time-window are regarded as being coincident and are counted. The detected coincidence events correspond to different lines 16 of projection throughthe reactor tank 2 (cf. Fig 3), and are used in tomographic reconstruction to create an image of the distribution of the radioactive isotope, which, according to the above, corresponds to the distribution of the coolant.

An advantage of the technique according to the second embodiment is that collimation of the gamma rays travelling to the detectors 11, 12, is not needed if the opposing detectors 11, 12, are sufficiently small or are placed at a sufficiently large distance from each other.An advantage of the technique according to the second embodiment is that collimation of the gamma rays traveling to the detectors 11, 12, is not needed if the opposing detectors 11, 12, are sufficiently small or are placed at a sufficiently large distance from each other.

Also, shielding against background radiation emitted from above and below the measurement plane is not needed as long as the rate of random coincidences is kept sufficiently low. A drawback with the technique according to the second embodiment is the inherent positional inaccuracy due to the finite positron range and the non- collinearity of the annihilation photons. Furthermore, the 511 keV photons will experience a stronger attenuation as compared with the higher energy photons that may be used in the method according to the first embodiment ofthe present invention.Also, shielding against background radiation emitted from above and below the measurement plane is not needed as long as the rate of random coincidences is kept sufficiently low. A drawback with the technique according to the second embodiment is the inherent positional inaccuracy due to the finite positron range and the non- collinearity of the annihilation photons. Furthermore, the 511 keV photons will experience a stronger attenuation as compared with the higher energy photons that may be used in the method according to the first embodiment of the present invention.

A disturbance may be initiated in the supplied electrical power, the coolant flow, the coolant temperature or the reactor tank pressure in the model reactor. The distribution of the coolant is determined during, and immediately after, the disturbance using one of the above described methods. 10 10 The embodiments described above may be amended in many ways without departing from the spirit and scope of the present invention which is limited only by the appended ciaims. it is possible to use other radioactive isotopes than ”F in a method according to the second embodiment of the present invention. lt is possibie to use other detectors than sodium iodide (Nal) scintillators.A disturbance may be initiated in the supplied electrical power, the coolant flow, the coolant temperature or the reactor tank pressure in the model reactor. The distribution of the coolant is determined during, and immediately after, the disturbance using one of the above described methods. 10 10 The embodiments described above may be amended in many ways without departing from the spirit and scope of the present invention which is limited only by the appended ciaims. it is possible to use other radioactive isotopes than ”F in a method according to the second embodiment of the present invention. lt is possibie to use other detectors than sodium iodide (Nal) scintillators.

Claims (12)

10 15 20 25 'so 35 11 CLAIMS10 15 20 25 'so 35 11 CLAIMS 1. A method of measuring the void (5) distribution in the coolant of a model nuclear reactor (1), which comprises a reactor tank (2) with a length axis (3), electrically heated rods (4) arranged in the reactor tank (2), and a coolant arranged in the reactor tank (2) surrounding the rods (4), characterized in that the method comprises the steps of providing a gamma emitting radioactive isotope into the coolant of the model nuclear reactor (1), providing an electrical current to the rods (4) for heating of the rods (4), measuring the gamma radiation in a number of different positions around the length axis on the outside of the reactor tank (2), and determining an image of the distribution of the radioactive isotope, and thus also the void (5) distribution, by using the measurements of the gamma radiation and tomographic reconstruction.A method of measuring the void (5) distribution in the coolant of a model nuclear reactor (1), which comprises a reactor tank (2) with a length axis (3), electrically heated rods (4) arranged in the reactor tank (2), and a coolant arranged in the reactor tank (2) surrounding the rods (4), characterized in that the method comprises the steps of providing a gamma emitting radioactive isotope into the coolant of the model nuclear reactor (1), providing an electrical current to the rods (4) for heating the rods (4), measuring the gamma radiation in a number of different positions around the length axis on the outside of the reactor tank (2), and determining an image of the distribution of the radioactive isotope, and thus also the void (5) distribution, by using the measurements of the gamma radiation and tomographic reconstruction. 2. The method according to claim 1, wherein the radioactive isotope is provided in the form of a chemical compound, called a radio- tracer, which comprises the radioactive isotope.The method according to claim 1, wherein the radioactive isotope is provided in the form of a chemical compound, called a radio-tracer, which comprises the radioactive isotope. 3. The method according to claim 1 or 2, wherein an image of the radioactive isotope distribution is determined for at least one plane being essentially perpendicular to the length axis (3), by using measurements from positions situated in the plane.The method according to claim 1 or 2, wherein an image of the radioactive isotope distribution is determined for at least one plane being essentially perpendicular to the length axis (3), by using measurements from positions situated in the plane. 4. The method according to claim 3, wherein an image of the distribution of the radioactive isotope is determined for a number of planes along the length axis (3).The method according to claim 3, wherein an image of the distribution of the radioactive isotope is determined for a number of planes along the length axis (3). 5. The method according to any one of claims 1-4, wherein the measurements from single positions are used as input for the tomographic reconstruction. 10 15 20 25 125. The method according to any one of claims 1-4, wherein the measurements from single positions are used as input for the tomographic reconstruction. 10 15 20 25 12 6. The method according to claim 5, wherein the radioactive isotope emits gamma photons having an energy of at least 500 keV, and preferably at least 1 MeV.The method according to claim 5, wherein the radioactive isotope emits gamma photons having an energy of at least 500 keV, and preferably at least 1 MeV. 7. The method according to claim 6, wherein the radioactive isotope is chosen from the group comprising 24Na, 14°La, 59Fe, “7Ca, 4680, 5800, mi, 65Zn, “Mm 22Na, and 6°Co.The method according to claim 6, wherein the radioactive isotope is chosen from the group comprising 24Na, 14 ° La, 59Fe, “7Ca, 4680, 5800, mi, 65Zn,“ Mm 22Na, and 6 ° Co. 8. The method according to any one of claims 1-4, wherein the measurement of the gamma radiation in each position is performed as a measurement of gamma photons which are coincident in time with gamma photons measured in an opposite position.The method according to any one of claims 1-4, wherein the measurement of the gamma radiation in each position is performed as a measurement of gamma photons which are coincident in time with gamma photons measured in an opposite position. 9. The method according to claim 8, wherein the radioactive isotope emits positrons. VThe method according to claim 8, wherein the radioactive isotope emits positrons. V 10. The method according to claim 9, wherein the radioactive isotope is wF.The method according to claim 9, wherein the radioactive isotope is wF. 11. The method according to any one of the preceding claims, wherein the gamma radiation is measured with a sodium iodide (Nal) scintillator.11. The method according to any one of the preceding claims, wherein the gamma radiation is measured with a sodium iodide (Nal) scintillator. 12. The method according to any one of claims 1-11, comprising the step of initiating a disturbance in the supplied electrical power, the coolant flow, the coolant temperature, orthe reactor tank pressure.12. The method according to any one of claims 1-11, comprising the step of initiating a disturbance in the supplied electrical power, the coolant flow, the coolant temperature, orthe reactor tank pressure.