WO2001095340A1

WO2001095340A1 - Method and apparatus for the determination of radioactivity distribution in nuclear fuel assemblies

Info

Publication number: WO2001095340A1
Application number: PCT/SE2001/001205
Authority: WO
Inventors: Ane HÅKANSSON; Peter Jansson; Anders BÄCKLIN; Staffan Jacobsson
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-06-05
Filing date: 2001-05-30
Publication date: 2001-12-13
Anticipated expiration: 2002-12-05
Also published as: AU6285301A

Abstract

A method and apparatus for tomographic determination of radioactivity distribution in a nuclear fuel assembly, comprising the steps of (a) detecting the radiation intensity in controlled angular and lateral positions about the fuel assembly, in one or more axial positions thereof, (b) defining a pattern of activity-elements, the shape and position of each activity-element being determined from the known and nominal internal geometry of the fuel assembly, said activity-elements representing portions that may emit radiations, (c) determination of a contribution coefficient from each activity-element in each controlled position, and (d) reconstructing the radioactivity of each activity-element. In a preferred embodiment, the method further comprises the step of detecting (e) possible displacement of individual rods in the fuel assembly and introducing the findings in the reconstruction process.

Description

TITLE

METHOD AND APPARATUS FOR THE DETERMINATION OF

RADIOACTIVITY DISTRIBUTION IN NUCLEAR FUEL ASSEMBLIES

TECHNICAL FIELD

This invention relates to methods and apparatuses for determination of radioactivity distribution in nuclear fuel assemblies. More specifically, the invention relates to a tomographic method for determination of the radioactivity and related fuel parameters such as e.g. thermal power of individual fuel rods in a fuel assembly. In a further development of the invention, possible displacements of individual fuel rods from their nominal position in the assembly are detected. The invention also relates to an apparatus that is designed for implementation of the method.

As used herein, the expression "tomography" refers to the reconstruction and representation of internal distribution of radiation sources based on external measurements of emitted radiation from an integral fuel assembly. Specifically, the tomographic representation referred to in this document involves Single Photon Emission Computed Tomography (SPECT) .

PRIOR ART

From EP-A1-0 684 611 is known a method for detecting the emitted radiation in a plane perpendicular to the longitudinal axis of a fuel assembly. The recorded radiation intensity data is computed to produce a tomographic im- age of the radiation intensity distribution over a section of the fuel assembly, thus revealing possible failure of individual fuel rods in the assembly and identification of the subject rods.

In the SKI Report 98: 17, "A tomographic method for verification of the integ- rity of spent nuclear fuel" published by the Swedish Nuclear Power Inspectorate, March 1998, a tomographic method is disclosed wherein physical properties of the set-up such as attenuation and solid angle of the exposed area of the detector are introduced in the reconstruction process.

It is also known in the art to estimate, by calculations, the thermal power of individual rods in a nuclear fuel core. In order to verify the accuracy of these calculations, individual rods are pulled out of the assembly and scanned for gamma radiation about the periphery of the rod. The procedure is time consuming and requires adequate security measures, and is thus costly to implement in full scale.

OBJECTS OF THE INVENTION

A main issue for producers of nuclear energy is to utilize the nuclear fuel to its maximum while maintaining the safety margins. To that end, new core physics calculation codes have been implemented that calculate power out- take, burn-up etc., on a fuel rod level. An obvious problem faced in this implementation is to validate and verify the codes against experimentally obtained data. Such measurements are difficult to perform, as they normally require dismounting of the fuel assemblies in order to allow measurements on individual fuel rods. In addition, the fuel may eventually be reassembled and inserted into the reactor core for further irradiation. This handling and reintroducing of the fuel is accompanied by a high risk of hazards and production failures. Further, the procedure is expensive and inefficient since only a limited number of fuel rods can be measured at each occasion, i.e. typically once per year. During the power cycle the fuel assemblies are subject to stress, which may lead to displacements of individual fuel rods and distortion of the entire fuel assembly. Such changes in the nominal geometry affect fuel parameters such as e.g. the power outtake. Currently, displacements of individual fuel rods can be detected by visual inspection. However, with such a method displacements of inner rods are hard to detect and quantification is difficult. To be able to obtain experimental data of sufficient quality and adequacy, a new method shall be featured to provide basic data for calculations of various fuel parameters, e. g., power outtake and burn-up. The method may advantageously provide the following features: • Ability to register radiation from several isotopes and determine the corresponding activity distribution within fuel assemblies with high accuracy (preferably in the order of one percent in each fuel rod) .

• No dismounting or other extraordinary handling of the fuel assemblies shall be necessary. • The measurements shall be performed within a reasonable amount of time, i. e. special care should be taken regarding fast counting detector arrangements and correspondingly for the electronics and data acquisition.

Advantageously, the measuring apparatus is designed to allow for trans- portation between various measuring sites. Also, the extremely high radiation field from newly irradiated fuel assemblies governs to some extent the selection of the various materials incorporated in the structure. This aspect is important in order to maximize the service intervals.

It is therefore an object of this invention, to provide a method for determining the source concentration of various radiation emitters in nuclear fuel assemblies with high accuracy.

It is another object to provide a method for such determination without the need of dismounting or other hazardous handling of the assembly.

It is further an object of the invention to provide a method and apparatus for fast measurements at reasonable cost. It is still another object of the invention to provide a method for determining various fuel parameters on the rod level, e.g. power distribution.

It is yet a further object to provide a method for determining possible dis- placements of individual rods and distortions of a fuel assembly.

It is still a further object of the invention to provide combined information on fuel parameters and displacements of individual fuel rods and/ or distortion of the assembly.

These and other objects are met by the method and apparatus of the attached set of claims.

SUMMARY OF THE INVENTION By the invention there is suggested a method and apparatus for reconstruction of the internal distribution of radioactivity and thermal power in a fuel assembly, based on external detection of emitted radiation.

The method involves detection of emitted gamma quanta in various axial, angular and lateral positions about the fuel assembly, and reconstruction of the activity distribution within the assembly. Based on the collected radiation data, a tomographic reconstruction of the activity distribution in the fuel assembly is produced. In calculations, the reconstructed activities are brought to match the detected intensities.

According to the invention, there is introduced a pattern of activity-elements determined from the known nominal geometry of the measured object. The activity-elements are determined in aspects of shape and position to define a portion of a fuel rod, e.g., and each activity element is assigned an activity value in the reconstruction process. In a further development of the invention, reconstructed activities in geometrically related activity-elements are compared in order to obtain information about possible rod displacement. The actual position of individual rods is then introduced in the reconstruction procedure by the employment of a position indicator P.

For implementation of the inventive method, there is provided a pressurized vessel for accommodation of the detector equipment. The vessel is mobile and adapted for operation also in a submerged condition, and may e.g. be lowered to the bottom of a water-filled basin during operation. An axial through hole in the vessel is formed to support an inserted fuel assembly while permitting natural circulation of the basin water in surrounding con- tact with the fuel. In a preferred embodiment one or several gamma quanta counters or detectors are driven and guided for vertical rectalinear movement relative to the inserted fuel assembly, and for horizontal rotational and lateral movement relative to the fuel assembly. More precisely, a slotted radiation shield or collimator is rotatable into several angular positions to- gether with each gamma detector, and driven and guided for translatory motion in each angular position relative to the fuel assembly. The collimator and detector system is controlled and positioned for radiation intensity measurements on numerous vertical levels, and on each vertical level, in numerous angular and lateral displacements about the fuel assembly.

The collected data of radiation intensity is transferred from the detectors to be processed outside the vessel. In operational setup, a digital processor unit is associated with the vessel. The processor unit receives the detected radiation intensity data for analysis and reconstruction of the activity distri- bution in the fuel assembly. The processor unit also provides control data for the positioning of the collimator and detector system in the vessel, and for detecting the accurate spatial location relative to the measured object in each measuring position.

The process of evaluating the collected data involves an algebraic tomographic algorithm wherein a contribution coefficient matrix W is introduced. For each detecting position, each coefficient in the matrix is significant for the activity in a specific activity-element of the fuel assembly and describes the fraction of the emitted gamma quanta that reaches the detector in that position.

The tomographic algorithms are introduced in an executable reconstruction sequence, computed in the processor unit for displaying the reconstructed distribution of radioactivity and possible displacement of individual fuel elements of the integral fuel assembly.

DRAWINGS

The invention will be further described below, with reference made to the attached drawings. In the drawings,

Fig. 1 is a block diagram illustrating the steps of a method for determination of radioactivity distribution in a nuclear fuel assembly according to the invention;

Fig. 2a is a diagrammatic cross-sectional view of a typical nuclear fuel assembly to which the method is advantageously applied;

Fig. 2b illustrates an example of a pattern of activity-elements and non- activity-elements, determined from the sectional and spatial layout of the fuel assembly of fig. 2a and introduced in the tomographic reconstruction process as radiating elements that contribute to the total amount of gamma quanta detected in a defined detecting position;

Figs. 3a, 3b and 3c are examples of two-dimensional representations of activity-elements adapted to cover the fuel in a single rod;

Fig. 4 is a view corresponding to figs. 3a-3c, wherein four additional activity- elements are defined about the outer periphery of the activity-elements in figs. 3a-3c; Fig. 5a is a diagrammatic longitudinal section through the center of an apparatus for implementation of the method, and

Fig 5b is a diagrammatic, transversal section of the apparatus of fig. 5a.

DETAILED DESCRIPTION OF THE INVENTION

As for all tomographic methods, an initial step of the present method is a data collection step. The data collection comprises detection of emitted radiation from a nuclear fuel assembly in various axial, angular and lateral positions relative to the assembly. For each position at which the radiation intensity is detected, the relative position is measured with high accuracy. The intensity data and the position data are stored in the memory of a proc- essor unit.

The recorded radiation intensity data is representative for the emitted radiation over the energy interval of interest. Preferably, at each position, spectrum analysis is performed over the recorded energy interval, to provide the best possible intensity values for the gamma-radiation caused by the decay of the isotope/ s of interest. By performing spectrum analysis, contributions from background noise and high intensity peaks close in energy are diminished.

The energy range of interest is typically between 0 and 3000 keV. In a preferred implementation of the method, the tomographic reconstructions are based on the decay of the isotope Ba-140 which generates radiation at the energy 1596 keV. As the activity of Ba-140 is nearly proportional to the power outtake during the period of time prior to the measurement, the > power distribution between individual rods in the assembly may be achieved, in addition to the activity distribution. In a preferred implementation, the present method comprises the following steps further described below, and shown in fig 1 :

-providing detector equipment for radiation intensity detection movable in a gas tight vessel adapted to be submerged in water;

-forming the vessel for vertical accommodation of a nuclear fuel assembly while permitting natural circulation of water in surrounding contact about the fuel assembly;

-controlling the detector equipment for angular and lateral positioning about the fuel assembly, and for axial positioning relative to the fuel assembly;

-collecting radiation intensity data in each controlled position of the detector equipment, and

- treating the collected data in a tomographic reconstruction of the radioactivity of individual fuel rods in the fuel assembly while employing information about the internal structural layout of the fuel assembly in the recon- struction process.

A high-accuracy tomographic reconstruction of the radiation- source distribution in the assembly may be performed according to the following steps:

A two- or three-dimensional representation of the assembly is defined, dividing the assembly into two- or three-dimensional elements, here called activity-elements and non-activity elements. Information of the nominal geometry of the assembly is employed in order to determine the shape and po- sition of the activity-elements. As used herein, an activity-element is intro- duced as a representation for a portion of the fuel assembly that may emit radiation, and the activity of which is determined in the reconstruction pro- cess. The remaining parts of the fuel assembly, including the space filled with water are defined as non-activity elements, the activity of which is permanently set to zero throughout the reconstruction process.

In fig. 2a an example of a conventional fuel assembly 40 is shown in cross section, the fuel assembly 40 comprises a fuel assembly box 42, a water channel 46 and fuel rods 47. A fuel rod 47 comprises fuel 48, which is supported and encased by a cladding 44.

In fig. 2b an example of an activity-element pattern corresponding to the fuel assembly 40 of fig 2a is shown. In this example, the fuel 48 in each rod 47 is represented by five activity elements 50 each, and the remainder is represented by one large non-activity element 52. Two-dimensional representations of alternative activity-element patterns for covering the fuel 48 in a single fuel rod 47 are visualized in Figs. 3a to 3c. As shown in figs. 3a to 3c, the number of activity-elements 50 used to cover the fuel 48 in each fuel rod 47 may be varied. If, in a section of the fuel assembly 40 the fuel 48 in each rod 47 is represented by one activity-element 50, information concerning the average activity for the fuel 48 may be obtained for that rod 47. If instead, the fuel 48 in each rod 47 is represented by two or more activity-elements 50, additional information concerning inhomogeneities within separate fuel rods 47, and information concerning displacements of fuel rods 47 may be obtained.

In this way the activity-elements 50 may be optimized to fit the actual radiating geometry of the fuel assembly 40, and a minimum number of activity- elements 50 will suffice for obtaining the desired accuracy of the reconstruction. By keeping the number of activity-elements 50 to a minimum, the total time for the overall tomographic reconstruction is substantially re- duced. Contribution coefficients are defined as the fraction of emitted gamma quanta from an activity-element 50 reaching the detector in a certain position. The coefficients are estimated for each activity-element 50 and every detector position. Preferably, each contribution coefficient is determined as the mean value of fractions of emitted gamma quanta reaching the detector, for a number of points of emission in the activity-element 50. E.g., in a two- dimensional implementation the mean value is preferably formed from at least one (1), and more preferably from at least eight (8) discrete radiation emission data points per mm², within each activity element 50.

The contribution coefficient is further determined by taking into account the known nominal geometry of the measured objects and the physical properties of the measurement setup.

Also, physical properties such as attenuation and solid angle of the exposed area of the detector may be introduced in the reconstruction process. Specifically, the area of the detector that is exposed to radiation from each point as well as the attenuation between each point and the exposed area, is cal- culated. Also, the gamma-ray transmission through the collimator may be treated, as well as the gamma quanta scattered into the detector.

The recorded radiation intensities, the defined contribution coefficients and the radiation-source concentrations in the activity-elements 50 are intro- duced in the reconstruction process:

Equation 1

where:

Wmn = contribution coefficient to measurement m from activity-element n An = radiation-source concentration in activity-element n

Im = intensity in detector position m

An algebraic reconstruction of the source concentration in each activity- element 50 is performed using the contribution coefficients and the recorded radiation intensities.

To achieve information concerning fuel rod 47 displacements, position indicators are defined for each rod 47. The position indicators are calculated for each rod 47 by comparing reconstructed activities in geometrically related activity-elements 50, e.g., according to:

Equation 2:

A_+i - A__;

A tot where Pi = the position indicator for direction i. A+i = the sum of the reconstructed activities in activity-elements

50 in the positive i-direction

A-i = the sum of the reconstructed activities in activity-elements 50 in the negative i-direction

Atot = the sum of the reconstructed activities in all activity- elements defined for the rod 47. For each rod 47, each position indicator Pi is interpreted as a deviation in direction i from the position presumed in the reconstruction. In this way information is obtained about possible displacements of single rods 47 from their nominal positions or distortions of the entire fuel assembly 40. The size of each displacement may be determined using a calibration curve obtained from e.g. simulations or measurements on a fuel model, or alternatively by performing iterative calculations.

A further reconstruction can be performed according to the preceding steps, based on the same set of recorded intensities using the rod positions as indicated by the position indicators Pi for the re-definition of a second and adjusted pattern of activity-elements 50.

In an alternative embodiment of the method, the displacement of a fuel rod 47, e.g., is detected using two or more activity-elements 54 with zero initial activity, which surround the outer periphery of the activity-elements 50 representing a rod 47, as shown in fig.4. If then, any of the surrounding activity-elements 54, after the reconstruction process, shows an activity signifi- cantly larger than zero, this may be interpreted as a displacement in the direction of that/ those surrounding activity-elements 54, and a position indicator Pi may be set.

The method offers high-accuracy in the final reconstruction by including in the model the actual geometry and all physical parameters contributing to the detected intensities.

The method according to the present invention may also comprise analysis of radiation originating from several other isotopes, whereby other fuel- parameters may be obtained, such as burn-up and power- and void history. Another example involves the analysis of radiation from Co-60. During operation Co-60 is deposited on surfaces in the fuel assembly 40. Such deposits are normally referred to as "CRUD" (Chalk River Unidentified Deposits). If the radiation emitted from Co-60 is detected and analyzed using the method according to the present invention, radiation signals will be obtained from the claddings 44 and from the fuel assembly box 42, e.g., whereby a high accuracy reconstruction of the complete fuel assembly 40 may be performed. In this implementation, the activity-elements 50 are designed .and positioned in adaptation to the above mentioned parts of the fuel assembly.

A gas-tight vessel, to be further disclosed below is provided for supporting the radiation intensity detector equipment movable about the fuel assembly. The vessel is movable to be submerged in water, and designed for a stable rest on a basin bottom.

The fuel assembly is transported in the water and inserted into a channel formed in the vessel, while permitting natural circulation of water in surrounding contact with the fuel.

In the vessel, the detector equipment is controlled for angular, horizontal positioning about the fuel assembly, i.e. about the channel enclosing the fuel, in an annular space that is peripherally defined by the vessel and the channel. The detector equipment is also controlled for vertical movement in the vessel, relative to the fuel, and for lateral positioning on each vertical level and in each angular position relative to the fuel.

The detector equipment records data of radiation intensities in a large number of controlled positions. For each recorded data point, the actual position of the detector is determined with high accuracy using position read-out > equipment. The obtained intensities and geometric data are utilized in the tomographic reconstruction process, performed as stated above. With reference to figs. 5a, 5b there will be disclosed the relevant and operative features of a novel apparatus suggested for implementation of the above method.

A vessel 1 is adapted for accommodation of detector equipment as is further described below. The vessel 1 is mobile and adapted for operation also in a submerged condition, and may e.g. be lowered to the bottom of a water-filled basin during operation. For this purpose, the vessel may include suitably formed external connections (not shown in the drawing) for engagement with hoisting equipment, and support structures (also not shown) may be formed in a bottom area of the vessel for securing a stable upright position during measurements. Ballast may be attached to the vessel in order to further secure its stable rest position.

The vessel 1 is advantageously rotationally symmetric about a longitudinal axis, and provides an axial through hole that may be realized as an exchangeable tube or channel 2. The channel wall 3 and the surrounding vessel wall 4 define a gas-tight inner space 5, axially enclosed by a vessel bottom 6 and a top lid 7. The inner space 5 may be pressurized in order to withstand pressure from surrounding water, and may e.g. contain a nitrogenous atmosphere in order to avoid condensation on internal surfaces and equipment.

The channel 2 is dimensioned to receive the full length of a fuel assembly 8, inserted to rest by its lower end in the bottom area of the channel while the upper end of the fuel assembly is arrested to secure that the fuel is centered in the channel. In the inserted position, an annular space is provided about the fuel assembly permitting natural circulation of basin water in surrounding contact with the fuel when measurements are conducted under^' water. In order to accommodate for thermal expansion of the channel 2, caused by heat transfer from the fuel, the fuel assembly is received by a flexible docketing structure 9 in the bottom area of the channel 2. E.g., the structure 9 may comprise an expandable portion in the lower end of the channel, realized through a bellows formation in the lower end of the metal tube that defines the channel 2.

In a preferred embodiment of the pressurized vessel 1, detector equipment 10 is movably supported for conducting radiation intensity measurements on numerous vertical levels, and on each vertical level, in numerous angular and lateral positions about the fuel assembly. The detector equipment 10 typically comprises collimator and detector systems as is further described below.

The detector equipment 10 is supported on a rotation table 11 that is driven for controlled angular positioning about the fuel assembly 8. The rotation table 11 is carried by an elevator table 12 that is driven and controlled to be accurately positioned on vertical levels in the axial extension of the fuel assembly 8. The elevator table 12 is ring shaped and guided in its vertical movement on longitudinal guide bars 13 that are angularly spaced about the periphery of the elevator table 12.

The detector equipment 10 comprises typically a collimator and a detector system, supported on the rotation table 11. Alternatively, at least two detector equipments 10 may be provided. Each detector equipment advantageously includes a collimator package formed by combining several separate collimators/ collimator slits in a modular assembly, wherein each collimator slit is associated with a separate detector system. When two detector equipments are used they may preferably be oppositely arranged on a common rotation table 11 for translatory motion relative to the table and relative to the fuel assembly 8.

Position read-out equipment is arranged to detect and introduce the actual measuring position in the reconstruction process. High accuracy information about current axial, angular and lateral positions may be determined with an accuracy in the order of about 10 micrometers or less, by using optical sensors.

All positioning movements of the detector equipment are preferably driven by means of electrically supplied drive units. Worm-drives, gear-racks, guide-rails and other mechanical structures as known per se by a man of ordinary engineering skills may be arranged for operating and controlling the movements in axial, angular and lateral positions about the fuel assembly.

A collimator for the gamma radiation emanating from the nuclear fuel under study is advantageously composed of a tungsten alloy. The collimator used in the invention displays preferably the following features:

-it provides background shielding by preventing radiation from entering the collimator from other directions than through the collimator slit. Further, the slit provides a well-defined direction from which gamma radiation emanating from the fuel can enter the detector system;

-it allows for exact spatial information by the use of a narrow slit and close positioning relative to the measured assembly; -it allows for measurements in high radiation fields by offering adequate - radiation shielding of the detector system within smallest possible volume; -it provides for a high-density volume that acts as a buffer for temperature variations. The temperature variation in the detector system due to radiation from the fuel is thereby minimized.

The collimator is further designed to:

-minimize the effect of back scattering. This is achieved by allowing for a low-density material behind the detector;

-minimize the effect of cross talk between adjacent detector systems. This is achieved by allowing for sufficient amount of heavy metal between adjacent detector systems. A detector system to characterize the gamma radiation energy spectrum is used in the invention. The detector system may advantageously comprise the following features:

-scintillation crystals are used in a preferred embodiment. They are small enough to fit within a limited available space. They are rugged enough to withstand the harsh environment around the nuclear fuel assemblies;

-photo-multiplier tubes are used to amplify the light output from the scintillation crystals.

A high-speed data acquisition system is used to collect energy spectra from the detector signals. This allows for studies of different gamma-ray energies (isotopes) and possible temperature drifts may be monitored.

Thus there has been suggested a method and apparatus for tomographic determination of radioactivity distribution in a nuclear fuel assembly, whereby information on the internal structural layout of the integral fuel assembly is employed for carrying out a high accuracy reconstruction of the activity distribution. In a further embodiment of the invention, possible dis- placement of individual rods in the fuel assembly is detected and introduced in the reconstruction process.

Claims

1. A method for tomographic determination of radioactivity distribution in a nuclear fuel assembly, comprising the steps of: (a) detecting the radiation intensity in controlled angular and lateral positions about the fuel assembly, in one or more axial positions thereof;

(b) defining a pattern of activity-elements, the shape and position of each activity-element being determined from the known and nominal internal geometry of the fuel assembly, said activity-elements representing por- tions that may emit radiation;

(c) determination of a contribution coefficient from each activity-element in each controlled position, and

(d) reconstructing the radioactivity of each activity-element.

2. The method of claim 1, wherein the step (c) of determining the contribution coefficient comprises determination of the fraction of emitted gamma quanta that reaches the detector, for a large number of points of emission within the said activity-element.

3. The method of claim 1, further comprising the step of re-defining the pattern of activity-elements by the employment of position indicators, obtained by comparing the reconstructed activities in geometrically related activity-elements defined in said pattern, and repeating the steps (c) and (d).

4. The method of claim 1, wherein the step (c) of determining the contribution coefficient comprises determining the area of the detector that is exposed to radiation from each activity- element, the attenuation between each activity-element and the exposed area, the gamma-ray transmis-' sion through the collimator, and the gamma ray scattering into the detector.

5. The method of claim 1, wherein the activity-elements defined in step (b) are adapted to the geometry of the fuel rods.

6. The method of claim 1, wherein the detected radiation intensity is emit- ted from Ba- 140.

7. The method of claim 1, wherein the activity-elements defined in step (b) are adapted to the geometry of the supporting structure of the fuel assembly.

8. The method of claim 7, wherein the detected radiation intensity is emitted from Co-60.

9. A method according to claim 1, further comprising the steps of:

-providing detector equipment for radiation intensity detection movable in a gas tight vessel adapted to be submerged under water; -forming the vessel for vertical accommodation of a nuclear fuel assembly while permitting natural circulation of water in surrounding contact about the fuel assembly;

-controlling the detector equipment for angular and lateral positioning about the fuel assembly, and for axial positioning along the fuel assembly, and -collecting radiation intensity data in each controlled position of the de- tector equipment.

10. An apparatus for detecting radiation intensities about a nuclear fuel assembly for the tomographic reconstruction of radioactivity in single fuel rods of the assembly, comprising > - a gas tight vessel adapted to be submerged in water, containing detector equipment for detecting radiation emitted from the fuel assembly; - said vessel defining a channel for insertion of the fuel assembly while permitting natural circulation of water in surrounding contact with the fuel assembly during detection;

- control means for relative angular and lateral positioning between the detector equipment and the fuel assembly, as well as control means for relative axial positioning between the detector equipment and the fuel assembly, and

- detection means for accurate determination of the actual relative position in each controlled position of the detector equipment.

11. The apparatus of claim 10, wherein the fuel assembly is stationary in the vessel, and the detector equipment is driven and guided for angular, lateral and axial positioning relative to the fuel assembly.

12. The apparatus of claim 11, wherein the channel (2) for accommodation of the inserted fuel assembly being structured for absorbing thermal expansion due to heat transfer from the fuel assembly.

13. The apparatus of claim 12, wherein the vessel has a circular section, and an annular space is defined by the channel wall and the vessel wall, resp., said space being pressurized and containing a nitrogenous atmosphere surrounding the detector equipment.

14. The apparatus of claim 13, wherein the detector equipment is carried by an annular elevator table, guided in axial movement on longitudinal bars disposed in angular spaced relation about the periphery of the elevator table.

15. The apparatus of claim 14, wherein the detector equipment is supported on a rotation table, driven and guided for angular positioning relative to a center of the elevator table.

16. The apparatus of claim 15, wherein the detector equipment is driven and guided for lateral positioning relative to the rotation table.

17. The apparatus of claim 10, wherein the detector equipment comprises at least one collimator system, each collimator having two or more gamma detectors.