Method and apparatus for detecting defective fuel elements in a nuclear reactor fuel assembly

Method and apparatus for detecting defective fuel elements in a nuclear reactor assembly. The assembly (20) is kept entirely immersed in a liquid such as water, ultrasonic waves are propagated successively in each of the fuel elements of the assembly or rods (21), an ultrasonic sensor (25) is disposed near the assembly (20) and the waves which may be scattered into the protective liquid by the defects in the fuel rod are picked up to determine the presence of a defective assembly and locate the defective rod in the assembly. The invention is particularly applicable to fuel assemblies of a pressurized water nuclear reactor.

FIELD OF THE INVENTION 
The invention concerns a method and apparatus for detecting defective fuel 
elements in a fuel assembly for a nuclear reactor. 
BACKGROUND 
Fuel assemblies for a nuclear reactor, particularly fuel assemblies for a 
pressurized water nuclear reactor, are constituted by a bundle of elongate 
fuel elements termed fuel rods, arranged parallel to each other in the 
longitudinal direction of the assembly. 
These fuel rods are constituted by tubes made of cladding material filled 
with fuel pellets. 
The different fuel rods are held in position in the assembly by spacer 
plates and end plates connected to support tubes in place of some fuel 
rods and allowing the rigidity of the assembly to be assured. 
During use in the core of the nuclear reactor, these assemblies can 
deteriorate under the action of various mechanical or thermal stresses or 
under the action of corrosion, so that the cladding material of the fuel 
rods can present fissures through which radio active material can pass 
into the cooling fluid of the reactor. 
When the nuclear reactor is reloaded, during which operation used 
assemblies located in one part of the core are replaced by new assemblies, 
it is necessary to detect assemblies including leaking fuel rods. 
During these operations, the core of the reactor is entirely immersed in a 
protective liquid such as water, and the assemblies involved in the 
reloading operations are conveyed under water from the vessel of the 
reactor to the fuel pond. 
Leaking assemblies must be detected and these assemblies put in a region of 
the core in which reloading is taking place or in a region of the core in 
which assemblies are kept ready. 
it is in fact essential to replace these leaking assemblies by new 
assemblies or to replace the defective rods inside the assembly by new 
rods. 
When the assembly must definitely be replaced by a new assembly, it is 
still essential to know whether the used assembly is leaking, since, if 
this is the case, special precautions must be taken for moving or storing 
it. 
To carry out the detection of a leaking assembly, the use of the 
apparatuses termed "crack detection cells" has been proposed, in which 
assemblies are disposed successively, one by one, inside the fuel pond. 
The temperature of the assembly is raised so that the pressure of the 
fission gases contained in the fuel rods increases and these gases escape 
inside the crack detection cell via the fissures in the rods, if these are 
defective. 
Measurement of gamma activity in the fluid occupying the crack detection 
cell allows the escape of fission gas and hence the presence of a 
defective assembly, to be detected. 
Such a crack detection cell is described in FRAMATOME French Pat. No. 
2.389.202. 
There is also a known method of detecting leaking fuel assemblies using 
sonic or ultrasonic acoustic phenomena connected with the impact of 
fission gas bubbles on a screen or with the build-up of the fission gases 
leaving the fuel rods through the faults in the cladding under the surface 
of the screen. 
To implement this method, the gases must be expanded by heating or 
depressurizing of the assembly. These two methods of finding defective 
assemblies which are simple to implement and integrate perfectly with 
operations for transferring fuel allow defective assemblies to be 
determined during these transfer operations. But these methods do not 
allow the leaking fuel rod to be exactly located inside the assembly. 
The defective fuel rods must obviously be identified when these rods are 
required to be replaced in the assembly by new rods before the assembly is 
reloaded in the core of the reactor. 
Methods have therefore been proposed which allow defective fuel rods to be 
identified in assemblies during reloading operations on the nuclear 
reactor. For example, French Pat. No. 2.222.732, discloses detection of 
the presence of water in defective fuel rods by heating each of the rods 
of the assembly by induction and detecting bubbles of vapor or 
condensation which can occur at the cap of the fuel rod, by means of an 
ultrasonic echo test. 
This method, which allows leaking fuel rods to be located, does necessitate 
partial dismantling of the assembly since each rod must be placed inside 
an apparatus for induction heating. 
It has also been proposed, in French Pat. No. 2.287.753, to propagate an 
acoustic signal along the cladding of each of the fuel elements and pick 
up the signal obtained after propagation along the cladding of the fuel 
elements. When there is a defect in the cladding, an attenuation of the 
signal is observed, due to the presence of this defect. 
To implement this method, an emitter and a receiver of acoustic waves must 
be placed on each rod, and the outer surface of the cladding of the fuel 
rod must be isolated from the cooling fluid, by placing the rod in a 
gaseous atmosphere. 
Also, when the defect to be detected is at a distance from the end where 
the acoustic wave receiver is located, there is a risk of the return 
signal being drowned in the acoustic background noise. 
There is therefore no known method allowing very reliable and easily 
implemented detection of defective fuel rods in an assembly which has not 
been dismantled. 
SUMMARY OF THE INVENTION 
The object of the invention is a method of detecting defective fuel 
elements in a nuclear reactor fuel assembly constituted by a bundle of 
elongate fuel elements or rods arranged parallel to each other in the 
longitudinal direction of the assembly, by detection of possible anomalies 
in the propagation of ultrasounds in the fuel rods, this detection method, 
which is very easily implemented, allowing reliable detection of defective 
fuel rods without dismantling the fuel assembly. 
To implement the method according to the invention: 
the assembly is kept entirely immersed in a protective liquid, such as 
water, 
ultrasonic waves are propagated in each of the fuel rods of the assembly 
successively, over their whole length from one of their ends, 
an ultrasonic sensor is disposed near the fuel rods of the assembly, 
the ultrasonic waves which may be scattered by the defects in the fuel rods 
into the protective liquid in which the assembly is immersed are picked up 
and, 
if such scattered waves are picked up, the presence of at least one defect 
in the rod in which the ultrasounds are propagated is thus determined. 
To provide a full understanding of the invention, an example of 
implementation of the method according to the invention will now be 
described, by way of non-limiting example, with reference to the attached 
drawings, in the case of the testing of fuel assemblies of a pressurized 
water nuclear reactor in the swimming pool of the reactor or in the fuel 
pond, with locating of the defect on the fuel rod.

DETAILED DESCRIPTION 
FIG. 1 shows a fuel rod 1 whose cladding has a fissure 2. 
The upper end of the rod is closed by a cap 3, with an ultrasonic generator 
4 disposed in contact, producing ultrasonic waves which are propagated in 
the whole fuel rod from the end represented in FIG. 1 to its opposite end. 
The fuel rod 1 is entirely immersed in the water which fills the fuel pond. 
An ultrasonic sensor 5 is disposed near the lateral surface of the rod 1, 
connected to a vertical displacement apparatus allowing it to take up any 
position along the length of the rods. 
Z designates the displacement axis of the sensor 5 which, in the case of 
the apparatus represented in FIG. 1, corresponds to the vertical 
direction. 
A connection between the ultrasonic generator 4 and the sensor 5 allows the 
time of emission of an ultrasonic wave to be established as time origin 
for the sensor 5. 
FIG. 2 shows the signal picked up by the sensor 5 in the case of an 
ultrasonic wave being emitted by the generator 4. 
This signal S can be resolved into two signals S1 and S2. 
The signal S1 corresponds to the ultrasonic waves scattered at the defect 2 
into the water surrounding the rod while the signal S2 corresponds to 
interference signals which arrive at the sensor 5 after the signal S1. In 
practice, when the ultrasonic wave is propagated in the cladding of the 
fuel rod 1, part of the energy associated with these waves is transmitted 
to the water surrounding the cladding in which it is propagated in the 
form of compression waves with a lower speed than the propatation speed of 
the wave in the metal of the cladding. 
The waves are reflected at the structures surrounding the fuel rod 1 and 
finally arrive at the sensor 5, which translates them into an interference 
signal S2. 
The principal signal S1 is the result of the waves which are propagated in 
the cladding of the rod as far as the fissure 2 where they are scattered 
into the water in which the rod is immersed, before arriving at the sensor 
5. 
These scattered waves, which have a shorter path in the water than the 
waves reflected by the elements surrounding the rod 1, arrive at the 
sensor 5 before the reflected waves. 
In the signal registered by the sensor 5, the part S1 of the signal 
therefore precedes the part S2 corresponding to the interference signals. 
The form of the signal S1 scattered at the defect is established by 
calculation or measurement on an isolated rod, so that a window can be 
isolated, like that represented in the hatched part of FIG. 2, 
corresponding to the part of the ultrasonic signal scattered into the 
water at the defect. 
During displacements of the sensors 5 in the direction Z, the signal S1 is 
shifted in time, since the path of the scattered waves in the water varies 
with the position Z of the sensor 5. 
Accurate measurement of the position Z of the sensor 5 allows the position 
of the window corresponding to the signal S1 in the measured signal to be 
determined. 
The maximum amplitude of the signal S1 is then measured and the variations 
in this maximum amplitude as a function of the position Z of the sensor 5 
along the height of the rod are determined. 
FIG. 3 shows the variation of this maximum amplitude as a function of the 
position of the sensor. 
It is quite clear that in the case of a defect being present in the 
cladding of the rod, i.e., in the case of the existence of the scattered 
signal S1, the curve representing the variations A (Z) has a maximum at 
the value Z=ZO corresponding to the position of the sensor at the exact 
height of the defect 2. 
The measuring and testing apparatus connected to the sensors 5, as shown in 
FIG. 1, consequently includes a preamplifier 7, a filter 8, an apparatus 9 
for recording or displaying the signal A (t), after amplification and 
filtering, a filter 10 associated with a unit 11 for recording or 
displaying the window S1 of the signal filtered by the filter 10, and a 
unit 12 for determining the maximum amplitude A of the signal S1. 
The apparatus also includes a unit 14 for measuring and recording the 
parameter Z determining the position of the sensor 5 over the height of 
the rods, a signal corresponding to this value of Z being transmitted to 
the filter 10 to determine the window corresponding to the signal S1 as a 
function of the position of the sensor. 
Last, the apparatus includes a unit 16 for recording and/or displaying the 
signal A (Z) and determining the maximum of the curve A (Z). 
The unit 16 receives on the one hand a signal representing the 
instantaneous value of Z and on the other hand the value of the maximum A 
of the signal S1 corresponding to this value of Z. The two values are 
recorded and allow recording and/or display of the curve A (Z). 
Determination of the maximum of this curve when the sensor 5 is moved along 
the rod over its whole length allows determination of the value ZO 
corresponding to this maximum. 
The apparatus therefore allows, on the one hand, determination of the 
presence of a defect in the rod 1 during examination and on the other, 
determination of the exact position of this defect on the rod. 
In practice, the presence or absence of a scattered signal S1 in the signal 
picked up by the sensor 5 allows determination of a defective rod or a 
non-defective rod, respectively. 
Accurate tracing of the curve A (Z) depends on the width of the window 
corresponding to the signal S1 in the signal picked up by the sensor 5. 
Either calculating or display units can be used for the different units 9, 
11 and 16, allowing determination and location of the defect by triggering 
of a signal associated with a numerical value, or by examination of a 
curve the maximum of which is determined. 
In the case of a non-defective rod, the signal recorded and possibly 
displayed by the unit 9 is characterized of interference signals, 
including only a part S2. The signal is easily recognizable if calibration 
has previously been effected on a non-defective rod in a particular way, 
for example a new rod, put in a comparable environment. 
When examination of an assembly is required, after an end plate has been 
removed allowing access to the end of each of the fuel rods of this 
assembly, a single sensor can be used, which is moved successively and 
automatically from one rod to another, or a set of ultrasonic emitters 
arranged in an array corresponding to the lattice of transverse sections 
of the rods in the assembly. 
FIG. 4 shows diagrammatically such an apparatus used for testing an 
assembly 20 constituted by a set of rods 21 arranged in a square-mesh 
lattice in a transverse plane. A set of ultrasonic emitters 22 is disposed 
on a plate with approximately the dimensions of the end plate of the 
assembly which is positioned over this so that each of the emitters 22 is 
over a rod 21 and in contact with its upper cap. 
An ultrasonic generator 23 is connected to each of the emitters 22 and 
associated with an electronic addressing apparatus allowing each of the 
emitters to be energized successively, i.e., ultrasonic waves to be sent 
successively into each of the rods 21 constituting the assembly. 
X, Y addressing, for each of the rods of the assembly, in a transverse 
plane with respect to the assembly allows the fuel rod in which the 
ultrasounds are propagated to be identified. 
The address of the emitter and the corresponding fuel rod is transmitted to 
a unit 26 which also receives the signal transmitted by a sensor 25 
movable in the direction Z of the assembly, at a fixed distance from the 
side wall of the latter. 
The assembly 20 is tested while the assembly is entirely immersed in the 
water of the fuel pond, and the sensor 25 is constituted by a bar of unit 
sensors the total length of which is at least equal to the side of the 
square constituting the transverse section of the assembly. In this way, 
the value of the signal received is increased because, when work on a rod 
disposed inside the assembly is involved, the wave scattered into the 
water is reflected at the components near the rod, so that the beam 
emitted outside the assembly has a greater width than that of a unit 
sensor, generally of the order of the side of the transverse section of 
the assembly. 
The measuring and testing apparatus associated with the sensor 25 and the 
unit 26 is also like that described for a single fuel rod and represented 
in FIG. 1. 
This apparatus includes a preamplifier 27, a filter 28, a unit 29 for 
recording and displaying the signal A (t), a filter 30 receiving a signal 
representing the measurement Z and allowing determination of the signal S1 
and its recording and/or display in the unit 31 and a unit 32 for 
calculating the maximum of the amplitude A of the signal A (t). 
This apparatus also includes a unit 34 for accurate measurement and 
development of a signal corresponding to the value of Z defining the 
position of the sensor 25 in the height dimension of the assembly and a 
unit 36 for recording and displaying the curve A (Z) and for determining 
the maximum of this curve corresponding to the value ZO defining the 
position of a possible defect in the rod during examination defined by its 
coordinates X, Y. 
To give the apparatus extra sensitivity, instead of a single bar disposed 
near one of the faces of the assembly, a set of four bars entirely 
surrounding this assembly can be used. 
The working of the apparatus represented in FIG. 4 is substantially 
identical to the working of the apparatus represented in FIG. 1, except 
that it allows identification of the defective rod or rods in an assembly 
by their coordinates X, Y. In practice, the coordinates X, Y can be 
identified at the unit 26, each time an ultrasonic signal scattered by a 
defect (S1) is identified on the curve A (t). 
If no scattered signal is found for the whole of an assembly, such assembly 
can be considered non-defective. 
One of the important advantages of the apparatus according to the invention 
is that it allows defective rods to be located with greater sensitivity 
than in prior techniques, since the signal picked up and undergoing 
discrimination is not drowned in the background noise of the signal 
produced in the rod, there being a time lag between the signal scattered 
by the defect and the background noise. 
In addition, examination can be carried out while the assembly is immersed 
in a protective liquid, as is always the case for irradiated assemblies 
during storing or moving. 
In addition, the detection method does not assume that the rod or the fuel 
assembly will be heated or that any other modification will be made in the 
physical conditions of the medium in which the assembly is immersed. 
The method according to the invention also has the advantage of allowing 
all the rods to be examined, even those not at the periphery of the 
assembly, since, with those rods disposed within the assembly, the energy 
scattered by the possible defect travels to the sensor after reflection on 
the elements adjacent to that element disposed within this assembly. 
In addition, the method according to the invention allows rods to be 
examined over their entire length and even in that part of them which is 
hidden by spacer grids, since the presence of the grids is translated 
simply into attenuation of the signal picked up which is compensated for 
by action on the amplitude of the signal emitted. In addition, the method 
according to the invention can be used on an assembly which has not been 
dismantled by using the free space between the caps of the rods and the 
upper plate for locating the sensors. 
The invention is not, however, limited to the embodiment described; it 
includes all the variants thereof. 
Thus, it is possible to use a fixed sensor positioned near the lateral 
surface of the rod or the assembly and receiving the waves which may be 
scattered into the water in which the rod or assembly is immersed by the 
defects thereof. 
Of course, in this case, accurate locating of the defect will not be 
possible, but discrimination and filtering of the signal corresponding to 
the scattered waves are still possible by using a sensitive enough sensor 
disposed at a distance from the assembly, allowing both measurement and 
discrimination of the signal possibly associated with waves scattered into 
the water. 
In the case of the testing of a complete assembly, instead of a bar of 
sensors corresponding in length to the length of the side of the assembly, 
a single sensor can be used, moved parallel to the side of the assembly's 
section so as to increase the extent of the region inspected. 
In this instance, the sensor is moved both parallel to the axis of the 
assembly and in a direction perpendicular to that axis. 
Use can also be made of a sensor which is moved inside guide tubes 
constituting the framework of the assembly instead of a sensor which is 
movable outside the assembly. 
Lastly, different ways of effecting discrimination of the signal scattered 
into the water in which the fuel rod or assembly is immersed from those 
described can be envisaged. 
The method and apparatus according to the invention is applicable to the 
detection and location of defects in any fuel assemblies constituted by 
fuel rods having a cladding within which the nuclear fuel is contained and 
which may have defects such as fissures.