EP0998745A1

EP0998745A1 - Device and method for permanently controlling the tightness of closing lids of containers for radioactive materials

Info

Publication number: EP0998745A1
Application number: EP98939710A
Authority: EP
Inventors: René CHIOCCA
Original assignee: Societe pour les Transports de lIndustrie Nucleaire Transnucleaire SA
Current assignee: TN International SA
Priority date: 1997-07-24
Filing date: 1998-07-20
Publication date: 2000-05-10
Also published as: JP2001511531A; SK792000A3; FR2766570A1; WO1999005686A1; US6223587B1; KR20010022166A; TW436813B; FR2766570B1

Abstract

The invention concerns a device for controlling the tightness of the lids of a heavy metal container for transporting and/or storing radioactive materials, said container comprising a cavity for nuclear materials delimited by a thick cylindrical shell closed at one end with a tightly fixed base and at the other end with at least two removable, thick superposed lids. The invention is characterised in that said lids are maintained supported on shoulders, arranged in the shell, by means of flanges provided with at least two concentric joints, and to each lid correspond at least two channels passing through said shell, emerging through a first control orifice at the shell outer surface in the proximity of the lids in an accessible place, and through a second orifice, one in the space between the lid concentric joints, the other in the space located between said lid, the shell and the immediately superposed lid, each orifice being optionally connected to a circuit measuring and controlling tightness.

Description

DEVICE AND METHOD FOR PERMANENTLY TESTING THE SEALING OF CONTAINER LID COVERS FOR RADIOACTIVE MATERIALS

TECHNICAL AREA

The invention relates to a device and a method for checking the tightness of covers, generally multiple, stacked on top of each other, closing the cavity of a container for transporting or storing radioactive materials, for example irradiated fuels or the vitrified residues from the reprocessing of these fuels, said device enabling the tightness of each of the lids to be checked, not only during their successive closure, but also thereafter, after complete closure of the container, during its lifetime while it is loaded during transport or storage.

STATE OF THE ART

Radioactive materials, in particular irradiated nuclear fuel assemblies or vitrified residues from reprocessing, are generally transported and / or stored in heavy containers (also called packaging) of cylindrical shape with thick walls (from a few cm to several tens of cm) , monolayers or multilayers, mainly based on forged or cast or rolled steel (possibly associated with lead), or based on cast iron, which in particular provide mechanical resistance functions (resistance to severe impacts, for example in the event of a fall ), radiological protection and thermal transfer.

These containers usually have a cylindrical shell closed at one of its ends by a bottom fixed in a sealed manner (for example by welding).

The cavity thus formed, in which the radioactive material takes place, is closed at the other end of the shell, sometimes by a single cover, but the more often by at least two watertight removable metal covers, stacked on one another.

To achieve sealing, it is known to use O-rings, either of elastomeric nature or of metallic nature, arranged in grooves whose geometry must be defined very precisely as a function of the characteristics of the seals that are used. . Usually each cover is equipped with two concentric seals bearing on a shoulder formed in the shell. This tightness must be able to be checked at any time, or even continuously for containers which, once loaded, are placed in a long-term storage location.

Figure 1 schematically shows an example of what is known to achieve to ensure and control the tightness of a container for nuclear material having either a single cover (1) or two overlapping covers (1) and (2) , i.e. three overlapping covers (1), (2) and (3).

A first thick cover (1), or primary cover, serves to confine the radioactive material taking place in the cavity (C) of the container.

The cover (1) is supported on a shoulder formed in the thick metal shell (4), generally cylindrical, constituting the body of the container, by means of two concentric seals (1 1), located in grooves cut in the cover flange (1) and tightened with bolts. It comprises a service channel (8) communicating the cavity (C) with the outside by opening onto the upper face of the cover (1) through a service orifice. This service channel is used to perform various manipulations in the cavity (C), for example introducing or extracting water, creating a vacuum, introducing or extracting a gas such as He, N ₂ ... The cover (1) further comprises a control channel (5) communicating the space between the two seals (1 1) with the outside by opening on the upper face of the cover through a control orifice to which one can adapt various control devices (manometers, qualitative and / or quantitative gas analyzer, for example mass spectrometer, vacuum pump, gas under pressure), as will be seen below, making it possible to check the tightness of the joints.

After use, the service orifice (8) is closed by a closure, not shown, comprising two concentric seals; a closable control tap, by a plug accessible from the top of the cover, opens between these two seals and makes it possible to check the seal. The control channel (5) is closed by a plug.

Once the primary cover (1) has been installed, its tightness checked, the service ports closed, their tightness checked, a second safety cover (2) or secondary cover, is put in place above the cover (1 ) in the same manner. Thus this secondary cover comprises two concentric seals (12) bearing on a shoulder formed in the shell, a service channel (9) and a control channel (6) used and closed in the same way as in the case of the cover. (1).

The service channel (9) is used to manage the space between the covers (1) and (2) and the control channel (6) to check the tightness of the seals (12).

After closing and checking the tightness of the cover and doing the same for the service openings, the control devices connected to the openings (5) and (6) having been dismantled, the container is ready. However, once placed in storage, it is sometimes covered with a thick metal protective cover (30) allowing it to withstand even better aircraft falls.

To check the tightness of the double seals, for example (1 1) (12), the following methods can be used: i) the cavity (C) having been filled with a gas, in general helium at 0.5 bar absolute, a vacuum can be created in the space between seals (1 1), at a pressure lower than those prevailing on either side of said seals (for example a few mbar) then observe and measure through the control orifice a possible rise in pressure in this space using a pressure gauge of the type (14). This method allows measurement of leak rates in the range of about 10 ^-5 to 10 ^-3 atm.cm ³ / sec;

ii) the space between seals can be put under overpressure with respect to the pressures prevailing on either side of said seals (for example 6 bars) and a possible pressure drop can be measured using also a pressure gauge of the type

(14). This method measures leak rates in the range of about 10 ^"ό to 10 ^{~ 3} atm.cmNsec;

iii) a helium test can be performed, which consists of making a vacuum in the space between seals and, the cavity being filled with helium at pressure PI, to count the amount of helium sucked by the possible leakage of the attached using a mass spectrometer previously calibrated using a calibrated leak. This method is much more sensitive and can detect leaks between 10 " ⁹ and 10 ^{~ 6} atm.cm ³ / sec.

By playing on different gases introduced on either side of the seals, we can then identify which is the seal, inside or outside, which is leaking.

Thus, during the fitting of the cover (1) and after filling the cavity (C) with gas at a pressure P1 below atmospheric pressure (in general helium at 0.5 bar absolute, as has been said) one can check the tightness of the double seals (1 1) and then that of the double seals of the closing of the service opening (s), using the control tappings opening into their spaces between seals. These verifications completed, the cover (2) is then put in place and the space between the covers (1) and (2) is filled with a gas at pressure P2 generally greater than PI (typically helium or l (nitrogen with an operating pressure of 6 bar) and the leakage checks of the various seals made as for the cover (1).

The pressure P2 can be continuously monitored by means of a pressure sensor. If, subsequently during long-term storage of the container, this pressure decreases, this is a sign of a leak either towards the atmosphere, or towards the cavity (C) of the container since the pressure P2 is significantly higher than the atmospheric pressure. external and a fortiori at the reduced pressure PI prevailing in the cavity (C). It is seen that thus the confinement of the radioactivity is ensured and that there is impossibility of release of radioactivity from the container cavity to the environment.

To implement an appropriate corrective action, it is important to know the origin of the leak by checking the tightness of each of the covers (1) and (2).

For this it is first necessary to remove the protective cover (30) to gain access to the control tap for closing the service orifice (9) and to the control orifice (6) for the seals (12) to check for leaks. In the event of leaktightness, it is deduced therefrom that the leak is located on the primary cover, which for example prohibits removing the cover (2), to avoid any risk of dissemination of radioactivity in the atmosphere.

If, on the contrary, a leak is noted on one of the joints tested previously, it must still be checked whether this leak is sufficient to explain the drop in pressure observed before being able to conclude that there is no other leak on the cover (1). When a leak is detected on the primary cover (1) of the container, the generally adopted solution consists in installing a guard cover (3) equipped, like the covers (1) and (2), with two concentric seals (13 ) resting on a shoulder of the ferrule, a control orifice (7) for testing the tightness of the concentric seals (13) of the cover (3) and a service orifice (10) intended to put the space between the covers (2) and (3) under a gas pressure P3, this orifice being in turn closed by a closure with double controllable seals.

We see that with such devices, it is not easy to locate with certainty the location of a possible leak on the covers, in particular to directly note a possible leak on the primary cover, and to be able to bring accordingly a suitable remedy. In addition to carry out these checks it is necessary, as has already been said, to remove the heavy protective cover (30) in order to have access to the various plugs of the service or control orifices.

Thus, the Applicant has sought a device and a method making it possible to locate possible leaks on each of the covers, independently of each other, both on the main seals and on the seals of the various service orifices, while simplifying the control operations. sealing, especially those made during storage of the container.

DESCRIPTION OF THE INVENTION The invention is a device for checking the tightness of the covers of a heavy metal container for the transport and / or storage of radioactive materials, said container comprising a cavity for nuclear materials delimited by a thick cylindrical shell. closed at one end using a bottom fixed in a leaktight manner and at the other end by at least two superimposed covers, removable and thick, characterized in that said covers are held in abutment on shoulders, practiced in the ferrule, by means of flanges fitted with at least two concentric seals, that each cover corresponds to at least two channels passing through said ferrule, opening through a first control orifice on the external surface of the ferrule near the covers in an accessible location, and through a second orifice, one in the space between the concentric joints of the cover, the other in the space between said cover, the ferrule and the immediately superimposed cover, each control orifice being optionally connected to a measurement and leakage control circuit.

When the end of the shell closed by the covers is protected by a thick cover, the accessible place is located outside the grip of said cover.

The leaktightness measurement and control circuit essentially comprises a buffer volume connected to generally common measurement and control devices, such as pressure gauges, vacuum pump, gas analyzer (for example mass spectrometer), gas cylinder. under pressure (e.g. He, N ₂ , Ar), using piping and valve sets.

This device for checking the tightness of the cover seals according to the invention essentially comprising channels passing through the thick shell of the container, can advantageously be supplemented by the addition of a service channel still passing through the shell and opening directly into the cavity.

It should be noted that the channels opening into the spaces between covers according to the invention can not only serve to check the tightness of the cover joints in cooperation with the channels opening into the spaces between joints, but also serve as a service channel. to manage the atmosphere of the space between covers (nature of the gas introduced, pressure, vacuum). In order to continue being able to use the existing installations and apparatus for loading the containers and checking the tightness of their closure from the upper face of the covers, it is recommended to maintain the presence of the control and service channels (such as as seen above in fig. l respectively marks 5,6,7 and 8,9,10) passing through the covers, with their closures accessible from the upper face of said covers.

But in this case said service channels of each cover must be connected, by connecting tubes, to the control channel opening into the space between the joints of the same cover, said space being connected according to the invention to a control channel sealing, opening to the outside after having crossed the shell, in order to be able to check the tightness of the closings of the service channels of said cover.

FIG. 2 illustrates a container with its device for checking the tightness of the covers, according to the invention.

The marks have the same meaning as in Figure 1.

We see that the container has three lids, the cover (3) being added, as has already been seen, in case of leakage of the primary cover (1). The channels 15, 1 7, 19 open into the space between the concentric joints of the covers 1, 2, 3 respectively, while the channels 1 6, 18 open into each of the two spaces between the said covers. The service channel 20, which can also serve as a control channel, opens into the cavity where the radioactive materials are placed.

The orifices of these channels 15, 1 6, 1 7, 18, 19, 20, located on the outer wall of the ferrule can be conventionally closed by closures (not shown) comprising two concentric seals; to check sealing a connection connects the space between these seals to the outer wall of the shell, connection which is then closed by a plug.

The orifice of each of the channels located on the outer wall of the shell is connected to a buffer volume V5, V6, V7, V8, V9. Each of them is in turn connected by a circuit of tubes and valves to the measuring devices used to perform the leakage control: pressure measurement (14), vacuum pump (21), mass spectrometer (22) , pressurized He bottle (23), etc.

Each of the covers also has control channels 5-6-7 opening into the spaces between the double seals (1 1, 12, 13) and service channels (8, 9, 10) opening into the cavity (C) and in the spaces between covers (P2, P3) inside the circle formed by the concentric joints (1 1, 12,13).

According to the invention, the control taps of the double seals of the closures (not shown) of the service channels (8-9-10) are connected, by connecting tubes (25-26-27), to the control channels ( 5-6-7).

In this case, the procedure for filling the container and closing the successive covers and then the protective cover (30), while ensuring their tightness when required, is the same as that used for the container in the figure. 1.

By cons, we see that with the device of the invention, there is access to all of the spaces between seals or between covers, or to the cavity, without any disassembly of the protective cover (30) or the covers (2) or (3) and that in this way there is a method enabling the tightness of each of the covers, including the primary cover (1) to be checked when all the covers and / or the protective cover is in place, and implement the corrective action adapted to the place where the possible leak is noted, which was not possible before. In general, the channels according to the invention are drilled in the metal shell (steel or cast iron); but when the latter is composite, that is to say when the internal steel ferrule is covered with lead, resin, etc., said channels drilled in the internal steel ferrule are usually extended by tubes passing through the successive layers. up to the surface.

The invention also relates to a method for checking the tightness of the various covers. Various procedures can be applied depending on whether the container has one or more lids.

Beforehand, the cavity was filled with gas, for example He, at a pressure PI lower than atmospheric pressure (generally 0.5 bar), the space between the covers (1) and (2) filled with a different gas. (for example N ₂ ) at a pressure P2 greater than atmospheric pressure (for example 6 bar), P2 being measured continuously via the channel (16), the buffer volume (V6) and the pressure indicator (14).

If the pressure P2 decreases, this indicates a leak either through the primary cover (1) or through the secondary cover (2).

To detect the leaking cover the following procedure can then be applied.

The vacuum is made using the pump (21) in the space between seals (1 1) via the channel (15), and the pumped gas is analyzed using the mass spectrometer (22 ).

If the analysis identifies He, it is the cover (1) which leaks either by the internal seal (1 1), or by the seal of the closure plug of the service channel (8). If the analysis reveals the presence of nitrogen, the leak is then on the outer seal (1 1) or on the outer seal of the service channel closure (8). A variant consists in replacing N ₂ by He once the internal seal (1 1) is tested as tight. The remedy for a leak detected on the primary cover (1), to avoid any risk of dispersion of radioactivity in the environment, can be to install a third cover (3) and thus to transfer the primary barrier initially constituted by the cover (1) on the cover (2).

If no leak is detected after having evacuated by (15), a vacuum is created in the space between the covers (1) and (2) using the channel (1 6) and after putting in l helium in the space between seals (12) of the cover (2) using the channel (17); the spectrometer should then confirm the presence of He.

The leak having been located on the cover (2), excluding a leak on the cover (1), it is then possible to intervene on the cover (2) without taking the risk of dissemination, which 'It was not possible to do before since it could not be concluded with certainty that there was no leakage on said cover (1), the seals of the latter not being accessible.

The same type of procedure can be applied subsequently to the inspection of covers (2) and (3).

The device according to the invention makes it possible to use other methods for detecting and locating possible leaks.

For example after having filled the space between the covers (1) and (2) with He at atmospheric pressure (P2 = 1 bar), we put the spaces between seals of the double seals (1 1) and (12) under the same pressure of N ₂ „for example at 6 bar. Any difference which appears later between the pressures of these spaces between seals indicates a leak in one of the corresponding double seals. The quantitative assessment of the leak can then be done by vacuuming the space between the leaking joints and analyzing the amount of He sucked in, using the mass spectrometer. Thus, by varying the nature of the gases used and their pressure, the device according to the invention makes it possible to measure continuously and at will from the outside, the possible leakage rate of each of the joints of each of the covers, including the deeper, and in addition without any disassembly being necessary.

Claims

. Device for checking the tightness of the covers of a heavy metal container for transporting and / or storing radioactive materials, said container comprising a cavity for nuclear materials delimited by a thick cylindrical shell closed at one end using a bottom fixed in a sealed manner and at the other end by at least two superimposed covers, removable and thick, characterized in that said covers are held in abutment on shoulders, formed in the shell, by means of flanges equipped with at least two concentric seals, to which each cover corresponds to at least two channels passing through said ferrule, opening through a first control orifice on the external surface of the ferrule near the lids in an accessible location, and through a second orifice, one in the space between the concentric joints of the cover, the other in the space between said cover, the ferrule and the cover immediately superimposed, each control orifice being optionally connected to a measurement and leakage control circuit.

2. Device according to claim 1 characterized in that at least one channel (20) passes through the ferrule (4) and opens into the cavity (C).

3. Device according to any one of claims 1 or 2, characterized in that the orifices of the channels (15 to 20) passing through the ferrule (4), located on the external surface of said ferrule (4), are closed by plugs and closures with their own tightness control system.

4. Device according to any one of claims 1 to 3 characterized in that the container has 3 overlapping lids (1, 2,3).

5. Device according to any one of claims 1 to 4 characterized in that each of the covers (1, 2,3) comprises a control channel (5,6,7) opening out between the concentric seals (1 1, 12, 13) covers (1, 2,3) and / or at least one service channel (8,9,10) opening inside the circle formed by the concentric seals (1 1, 12,13), said channels service (8,9,10) passing through each of said covers from its upper face to its lower face, and the orifice of each service channel located on the upper face of the corresponding cover being closed by a closure having two seals between which opens a leakage control nozzle itself closed by a plug.

6. Device according to claim 5 characterized in that the tightness control nozzles for the closings of the service channels (8,9,10) are connected to the control channel (5,6,7) of the corresponding cover.

7. Device according to any one of claims 1 to 6 characterized in that the orifice of at least one channel (15 to 20) is connected to measuring or control devices.

8. Device according to claim 7 characterized in that the connection between the orifices of the channels (15 to 20) and said devices is made by means of a buffer volume (V5-V9) and that said devices are common to several orifices (15 to 20).

9. A method of sealing control using the device of any one of claims 1 to 8.

10. Method for checking the tightness of the primary lid of a container of radioactive materials comprising at least two overlapping lids (1, 2), each of them being closed with concentric seals

(1 1 -12), the primary cover (1) being in contact with the materials radioactive, characterized in that it is carried out when all the covers and / or the protective cover of the container is in place.