GB2119157A - In-core irradiation assembly for a nuclear reactor - Google Patents

Abstract

The invention relates to an in-core irradiation assembly for a nuclear reactor through which a coolant flows downwards, in which, in operation, reactor coolant flows directly around the irradiation specimens but, nevertheless, charging and discharge of the irradiation duct is possible during full reactor operation. This is achieved in that the irradiation duct, which surrounds the irradiation specimens and through which coolant flows, can be sealed above the coolant- inlet slots (7) by a plug (16'') releasably couplable with the lower end of the irradiation capsule, and able to seal the duct in the region of the top cover 4 during removal/replacement of a capsule. The invention is applicable to all types of irradiation, for example for isotope production. <IMAGE>

Description

SPECIFICATION In-core irradiation assembly for a nuclear reactor The invention relates to an in-core irradiation assembly for a nuclear reactor through which a coolant flows, having an incasing tube anchored securely in the reactor core and surrounding at least one duct for accommodating a cylindrical iradiation capsule.

An assembly of this type is known, for example, from FR-OS 2 079. The encasing tube is closed at its underside and on its upper side, still in the region of the reactor core, has a funnel-shaped opening. For the insertion of an irradiation capsule into the encasing tube, it is necessary for a charging arm, which passes through the cover of the reactor vessel, to be brought into alignment with the encasing tube, whereupon a vessel surrounding the irradiation material is lowered into the encasing tube.

An irradiation assembly of this type can carry out charging and discharge operations only when the reactor is shut down. In addition to this, because of the unilaterally - closed encasing tube, difficulties arise in the cooling of the irradiation material and thus temperature conditions occur which are neither clearly defined nor can be effectively influenced from outside.

The FR-OS 2417 828 further discloses an in-core irradiation assembly in which the irradiation specimens can be inserted in an encasing tube installed securely in the reactor. The specimens are suspended from an arresting head in which there engages a manipulating arm actuatable from above the reactor cover. This head has means for engaging in corresponding grooves on the upper edge of the encasing tube and closes it tightly in the engaged position.

Here, too the charging and discharge of the irradiation capsule can only be effected during reactor shut-down, since the manipulator has to be passed through the cover of the pressure vessel.

Furthermore, the same difficulties arise here for cooling the capsule as in the previously-mentioned case.

The GB-OS 2015239 discloses an irradiation assembly with an encasing tube which has numerous openings for the passage of the reactor coolant.

The encasing tube is situuated in the lateral annular gap between the reactor vessel and the reactor core and cold primary coolant flows around and through the tube. Admittedly, the cooling problem is satisfactorily splved here but, as previously, any exchange of the irradiation capsules is only possible during reactor shut-down, since only then can the reactor cover be opened and access gained to the irradiation assembly.

The present invention seeks to provide an irradiation assembly through which the primary coolant of the reactor flows effectively and in which the charging and discharge procedures can take place during reactor operation and without interrupting the latter. This advantage is of particular significance for research reactors, wherein very long operating periods are required between two fuel-element changes, whereas the irradiation time of the individual irradiation capsules is variable and as a rule is chosen to be substantially shorter (for example for the production of radioisotypes).

This advantage is achieved by the irradiation assembly defined in the characterising part of Claim 1.

Advantageously, the upper side of the irradiation capsule according to the invention is releasably coupled to a rod-like implement.

In a preferred embodiment of the invention, the encasing tube surrounds three irradiation ducts which are fully independent of one another and which can thus also be charged and discharged completed independently of one another.

Reference is made to the sub-claims with regard to advantageous features of one embodiment of the invention.

The invention will now be described on the basis of one example of an embodiment, with reference to the drawings, where: Figure 1 shows a section through a reactor, in which the irradiation assembly according to the invention is inserted, Figure 2 to 5 each show on an enlarged scale cut-away portions from the illustration according to Figure land Figure 6 shows a cross-section through three irradiation ducts according to Figure 1, at the level of the irradiation zone.

The irradiation assembly shown in longitudinal section in Figure 1 is inserted in a reactor, of which only the lower grid plate 1, the upper grid plate 2 and the upper side of the pressure vessel 3 with a cover 4 are illustrated in the Figure. The actual reactor core is situated between the two grids 1 and 2 and comprises fuel elements, control rods and cooling channels (not illustrated). A coolant, e.g. water, is fed in at the top through the upper grid 2 to the reactor core 5 and passes through the lower grid 1 to reach a heat exchanger (not shown? where the heat absorbed in the reactor core is given off.

In the desired irradiation zone of the reactor core there is situated a securely-installed encasing tube which is supported partly on the lower core grid 1, which passes through the upper core grid 2, and extends just into the cover 4. Above the upper grid 2, the encasing tube 6 has coolant-inlet slots 7. The normal reactor coolant enters the encasing tube through these slots and leaves the encasing tube through openings 8 at its bottom end.

The encasing tube surrounds three irradiation asemblies which are independent of one another, as shown in cross-section in Figure 6. Each of the irradiation assemblies is situated in a guide tube 9 installed securely in the encasing tube, which guide tube likewise has slots in the region of the encasingtube slots 7 and which likewise is open at the bottom.

The material to be irradited consists of stacks of tubular pieces 10. Each stack is mounted on a central support rod 11 and is centred coaxially relative to this rod by means of suitable centering projections.

The support rod is hollows and a retaining pin 12 passes through it. In the absence of other forces, the retaining pin is held by a spring 13 in its upper end position, in which a spring abutment 14 is applied against an inner shoulder of the support rod. At its lower end, the retaining pin has a locking plunger 34 which cooperates with locking balls 15 at the top end of a plug 16 in such away that the plug is connected in form-locked manner with the lower edge of the support rod when the locking plunger 34 presses the balls 15 into corresponding recesses in the lower edge of the support rod.

The plug 16 is hollow and itself has a retaining pin 17 which has a locking plunger 18 and, in the absence of external forces, is pressed upwards by a spring 19 so that further locking balls 20, which are situated on the plug, are pressed radially outwards.

In Figures 2 to 6, the irradition assembly is shown in the condition in which it is to be found during irradiation. In this condition, a central actuating part 21 of an implement (yet to be described) presses on the retaining pin 12 in the support rod 11 and urges it into its lower end position, against the force of the spring 13. In the same way, the retaining pin 17 of the plug 16 is pressed downwards by this retaining pin 12, against the force of the spring 19. In this position, the locking plunger 34 of the retaining pin 12 is engaged and couples the plug 16 to the support rod, whereas the locking balls 20 are not engaged by the locking plunger 18 of the plug 16.

The aforementioned implement is connected via further locking balls 22, which engage in the upper edge of the support rod 11, in form-locked manner with this support rod, since they are prevented from being deflected radially outwards by a slide member 23 engaging over them. The slide member 23 forms the lower end of the displacable part 21 of the implement. If this displaceable part is situated in its bottom end position, the slide member 23 engages over the balls 22 and, at the same time, the retaining pin 12 of the support rod 11 is pressed downwards.

The displaceable part 21 of the implement can be displaced relative to a tubular housing 24 and, by means of a spring 25 - the force of which exceeds that of the springs 13 and 19 is held in the bottom end position, in the absence of other forces. The tubular housing 24 of the implement embraces with its bottom end 26 the top edge of the support rod and, as already mentioned, can be connected thereto in form-locked manner by means of the balls 22.

In an intermediate position between the bottom end 26 and the head 27 of the implement, a resilient seal 28 is situated between the tubular housing 24 and the displaceable part 21 of the implement, which seal can follow, because of its resilience, the manipulating movements of the displaceable part relative to the housing. Coolant fluid cannot pass upwards between the tubular housing 24 and the encasing tube 9, since the implement is sealed by an annular seal 29 in the irradiation zone and during the charging and discharge operations.

The displaceable part of the implement terminates at the top outside the reactor-vessel cover 4 in a handle 30 which is adjacent a handle 31 securely connected to the housing 24 of the implement.

In preparation for an irradiation cycle, the irradiation material is mounted in the form of tubular pieces 10 in a support rod 11.

In the meantime, the relevant irradiation duct is tightly sealed by means of a plug 16", which in the vicinity of the reactor cover 4 is itself retained in the guide tube 9, by means of the pressure of the spring 19, in a locking position in which the locking balls 20 engage in a corresponding annular groove in the guide tube 9. In this position, one sealing surface 35 of the plug 16 is applied against a corresponding sealing shoulder of the guide tube 9 in the vicinity of the cover 4 and tightly seals the irradiation duct.

During this period, it is possible for coolant to pass unimpeded through the inlet slots 7 into the irradiation duct and then emerge therefrom again through the openings 8.

If the irradiation capsule, comprising the irradiation material 10, the support rod 11 and the retaining pin 12, is now mounted on such a plug 16" locked in a cover 4, the locking between the plug 16" and the support rod only takes place when the implement, comprising the displaceable part 21 and the tubular housing 24, is mounted at the top on the irradiation capsule, in which case temporarily the two handles 30 and 31 have to be brought close together. If the handles are then released, the spring 25 presses the retaining pin downwards to initially effect the locking of the plug 16" with the support rod and the locking of the support rod with the implement, and, subsequently, the unlocking of the plug 16" in the guide tube.The sealing of the duct, in which the pressure of the reactor coolant prevails, is then no longer effected via the sealing surface 35 of the plug, but via the annular seal 29, that is both during the lowering of the capsule into the irradiation position and also during the subsequent irradiation period and discharge.

Figure 1 indicates three alternative positions of the irradiation capsule in the duct, namely a charging position (plug 16"), an irradiating position (plug 16) and so-called 'parking' position (plug 16').

When the irradiation capsule is situated in the irradiating position, the cooling water, which passes through the slots 7 into the guide tube above the upper core grid 2, flows past the lower part of the implement, past the irradiation material 10 and past the plug 16, in which case a portion of the cooling water passes through slots 33 at the top end of the tublar stack forming the irradiation material 10 and penetrates into the tube interior and flows out again at the bottom end of the stack through corresponding openings 32. Accordingly, the irradiation material is effectively cooled on either side by the reactor coolant. This cooling flow is not interrupted, upon raising the irradiation capsule, while thecapsule is still situated beneath the slots 7. Therefore, by suitable locking means on the head of the implement, it is possible to arrest the irradiation capsule in a parking position, in which it is in fact already above the upper core grid 2 but still lies below the inlet slots 7. In this zone, the rapidly decaying isotopes break up, without exposing the environment to radiation. In this parking position the irradiation duct is also seald against escaping cooling water, i.e. via the annular seal 29.

In Figures 1 to 5, only one irradiation duct is shown within the encasing tube 6. This irradiation duct could also take up the entire cross-section of the encasing tube 6. However, as shown in Figure 6, in the chosen exemplified embodiment three mutuallyparallel irradiation ducts are provided, each with a stack of tubular pieces forming the irradiation material 10, each with a support rod 11,a retaining pin 12 and a guide tube 9. The three irradiation ducts are each attended by a separate implement and can be charged and discharged completely independently of one another and of the fuel-element cycles. This results in optimum flexibility of the irradiation cycles, such as are required for the production of the most diverse radionuclides.

As described above, the invention offers for the first time the possibility of carrying out the irradiation of specimens in a pressure vessel, wherein the specimens are cooled by the reactor coolant during irradiation and, independently of the fuel-element cycle, can be charged, discharged and brought into a parking position. The pressure vessel may be situated in a water tank, the water of which cannot come into contact through the irradiation duct with the reactor coolant in any operating condition.

Claims (8)

1. An incore irradiation assembly for a nuclear reactor through which a coolant flows downwards, having an encasing tube anchored securely in the reactor core and surrounding at least one duct for accommodating a cylindrical irradiation capsule, characterised in that the encasing tube rests partly on a lower core grid and is guided through an upper core grid into the cover of the reactor vessel in that, between the cover and the upper grid, coolant-inlet slots are provided in the encasing tube, while an outlet of the coolant from the encasing tube is provided in the region of the lower core grid, passages for the coolant being provided between the irradiation capsule and the wall of the duct, in that the irradiation capsule can be coupled on its underside to a plug of slightly larger diameter than the capsule which plug has engagement means which cooperate with corresponding means in the region of the reactor cover, in the sense of tightly sealing the duct in respect of the coolant.

2. An irradiation assembly as claimed in Claim 1, wherein the upper side of the irradiation capsule is coupled to a rod-like implement.

3. An irradiation assembly as claimed in Claim 2 wherein the irradiation capsule is provided for annular irradiation specimens and has a central support rod, through which passes a retaining pin which is movable relative to the specimens in the direction of the duct axis and which effects the coupling of the plug to the capsule.

4. An irradiation assembly is claimed in Claim 3, wherein the implement comprises two parts which are mutually displaceable in the axial direction of the duct and one of which forms a tubular housing coupable to the irradiation capsule, and the other acts on the retaining pin of the support rod in axial direction.

5. An irradiation assembly is claimed in any one of the preceding claims wherein the encasing tube surrounds three irradiation ducts which are independent of one another.

6. An irradiation assembly as claimed in any one of the preceeding claims wherein the irradiation capsule can be locked in an intermediate position in which the irradiation capsule is already situated above the upper core grid, but below the coolantinlet slots.

7. An irradiation assembly is claimed in Claim 1 substantiaily as hereinbefore described with reference to and as illustrated in the accompanying drawings.

8. A nuclear reactor having an irradiation assembly as claimed in any one of the preceeding claims.