Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Having particular end closure or seal
Patent
1986-10-14
1988-03-08
Walsh, Donald P.
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Having particular end closure or seal
376204, G21C 310
Patent
active
047298707
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a nuclear fuel element for use in a nuclear reactor, preferably of the fast neutron type.
The nuclear fuel elements used in fast neutron reactors are generally in the form of rods, which are then brought together in bundles or groups in nuclear fuel assemblies constituting the fissile part of the reactor core.
Each nuclear fuel rod essentially comprises a tubular metal can or jacket, in which are stacked fissile material pellets constituting the actual fuel. The can is sealed at each of its ends by a plug, which is forced into the interior of the can and then welded level with the junction plane between plug and can.
Due to the tolerances inherent in the manufacture of tubular cans, the internal diameter thereof can vary by approximately 1/100 mm within the same production batch and by approximately 6/100 mm in different production batches.
To take account of these tolerances, the presently used plug fitting process consists firstly of pairing the plugs as a function of the internal diameter of the cans. When this pairing has taken place, the plugs are fitted into the cans with a force of approximately 80 daN for an average tightening of 2/100 mm, prior to being welding.
However, despite the precautions taken, there are still mixtures of cans obtained from different batches or non-conforming parts, which often makes it necessary for manufacturers to manually complete the plugging operation using a hammer and the elimination of plugs which do not hold in the cans. This leads to a very low working speed and to considerable wastage at the production stage.
Moreover, the approximate nature of the tightening between the plugs and the can often leads to excessive tightening, which can cause an abnormal swelling of the can and to a pulling away of metal, which often leads to defects in the welded area. This is another cause of wastage when these are latent defects. Among these defects, reference can be made to the plastic deformation of the bearing face during the stopping of the upper plug leading to a flattening which is prejudicial to the fitting of the lower plug and to the quality of its weld. A deformation at the weld of the lower plug also causes problems for fitting the rod bundles within the assemblies. Moreover, excessive tightening of the plugs in the can can lead to buckling of the latter, bearing in mind the considerable difference between its diameter and its length.
A consequence of all these difficulties is that it is not possible with the present technology to envisage an automation of the production of nuclear fuel elements.
SUMMARY OF THE INVENTION
The main object of the invention is to permit a high speed automatic production of nuclear fuel elements. To this end, a nuclear fuel element is proposed, whose plugs are produced in such a way that the fitting force is limited to a minimum value, so as to avoid any deformation, whilst still ensuring a good hold of the plug, without it being necessary to pair plugs and cans and whilst completely guarantying the conformity of fitting, the defects referred to herein before being eliminated.
The present invention therefore specifically relates to a nuclear fuel element comprising a tubular can having an internal diameter between a maximum diameter and a minimum diameter, which is sealed at at least one end by a plug having a cylindrical portion force fitted into said can, wherein the cylindrical portion has a diameter equal to the minimum diameter of the can and has at least three serrations oriented in accordance with the generatrixes of said cylindrical portion and which are regularly spaced, said serrations forming overhanging beads with respect to said cylindrical portion over a thickness equal to half the difference between the maximum diameter and the minimum diameter of the can.
Preferably, only three serrations are formed on the cylindrical portion of the plug, whilst the driving in force is reduced to a very low value of approximately 15 daN.
BRIEF DESCRIPTION OF THE DRAWINGS
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REFERENCES:
patent: 3079322 (1963-02-01), Stockdale
patent: 3392438 (1968-07-01), Coulter
patent: 4587094 (1986-05-01), Bucker
patent: 4591068 (1986-05-01), Tolind
patent: 4657733 (1987-04-01), Millot
Mabboux Marcel
Mercier Claude
Commissariat a l''Energie Atomique
Walsh Donald P.
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