| 摘要 |
A rotatable plug or cover for a nuclear reactor has a trough and crown construction, the trough into which the crown dips being filled with a low melting alloy, preferably a Bi/Sn eutectic (56-60% Bi, the remainder Sn), which acts as a seal. The cover rotates on ball bearings, and can only be turned after the alloy is rendered molten by means of associated heating elements. The surfaces of the trough and crown preferably consist of stainless steel and these surfaces must be treated to ensure the alloy sticks to them. Such a surface, i.e. one having at least a portion thereof formed from a non-oxidizing metal, e.g. stainless steel, is treated so that a readily liquefiable alloy, e.g. the above eutectic, will stick to it by tinning the metal surface with a thin layer of said alloy in the presence of a chemically active or strong flux, then covering this surface with a thin layer of a soft flux. The surface thus covered is preheated to 80 DEG C., and the said alloy poured on to the prepared and preheated surface at a temperature of about 200 DEG C., and the whole is heated slowly at about 150 DEG C. to homogenize the various substances of the alloy and to remove the soft flux, whereafter the whole is cooled. Prior to tinning, the surfaces may if necessary be cleaned with an aqueous solution containing 2% HF and 10% HNO3 and heated to 60 DEG C., degreased with ethyl acetate and thoroughly washed with water. The tinning operation is performed by means of shaped, heated (340-400 DEG C.) irons), and the coating applied is 0.5-0.7 mm. thick. The surface being tinned is maintained horizontal by an appropriate jig. After tinning the surface is washed and a coating 0.3-0.4 mm. thick of the soft flux is applied. The chemically active strong flux used is preferably an aqueous solution of phosphoric acid containing a small quantity of ammonium chloride, and the preferred soft flux is alcoholized colophony. Other alloys suggested for use are Bi,Pb,SnCd, and Bi,Pb,Sn. |
