Patent Number: 
Section: claims

1. A double strip mixing grid for nuclear reactor fuel assemblies, comprising a plurality of inner double strips, each fabricated by integrating two thin sheets together into a single structure having a plurality of coolant channels and intersecting each other at a predetermined angle to form a desired mixing grid having a plurality of four-walled cells, wherein each of said strips is partially cut away at predetermined portions from its top edge to form nozzles of said coolant channels, a plurality of mixing blades are provided at top portions of the strips for forcing coolant flowing inside the channels to be actively mixed with coolant flowing outside the channels, thus improving coolant mixing effect of the grid, and said two thin sheets are integrated in such a manner that each of said plurality of mixing blades provided at top portions of the strips faces one of said predetermined portions cut away from top edge to form nozzles for said coolant channels. 2. The double strip mixing grid according to  claim 1 , wherein a plurality of lateral flow blades are provided at the strips for creating a lateral flow of coolant between the four-walled cells. claim 1 3. The double strip mixing grid according to  claim 1 , wherein a plurality of swirling flow blades are provided at intersections of the strips for creating a swirling motion of coolant at positions around said intersections. claim 1 4. The double strip mixing grid according to  claim 1 , wherein a plurality of lateral flow blades and a plurality of swirling flow blades are provided on the strips for creating both a lateral flow of coolant and a swirling motion of coolant at the same time. claim 1 5. The double strip mixing grid according to  claim 1 , wherein said two sheets of each of the strips are continuously welded together at their junctions. claim 1 6. The double strip mixing grid according to  claim 1 , wherein said two sheets of each of the strips are provided with a plurality of vertical slots at their portions, corresponding to the coolant channels and acting as fuel rod positioning springs. claim 1 7. The double strip mixing grid according to  claim 6 , wherein a cross-sectional area of each of said coolant channels is gradually enlarged in a direction from an inlet to outlet of said channel or varies such that it is maximized at a middle portion of the channel supporting a fuel rod within each four-walled cell. claim 6 8. The double strip mixing grid according to  claim 1 , wherein each of said strips has a thickness ranging from 0.25 mm to 0.40 mm, and each of said channels has a width ranging from 7 mm to 10 mm. claim 1