Patent Number: 
Section: claims

1. A fuel assembly for a boiling water reactor adapted during operation of the reactor to allow coolant to flow upwards through the fuel assembly while absorbing heat from a plurality of fuel rods to transform a portion of the water into steam, said fuel assembly comprising: a first steam pipe arranged with a longitudinal axis parallel to a longitudinal axis of the fuel assembly,  wherein the first steam pipe comprises an inlet for the steam arranged in a first end of the first steam pipe, and an outlet for the steam arranged at a second end of the first steam pipe;  a second steam pipe arranged above and spaced from the first steam pipe along the longitudinal direction of the fuel assembly such that an opening is formed between the first and second steam pipes,  wherein the outlet of the first steam pipe has an outlet diameter which is larger than an inlet diameter of the inlet of the second steam pipe,  wherein fuel rods are arranged laterally with respect to said first and second steam pipes in said fuel assembly,  wherein a second end of the second steam pipe comprises means for collecting water on an inside of the second steam pipe and conducting the collected water towards the outlet of the second steam pipe. 2. A fuel assembly according to  claim 1 , further comprising a third steam pipe arranged above and spaced from the second steam pipe such that an opening is formed between the second and third steam pipes, claim 1 wherein an outlet of the second steam pipe has a diameter which is larger than a diameter of an inlet of the third steam pipe. 3. A fuel assembly according to  claim 1 , wherein the second end of the first steam pipe is arranged tapering towards the outlet of the first steam pipe and the first end of the second steam pipe is arranged tapering towards the inlet of the second steam pipe for achieving a venturi effect. claim 1 4. A fuel assembly according to  claim 1 , wherein a distance between the outlet of the first steam pipe and the inlet of the second steam pipe is less than half a diameter of the inlet of the second steam pipe. claim 1 5. A fuel assembly according to  claim 1 , wherein the second end of the first steam pipe comprises means for collecting water on an inside of the first steam pipe and conducting the collected water towards the outlet of the first steam pipe. claim 1 6. A fuel assembly according to  claim 5 , wherein said means for collecting water comprise a plurality of elongated grooves on the inside of the first steam pipe. claim 5 7. A fuel assembly for a boiling water reactor adapted during operation of the reactor to allow coolant to flow upwards through the fuel assembly while absorbing heat from a plurality of fuel rods to transform a portion of the water into steam, said fuel assembly comprising: a first steam pipe arranged with a longitudinal axis parallel to a longitudinal axis of the fuel assembly,  wherein the first steam. pipe comprises an inlet for the steam arranged in a first end of the first steam pipe, and an outlet for the steam arranged at a second end of the first steam pipe;  a second steam pipe arranged above and spaced from the fist steam pipe along the longitudinal direction of the fuel assembly such that an opening is formed between the first and second steam pipes,  wherein the outlet of the first steam pipe has an outlet diameter which is larger than an inlet diameter of the inlet of the second steam pipe,  wherein fuel rods are arranged laterally with respect to said first and second steam pipes in said fuel assembly,  wherein the second end of the first steam pipe comprises means for collecting water on an inside of the first steam pipe and conducting the collected water towards the outlet of the first steam pipe,  wherein said means for collecting water comprise a plurality of lugs arranged around the outlet of the first steam pipe. 8. A fuel assembly according to  claim 1 , wherein the fuel assembly comprises at least two fuel units stacked on top of each other, claim 1 each fuel unit comprising  a top tie plate,  a bottom tie plate,  a plurality of fuel rods extending between the top tie plate and the bottom tie plate, and one of said first and second steam pipes. 9. A fuel assembly according to  claim 8 , wherein the first end of the first steam pipe is attached to the bottom tie plate and the second end of the first steam pipe is attached to the top tie plate. claim 8 10. A fuel assembly for a boiling water reactor adapted during operation of the reactor to allow coolant to flow upwards through the fuel assembly while absorbing heat from a plurality of fuel rods to transform a portion of the water into steam, said fuel assembly comprising: a first steam pipe arranged with a longitudinal axis parallel to a longitudinal axis of the fuel assembly,  wherein the first steam pipe comprises an inlet for the steam arranged in a first end of the first steam pipe, and an outlet for the steam arranged at a second end of the first steam pipe;  a second steam pipe arranged above and spaced from the first steam pipe along the longitudinal direction of the fuel assembly such that an opening is formed between the first and second steam pipes,  wherein the outlet of the first steam pipe has an outlet diameter which is larger than an inlet diameter of the inlet of the second steam pipe,  wherein fuel rods are arranged laterally with respect to said first and second steam pipes in said fuel assembly,  wherein the second end of the first steam pipe comprises means for collecting water on an inside of the first steam pipe and conducting the collected water towards the outlet of the first steam pipe,  wherein a second end of the second steam pipe comprises means for collecting water on an inside of the second steam pipe and conducting the collected water towards the outlet of the second steam pipe.