Patent Number: 
Section: claims

1. A control rod for a boiling water reactor which control rod is submerged under water during operation, said control rod comprising: a top piece;  a bottom piece; and  an absorber part fixed between said top piece and said bottom piece, said absorber part including neutron absorbing metal sheets arranged opposite each other forming a cruciform cross-section comprising a central part and four wing parts;  a plurality of indentations being distributed intermittently in the longitudinal direction on each said metal sheet, a connection being formed between corresponding indentations on opposite metal sheets in order to space said metal sheets apart; and  said central part and said wing parts being hollow and containing no other support structure or neutron absorbing material except for said connections, said central part and said wing parts extending between said top piece and said bottom piece and being open at least at their top and bottom so as to fill the interior of said absorber part with water when submerged whereby a water volume is surrounded by said metal sheets. 2. A control rod for a boiling water reactor which control rod is submerged under water during operation, said control rod comprising: a top piece;  a bottom piece; and  an absorber part fixed between said top piece and said bottom piece, said absorber part including neutron absorbing metal sheets arranged opposite each other forming a cruciform cross-section comprising a central part and four wing parts;  said central part and said wing parts being hollow and containing no other support structure or neutron absorbing material, said central part and said wing parts extending between said top piece and said bottom piece and being open at least at their top and bottom so as to fill the interior of said absorber part with water when submerged whereby a water volume is surrounded by said metal sheets. 3. A control rod according to  claim 1 , wherein said neutron-absorbing material in the absorber part comprises hafnium. claim 1 4. A control rod according to  claim 2 , wherein said neutron-absorbing material in the absorber part comprises hafnium. claim 2 5. A control rod according to  claim 1  wherein said metal sheets are L-shaped metal sheets. claim 1 6. A control rod according to  claim 2  wherein said metal sheets are L-shaped metal sheets. claim 2 7. A control rod according to  claim 1 , wherein the thickness of the metal sheets varies. claim 1 8. A control rod according to  claim 2 , wherein the thickness of the metal sheets varies. claim 2