referring now to the figures of the drawing in detail and first , particularly , to fig6 thereof , there is shown a cladding tube 10 of the fuel rods 6 is in each case closed in a gastight manner at upper and lower ends by an end plug 11 . at the upper end a spring 12 subjected to compressive stress ensures that a corresponding plenum 13 is maintained at least at the upper end . a column of fuel pellets 14 contains in each case , at its upper and lower end , a body 15 that contains virtually no fissionable material and may consist , for example , of aluminum oxide or else natural uranium or depleted uranium . in this case , in order to increase conductivity between the pellets 14 and the cladding tube 10 , the cladding tube 10 is filled with a high - pressure gas ( for example , helium ). in the present case , a supporting body 16 at the lower end of the fuel rod also keeps free a corresponding plenum . the outside diameter of the cladding tube is about 9 . 55 mm , and its wall thickness is about 0 . 61 mm . according to fig7 , a cladding tube 20 is formed of a matrix 21 , the thickness of which is about 75 to 95 % of a cladding tube wall . a protective layer 22 is bound metallurgically to the matrix 21 on the inside of the cladding tube 20 , and it is also indicated that a further protective layer 22 ′ may also be attached to the outside . table 2 indicates the lower and upper limit values for the composition i of the matrix 21 . here , the values given in brackets in each case describe preferred relatively narrow limits for the contents of the individual alloy constituents or the particularly preferred limit values for the accompanying elements of the alloy constituents which are already contained as impurities in the zirconium of industrial purity (“ sponge ”, see table 1 ) and can be maintained for the lower limits which are also advantageous , as in the case of oxygen or silicon . in the preferred exemplary embodiment , the matrix 21 contains 1 . 3 ± 0 . 1 % sn ; 0 . 28 ± 0 . 04 % fe ; 0 . 16 ± 0 . 03 % cr ; 0 . 01 ± 0 . 002 % si and 0 . 14 ± 0 . 02 % o . the size of the precipitated secondary phases is in this case 30 · 10 − 18 h . the protective layer 22 consists of 0 . 4 ± 0 . 04 % fe and zirconium sponge , the precipitation size being determined by a = 1 · 10 − 18 h . in the second phase , the precipitations consist virtually of intermetallic zrfe compounds , in the case of the matrix 21 of mixed compounds of zirconium with iron and chromium , and , in fig8 , it can be seen under i that , up to temperatures of about 820 ° c ., there is an α - phase of zrsn in addition to the corresponding secondary phase γ of these precipitations . in the range between about 820 and 960 ° c ., there is also a β - phase of zrsn in addition to the α - phase , and at about 840 ° c . (“ solution temperature ”) the γ - phase of the intermetallic compounds becomes a solution . above 960 ° c ., only the β - phase with the dissolved precipitations is still stable . if , therefore , the matrix is heated into the β - range ( temperatures of above 960 ° c .) and is then rapidly cooled , a fine - grained α - phase is first formed , in which part of the iron is distributed in a finely dispersed manner as precipitations of the γ - phase , while the rest of the iron remains bound as metastable supersaturation in the α - phase . in this case , the finely dispersed precipitations correspondingly form nuclei , on which the excess iron fraction is accreted the more rapidly and the more highly , the higher the temperature and duration in which the matrix material is exposed to further thermal treatments in the α - range ( temperatures of below 820 ° c .). to produce the cladding tube 20 , first , the first alloy of zrsnfecr , provided for the matrix 21 , is remelted a plurality of times under a vacuum in a step 30 a ( fig9 ) to homogenize the alloy constituents , in a step 31 a the alloy is forged to a shape suitable for the processing of a tube blank and , in a further step 32 a , the alloy is rapidly cooled (“ β - quench ”) from a temperature in the β - range ( above 960 ° c .). this may be followed by further forging ( step 33 ), the first tube blank ra being produced at the latest during a step 34 . the step 34 is also followed by further annealings , in order to set the parameter a = 30 · 10 − 18 h in the first tube blank . in a similar way , the second zirconium alloy ( zrfe ) provided for the protective layer 22 is likewise remelted in a step 30 b , in a step 31 b is heated into the β - range ( temperatures of above 960 ° c .) and in the step 32 b is rapidly cooled . during these steps , a second tube blank rb is also produced . in this case , the β - quenching ( step 32 b ) is followed by virtually no further heating , instead the two blanks , the shapes of which have been adapted to one another , are placed one into the other , welded to one another and jointly extruded in a step 35 . this coextrusion does not , in practice , contribute to the ripening of the precipitations , so that , in the composite tube obtained , the matrix material possesses the value a = 30 · 10 − 18 h and the second zirconium alloy possesses virtually the value a = 0 . subsequently , a plurality of pilgrim steps 36 are carried out , between which brief annealings at temperatures well below 820 ° c . are carried out in each case , in order to recover the cold - formed material and prepare it for the next pilgrim step . what is then achieved by terminal annealing 37 is that the parameter a = 1 · 10 − 18 h is set for the entire processing of the composite tube to form the finished cladding tube , that is to say the first zirconium alloy of the matrix has the value a = 31 · 10 − 18 h , but the second zirconium alloy of the protective layer has the value a = 1 · 10 − 18 h . for steps 33 and 34 , a range a = 2 to 80 · 10 − 18 h is maintained , values of above 5 · 10 − 18 h being advantageous . values of above 60 · 10 − 18 h signify long annealing durations at high temperatures which do not seem necessary . for steps 35 to 37 , in general , values a of below 2 · 10 − 18 may be maintained . for the finished zirconium alloy of the matrix , therefore , values a = 5 to 60 · 10 − 18 h seem advantageous , while a = 1 to 3 · 10 − 18 h should be maintained for the second zirconium alloy of the protective layer . in the second exemplary embodiment according to fig1 , a cladding tube 40 is formed of the matrix 41 with the composition 1 . 0 ± 0 . 2 % nb , 0 . 14 ± 0 . 02 % o , remainder : zirconium of industrial purity , see table 2 indicating under ii the preferred limits for the constituents in similar compositions . it can be seen in fig8 , under ii , that , in the phase diagram of the alloy , at temperatures of up to 580 ° c . there is a stable α - phase in which about half the niobium is dissolved , while the remainder is precipitated as a stable β - phase of the niobium . at 580 ° c ., there is a mixed phase α + β , in which virtually all the niobium is dissolved , while , at temperatures of above 960 ° c ., only a β - phase of the zirconium , with the completely dissolved niobium , still exists . the second zirconium alloy in a protective layer 42 of the cladding tube 40 is formed of the same zrfe alloy as in the first preferred exemplary embodiment already described . to produce the cladding tube 40 , a diagram according to fig1 , similar to that of fig9 , is obtained . in this case , however , after multiple remelting under a vacuum ( step 50 a ) and forging in the β - range ( temperatures of above 960 ° c .) ( step 51 a ), the first zirconium alloy zrnb of the matrix is quenched ( step 52 ), a first tube blank rc being produced from the matrix material , without the β - quenching ( step 52 a ) being followed by thermal treatment with an appreciable parameter value a . a step of this kind is provided only for the second zirconium alloy of the protective layer , in which multiple remelting in a vacuum ( step 50 b ) and forging in the β - range ( step 51 b ) and annealing at temperatures of below about 600 ° c ., in particular below 580 ° c . ( α - range ), take place . in this case , the second tube blank rd is produced , which is inserted exactly into the interior of the first tube blank rc . for the first tube blank rc produced in steps 51 a and 52 a , virtually the parameter value a = 0 is obtained , while , in steps 51 b , 52 b and 53 , the second tube blank rd can be produced with a parameter value below 2 · 10 − 18 h . in the exemplary embodiment , a = 1 · 10 − 18 h was set in step 53 . the two tube blanks inserted one into the other are welded to one another and extruded together , subsequently brought to the final dimensions of the cladding tube ( step 55 ) in a plurality of pilgrim steps , with recovery annealings interposed between them , and subjected to terminal annealing 56 . in steps 54 to 56 , a lower than 0 . 5 · 10 − 18 h is maintained , even values a lower than 0 . 1 · 10 − 18 h being possible ( here : a = 0 . 9 · 10 − 18 h ). according to the value a being lower than 0 . 5 · 10 − 18 ( preferably , a lower than 0 . 2 · 10 − 18 , at all events at least lower than 0 . 3 · 10 − 18 ) for steps 50 a to 52 a , in the finished cladding tube preferably a value a lower than 0 . 1 · 10 − 18 h is obtained for the first zirconium alloy of the matrix 41 , whereas a value a = 0 . 1 to 3 · 10 − 18 h , preferably between 0 . 2 and 1 . 5 · 10 − 18 h , is obtained for the second zirconium alloy . the cladding tubes produced in this way are filled with the columns of relatively highly enriched fuel pellets and with the high - pressure gas , are closed in a gastight manner by the end plugs and are inserted into the skeleton mentioned . they have a high burn - up which makes it possible to have a long period of utilization in the pressurized water reactor . when the pressurized water reactor is in operation , in the control of the power output the permissible rates of change need to be coordinated essentially only with the time constants defined by the physics of the fuel and of the reactor , only minor account needing to be taken of possible material damage which , even after lengthy operating times under part load , could occur on the cladding tubes when the reactor power output is being run up .