Patent Number: 046559990
Section: description

PREFERRED EMBODIMENTS OF THE INVENTION Before explaining the embodiments of the present invention, the typical neutron radiation amount distribution of a cruciform control rod for use in a boiling water reactor will be described with reference to FIG. 1. In FIG. 1, the neutron radiation amount distribution is illustrated for one of the four wings of the cruciform control rod. The neutron radiation amount decreases gradually from the top end toward the bottom end of the control rod, and in the top end part, it is large at the center of the control rod and the outermost end of the wing. In particular, the neutron radiation amount at the wing outermost end in the top end part of the control rod is large. Since, as stated before, the decrease of .sup.10 B at the control rod top end of the large neutron radiation amount is fast, the lifetime of the control rod is determined by the reactivity degradation of the top end part of the control rod. Next, one embodiment of the control rod for a nuclear reactor according to the present invention will be described with reference to FIGS. 2 and 3. In FIG. 2, a cruciform control rod 1 is cruciform in cross section, and it is composed of a neutron absorbing rod-receiving portion 3, an upper supportion portion 5 which overlies the portion 3, and a lower supporting portion 7 which underlies the portion 3. The neutron absorbing rod-receiving portion 3 is made up of four control blades or wings which are fixed to a central supporting member 9. As shown in FIG. 3, each of the control blades is constructed of a control rod sheath 11 which is mounted on the central supporting member 9 made of stainless steel, twenty-one neutron absorbing rods 13 which are arranged in a row inside the control rod sheath 11, and a pair of members 19 which are made of a second neutron absorber and which are respectively inserted between the control rod sheath 11 and the row of the neutron absorbing rods. The neutron absorbing rod 13 consists of a clad tube 15 of stainless steel, and boron carbide powder 17 packed in the clad tube, and it forms a principal neutron absorber. The member of the second neutron absorber 19 is a hafnium sheet which has a thickness of 0.3 mm, a width substantially equal to that of the neutron absorbing rod row, and a length substantially equal to that of the neutron absorbing rod row. The sheets 19 are carried along with the neutron absorbing rods 13 by the supporting plate of the lower supporting portion 7 so as not to be subjected to any external mechanical force. The upper supporting portion 5 is formed of an upper supporting member with a handle, and it is fixed to the sheath 11 and the central supporting member 9 after the neutron absorbing rods 13 and the hafnium sheets 19 have been inserted. The hafnium sheets 19 can be readily inserted into and drawn out of the control rod sheath by detaching the upper supporting member 5. Shown in FIG. 4 is a modification of the control rod for a nuclear reactor in FIG. 3. In this modified embodiment, the neutron absorbing rod located at the wing outermost end of the largest neutron radiation amount is a rod 21 which is made of hafnium. The others are quite the same as in the embodiment of FIGS. 2 and 3. That is, the same neutron absorbing rods 13 as in the foregoing, but numbering twenty, and one hafnium rod 21 at the wing outermost and are inserted in the control rod sheath 11 which is fixed to the central supporting member 9, and one pair of hafnium sheets 19 each of which is 0.3 mm thick are inserted between the neutron absorbing rods 13, 21 and the sheath 11. FIG. 5 shows a modification 19a of the hafnium sheet 19 in the embodiment of FIGS. 2 and 3 or the embodiment of FIG. 4, along with a mounting example thereof. The hafnium sheet 19a is mounted on an upper supporting member 5 unitary with a handle 6, by such joining means 23 as screws. The hafnium sheet of 19a is in the shape of a rectangle whose length is about 1/4 of the effective length of the neutron absorbing rod. However, the shape of the hafnium sheet need not always be rectangular, but it can be put into a shape which covers parts of large neutron radiation amounts as shown in FIG. 6 in accordance with the neutron radiation amount distribution in FIG. 1. The upper end of this hafnium sheet 19b is fixed to the upper supporting member 5 by the joining means 23, while the lower end conforms with the distribution of the neutron radiation amounts. That is, the hafnium sheet 19b is so shaped that its parts corresponding to the central part of the control rod and the outermost end of the wing are long, whereas its part between them is short. The longest part has the length of about 1/4 of that of the neutron absorbing rod. FIG. 7 shows the relative absorption rate of the neutron absorbing rod owing to the hafnium sheet 19, 19a or 19b described above. When a sheet being 0.3 mm thick is used as the hafnium sheet, the neutron flux within the neutron absorbing rod decreases by about 20%. Accordingly, the degradation of the neutron absorbing substance in the neutron absorbing rod equalizes so that of the prior-art control rod having no hafnium sheet with a neutron radiation amount which is about 1.2 times larger than the neutron radiation amount that the prior-art control rod undergoes till arrival at its lifetime. That is, under the same irradiation condition, the lifetime of the control rod of the present invention becomes about 1.2 times longer than that of the prior-art control rod. The weight of one hafnium sheet shown in FIG. 5 is about 370 gr., and that of the same shown in FIG. 6 is lighter. The hafnium sheets have slight influence on the weight of the control rods because one hafnium sheet is light even when compared with one neutron absorbing rod of hafnium which is about 870 gr. in weight. FIG. 8 shows an embodiment of the control rod for a nuclear reactor according to the present invention wherein the inner side of a control rod sheath 11 is lined with a neutron absorber. The neutron absorber 25 may be, for example, an alloy which contains amorphous boron or cadmium. When the inner side of the control rod sheath is lined with the neutron absorber, there is the merit that the neutron absorber can be distributed in the control rod as is necessary as illustrated in FIG. 9. By way of example, a structure can be formed wherein neutron absorbing plates 25a of comparatively small width are arranged in the inner end part of the wing and a neutron absorbing plate 25b of comparatively great width in the outer end part of the wing, with no neutron absorber disposed between them. FIG. 10 is a view which shows another embodiment of the control rod for a nuclear reactor according to the present invention. In the present embodiment, a hafnium sheet has its thickness varied in steps so as to be thicker in the end parts of the control rod wing and to be thinner in the central part thereof and has a U-shape. Excepting this hafnium sheet 19c, the embodiment is the same as that of FIG. 4. When, in this manner, the thickness of the hafnium sheet is varied in the wing direction as indicated at A in FIG. 11, the neutron absorption rates of the neutron absorbing rods are flattened as illustrated at B in FIG. 11. When such hafnium sheet is employed and the absorbing rod at the wing outermost end is rendered the hafnium rod, the control rod lifetime becomes about 1.3 times that of the control rod in which all the neutron absorbing rods are the B.sub.4 C rods. In addition, while the quantity of use of hafnium in the present embodiment is equal to that in the control rod in which the hafnium sheet having a constant thickness of about 0.2 mm is arranged, the control rod lifetime is prolonged by 5% by distributing the hafnium sheet thickness as in the present embodiment. Thus, the prolongation of the control rod lifetime can be effectively realized by varying the quantity of hafnium being the second absorber along the control rod wing. While, in the present embodiment, the hafnium sheet thickness has been varied as the steps, similar effects are produced even when it is varied continuously. FIG. 12 shows another embodiment of the control rod for a nuclear reactor according to the present invention wherein the outer surface of a cladding is coated with a neutron absorber. A neutron absorbing rod 27 is composed of the boron carbide 17 which is a neutron absorbing substance, and the cladding 15a the outer surface of which is coated with the neutron absorber 29. The neutron absorbing rods 27 are arranged in a number of one in the central part of the control rod and in a number of four in the outermost end part of the wing, while the boron carbide-containing neutron absorbing rods 13 as in the foregoing are arranged between both the parts. The neutron absorber 29 in the present embodiment delays the degradation of the neutron absorbing substance, and also takes partial charge of the cladding. The boron carbide encloses helium produced by the (n, .alpha.) reaction of .sup.10 B, to swell and to incur the mechancial interaction with the cladding. A stress to act on the cladding increases with the proceeding of the burn-up of the .sup.10 B, but the strength of the cladding can be enhanced by the application of the neutron absorber. Even in a case where the cladding has cracked due to the mechanical interaction, the outflow of the boron carbide through the crack can be hindered to prevent the lowering of the control rod lifetime attendant upon the outflow of the boron carbide. In a case where the quantity of .sup.10 B in the neutron absorbing rod is increased by enrichment or increase in the density, the neutron absorption rate rises, and the quantity of helium to be produced increases. The cladding is therefore required to have a higher strength. In this regard, when the cladding coated with the neutron absorber is employed, the neutron flux in the neutron absorbing rod lowers, so that the production of the helium can be suppressed to reduce the stress on the cladding. In addition, since the strength of the cladding can be increased as stated above, a control rod having a .sup.10 B quantity distribution as shown in FIG. 13 can be constructed besides the arrayal of the neutron absorbing rods 13, 27 as described above. In the control rod shown in FIG. 13, the neutron absorbing rods in which .sup.10 B degrades fast has the quantity of .sup.10 B increased, and hence, the control rod lifetime prolongs. An embodiment of the present invention for raising the reactivity worth of a control rod having a low reactivity worth will be described with reference to FIGS. 14 and 15. The control rod of this embodiment is such that neutron absorbing rods are hafnium rods 21 and that sheets 19d of boron-containing stainless steel are inserted in the clearances between a control rod sheath 11 and the hafnium rods. FIG. 15 shows a method of mounting the sheet in the control rod of the present embodiment. The sheet is fixed to a lower supporting plate 7 and an upper supporting plate 5 and is fitted over the full length of the control rod. The control rod composed of the hafnium rods is about 1% lower in the control rod reactivity worth than the control rod employing boron carbide, but it can be endowed with a reactivity worth equal to that of the control rod employing boron carbide by the present invention described above. Moreover, the sheets can be taken out of the control rod by detaching the upper supporting plate 5 unitary with a handle 6, so as to be replaced with new ones every fixed period (the inserting and drawing-out structure is the same as that of the embodiment in FIG. 3). Thus, when the control rod reactivity is compensated by such method, the expensive hafnium can be used for a long term. As thus far described, when the present invention is applied to a control rod employing boron carbide, the prolongation of the lifetime of the control rod can be achieved without spoiling the merits of low price and light weight. Further, when the invention is applied to a control rod of small reactivity worth, the reactivity can be supplemented without greatly increasing the weight of the control rod, and when the invention is applied to a hybrid control rod containing boron carbide and hafnium, the effective utilization of the expensive hafnium is permitted.