Patent Number: 039716988
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 of the drawings this shows a nuclear reactor fuel element assembly comprising an outer tubular casing 1 of hexagonal cross-section; the casing 1 is of stainless steel. Within the casing 1 there is arranged an assembly of thirty-six breeder pins 2 and one hundred and twenty-seven fuel pins 3. The fuel pins 3 extend longitudinally through the casing 1 parallel one to another and are arranged in hexagonal lattice form. The breeder pins 2 are arranged in an outer row surrounding the assembly of fuel pins 3. Each of the fuel pins 3 comprises a tubular sheath of stainless steel containing fissile nuclear fuel material which is of ceramic form. Spacing means for the fuel pins 3 comprises a helically wrapped wire 4 on the sheath of each fuel pin 3. Likewise each of the breeder pins 2 comprises a tubular sheath of stainless steel containing nuclear breeder material which also is of ceramic form. The sheaths of the breeder pins 2 are also provided with helically wrapped wires 4 serving as spacing means. The breeder pins 2 and the fuel pins 3 are supported at their lower ends within the casing 1 by a bottom support grid 5 which is mounted from a support ring 6 fitted inside the lower end of the casing 1. Referring now especially to FIG. 2, the outer ring of breeder pins 2 is separated from the inner assembly of fuel pins 3 by a barrier construction 7. Being of hexagonal cross-section the casing has six flat sides 8. The barrier construction 7 comprises six individual separator baffles 9 which extend longitudinally inside the casing 1 parallel to the inside faces of the sides 8. Each of the separator baffles 9 is formed from a strip of stainless steel sheet the longitudinal edges of which are bent over to form flanges 10. The flanges 10 are welded along the internal corners between adjacent sides 8 of the casing 1. Passageways 11 are thus defined between the separator baffles 9 and the inside faces of the sides 8 of the casing 1. Each passageway 11 contains the six breeder pins 2 which extend adjacent the corresponding side 8 of the casing 1. As shown in FIG. 1 an extension sleeve 12 is fitted into the lower end of the casing 1. The extension sleeve 12 is of hexagonal cross-section corresponding to the cross-section of the casing 1. The end of the casing 1 engages an external rebate 13 on the extension sleeve 12 and is secured to the sleeve by a circumferential edge weld 14. The extension sleeve 12 has a bore 15 of circular cross section. At its upper end the bore 15 opens into a conical throat 16. At its lower end the bore 15 has a counterbore 17 of large diameter. A main gag assembly 18 is fitted in the bore 15 of the extension sleeve 12. The gag assembly 18 comprises an outer sleeve 19 which is a close fit in the bore 15, a collet sleeve 20 which is split longitudinally in two halves 21 and fits inside the outer sleeve 19, and a series of toroidal rings 22 having wire mesh discs 23 and fitting in longitudinally spaced annular grooves 24 inside the collet sleeve 20. The lower end of the barrier construction 7 is shaped to fit about the upper end of the outer sleeve 19 of the gag assembly 18 the outer sleeve 19 forms a lower extension of the barrier construction 7. Six longitudinal slots 25 in the bore 15 of the extension sleeve 12 connect between the conical throat 16 at the upper end of the bore 15 and the counterbore 17 at the lower end of the bore 15. The longitudinal slots 25 in the bore 15 of the extension sleeve 12 are disposed in alignment with the passageways 11. As shown in FIG. 3 a gag ring 26 is fitted in the counterbore 17 at the lower end of the bore 15 in the extension sleeve 12. The gag ring 26 fits about a raised circumferential land 27 on the outer sleeve 19 of the gag assembly 18. The gag ring has six ports 28 which correspond to the six longitudinal slots 25 in the bore 15 of the extension sleeve 12. A spike member 29 at the lower end of the fuel element assembly comprises an upper end adaptor sleeve 30. The adaptor sleeve 30 has a head 31 corresponding in shape to the lower end of the extension sleeve 12 with which it is cojoined. The adaptor sleeve 30 has an external cylindrical bearing surface 32 of smaller diameter than the head 31 and a bore 33. The head 31 of the adaptor sleeve 30 has a circumferential rebate 34 which fits inside the lower end of the extension sleeve 12. As shown in FIGS. 4 and 5 the extension sleeve 12 and the upper end adaptor sleeve 30 are fastened together by twelve high tension socket headed screws 35 which, as shown in FIG. 3 extend through external longitudinal slots 36 in the gag ring 26. As shown in FIGS. 1 and 4 six longitudinal slots 37 inside the head 31 of the adaptor sleeve 30 lead from the bore 33 therein and are disposed in alignment with the ports 28 in the gag ring 26 and with the longitudinal slots 25 in the bore 15 of the extension sleeve 12. The spike member 29 also includes a strut member 38 which extends co-axially from the adaptor sleeve 30. The strut member 38 has three radially extending spider arms 39 at its upper end and the spider arms 39 fit inside the upper end of the bore 33 in the adaptor sleeve 30. The bore 33 of the adaptor sleeve 30 has an internal circumferential land 40 and the spider arms 39 of the strut member 38 have rebates 41 at the ends of their outer faces 42, which rebates 41 engage with the internal land 40 in the bore 33 of the adaptor sleeve 30 and thus locates the strut member 38 longitudinally with respect to the adatpor sleeve 30. The lower end of the strut member 38 has a cylindrical boss 43 over which there is fitted a lower nose piece 45. The nose piece 45 has an external cylindrical bearing surface 46 and is located on the cylindrical boss 43 of the strut member 38 by two straight pins 47 secured in holes in the body of the lower nose piece 45 and serving to hold it loosely to the strut member 38. The pins engage with a groove 48 around the cylindrical boss 43 of the strut member 38. A filter assembly 49 forming part of the spike member 29 comprises upper and lower toroidal ring members 50 and 51 which are joined by longitudinal rods 52. Intermediate spacing of the rods 52 is by inner ring members 53 which engage with grooves 54 in the rods 52. The upper ring member 50 of the filter assembly 49 fits around a rebate 55 at the lower end of the adaptor sleeve 30 of the spike member 29. The lower ring member 51 of the filter assembly 49 fits around a rebate 56 at the upper end of the nose piece 45 of the spike member 29. The sub-structure of the filter assembly 49 comprising the upper and lower ring members 50, 51 and the longitudinal rods 52 is covered by a wire gauze filter sleeve 57. In use the fuel element assembly of FIG. 1 forms part of the core structure of a nuclear reactor with the spike member 29 plugged into a bottom core support structure or diagrid 58. The diagrid 58 comprises upper and lower plate members 59 and 60. The spike member 29 fits in the diagrid 58 with the cylindrical bearing surface 32 of the adaptor sleeve 30 fitting in an aperture 61 in the upper plate member 59 of the diagrid 58 and with the lower nose piece 45 of the spike member 29 fitting in an aperture 62 in the lower plate member 60 of the diagrid 58. In operation of the reactor liquid sodium coolant is passed into the fuel element assembly from the interspace between the upper and lower plate members 59 and 60 of the diagrid 58. Sodium flows into the fuel assembly through the filter assembly 49 of the spike member 29. The main sodium flow is through the main gag assembly 18 and then over the assembly of fuel pins 3 inside the barrier construction 7 within the casing 1. However a proportion of the sodium passes through the ports 28 in the gag ring 26 and then passes through the longitudinal slots 25 in the bore 15 of the extension sleeve 12 to enter the passageways 11 which are defined between the separator baffles 9 and the inside faces of the sides 8 of the casing 1. The sodium passes up the passageways 11 over the breeder pins 2 contained therein. It is arranged that the rate of sodium flow through the passageways 11 over the breeder pins 2 is greater than the rate of main sodium flow over the fuel pins 3. Thus the sodium flowing over the breeder pins 2 will be at a lower temperature than the sodium flowing over the fuel pins 3. The sodium flowing over the breeder pins 2 will also be inherently at a lower temperature because of the lesser heat generation in the breeder pins 2. This means that the casing 1 of the fuel element assembly will be operated at a lower temperature than would be the case if the casing were subjected to the temperature of the main sodium flow over the fuel pins 3. Operation of the casing 1 of the fuel element assembly at a lower temperature reduces the amount of irradiation induced voidage growth which occurs in the casing 1 of the fuel element assembly. Therefore where the fuel element assembly is located in a position in the reactor core structure where it will be subjected to a transverse gradient in the neutron flux the amount of bowing of the fuel element assembly due to differential growth of the casing under irradiation will be reduced. The gag ring 26 can be adjusted in angular position before charging of the fuel element assembly into the reactor core structure so as to adjust the degree of overlap of the ports 28 in the gag ring with the longitudinal slots 25 in the bore of the extension sleeve 12. By this means the rate of sodium flow over the breeder pins 2 can be preset to the required amount. In FIG. 6 a fuel assembly with a casing 1 of circular section is shown. The separator baffles 9 are fitted with additional pad members 63 which locate the breeder pins 2 against the inside faces of the casing 1. The pad members 63 are continuous over the full length of the baffles 9 but they may be made discontinuous along the length of the baffles 9 to serve as local supports only. The continuous pad members 63 form open ducts for coolant flow but alternatively they may be arranged to enclose stagnant columns of coolant. At the upper end of the fuel element assembly the separate sodium flows from each side of the separator baffles 9 are allowed to mix giving a uniform coolant outlet temperature from the fuel element assembly. This can be achieved by means of V-slots cut in the upper ends of the separator baffles 9 allowing gradual mixing of the two flows.