GB1373677A - Refuelling of nuclear reactors - Google Patents

Abstract

1373677 Fuelling reactors UNITED KING- DOM ATOMIC ENERGY AUTHORITY 28 March 1973 [29 March 1972] 14912/72 Heading G6C A reactor has a core constructed and arranged so that fuel may be added to the core whilst the reactor is on-load without the need for discharging irradiated fuel from the core. As shown, the reactor 10 is of the high-temperature, gas-cooled type having a core 11 contained within a steel-lined pressure vessel 12 of prestressed concrete. Standpipes 13 afford access to the top face of the core 11 by means of refuelling machines 14a, 14b, for on-load and offload refuelling, respectively. The core 11 is formed of columns of interchangeable units comprising prismatic graphite blocks 15, 16, the blocks 15 containing nuclear fuel in the form of fission product-retaining coated particles while the blocks 16 are devoid of nuclear fuel and are disposed around the fuel-containing blocks 15 to serve as neutron reflectors. Vertical channels 17, 18 extend vertically through the blocks 15, 16 to provide channels for the reactor coolant (helium), the channels 18 being of larger diameter than the channels 17 and accommodating control rods 19 operated by drive mechanisms 20. The coolant is circulated down through the reactor core 11 and up through heat exchangers 23 housed within the wall of the pressure vessel by means of circulators 22. Each block 15 (Fig. 3) is penetrated by a plurality of coolant channels 17 and a single control channel 18, the fuel being contained in the form of cylindrical compacts 29 within holes 30, each compact 29 comprising spherical fuel particles coated with layers of pyrocarbon and SiC in a graphite matrix. The blocks 16 are of similar form but without the fuel (Fig. 4, not shown). The upper face of the core 11 is initially bounded by a double layer of reflector blocks 16 and progressively, in an on-load operation, the lower of the two upper blocks 16 of each column is replaced by a fuel-containing block 15. After a further period of operation, the reactor is shutdown for off-load refuelling, after which the core columns are as they were at start-up. By means of this scheme, the shut-down frequency for refuelling can be half that for an entirely off-load refuelling scheme, only new fuel and unfuelled reflector blocks are handled on-load thus eliminating many of the hazards and difficulties of normal on-load refuelling, and new fuel is always concentrated near the top of the core where, with the downward coolant flow, the coolant temperature is lowest and where partially inserted control rods can be employed to suppress local power peaks.