Patent GB-1107384-A (GB)
Title: Nuclear reactor fuel elements
Inventor: GATLEY JOHN ANDREW
IPC: G21C3/04
Filed: 1964-08-28
Abstract: 1,107,384. Fuel elements. UNITED KING- DOM ATOMIC ENERGY AUTHORITY.' 12 Aug., 1965 [28 Aug., 1964], No. 35424/64. Heading G6C. A vented fuel element has a vent path of extended length for promoting by delay the decay of short-lived gaseous fission products before they are discharged into fluid coolant contacting the element. Such an element for use in a liquid metal-cooled fast reactor comprises, as shown in Fig. 1B, an assembly of elongated fuel pins 11 housed within an open-ended hexagonal casing 12. The fuel pin sheathing is of stainless steel and the fuel a U0 2 /PuO 2  mixture. Two superposed gratings 13, 14 are located at the lower end of the casing 12, one set of alternate rows of pins 11 being brazed into one of the gratings while the other set are brazed into the other grating. Passing through each of the grating bars is a passage 16 which connects the interiors of the corresponding row of fuel pins with an annular chamber adjacent the casing 12 which in turn is connected by a gas entry bore to a collector chamber 17 formed by a sleeve 18 extending coaxially inside the lower end of the casing 12. A vent tube 23 extends upwardly from the bottom of the collector chamber 17 through the grating 13 to occupy one position of the fuel pin lattice. Above the fuel pin lattice, the vent tube 23 continues into an upper fixture joined to the casing 12 by legs leaving space for the discharge of coolant from the casing 12. On charging the fuel element into the reactor core, the coolant being under pressure to some extent in the region of the upper fixture will enter through holes in the latter into a gas trap chamber and if the initial gas content of the element remains cool, compression of such gas allows the coolant to reach the collector chamber 17 and form a free surface therein. When the initial gas content is raised in temperature corresponding to on-load operation of the reactor, the expansion of the gas expels coolant from the vent tube 23 and the gas trap chamber. Thereafter, gas liberated by the fuel material during operation finds its way through the passages 16 of the gratings 13, 14 to the collector chamber 17 and from there can ultimately escape to the surrounding coolant through the vent tube 23 and the gas trap chamber. The chief difference in a second embodiment lies in the construction of the collector chamber 17. Outer and inner concentric sleeves having a small clearance between them are disposed in the chamber 17 to form a labyrinth between the gas entry bore and the vent tube 23. A pool of sodium is introduced into the bottom of the delay space to immerse the lower end of the outer sleeve and so form a dip seal. This is effective to retain any Cs occurring as a decay product. Other liquids may be used to arrest a wider range of decay products. In a third embodiment, as shown in Fig. 3, a fuel element for a gas-cooled thermal reactor comprises a cluster of pins each comprising stacked pellets 41, 42 of slightly enriched U0 2 . At the lower end of the pin, an end cap 43 is separated from the pellet stack by two Al 2 O 3  discs 44, 45 while at the upper end there is only one Al 2 O 3  disc 46, a chamber 47 formed as an extension of the end cap 48 taking the place of the second disc and having open communication with the fuel-containing space through a hole 49. A length of capillary tube 50 longer than the overall length of the pin is coiled helically within the chamber 47. One end of the tube 50 is within the chamber 47 while the other projects externally of the pin and is sealed by brazing in passage through the end cap. Alternatively, the tube could extend along the fuel axis as a spine from one end of the fuel-containing space to the other and be open to this space at one end and open to the exterior of the pin at the other.