Patent Number: 059784318
Section: description

Embodiments of the present invention will now be described by way of example only with reference to the following Examples. EXAMPLE 1 A powder mixture comprising 96% by weight UO.sub.2 formed by the process described in Applicants' GB 2064503B and corresponding U.S. Pat. No. 4,397,824 and 4% by weight PuO.sub.2 is produced in a conventional blender. The powder is milled in an attritor mill which has already been run for about one minute with approximately 0.1% by weight zinc stearate lubricant powder before introduction of the UO.sub.2 /PuO.sub.2 powder. The product of the milling step is transferred to a spheroidiser of the kind described in Applicants' EP 331, 311B and corresponding U.S. Pat. No. 4,936,766 and treated for a period of around 30 minutes using a paddle speed of approx 20 revolutions per minute. Neutron poison, such as gadolinia is added to the free flowing powder produced following the spheroidising step to form a mixture in which the gadolinia concentration is about 8 percent by weight. Large grain single crystal UO.sub.2 seed crystals made by the method described in the Applicants' EP 416,778A and corresponding U.S. Pat. No. 5,061,434 are added to the gadolinia containing powder, the seed content being approximately 2 percent by weight. The overall mixture is then pressed at a pressure of approximately 4 Te cm.sup.-2 in a die to provide a suitable pellet shaped body. After the body has been through the pressing stage, it undergoes a sintering stage. The sintering is at an atmosphere of about 5 percent by volume H.sub.2 in an H.sub.2 --Ar or H.sub.2 --N.sub.2 atmosphere for 6 to 12 hours at a maximum temperature of from 1650.degree. C. to 1750.degree. C. MOX pellets produced in the above manner may be loaded into cans to form fuel pins under remotely controlled handling operations in the manner described for example in Applicants' EP 627,743. EXAMPLE 2 The same milling and spheroidising steps are performed as in example 1. However 10 wt % Gd.sub.2 O.sub.3 is added to a 5 wt % PuO.sub.2 and 95 wt % UO.sub.2. To avoid the resulting product undergoing bloating and micro-cracking and also to eliminate the very small grain size resulting from the addition of a neutron poison such as gadolinia the sintering conditions and additives required are as follows. The pressed fuel is sintered by heating it to a temperature of approximately 1000.degree. C. at a rate of 200.degree. C. hr.sup.-1 and then increasing the temperature to 1650.degree. C. at a rate of 50.degree. C. hr.sup.-1. This temperature 1650.degree. C. is maintained for approximately four hours, after which the temperature is ramped down to room temperature. The grain size is increased to 10-15 .mu.m by the addition of aluminosilicates. EXAMPLE 3 A powder mixture comprising 92 wt % UO.sub.2, formed by the process described in Applicants GB2046503 and corresponding U.S. Pat. No. 4,397,824 and 5 wt % PuO.sub.2 and 3 wt % Gd.sub.2 O.sub.3 which are produced in conventional blender are milled together in an attritor mill. The mill is run for 30 minutes at 30 rpm to warm up. The UO.sub.2, PuO.sub.2 and Gd.sub.2 O.sub.3 are added whilst the mill is running at 30 rpm for 2 minutes in the following sequence : 1/2 the UO.sub.2 plus 0.1 wt % Zinc stearate plus 0.5 wt % CONPOR; PA1 the PuO.sub.2 ; PA1 the remaining UO.sub.2 and the Gd.sub.2 O.sub.3. The powder mixture is then milled for a further 22 minutes at 180 rpm before being discharged. The milled powder is then transferred to a spheroidiser of the kind described in Applicants EP331311, where it is conditioned for 25 minutes with an additional 0.1 wt % Zinc stearate. The homogeneous free flowing granules are then pressed into pellets and sintered in a 4%H.sub.2 :Ar atmosphere for 4 hours at a maximum temperature of 1650.degree. C. EXAMPLE 4 Pellets are produced in the same manner as in Example 1 or 2 or 3 except that, instead of UO.sub.2 seed crystals, particles comprising one or more of Ti, Al, Nb, Cr and Mg are added in a concentration of 0.01% to 1% to the mixed oxide powder prior to the pressing step. EXAMPLE 5 Pellets are produced in the same manner as in Example 1 or 2 or 3 except that particles of one or more of Ti, Al, Nb, Cr and Mg are added in a concentration of 0.01% to 1%, as well as the UO.sub.2 seed crystals, to the mixed oxide powder. EXAMPLE 6 Pellets are produced in the same manner as in Example 1 or 2 or 3, except that in the sintering step the sintering temperature is at least 1800.degree. C. EXAMPLE 7 Pellets are produced in the same manner as in Example 1 or 2 or 3, except that in the sintering step the sintering temperature is first at the reduced temperature to 1400.degree. C.-1600.degree. C. with oxidising atmosphere, followed by treatment at a higher temperature, for a period of typically 1-4 hours using a reducing sintering atmosphere of hydrogen/argon.