GB2107691A

GB2107691A - Production of ceramic nuclear fuel pellets

Info

Publication number: GB2107691A
Application number: GB08131497A
Authority: GB
Inventors: Frank Rigby
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1981-10-19
Filing date: 1981-10-19
Publication date: 1983-05-05
Also published as: GB2107691B

Abstract

In the production of nuclear fuel pellets consisting essentially of uranium dioxide formed into compacts and sintered, magnesia is included in the compacts and the compacts are sintered first in a reducing atmosphere and then in an oxidising atmosphere, the reducing atmosphere being maintained until the temperature is high enough for grain growth to occur. By delaying the introduction of the oxidising atmosphere grain growth may be enhanced more than is to be expected from the resulting hyperstoichiometry in the uranium dioxide and the presence of the magnesia.

Description

SPECIFICATION Production of ceramic nuclear fuel pellets This invention relates to the production of ceramic nuclear fuel pellets.

In the production of ceramic nuclear fuel pellets from powdered oxides of uranium, plutonium or thorium, or mixtures of these oxides, additives are often mixed with the powdered oxide. The additives may also be oxides, of magnesium, niobium, yttrium or titanium, for example, and one effect of the additives is on grain size in the sintered pellets.

According to the present invention, in the production of nuclear fuel pellets consisting of essentially of uranium dioxide or a mixture of uranium dioxide with an oxide of thorium or plutonium, formed into compacts and sintered, magnesia is included in the oxide compacts and the compacts are sintered first in a reducing atmosphere and then in an oxidizing atmosphere, the reducing atmosphere being maintained until the temperature is high enough for grain growth to occur.

By delaying the introduction of the oxidising atmosphere grain growth may be enhanced more than is to be expected from the resulting hyperstoichiometry in the uranium dioxide and the presence of the magnesia. It is considered that this is due to the relative insolubility of magnesia in uranium dioxide unless the uranium dioxide is hyperstoichiometric so that, while a reducing atmosphere is maintained grain growth is retarded by insoluble magnesia but, on allowing the atmosphere to become oxidising, rapid dissolution of magnesia activates abnormal grain growth provided the temperature is above grain growth temperature.

The invention is illustrated by the following examples in which reference is made to the accompanying drawings Figures 1-4 which are all graphs.

The required amount of magnesia powder up to 1% was added to uranium dioxide powder by sieving the powders together and then milling. The homogenised powder was mixed with an organic binder and after drying and granulating pressed into cylindrical compacts. Binder was removed from these compacts by heating for four hours at 600"C in hydrogen, and then decarbonising by holding for a further four hours at 11 00'C in 70% CO2/30%H2. The decarbonised compacts were sintered for various periods in hydrogen at 16500Cto produce sintered pellets of different grain-sized material. Each pellet was cut in half and, after mounting, polishing and etching the grain size of specimens from each batch was measured by a linear intercept method.No correction was applied to convert these intercept length to so-called true grain sizes. In Figure 1 grain size is plotted as a function of time at 1 6500C for the undoped uranium dioxide and for all the magnesia doped compositions.

One-half of each of these hydrogen-sintered pellets were annealed for 12 hours in 98% CO2/2% CO at 1 200 C. Under these conditions uranium dioxide is oxidised up to UO2+x and magnesia dissolves in it but the temperature is too low for any grain growth to occur. Both halves of each pellet were then heated to 16500C in carbon monoxide and, as soon as temperature was reached, the atmosphere was changed to 50% CO2/50% CO. These conditions were maintained for four hours. Before allowing the pellets to cool, the atmosphere was again changed to carbon monoxide, and the temperature lowered to 1 5000C for 4 hours to allow reduction to stoichiometry.The grain sizes in both halves of each pellet were again measured and increases which occured are shown in Figures 2 and 3.

Figure 1 shows that the long term effect of added magnesia on the grain size in hydrogen-sintered uranium dioxide is one of inhibition. In the very early stages of grain growth doping with magnesia, irrespective of concentration in the range up to 1 wt.

%, increases the grain size by about 30% relative to that in the undoped uranium dioxide. However, as can be seen from Figure 1, this effect is only transient and eventually a limiting grain size is reached which depends on the magnesia concentration.

Figures 2 and 3 show first the effect of hyperstoichiometry in uranium dioxide on grain growth.

By comparison with the results in Figure 1 it can be seen from Figures 2 and 3 that the grain size of undoped uranium dioxide is approximately 8 microns larger in the hyperstoichiometric material and the magnitude of this increase is unaffected by the inclusion or omission of the 1200"C anneal. The increase in grain size is also independent of the pre-anneal grain size and has been shown to be unaffected by repeated recycling between stoichiometric and hyperstoichiometric conditions.

Figure 2 also shows the effect of dissolved magnesia on grain growth, summarising results obtained on specimens in which the magnesia had been dissolved in hyperstoichiometric uranium dioxide at 1200"C before receiving a 16500C oxidising anneal.

The increase in grain size during this high temperature anneal will be due to a combination of hyperstoichiometry and dissolved magnesia and the family of curves in Figure 2 shows an increase in grain size with the presence of magnesia in addition to the increase with hyperstoichiometry alone, the increase in grain size due to the presence of magnesia rising with increasing concentration of dissolved magnesia to a maximum of approximately 14 microns at 0.8 wt. % magnesia.

Comparison of Figure 2 with Figure 3 provides a measure of the increase in grain growth which can be achieved by abnormal grain growth in accordance with the invention. The results plotted in Figure 3 were obtained by allowing magnesia to dissolve in hyperstoichiometric uranium dioxide at 16500C,that is at a temperature high enough for grain growth to occur. Thus, in the absence of magnesia, the graph lies parallel to the x axis indicating no abnormal grain growth but, in the presence of magnesia, grain size increases with increasing preanneal grain size.

The maximum increase (due to abnormal grain growth) is approximately 11 microns over the range 0.4 - 1.0 wt. % magnesia. In contrast, at a magnesia concentration of 0.2 wt. % the maximum increase in grain size is only 6 microns. This is explained by further reference to Figure 1 which shows that the specimens containing 0.2 wt. % magnesia had not reached their limiting grain size prior to dissolution of the magnesia. It is as the limiting grain size is approached that abnormal grain growth becomes significant.

Evidence of abnormal grain growth is also provided by examination of the grain boundaries in the specimens, highly irregular grain boundaries and grain shape suggesting that a large number of small grains have been "consumed" by a large grain during growth. This is the mechanism of abnormal grain growth.

To determined the time for abnormal grain growth to reach completion, three sets of pellets, containing 0.13,0.4 and 1.0 wt. % MgO respectively, were first sintered for 32 h in hydrogen to grain sizes of 9.4, 8.9 and 6.2 Fm. respectively and then annealed for up to 4 h in a 50% CO2 - 50% CO mixture, after being brought to temperature in carbon monixide. Other, similar specimens were treated in the same way, but in addition they received an intermediate anneal at 1200"C in 98% C02 - 2% CO for 12 h. After completion of the 1650"C anneal, each pellet was sectioned, and its grain size measured. Results are shown in Figure 4.Comparing the grain sizes of pellets which had received the 1 2000C anneal with those which had not, it can be seen, since the two curves at each magnesia concentration are paraliel, that abnormal grain growth is complete within the first 0.5 h at 1650"C, during which time the oxygen potential of the annealing atmosphere has reached a new equilibrium as carbon monoxide is replaced by the CO2/CO mixture. It appears, therefore, that abnormal grain growth occurs very rapidly and simultaneously with the dissolution of the magnesia.

Two other features of Figure 4 may be noted. The slope of the curves increases with increasing magnesia concentration, showing that the rate of normal grain growth is proportional to the concentration of dissolved magnesia, a conclusion which can also be deduced from Figures 2 and 3. Secondly, normal magnesia-enhanced grain growth continues at a steady rate even after abnormal grain growth is complete.

Claims

1. The production of nuclearfuel pellets consisting essentially of uranium dioxide or a mixture of uranium dioxide with an oxide of thorium or piutonium formed into compacts including magnesia and sintered characterised in that the compacts are sintered first in a reducing atmosphere and then in an oxidising atmosphere, the reducing atmosphere being maintained until the temperature is high enough for grain growth to occur.

2. The production of nuclear fuel pellets as claimed in claim 1 characterised in that said temperature is at least 16500C.

3. The production of nuclearfuel pellets as claimed in claim 1 characterised in that the pellets contain up to 1 per cent by weight of magnesia.

4. The production of nuclear fuel pellets as claimed in claim 1 characterised in that the reducing atmosphere is essentially carbon monoxide.

5. The production of nuclear fuel pellets as claimed in any preceding claim including the further step of heating the pellets in a reducing atmosphere following sintering in the oxidising atmosphere.