GB2177249A

GB2177249A - Nuclear fuel bodies

Info

Publication number: GB2177249A
Application number: GB08517110A
Authority: GB
Inventors: Geoffrey Alan Wood; Ronald Henry Watson
Original assignee: British Nuclear Fuels PLC
Current assignee: Sellafield Ltd
Priority date: 1985-07-05
Filing date: 1985-07-05
Publication date: 1987-01-14
Also published as: GB8517110D0; GB2177249B

Abstract

Nuclear fuel pellets are made by incorporating seed particles, eg. UO2 seed crystals into a matrix of powdered ceramic material having nuclear characteristics, eg. UO2, and annealing. The UO2 seed crystals can be obtained by heating compacts of UO2 powder or presintered material in dry hydrogen above 1750 DEG C, holding at temperature, cooling and sifting to obtain predominantly single crystals which are used as the seed material. The seed material may also be PuO2, a mixture thereof with UO2, or enriched UO2, and the matrix material may be depleted UO2. A two layered pellet may be produced. <IMAGE>

Description

SPECIFICATION Nuclear fuel bodies This invention relates to nuclear fuel bodies and in particular to the production of nuclear fuel bodies with a large mean grain size and to novel fuel bodies produced thereby.

Hitherto, nuclear fuel bodies made from ceramic material, particularly uranium dioxide (us2), have tended to be made from powder particles of uniform small size, the accent having been on achieving densities close to maximum theoretical on subsequent sintering.

A demand has arisen for fuel bodies which allow higher burn-up to be achieved. Higher burn-up in turn calls for fuel bodies with enhanced fission gas retention properties. It has been recognised that fission gas retention can be enhanced if the fuel bodies, after sintering, contain uniform grains of largersize than conventional fuel bodies. Proposals have been made to achieve larger grain sizes involving, for example, prolonged treatment of the fuel bodies at elevated temperatures in excess of 2000"C which is expensive in terms of operating costs and installation of suitable furnace plant.

It is an object of the present invention to provide novel nuclear fuel bodies which exhibit the desirable property of enhanced fission gas retention at high burn-up and which can be produced without resorting to prolonged treatment at elevated temperatures.

According to the invention there is provided a ceramic fuel body formed by sintering a compacted matrix of powdered ceramic material having nuclear characteristics in which are dispersed seed particles which, during sintering, act preferentially as nuclei for growth of grains by consumption of matrix particles surrounding each seed particle.

The individual grain sizes produced by seedinduced growth may range up to 300 microns.

Large grain size fuel can be prepared by introducing seed crystals into a UO2 matrix and annealing. Such crystals will induce secondary recrystallisation and will grow epitaxially at an accelerated rate (inversely proportional to the matrix grain size) by consuming the smaller matrix grains and resulting eventually, if allowed to continue, in a fully recrystallised large grain structure. However, once the abnormally large grains impinge on one another, secondary recrystallisation is expected to largely cease, and the result will be a mixed grain size distribution structure consisting of the large seed-grown grains and the residual matrix grains.It is thus possible to produce a nuclear fuel body with a structure which includes recrystallised grains factors larger than the matrix grains without recourse to prolonged sintering times, additions of chemical additives, active atmospheres or elevated temperatures of of greater than 2000"C.

The seed crystals can be prepared by heating UO2 powder compacts or pre-sintered UO2 in dry hydrogen up to a temperature above 1750"C leading to the formation of hypostoichiometric UO2,. On cooling, uranium hydride forms on grain boundaries and the associated volume increase results in disintegration of the pellet into particles, a large proportion of which are essentially single crystals.Typical conditions were heating presintered UO2 pellets having initial mean grain size, after primary sintering, of approx. 10 microns to 2000"C at 200cumin in dry hydrogen, maintaining at 2000"C for 30 minutes, cooling to ambient at 40"C/min, sieving the fine powder so produced using a sonic sifter and obtaining fractions of and above, and less than, 53 microns. The undersized fraction was predominantly single crystal material with occasional bicrystals, and the oversize fraction was mostly polycrystalline agglomerates. The undersized fraction was employed as seed material, the mean size of the individual crystals being about 30 microns.Green pellets of UO2 powder with a median size less than 3 microns were produced by binderless technique with for example 5 wt/o seed crystals dispersed throughout the green pellets, and the pellets were sintered for 4 hours at about 1720"C (preferably not less than 1700 C) in moist hydrogen. The resulting product had a mean grain size of 44 microns and it was found that the recrystallised grains occupied in the region of 90% of the total volume, with the recrystailised grains approximately five times as large as the matrix grains. Seed additions can vary between 0.1 and 25wt/o, depending upon the choice of process parameters of temperature, sintering time and heating/cooling rates.

Pellets containing seeded grains exhibit lower densities than unseeded pellets depending upon the quantity of seed used. This is considered to be the result of porosity trapped within grains during the growth of the seeded grains. This porosity remains thermaliy stable and can therefore be employed for accommodation of fission product swelling in zones where diffusional creep of UO2 can occur.

Novel fuel structures which the seeding technique gives rise to are, firstly, a mixed enrichment UO2 with micro encapsulation. By nucleating a depleted UO2 matrix with enriched UO2 seeds it is considered possible to obtain a fully or nearly fully recrystallised microstructure. The depleted UO2 shell surrounding each seed provides a long diffusion path between the source of fission products and the sink-the grain boundaries. In addition the shell would act as a reservoir for containing fission products and at the same time because of its much lower operating temperature help restrain the kernel, minimising fission gas swelling. This new form of dispersed fuel is expected to offer benefits from fission gas retention and swelling considerations.

A second type of novel micro encapsulated mixed oxide fuel body can be envisaged employing plutonium dioxide (PuO2) seeds, manufactured by a disintegration process or by mechanical attrition, dispersed in a depleted UO2 matrix and then sintered.

It is expected that an interface would form between the two materials. The subsequent growth of the seeded grains would proceed by movement of UO2 grain boundaries through the UO2 matrix and away from the UO2/PuO2 interface. The resulting microstructure would comprise large recrystallised grains centred on Pu02 kernels. Full recrystallisation would not be necessary as fission would be confined to the PuO2 nuclei. Each recrystallised grain then offers a lengthened diffusion path and a possibly beneficial diffusivity gradient. However full recrystallisation gives a more uniform grain size distribution and is therefore desirable.

A further type of novel fuel body can be envisaged in which the seed crystals employed for growth are formed from mixtures of UO2 and Pu02 in mutually soluble proportions.

In a further application of the invention, the fuel bodies may have a duplex structure comprising an inner portion (which may be cylindrical or annular) and an outer portion (which may be annular). Both portions may, in the green state, comprise a matrix of powdered ceramic material, such as UO2, possibly with different degrees of enrichment, and at least one portion may be seeded in the manner described hereinbefore to produce a large grained structure. If both portions are seeded, the mean seed particle sizes or concentrations may differ.

Claims

1. A ceramic fuel body formed by sintering a compacted matrix of powdered ceramic material having nuclear characteristics in which are dispersed seed particles which, during sintering, act preferentially as nuclei for growth of grains by consumption of matrix particles surrounding each seed particle.

2. A fuel body as claimed in Claim 1 in which said grains produced by seed-induced growth range in size up to about 300 microns.

3. A fuel body as claimed in Claim 1 or 2 in which said seed particles are composed of the same ceramic material as the matrix.

4. A fuel body as claimed in Claim 1 or 2 in which said seed particles are composed of a different ceramic material, also having nuclear characteristics.

5. A fuel body as claimed in Claim 4 in which said seed particles are composed of Pu02 and said matrix is composed of UO2 powder.

6. A fuel body as claimed in Claim 3 in which said ceramic material comprises UO2.

7. A fuel body as claimed in Claim 6 in which the matrix is composed of depleted UO2 and said seed particles comprise enriched uO2.

8. A fuel body as claimed in Claim 1 or 2 in which said matrix comprises UO2 and said seed particles are formed from a mixture of UO2 and PuO2 in mutuall soluble proportions.

9. A fuel body as claimed in any one of Claims 1-8 in which the body comprises an inner portion surrounded by an outer portion both having a matrix composed of ceramic material or materials having nuclear characteristics and in which at least one of said portion contains said seed particles.

10. A fuel body as claimed in Claim 9 in which said inner portion contains said seed particles.

11. A fuel body as claimed in any one of Claims 1-10 in which said seed particles are constituted essentially by single crystals.

12. A fuel body as claimed in any one of Claims 1-11 in which essentailly all of said seed particles have a size exceeding the mean particle size of the matrix powder particles.

13. A process for the preparation of ceramic nuclear fuel bodies with a large mean grain size including the introduction of seed particles into a matrix of powdered ceramic material having nuclear characteristics and heat treating the matrix, said seed particles acting preferentially as nuclei for growth of grains by consumption of the matrix particles surrounding each seed particle.

14. A process as claimed in Claim 13 in which the seed material constituent within the matrix lies in the range of 0. 1 to 25 wt/o.

15. A process as claimed in Claim 13 or 14 in which the seed particles essentially comprise single crystals of UO2 derived by heating a compact of UO2 powder or pre-sintered UO2 powder in dry hydrogen to a temperature above 17500C, cooling the same so that the material disintegrates into particles, a large proportion of which are essentially single crystals and sifting the particles to isolate the essentially single crystal fraction.

16. A ceramic fuel body substantially as hereinbefore described.

17. A process for the preparation of ceramic nuclear fuel bodies with large mean grain size, substantially as hereinbefore described.