GB2216711A - Process of filling cavities - Google Patents

Description

1 221671 1 PROCESS FOR FILLING CAVITIES This invention relates to a

process of filling cavities, more particularly the invention relates to the encap ulation of hazardous waste, e.g radioactive nuclear waste in reinforced concrete containers sealed with a concrete like sealant.

In prior unpublished German Patent Application P 38 02 910 3, a process is described in which scrap nuclear power plant material is encapsulated in a reinforced concrete container, which has a cylindrical outer shell, a hemispherical dome and a soleplate consisting of a circular disk The cavities left in the container described in German Patent Specification 28 54 330 If the sealant consists of a conventional concrete, the latter will expand owing to the heat of hydration If the concrete is used to encadulate nuclear reactors or radioactive wastes, additional heat will be generated by the decomposition of the radioactive substances and will be retained in the container so that the concrete used as a sealant will expand further But a conventional filling concrete which expands under the action of heat will apply such a high pressure to the walls of cavities or containers that the walls may burst or crack.

Such cracks in cavity-defining walls will be intolerable, particularly if hazardous materials, such as radioactive waste, are stored in said cavities.

For this reason a concrete like sealant is now proposed which is intended to use in a process of the kind described first hereinbefore and which under the action of heat will not apply to the cavity-defining walls a pressure which is so high that it could cause the walls to burst or crack.

Accordingly the sealant now proposed consists of an earth concrete mixture consisting of ground clay and/or fly ash, aggregates, a binder in such a small quantity that the plastic nature of the earth concrete will be preserved, and an air or gas-entraining agent.

Earth concrete mixtures consisting of cement, aggregate and ground clay and having a very low modulous of elasticity and a high C deformability have been used for many years As the cement content of said mixtures is decreased relative to the content of ground clay, such mixtures increasingly approach the nature of a cohensive soil Fly ash may also be used as a binder and will set when calcium oxide or hydrated lime has been added Because the solidification takes a long time, the strength properties of a soil-cement mixture which contains fly ash will change during a long time This ( would be disturbing in the process in accordance with the invention.

In accordance with a feature of the invention an inert fly ash, i e, a fly ash having no hydraulic properties, is added to the earth concrete mixture The calcium oxide is added in a smaller quantity than the fly ash For instance, a small quantity of blast furnace cement having a high slag content is added so that only a part of the fly ash can react with the hydrated lime.

The reaction will be stopped when the lime content has been consumed The strength of the mixture remains low.

The high content of fly ash increases the thightness of the mixture Another advantage afforded by the mixture resides in that only a small amount of heat of hydration is generated.

The deformability can be further improved and the compressive strength of the earth concrete and its tensile strength under a bending load as well as its tightness will be preserved if air-entraining agents are added to the mixture by intense mixing to form air mi- cropores in an amount of 3 to 10/% by volume Said cellike artificial air micropores will not be destroyed even under relatively high pressures and even when the grain structure is entirely saturated with water the volume of such air micropores will be changed under pressure be- cause the concrete can expand into the pores The addi- tion of an air-entraining agent will increase the vis- cous component of the visoelastic behavior of the earth concrete.

The earth concrete mixture used in accor- dance with the invention is particularly suitable as a 4 - sealant in reinforced concrete containers having rela- tively rigid walls or in rock cavities in which radio- active waste is stored The radioactive waste is em- bedded in the earth concrete, which fills the entire cavity Even in case of an expansion of the earth con- crete owing to the heat of hydration and to the radio- active decomposition of the stored material, the earth concrete owing to its low modulus of elasticity and its high viscoelasticity will not be able to apply such a high pressure to the walls that they might crack The earth concrete in accordance with the invention can be used to high advantage as a sealant in the cores of dams and in other sealing layers provided in the ground.

The earth concrete in accordance with the invention contains a sufficiently large void volume which can be compressed without a squeezing of water out of the concrete and without an application of an excessively high pressure to exterior walls if the con- crete is used as a filling concrete Owing to its good viscoelastic properties the earth concrete in accordance with the invention will preserve its high strength pro- perties also when it has been compressed It has a good sealing action and stability in a compressed state.

The earth concrete mixtures used in accor- dance with the invention may have, e g, the following compositions:

1 20 kg blast furnace cement designated HOZ 35 L, i e, a slow setting blast furnace cement having a strength of at least 35 N/mm 2; kg fly ash; kg ground clay; ( 1300 kg aggregate, a sand-gravel mixture having a particle size range from O to 16 mm; 350 kg water 0.2 kg air-entraining agent LPS from Gr Unau (foam- able products based on soap).

The earth concrete made from that mixture has the following properties:

-11 Coefficient of permeability k = 12 x 10 11 m/sec Air pore content 5 8 % by volume Modulus of compression K = 30 MN/m 2 Modulus of elasticity '_= 55 MN/m 2 2 50 kg HOZ 35 L; 120 kg fly ash; kg ground clay; 1400 kg aggregate, a sand-gravel mixture having a particle size range from O to 4 mm; 330 kg water; 0 2 kg LPS Properties of the mixture k = 2 4 x 10-9 m/sec.

Air pore content: 7 % by volume K = 90 MN/m 2 = 190 MN/m 2 Owing to the binder contained in the earth concrete mixture the latter can be placed in a liquid state and without being compacted and as the binder sets will lose its liquid properties and will become a solid.

If fly ash is used, such a content of lime or cement will be chosen that only a part of the inert ( 6 - fly ash can react with the calicum oxide so that a major part of said fly ash will be preserved as an aggregate and the earth concrete will not harden fur- ther even when the calcium oxide has been consumed.

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Claims (5)

1 A method of encapsulating hazardous waste material which comprises placing the waste material in a reinforced concrete container and filling the voids therein with a sealant, wherein the sealant used comprises a concrete mixture containing ground clay and/or fly ash, aggregates, a binder in such a small quantity that the plastic nature of the concrete is preserved, and an air or gas- entraining agent.

2 A method according to claim 1, wherein the binder is cement or lime.

3 A method according to claim 1 or 2, wherein the sealant comprises fly ash, and the lime or cement content of the sealant is such that only part of the fly ash reacts with the calcium oxide provided by the binder.

4 A method according to claim 1, wherein there is used as the sealant substantially as hereinbefore described.

5 A method according to any one of claims 1 to 4, as applied to the encapsulation of radioactive nuclear waste material.

Published 1989 &t The Patent Office,State House, 66/71 High Holborn, London WC 1 R 4 TP Further copies maybe obtained from The Patent Office.

Sales Branch, St Mary Cray, Orpington, Kent BBS 3 BD Printed by Multiplex techniques ltd St Mary Cray, Kent, Con 1/87 C