A.E. Arustamov, M.I. Ojovan, M.B. Kachalov,
V.V. Shiryaev, I.A. Sobolev, E.M. Timofeev
Scientific and Industrial Association "Radon"
The 7-th Rostovsky Lane, 2/14 Moscow, 119121, Russia
Tel. (095)248 1680, Fax (095)248 1941,
E-mail: Oj@nporadon.msk.ru

Metals and alloys with relatively low melting temperature are considered in various countries as perspective matrices for immobilization (encapsulation) of spent nuclear fuel in containers in preparation for the final disposal at underground repositories. Lead and lead-based alloys are used for conditioning spent radionuclide sources at radioactive waste disposal facilities in Russia.

A rich variety of investigations of the possibility of using lead as matrix for immobilization of spent fuel and high-level radioactive waste containing long-lived radionuclides shows that lead holds much promise for radioactive waste disposal.

One of the basic concepts currently under AECL discussion is a placing spent fuel assemblies into corrosion resistant metal containers, pouring of containers by matrix metal with low melting temperature and then disposal of containers in rock at a depth of 500-1000 m. Results of comparative investigations have shown the best compatibility of lead with material of fuel assemblies and containers which provides maximum corrosion and mechanical resistance of fuel containers [1].

The most detailed comparative investigations were carried out in SKB. Assessment of efficiency of spent fuel immobilization method was performed based on a number of parameters: cost, technological effectiveness, corrosion resistance and long-term safety. It was established that use of copper tanks filled with lead would make it possible to isolate fuel for several millions of years. Lead as a filler for steel tanks provides mechanical stability during disposal and increases significantly corrosion resistance of fuel containers [2].

Spent sources delivered to disposal at SIA "Radon" are extremely hazard waste with high level of radioactivity. Specific activity of some sources can achieve 100 Ci/g and higher. With the aim to provide long-tern safety of spent sources disposal there has been developed technological scheme which allows to seal sources off from the environment by additional barrier from lead matrix. This technology allows solving a problem of safety of sources disposal in underground borehole repositories.

High radiation stability and heat conductivity render the metals the most attractive for use as matrix material in spent sealed radiation sources immobilization.

In practice, the necessity of limitation of duration and value of heat effect on sources during conditioning as well as necessity of simplification of equipment design of technological process constrain the choice of metal to be used. Moreover, necessity of decreasing radionuclide carry-over during conditioning as well as effects of interactions of metal melts with source shells at high temperatures have an important impact on choice of matrix material. Metals with melting temperature not higher than 973°K are considered as the basic ones.

To study matrix material behavior and to develop technological aspects of conditioning technique there was constructed a laboratory-scale facility reproducing the conditions of actual disposal of spent sources at the scale. A number of metals and alloys with different melting temperatures were studied using this facility (see Table I).

Technological process of encapsulation involves two steps with melted metal portions varying in volume. In the first stage the sources in underground repository vessel are poured by melted metal of temperature 1.5 times higher than melting temperature. Owing to difference in the specific weight, the sources rise to the surface of the melt, and on its crystallization they are fixed on the surface. In the second stage of the technological process a thin layer of melt is poured on the surface of metal block. This layer covers the sources and provides their reliable immobilization (Fig. 1).

In order to obtain a high-performance metal block the melt quantity is calculated depending on the temperature of the melt, sources activity, repository thermal properties from the following expressions:

where æ - effective repository heat conductivity, e_p - average energy of one decay, D T₀ - permissible overheating of the repository, A_i - radionuclide activity in specific set of sources, b - in this coefficient the effect of repository geometry is included. For typical underground borehole repositories D T₀=503 ° K, æ =(2.8+0.3)W/m*K [3].

Structure of the block obtained and coupling between layers were controlled by the use of ultrasonic defectoscopy and testing of the samples for fracture. The measurements showed that some aluminum and tin matrix samples had structural defects in the form of even boundaries between layers. This was caused by rapid oxidation of surface of the metals having the sufficient high reactivity. The best results were obtained for lead and lead-based alloys. No uniformity rupture of the structure of the block obtained was found.

If sources contain high volatile radionuclides it is essential to lower melting temperature as much as possible and at the same time to provide reliability of cohesion between matrix metal layers. To attain this aim at first sources are poured by lead-based alloy melt (e.g. Bi50.1-Sn14.6-Pb24.9-Cd10.8) having low melting temperature. Upon crystallization of the latter the technological operations above-mentioned are performed. Investigation of the block obtained by ultrasonic defectoscopy has shown no structural defect in the block. During examination of the samples surface by scanning microscopy there was found a transition layer in the area of lead layers joining about 2 mm in width. Element composition of this layer was determined by laser energy mass analyzer EMAL-2 (Table II).

Thermographic analysis (Fig. 2) has shown that material melting temperature in a layer decreases from border of transition layer to center not going below temperature restriction established for specific type of a repository.

Low-melting alloys with melting temperature below 373 ° K can be used for spent sources immobilization buried in the old repositories flooded by water. For this purpose sources are initially poured by melted alloy and if required, water in the repository is heated to temperature higher than alloy melting temperature. Owing to significant difference in the specific weight, water or water-clay suspension is displaced from sources layer by the melt. Upon the pumping out water and drying the repository, metal block melting temperature required is achieved by successive pourings of several melted lead portions.

Repositories with spent sealed radiation sources are placed at shallow depths. Long-standing observations of spent sources repositories state have shown that a repository contacts with underground water containing clay suspension in specific cases [4]. Under such conditions it is essential to use metals with sufficient high corrosion resistance. Complex system resulting from spent sources immobilization involves stainless steel container and additional corrosion barrier from lead placed in argillaceous or sandy-argillaceous medium in aqueous equilibrium with surrounding soils. Investigations of lead corrosion under direct contact with clay [5] and in natural underground water [6] are of specific interest. Results of the experiments have shown that corrosion rate depends heavily on the chemical composition, pH and salt concentration in underground waters as well as the temperature in the area of lead matrix contact with the environment.

Under direct contact with clay the corrosion rate ranges from 0.005 to 0.01 mm/yr [5]. It has been noted that oxidation of pyrite present in clay to sulfate or increase of sulfate concentration decreases suddenly the lead corrosion rate.

From data [6] the corrosion rate in natural underground water at the temperature T=348 ° K is 0.0013 mm/yr. Under the most severe conditions when underground water is saturated with oxygen, the corrosion rate increases to 0.007 mm/yr. The time-dependence of the lead corrosion rate is presented in the Fig. 3 [7].

Corrosion rate decreases with increasing hold time for the samples which is due to formation of a dense layer of lead corrosion products on surface of the lead samples. This layer passivates the surface and inhibits corrosion. It has been found by X-ray diffraction analysis that this layer consists of difficultly soluble salts of anions Cl^-, HCO₃^-, SO₄^-2 which present in the underground water. It has been confirmed by the experiments that increase of concentration of the ions Cl^-, HCO₃^- [2] and SO₄^-2 [6] results in decrease of lead corrosion rate.

Comparison of underground water compositions (Table III) allows the conclusion that underground waters sampled from the area of shallow ground repositories with spent radiation sources contain a great quantity of passivating ions which create the conditions favorable for lead matrix corrosion reduction.

In view of the fact that shallow ground repositories are used for the disposal of short-lived radionuclides with half-life T_1/2 less than 30 years and based on the maximum corrosion rate, it may be concluded that a lead layer about 5-10 mm in thickness provides a high degree of long-term safety of spent sources disposal.

Spent sealed sources are encapsulated into metal matrices just in the shallow ground borehole repositories. Such repository is a cylindrical vessel from stainless steel with diameter of 400 mm and height of 1500 mm placed in the reinforced concrete shaft at a depth of 6 m. The spent sources are fed from the surface to the vessel through curved charging pipe 108 mm in diameter. The concrete walls of the repository are surrounded by sandy-clay mixture or clay [4]. Due to generation of heat during radioactive decay the temperature in the repository increases. Therefore there is a limitation on the activity of spent sources by this parameter for the standard typical repositories. Overheating D T=503 ° K in the center of the repository (metal block with sources) should not be exceeded. For determination of thermal properties of actual repositories for spent sources there has been developed a bench-scale facility. This facility is an actual repository on three sides around which observation holes have been drilled same distance apart. The scheme of the bench-scale facility is presented in Fig. 4.

1. head part
2. observation hole
3. temperature-sensitive elements
4. isolation plug
5. uderground vessel
   L- distance between observation holes

Heat generation of sources (metal blocks with sources encapsulated) was modeled using linear electric heater 1.3 m in length. Temperature fields change was registered by temperature-sensitive elements which were set up at a depth 5.5 m in the observation holes of bench-scale facility. To eliminate the influence of environment temperature the upper parts of the holes were closed by heat-insulating material.

Based on the data from simulation experiments, the quasistationary temperature fields in the repository vessel and near the repository were calculated [8]. Obtained experimental data made it possible to determinate the effective coefficient of heat conductivity for typical repository æ _k=(2.8± 0.3)W/m*° C and to calculate the minimum heat conductivity of metal of the matrix (æ _m=28 W/m*° C). This matrix permits reducing heat load on the repository as compared with standard ones. When employing the lead (æ _Pb=30 W/m*° C) as a matrix material for immobilization, the activity of sources to be disposed and encapsulated in a metal matrix should be within the permissible limits of 180. 000 Ci by Co⁶⁰ per one repository. This value far exceeds previously allowed limit of disposal of sources without conditioning which was 33. 000 Ci by Co⁶⁰ per one repository.

Predictions based on the results of simulation experiments using the bench-scale facility were completely confirmed under conditions of actual sources disposal. In a repository of SIA "Radon" spent radionuclide sources with total activity of 128. 000 Ci by Co⁶⁰ were encapsulated into lead matrix. Analysis of repository state showed that the temperature in the repository was 423 ° K, dose rate was of the order of 3 R/s. No radiolysis hydrogen was found in the samples of gas phase.

For realization of technique of spent sealed sources conditioning using metal matrices the mobile facility was developed. The use of this facility allows to follow traditional technology of spent sources disposal and to condition spent sources just in the underground vessel of repository [8]. The facility contains a joining unit and gas purification system which provide insulation of inside volume of the repository from the atmosphere. In addition, producing of reduced pressure eliminates the possibility of radionuclide release into the environment during conditioning. Matrix metal melt is prepared outside of the repository volume in the special technological unit. In this case duration of thermal effect on spent sources is minimized. Prepared melt is fed to underground vessel of the repository by flexible heat-resistant hose lowered down the repository through the charging pipe. The own biological protection of the repository serves the function of ionizing radiation shield.

The facility performs all works on conditioning of sources , beginning with inspection of repository state and including pumping out of water or condensate from the repository, drying of underground vessel by hot air and encapsulation of sources into metal matrix. Upon execution of all technological operations the repository is ready for further operation, accompanied by encapsulation of sources into metal.

The technology of encapsulation of spent sources into metal matrix has been used since 1986 at SIA "Radon". Since 1990 a new technique has come into use at the regional centers of radioactive waste disposal such as Volgograd, Nizhny Novgorod and Yecaterinburg regional repositories. By now spent sealed sources with total radioactivity over 1 million Ci have been encapsulated into metal matrices [9].

Metal matrices hold the greatest promise for not only immobilization of spent sources. This technology is considered as one of the most promising methods for spent nuclear fuel preparatory to long-term storage or final disposal. Preliminary investigations in this field are being carried out at SIA "Radon".

1. P.M.Mathew, F.Weinberg et al., Filler metals for containers holding irradiated fuel bundles.Metal Technology. September, 1982, v.9. p.375-380.
2. SKB TR 93-04. Project on Alternative Systems Study (PASS). Final Report. Stockho;m, October, 1992.
3. V.A.Kascheev, M.I.Ojovan, P.P.Poluectov et al., Soviet Atomic Energy, 1990, v.69, #1, p.572-576.
4. I.A.Sobolev, A.E.Arustamov, M.I.Ojovan et al., Soviet Atomic Energy, 1989, v.66, #5, p.340-342.
5. P. De Regge, F.Casteels. Compatibility between compacted cladding waste and clay formation. Ber. Kern. Julich Conf., 1985, #54, p.455-469.
6. O.Cassiba and S.Fernandez. Lead corrosion behaviour in simulated media of an underground repository. Journal of Nuclear Materials, 1989, #161, p.93-101.
7. H.Rachev, S.Stefanova. handbook on corrosion, "Technika", Sofia, 1977.
8. M.I.Ojovan, I.A.Sobolev, M.B.Kachalov et al., Proc. 1993 Int. Conf. On Nuclear Waste Management and Envir. Remed., Prague, v.1, p.155-157.
9. I.A.Sobolev, E.M.Timofeev, M.I.Ojovan et al., Mat. Res. Soc. Symp. Proc., 1988, to be published (Proc. Int. Symp. Sci. Bas. For Nucl. Waste Manag., Davos, Switzerland, Sept.28-Oct.3, 1997).

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