Georgia:  Long-Term Storage of Spent Fuel

 

This paper was originally presented at a conference held at IPPE, Obninsk in March 1997.  It is included in the NIS Nuclear Profiles Database by permission of the authors.

The Research Nuclear Reactor IRT-2000 (since 1968 - IRT-M; M - modernized) of the Institute of Physics, the Georgian Academy of Sciences, was put in operation in 1959 and worked up to 1988 when it was stopped according to the order of State Controlling Body in connection with increased demands on nuclear and radiation safety of nuclear objects.

1. Upper platform 2. Central vertical experimental channel, 3. Channels of ionization chambers, 4. Channels of emergency protection rods, 5. Impringment hood (buffle), 6. Cassette holder, 7. Tangent horizontal experimental channel, 8. Lead shield, 9 Circular gap between channel and thermal column, 10. Exhaust pipe, 11. Channels of mobile ionization chambers, 12. Exhaust valve.

In March 1990 taking into account the insufficiency of residual operating resources of reactor and pointed antireactor spirits of society the Georgian Academy of Sciences made the decision to put reactor finally out of operation and dismantle it. At present the Institute of Physics together with the Institute of Building Mechanics and Seismic Stability of the Georgian Academy of Sciences completes the elaboration of the project on conversion of the reactor into completely radiation-safe state for its long-term storage until beginning its dismantling. This project together with other measures envisages also the solution of the problem with the part of spent and fresh fuel remained at the reactor.

The research reactor IRT-2000 of the Institute of Physics, the Georgian Academy of Sciences, with thermal power of 2000 kW was designed in the Soviet Union and belongs to the group of light water pool-type reactors in which the ordinary water (light, distilled) is used as heat carrier and moderator of neutrons and biological protection as well.

The pool of reactor represents an aluminum tank (after the second reconstruction, 1973, of stainless steel) surrounded by 2 m thick reinforced concrete layer of biological shield. The height of the pool is 7.8 m and the area of horizontal cross-section - 7.2 m². The volume of the pool is 56 m³ and the volume of water in operating state - 52 m³ (see Fig. 1 and Fig. 2).

Figure 2: Horizontal Section of IRT Reactor

1. Storage of spent fuel elements, 2. Loading pipes, 3. Thermal column, 4. Channels of mobile ionization chambers, 5. Reactor core, 6. Valve, 7. Generator of the g-radiation contour, 8. Ejector, 9. Pressure line of first contour, 10. Irradiator of g- radiation contour, 11. Hot chamber.

Reactor IRT-2000 was twice subjected to large-scale reconstruction (in 1968 and 1973) during the whole period of its operation. As a result the power of the reactor was successively increased first up to 5000 kW and then up to 8000 kW. Owing to these reconstruction the experimental possibilities of the reactor were significantly increased and its thermophysical, operating and technological characteristics were considerably improved.

The main scientific directions developed on the basis of the reactor are: neutron physics, radiation chemistry and biology, neutron-activation analysis and, especially, low temperature radiation physics of solids and low temperature radiation materials science. The reactor was equipped with special facilities elaborated at the Institute of Physics allowing to irradiate samples at temperatures down to 10 K and to transport them without heating for post-radiation measurements and also to carry out intra-channel measurements at low temperatures directly in the course of irradiation.

The operation of the reactor IRT-2000 till September 1969 was carried out on the basis of IRT-1000 type fuel assemblies consisting of rod-like EK-10 type fuel elements with 10% enrichment of the fuel in Uranium-235 (Fig. 3).

 
FIGURE 3. Fuel Element EK-10

1. Plag [Plug?] 2. Uranium core 3. Protective aluminum jacket

The IRT-1000 type Fuel assembly is a cylindrical square cross-sectioned tube-type cassette made of aluminum alloy, within of which in cells of special grid with definite distances between them 16 or 15 EK-10 type fuel elements are located depending on configuration of the IRT-1000 type fuel assembly (Fig. 4). The amount of Uranium-235 in each EK-10 type fuel element is 8g, and in the assemblies of IRT-1000 type with 16 or 15 fuel elements it is 128g and 120g, respectively.

Figure 4. Disposition of fuel elements EK-10 in assemblies IRT-1000 of different configuration. 1. EK - 10 fuel elements, 2. Assembly with 2 chamfers, 3. Assembly without chamfers, 4. Assembly with 1 chamfer, 5. Assembly with 3 chamfers, 6. Regulation rod.

From October 1969 to January 1988 the exploitation of the reactor was based on the new fuel assemblies of IRT-2M type consisting of four concentric tube-type fuel elements of square cross-sections with 90% enriched nuclear fuel in Uranium-235 (Fig. 5). The IRT-2M type fuel assemblies have more developed surface of convection heat transfer (2.6 times) and contain more Uranium-235 as compared to assemblies of IRT-1000 type. The central fuel element of the assembly of IRT-2M type is removable allowing to locate channels with diameter up to 28 mm inside the assemblies for regulation and emergency protection rods or experimental channels for irradiation of various samples in them. The amount of Uranium-235 in three-tube assembly is 140g and in four-tube assembly is 170g.

It should be also noted that in the nuclear reactor IRT-M of the Institute of Physics nonstandard experimental (pilot) TTR type fuel assembly, consisting of TTR type rod-like fuel elements with 90% enriched nuclear fuel in Uranium-235 was tested in 1974-1975.

Frame of assembly (hexagonal cross-section) with tips and grids was fabricated at the Institute of Physics. The assembly contains 91 fuel elements with 4.5 g Uranium-235 in each, while its total amount in the assembly itself is 409.5 g (Fig. 6).

Reactor IRT-M of the Institute of Physics, as a rule, was operated in uninterrupted regime for 5-day working cycles. During the whole time of operation 201 fuel assemblies were used, including 95 of IRT-1000 type, 105 of IRT-2M type and one assembly of TTR type. Average annual consumption of fuel assemblies was 7.

The most part of spent fuel assemblies (in total 196, including 92 of IRT-1000 type and 104 of IRT-2M type) was transported to reprocessing enterprise at several times. At present there are 5 spent fuel assemblies of various types in a special storage of the reactor.

The complete list of spent fuel assemblies used during the whole operation time of nuclear reactor of the Institute of Physics is given in Table 1.

Table 1.

Table 2 presents the main characteristic data of all spent fuel assemblies existing at present in the storage of reactor, including their activity, calculated according to the power measurements of g -irradiation doses carried out in January 1997.

Table 2.

N	Type of assembly	Number of assemblies	Total mass of U-235, g	Loading date	Unloading date	burn up, %	Specific energy release MW× day/kg U235	Gamma-equivalent of fission products, g-eqv× Ra
1	IRT-1000	93	120	09.68	09.69	18	148	1.4
2	IRT-1000	94	128	09.68	09.69	3	23.8	0.15
3	IRT-1000	95	128	09.68	09.69	18	148	1.3
4	IRT-2M	237	170	12.87	01.88	1.25	10.2	1.8
5	TTR	1	409.5	10.74	05.75	6.8	54.8	3.5

In early years of exploitation of the reactor the spent fuel elements and assemblies were stored in special storage of reactor which is located in reinforced concrete block of biological shield (next to reactor pool) and is designed for keeping only 50 assemblies in it. As the problem of spent fuel shipment to the corresponding reprocessing enterprises for a number of years had not been solved (up to 1980) the limited capacity of storage put under question the long-term exploitation of the reactor under normal conditions. Since it was necessary either to decrease significantly the reactor power with the aim to reduce the fuel consumption or to build up additional storage by ourselves. The decrease of reactor power and consequently the intensity of neutron fluxes deprives the research reactor of its main functions from viewpoint of scientific and applied aspects. Therefore, in 1970-1973 the Institute of Physics similar to other research nuclear centers built up the additional storage outside the reactor building according to the designed storage for keeping 128 spent fuel assemblies.

The first shipment of spent fuel to the reprocessing factory was carried out only in 1984, i.e. on the 24-th year of reactor operation. In table 3 the data of all shipments of spent fuel assemblies to the reprocessing enterprise by the Institute of Physics from 1984 to 1991 are given.

Five remained spent fuel assemblies of various types (see table 2) are stored at present in the reactor storage in which the water parameters had been and are still maintained at the prescribed level:

- pH=5.5¸ 6.5;

- electric conductivity: s =1.5¸ 2.0 m Sm/cm;

- chloride ions: 50 m g/l;

- Activity due to impurities Cs and Co: 3.7 Bq/l;

- Activity due to Tritium: 7.0 kBq/l.

Table 3 

Table 4

N	Fuel element type	Element form	Enrichment in Uranium-235, %	Content of Uranium-235, g	Number of fuel elements	Total mass of isotope Uranium-235, g
1	EK-10	rod	10	8	73	584
2	TTR	rod	90	4.5	594	2673
3	IVV-2	rod	90	4.2	83	348.6
4	Internal ele-ment of the assembly of IRT-2M type	square cross-section tube	90	24	29	696
	Total:					4301.6

The fresh nuclear fuel is stored in a special blank-walled building equipped with alarm system of ADVANTOR type, TV-system for remote watching of the building and special barrier erected in front of the only entrance. The building also has a special drain to exclude the possibilities of water gathering in it. Containers with fuel elements are fixed in a special iron framework to provide the necessary distances between them for safety. Besides, the whole object is guarded by the special personnel of the Chief Police Board of the Ministry of Internal Affairs of Georgia.

In 1986 the Institute of Physics, the Georgian Academy of Sciences received a batch of 17 new technological fuel assemblies of IRT-3M type which in 1995 was handed over to the Institute of Nuclear Physics, the Uzbekistan Academy of Sciences for research nuclear reactor of VVR-SM type.

As far as in Georgia there is no Special State Depository of radioactive waste for their reliable isolation from environment, the existence of remainder of spent fuel containing the fission products and transuranium elements (the time necessary for their isolation from biosphere being correspondingly hundreds and tens of thousand years) makes the problems for us the solution of which under our conditions is connected with certain difficulties.

In spite of the fact that the conditions, under which the spent fuel assemblies are stored at present, can be considered as satisfactory, still this is not the solution of the problem as they are in water for a long time (more than 20 years) and there is a real danger of corrosion on their surfaces which can lead to their seal failure in time.

It is natural that the most drastic solution of this problem for us would be the shipping of spent fuel beyond Georgia. However, its shipping on condition the radioactive waste formed in the process of their reprocessing returns to Georgia, as it is usual in practice, unfortunately is not acceptable for us just because of the absence of appropriate burial place. Therefore, the Institute of Physics develops the alternative versions of long-term keeping of remained spent fuel assemblies in situ, i.e. on the territory of the nuclear reactor.

The most simple version of long-term storage of the spent fuel assemblies in dry form is the possibility of application of standard dry channels located in biological shield next to the reactor pool and intended usually for keeping radioactive materials (Fig. 2). The given version foresees the preliminary disposition of each separate spent fuel assemblies in hermetic container of stainless steel, which can be located in corresponding dry channels of biological shield supplied with safety plugs and locks. At the same time for complete exclusion of possible increase of g -irradiation dose rate on the upper platform of the reactor the closing of neighborous dry channels with the similar safety plugs or their filling with absorbing material is foreseen.

The other possible version of long-term storage of the spent fuel assemblies, in our opinion, is the creation of a special hermetically closed lead container similar to transport packing set of 19-type with a basket designed for location all five spent fuel assemblies in it. Such container capable to decrease g -irradiation dose rate down to background level is maximum 5 tone in weight and by means of standard bridge crane can be located in any place of the physical hall of the reactor or in the technological building of the first cooling contour. Such container can be also supplied with the special lock system preventing access to the assemblies.

It should be emphasized that the given versions of long-term storage of the spent fuel assemblies allows periodic examination and control of the assembly states if necessary.

As the complete dismantling of the nuclear reactor of the Institute of Physics of the Georgian Academy of Sciences envisaged by the accepted decision on its removal from the operation is practically impossible under our present conditions due to the high price and extreme complexity of this process, as well as to the absence of the depository for radioactive waste. The Institute of Physics developed the project of converting the reactor into completely radiation-safe state for its long-term storage. This project foresees concreting of the lower high-radioactive part of reactor tank with simultaneous burying of large-sized radioactive wastes existing on the reactor after exploitation of experimental devices and systems in the form of separate units and constructions in it. Besides, this project also foresees disposition in the upper part of reactor tank of the "zero" power reactor, which can be used both for solving a number of applied tasks and as a trainer for training the young specialists in the field of physics and technology of nuclear reactors.

In connection with this project we have the third version of long-term storage of the spent fuel assemblies. According to this version in a new reinforced concrete block inside the reactor tank there is a special stainless steal plated socket covered with a special reinforced concrete plate in which the hermetically closed container is located, in five cells of which the spent fuel assemblies are preliminary arranged. After placing the container with the spent fuel assemblies into the socket and its covering with reinforced concrete plate the whole reinforced concrete block in the tank is covered with a sheet of stainless steal welded to the tank walls thus becoming a new bottom for the other part of the reactor tank designed for placing "zero" reactor in it (Fig. 7).

The advantages of this version as compared to those two mentioned above are a high reliability, long-term storage of spent fuel assemblies and practically their complete safety even in extreme situations or any hypothetical emergencies. However, the disadvantage of this version is the impossibility of controlling of the assembly state in the process of their long-term storage as well as of their timely extracting if necessary.

It should be especially noted that while suggesting nonstandard versions of handling the spent fuel we take into account the following peculiarities of the remainder of spent fuel assemblies:

The choice of the long-term storage version of the reminded spent fuel assemblies at the nuclear reactor of the Institute of Physics will entirely depend on the solution of the problem connected with their shipment beyond Georgia. If the problem of their shipment will not be solved appropriately, their long-term storage in situ will be accomplished on the basis of realization of one of the considered versions. We hope that the present Conference enables us to deduce more correct solution of the problem.

Comments or questions? Contact Kenley Butler at MIIS CNS: Kenley.Butler@miis.edu