OPERATION, STORAGE AND RECYCLING
2. RADIONUCLIDE POWER ENGINEERING
Radionuclide power engineering using 90Sr radionuclide thermoelectric generators (RTGs) started in the Soviet Union in 1962. Its origin was due to the desire to utilize reactor wastes, and to the need for independent sources of electrical power in the range of a few watts for supplying autonomous equipment in outlying and sparsely populated districts.
By the mid-1970s, the scientific and technical, structural and techno-logical problems of development and industrial production of the main elements of 90Sr RTGs and of radionuclide power installations (RPIs) based on them had been solved, as had the problems of preventing radiation exposure and environmental accidents associated with their application.
Production was started of a wide range of these products with electrical capacities of from several watts up to some hundreds of watts, and a nominal service life of ten years. Industrial production of RTGs and RPIs was carried out up to the mid-1990s.
The main areas of application of 90Sr RTGs and RPIs are:
(a) Sources of electrical power for navigation equipment located in outlying and hard to reach districts away from settlements, on the coast and islands of the Russian Federation; in particular, about 380 RTGs now operate on navigation objects on the Northern Sea Route.
(b) Sources of electrical and thermal power for seismic measuring equipment in the seismic inspection groups which have been deployed within the territory of the Russian Federation under the Nuclear Test Ban Treaty.
(c) Sources for supplying power for special purpose equipment.
The advantages of RTGs (RPIs) include:
(1) High power and autonomous operation for a long time (in practice more than 20–25 years);
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(2) High operational reliability and ecological safety throughout the life cycle, as confirmed by long term practice;
(3) Simplicity of design and absence of moving parts.
A basic shortcoming of RTGs (RPIs), which has become most urgent in the last few years, is their vulnerability, while in service in outlying areas of the country, to the possibility of unauthorized actions by malefactors, including terrorists.
Furthermore, since the 1990s, in connection with well known political and economic changes in our country, financing of work on radionuclide power engineering has been stopped. As a result, practically all the 90Sr RTGs and RPIs remaining in operation (about 1000) are products which have gone beyond the nominal ten year service life.
Under these conditions, the task of ensuring radiation and environmental safety and physical protection of the RTGs and RPIs remaining in operation and containing 90Sr with a total activity of about 45 MCi has become a topical problem requiring an urgent solution.
The need to solve this problem promptly, as was already mentioned, is due to the fact that the products — as a result of the specificity of their use — are essentially unprotected against unauthorized actions and actions by malefactors, even though in the design of RTGs and RPIs some measures to hinder such actions had been stipulated.
Within the framework of solving the above problem and with the purpose of answering questions connected with the handling of 90Sr RTGs and RPIs at all stages of their life cycle, VNIITFA has developed, in co-ordination with interested organizations, “Regulations for the operation and withdrawal from operation of radionuclide power installations using 90Sr radionuclide heat sources (RHSs)”, which were authorized by the Ministry of the Russian Federation for Atomic Energy (Minatom) at the end of 1999 and are obligatory for all organizations. This document regulates the activity of those organizations connected with RTGs and RPIs at all stages, from development and manufacturing to recycling and burial.
In accordance with these regulations a total of 254 90Sr RTGs and RPIs which had reached the end of their service life were surveyed during the last three years at their places of operation from the point of view of their technical, radiation and ecological condition. The results of the inspections were presented to an interdepartmental commission for its decision on the further destiny of the RTGs and RPIs: prolongation of service life or return to VNIITFA for dismantlement and subsequent recycling of the 90Sr radionuclide sources and their burial at the facilities of the Mayak Production Association.
It must be emphasized that during the inspection at their places of operation of hundreds of RTGs and RPIs which have reached the end of their lifetime, and of tens of RHS-90s which have had a service life of up to 30 years and were taken from returned RTGs and surveyed in VNIITFA, not a single RHS-90 failure (breach of capsule tightness with release of radionuclide into the environment) was observed. At the same time, a number of surveyed RTGs (RPIs) had been exposed to fire or to unauthorized dismantling, including extraction of the RHS-90 containing tens of kilocuries of 90Sr.
The RHS-90 service life attained in an RTG (RPI) structure was as expected and provided experimental confirmation of a significant reserve both in radiation and in ecological safety in the RHS-90 design.
The objective factors allowing one to expect such a reserve in RHS-90 service life are:
— Technical and technological solutions incorporated into the design;
— Conditions of operation in the RTG (RPI) structure.
In the RTGs (RPIs) now in operation, in which the temperature of the hot junction of the thermoelectric converter is limited to no more than 300oC, because of the properties of the materials used, the working temperature of the surface of an RHS-90 capsule cannot exceed 400–500oC. Moreover, the RHS-90 is enclosed in the hermetically sealed cavity of the RTG (RPI) in a rare gas environment and is not mechanically loaded. The calculated value of mechanical reliability for the safety of an RHS-90 in the RTG (RPI) structure under conditions of regular operation and possible accidents (shock impacts, fire, impact of sea water) confirms the existence of a significant reserve. The process of radioactive decay of 90Sr into the stable 90Zr in the fuel composition matrix of the RHS-90 is not accompanied by gassing or volume changes, and the thermal loading on the RHS-90 elements decreases with time. Under such conditions, and in view of the known heat, mechanical and corrosion properties of the materials used in the active part (ceramics) and of the constructional materials of RHS-90 capsules and their double hermetic sealing by an argon arc welding method, it is clear that the time factor is not critical for the stability and safety of RHS-90s, as fully proven by experience of their actual long operation.
The limit of RHS-90 service life in the RTG (RPI) structure is set by the limit of the reduction in the thermal flux from the RHS-90, which is connected with the decay of 90Sr with time, i.e. with its functional (thermal) reliability.
The above mentioned RHS-90 service life of ten years in the RTG (RPI) structure was determined first of all by the fact that in this period of time the
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thermal flux is reduced by approximately 22% from its initial value, and in view of the efficiency of the thermoelectric converter at the time of manufacture, the electrical output parameters of the RTG (RPI) remain at an allowable level up to the end of the specified term of operation.
An increase of the nominal and guaranteed RHS-90 service life in the RTG (RPI) structure to 25 years is now technically and ecologically proven possible. For the purpose of experimental confirmation of the radiation and ecological safety of the RHS-90 over longer periods of time, prolonged operation is proceeding.
The results obtained on the prospect for uninterrupted functioning of existing navigation systems in the Russian Federation have suggested continuation of the use of the RTGs (RPIs), including those already beyond their initial lifetime (ten years), for a further 10–20 years; this is made possible by their overhaul at the specialized VNIITFA enterprise and by modernization, including in some instances work carried out at the place of operation and the use of modern thermoelectric converters of increased efficiency without replacement of the RHS-90, with a guarantee of the necessary level of safety.
The electrotechnical components of the RTGs were often replaced at the place of operation in order to ensure the power parameters required of an autonomous power supply source.
The realization of this concept has made it possible to:
(i) Prolong the period of operation of more than 100 end of service life RTGs such as the Beta-M and RPIs such as the IEU-2 for a further 10–15 years;
(ii) Ensure the return to operation of tens of RTGs (RPIs) of various types after overhaul, reusing the RHS-90;
(iii) Develop variants of the structure of basic RTGs with a service life of close to 40 years and more. In the basic RTGs, the element base, com-ponents and units taken from ‘primary’ RTGs should be utilized to the extent possible. The RHS-90s also continue to be used if the regular inspection at each stage of modernization gives positive results.
At the same time, it is necessary to recognize that the final stage of the life cycle of any RTG (RPI) is obligatory recycling and burial of the RHS-90 contained in it.
It is necessary to bear in mind that the weight of one RTG (RPI), depending on the type, is between 560 and 2500 kg, and taking into account the locations of their operation, this complicates and makes more expensive the work of recycling.
Mention must be made of what are, fortunately, isolated emergency instances, requiring a special approach and, of course, involving serious financial consequences. The first case was of damage to the biological shielding of one RTG in the Far East. The biological shielding was made of depleted uranium. As a result of excessive mechanical forces during transport by helicopter, there was depressurization of the radiation shielding block, which caused an intensive process of oxidation of the uranium, sloughing of uranium oxide powder, and, as a consequence, a significant local increase of the external dose characteristics of the RTG. A complex of organizational and technical measures, including all necessary measures for safety, localization of the emergency and RTG removal from the place of operation, were implemented.
The costs were significant.
Secondly, there was a case of the disappearance of two RTGs of one of the coastal navigation systems on the Northern Sea Route in the area of the Laptev Sea, which the experts of the operating organization connected with thermokarstic processes in the coastal zone. The objects sank into the ground, as a result of which there was a landslide of coastal ground with gully formation. It is believed that at the bottom of this gully, under a layer of liquid soil, there are the two RTGs with the rest of the equipment. This situation may be aggravated by the fact that the heat generating capability of the RTGs can promote further sinking through the locally thawing permafrost, which will seriously complicate the search for the objects.
The final stage of the life cycle of the RHS-90s used in the RTGs (RPIs) is burial. The technology for RHS-90 burial has been developed and is carried out at the Mayak facilities. During 2001 and 2002, VNIITFA carried out the recycling of 100 RTGs and RPIs of various types. Up to the present time, Mayak has carried out burial of 100 RHS-90s from RTGs (RPIs) with a total
90Sr activity of about 2.3 MCi.
The problem of RTG (RPI) recycling also raises the question of recycling of the depleted uranium used in a number of RTGs (RPIs) as radiation shielding blocks. Certainly this problem should be considered in combination with the recycling of the depleted uranium used as biological shielding in other products.
Thus for the approximately 1000 90Sr RTGs still in operation, the process of their safe removal from operation at the end of their service life for recycling and final burial has been worked out in detail organizationally, technically and technologically, but is hampered by the lack of the necessary financial resources under the economic conditions now existing in the Russian Federation. This circumstance is fraught with serious consequences, in view of the possibility of unauthorized actions affecting the integrity and safety of sources of high potential danger.
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3. RADIATION TECHNOLOGY EQUIPMENT WITH USE OF