by Anatoly ZRODNIKOV, Dr. Sc. (Tech.), General Director, RF Research Center - Leipunsky Institute of Physics and Power Engineering;

Alexander YEFANOV, Dr. Sc. (Tech.), Director of Heat Physics Department of the same Institute;

F. KOZLOV, Dr. Sc. (Tech.), Deputy Director of the Department;

Valery YUGAI, Cand. Sc. (Tech.), Assistant Director of the Department

Most, if not all, expectations for the progress of atomic power engineering in the 21st century are associated with fast, or fast-neutron, reactors. These units process their uranium fuel practically completly and not just a small fraction as is the case with their thermal-neutron analogues, something which broadens the scale of raws utilization in a closed cycle.

When scientists were just starting working on fast reactors they knew already that the high density of energy release in the fuel core (10 to 100 times greater than in thermal reactors) and the possibility of using within it materials with low neutron capture- these are the main criteria for the selection of the coolant, or heat- transfer agent. But coolants like water, or organic and silico- organic compounds with appropriate heat-transfer parameters were rejected as inappropriate because of their high rates of neutron capture. This being so, a whole range of substances were tested until in the late 1940s and early 1950s.

Academician A. Leipunsky came up with the original idea of using liquid metals as heat-transfer agents, or coolants. The idea was confirmed by the subsequent experiments because fast neutron capture by potassium, sodium, lead, bismuth, iron, chromium and nickel proved to be quite insignificant.

Later on scientists of the Institute of Physics and Power Engineering carried out a program of studies into the properties of liquid metals, like sodium, eutectic sodium-potassium alloys and lead-bismuth and lithium, which laid the foundation of heat hydraulics, physical chemistry and of the technologies of their application as coolants in nuclear power plants. Working in conjunction with R&D centers and industrial units, experts were able to develop systems and apparatuses providing for effective applications of fundamentally new units and devices unprecedented in other countries.

The list of these novelties is topped by an industrial atomic power station with a BN-600 fast reactor using a sodium coolant which has now been in operation for more

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than 20 years*. Another example are the power plants code- named Buk, Topol, Topaz** and Enisei (with coolants of sodium- potassium, cesium and gallium) used on our space probes. Finally there came submarine reactors using lead-bismuth coolants.

Having said that, let us take a closer look at the aforesaid coolants and their features.

As was said before, nuclear power engineering reaches its maximum efficiency when it makes use of the total energy potential of uranium-something which can be done only by using fast reactors with sodium coolants. This choice was confirmed by studies conducted at the start of the 1950s in this (years of operation of the BR-5 pilot plant) and other countries. Apart from meeting high requirements in terms of heat physics, hydrodynamics and technical parameters, sodium also features high chemical activity. As we continued to gain operation experience with the BR-5 and later BR-10 units, we began designing and studying the system as a hole, that is the coolant - admixtures - structural materials - inert gas.

The main problem associated with the normal operation of an atomic power station is associated with what we call reactor purity of the coolant.

This parameter is achieved in two stages. In our case before filling the system, potassium was cleaned mechanically from a conservant organic mass with the subsequest "washing" in kerosene followed by distillation leading to the "reactor purity" level.

However, in the process of operation the coolant is "polluted" with sodium oxide and hydride and with tritium. Their amounts largely depend on the structural materials used. For maintaining sodium in the proper condition engineers are using now what they call cold, or freeze traps. In these units, with a capacity of 1-2 m ³ , the admixtures are accumulated and crystallized to be later cleared by regeneration.

* See: V. Subbotin, "Nuclear Power: From the Past Into the Future", Science in Russia, No. 1, 1997 . - Ed.

** See: A. Zrodnikov, A. Sorokin, "Obninsk - Naukograd of the 21st Century", Science in Russia, No. 2, 1999. - Ed.

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Studies of physico-chemical and heat and mass-exchange processes in cold traps conducted by our Institute's experts in conjunction with specialists of the R&D Bureau of Mechanical Engineering (Nizhni Novgorod) have made it possible to design and build a unit which surpasses its foreign analogues by 3 to 5 times by its main parameter (admixtures capacity).

Finally, the system also uses a shielding gas which fills the reactor interior (in our case this is nitrogen, sometimes argon).Their use is dictated by hydrodynamics of the coolant and reactor safety as a whole.

Thus the applied methods of admixtures control in the coolant and shielding gas are subordinated to the central objective of ensuring long and trouble-free operation of industrial reactors and conduct of research.

Apart from the above positive properties of sodium it also has one weak spot - high chemical activity with respect to air and water. Therefore in designing atomic power plants special attention was given to the means of early detection of water leakage into the coolant and ensuring fire safety of the plants*.

The more than 50 years of experience with sodium provided the basis of a technology of "washing out" and deactivation of the equipment, disposal of the used coolant and wastes. But still and all, a lot is still to be done for improving the existing and developing new specialized systems of this kind.

Over the years, we know, nuclear power plants began to be used not only on terra firma, but also out in space. In 1956-1958 experts started work on nuclear power plants for use in orbit with thermoelectric and thermoemission modes of energy conversion which used sodium-potassium as coolant. And the problem of its cleaning was again top of the agenda. This was done in two stages - during preparation of the system before charging and during its subsequent operation.

In 1967 our Institute's experts also studied the solubility of both oxygen and hydrogen in eutectic media and other blends of sodium and potassium. They established that while interacting with the alloy at temperatures above 180 ⁰ C these gases form a hydroxide of an almost equimolar composition (equal quantities of NaOH and KOH) due to which aggressiveness of the medium increases many times over at high levels of admixtures. At lower temperatures it disintegrates and a deep cleaning of the alloy contaminated by oxygen and hydrogen at one and the same time is possible only with a marked prevalence of the one over the other. And the obvious question was how one can get rid of one of the gases. Subsequent studies proved that it was more practical getting rid of hydrogen. And its most effective getter (absorber) is an alloy of 99 percent of zirconium and one percent of niobium. A strong non-pealing oxide film is formed on its surface which can contain large amounts of hydrogen. Subsequently the alloy was used for making mesh with a large surface of absorption (so-called thermal, or "hot" traps) used both in aerospace and ground units.

It was in this way that the cleaning of coolant was done, and the filling with it, of the atomic power plants Buk, Topaz and Enisei. And there were no emergencies during the tests and the operation of their numerous ground prototypes and 35 orbital nuclear power plants with a sodium-potassium coolant. Incidentally, these experiments were unparalleled anywhere in the world.

Apart from the aforesaid coolants, pioneering studies were carried out by our experts in 1964 of using cesium as the working substance in thermoemission energy converters of the Topaz type. Studies of the interactions of this metal with oxygen and hydrogen, which are mainly responsi-

* See: Subbotin, "Nuclear Power Safety", Science in Russia, No. 1, 1998 . - Ed.

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ble for its purity, continue to this day. The results obtained have made it possible to design and build units which have 3 to 4 times longer service life than their analogues.

And there is one more point in our plans for the future. In 1963 American scientist D. Grover came up with the idea of what was called a thermal pipe. It is a closed-circuit device with natural circulation of coolant within it in which heat is carried by steam from the input to the discharge zone. The return of the coolant takes place in the liquid phase under the effect of capillary or gravitational forces and without any external effects. The heat conductivity of the pipe, made of stainless steel, and with, let us say, sodium coolant is tens of thousands of times greater that in a silver and copper heat conductor of the same size. These devices boast high isothermal effect, unilateral transfer of heat, thermoregulation and other advantages which make them attractive for different areas of technology.

Despite their apparent simplicity, the process of heat and mass transfer in such pipes is very complicated, involving evaporation and condensation, flow with injection and sucking-off, transition across the speed of sound in the steam, separation of the mixtures into components (the first two processes are strongly influenced by even insignificant admixtures of gas, corrosion products, etc.). Studies in this area were initiated at the Institute in 1967, and it was soon established that thermal pipes are simply indispensable in the making of heat exchangers, for example in Stirling motors*. At that time our experts designed a universal system of heat supply for this unit which could be used with different energy sources (organic fuel, nuclear reactor, solar radiation) and a testing stand. It was demonstrated that using this device the motor efficiency could be boosted from 17 to 26 percent in using organic fuel and its service life could also be increased. In the subsequent models of heat exchangers the efficiency reached 30 to 32 percent at a temperature of the heated surface approaching 700 ⁰ C.

Later on it became clear that high-temperature thermal pipes can be used in nuclear power plants and also in space with the only requirement of their trouble-free operation for no less than 10 years. This called for considerable technological improvements which we introduced in the late 1990s. Pipes began to be made of stainless and refractory steels and alloys, filling them with what are called wick cores-fine-mesh grids of metal felting. The whole structure was designed for a long service life at temperatures of up to 750 ⁰ C. At the present time we keep perfecting the technique of making such units from infusible alloys filling them this time with lithium. And we have already produced a molybdenum pipe for operating temperatures of 1,300- 1,400 ⁰ C. And we also have a conceptual diagram of a system of emergency cooling of fast sodium reactors which should operate on the principle of thermal pipes and provide for the possibility of "operator-free" starting and functioning of such units in case of emergencies.

As has been said before, one of the main areas of our Institute since the middle of the 20th century has been the development of nuclear power plants for submarines. The first circulatory stand with a lead-bismuth coolant was built in 1951. In making this choice we took into account the fact that the low chemical activity of both lead and bismuth in the interaction of the alloy with air, water and steam rules out the risk of explosions and fires; high working temperature of the coolant precludes its boiling at energy- intensive sections of the unit. And, last but not least, the low working pressure in the circuit increases the reliability and safety of the unit, simplifies the design and manufacture of the equipment and considerably facilitates its operation.

Early in 1959 our experts put into operation a ground prototype of a nuclear power plant (code-named "27/VT") and put into operation in

* Internal combustion motor in which the working substance (helium or hydrogen) remains within a closed space and changes its volume from heating or cooling. - Ed.

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1963 the first nuclear sub (Project 645) with lead-bismuth coolant. It seemed that all of the associated technical problems have been resolved, but our hopes were dashed by a breakdown of the reactor in 1968. It was caused by the incomplete knowledge of the coolant and its negative properties, and there were yet no proper regulating and control procedures and no means of cleaning the coolant and the working circuit.

The heart of the matter was that on the first voyage of the sub impurities were accumulated in the circuit, mainly oxides of the coolant components, which entered the reactor core on the second voyage, causing a sharp deterioration of heat take-off. In a word, it was necessary to study more deeply the alloy used, and it turned out that the lead-bismuth alloy is aggressive enough with respect to the structural materials, that is it can be polluted in the process of interaction with them and also with oxygen. To ensure trouble- free operation of such units it was necessary to cope with two problems: provide for the necessary purity of the coolant itself and of the inner surfaces of the circulation circuit equipment, and, secondly, it was necessary to develop corrosion-resistant structural materials which come into contact with the alloy.

In dealing with the latter problem we used what we call the method of inhibition in a liquid-metal medium. After long searches and tests of materials reducing the rate of chemical reaction we picked up oxygen which is also naturally present in the alloy Subsequent experience confirmed our choice and that determined the direction of our further studies, and a set of systems and technologies of coolant manufacture.

We found out later, however, that of great importance for the operation of a reactor are the amounts of oxygen dissolved in the alloy. For its level control we developed a sensor of thermodynamic activity (activometer). Its advent and use on all of the test and later industrial units served to increase the "factor of authenticity" of our investigations. And the new device helped us keep in check all technological processes involved in the making of coolant.

Later on special attention was given to the purity of the alloy and methods of its purification. This can be achieved in two ways. In one impurities are transformed by the hydrogen reduction of metal and the product is returned into the coolant (thus we get rid of the oxides of lead and bismuth), and in the second case the impurity is removed from the circuit and collected in special devices from which it can be removed and replaced or stored therein during the whole service life.

Apart from the above chemical compounds, also present in the coolant are suspended impurities which cannot be regenerated. For their removal we developed filters of special woven materials consisting of glass, metal or carbon fibers.

Another condition of a normal operation of such units is maintaining in the coolant an optical level of dissolved oxygen. A special feature of such sealed lead-germanium circuits consists in the fact that in the course of their operation the volume of this gas diminishes due to the escape into the coolant of components of structural materials (chromium, iron, etc.) and also of elements formed under the impact of neutrons and protons which have closer "affinity" with oxygen than the components of the alloy All of these substance interact with oxygen, producing oxides. As a result at certain temperature regimes the process of metal transfer is intensified with the "plugging" of the cool sections of the unit or a dissociation of protective coatings covering structural materials at hot spots. To prevent this institute specialists have developed systems and devices which supply into the circuit oxygen gas, mixtures of steam and hydrogen and also provide for the dissolution of the solid oxides of lead and bismuth.

Solving all of these problems, which were mostly of pioneering nature for our contemporary science and technology, has made it possible to design and build nuclear subs which boast higher speeds than their foreign rivals and can even "run away" from anti- sub torpedoes*.

Over the past few years scientists of the Leipunsky Institute of Physics and Power Engineering have been working on a fast reactor with a lead coolant. And the reason for this choice is that, alongside such positive properties of lead as relatively low chemical activity and a high boiling point, it is available in large quantities and is relatively inexpensive. Its main flaw is its high melting point (327 ⁰ C) which is causing problems with its application. These include greater quantities of impurities and the intensity of the physical-chemical processes and mass transfer - something which is bound to create problems with the quality of such coolant and its purification. Finally, using the familiar structural materials will reduce the working range of temperatures and cause greater technological problems with starting, running and repairs of such units.

As we know from practical experience, the technologies tested with lead make it possible to remove from the circuit any excessive solid phase on the basis of lead oxides; control and regulate the quality of the coolant; form protective oxide coatings on the surface of steel. All of the above considerations give grounds to hope and belief that a fast reactor with a lead coolant will finally materialize.

* See: G. Gladkov, "A-Fleet Pioneers", Science in Russia, No. 3, 1999 . - Ed.

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