V.M. POPLAVSKY*, A.D. EFANOV*, F.A. KOZLOV*, Yu.I. ORLOV*, A.P. SOROKIN*, A.S. KOROLKOV**, Yu.Ye. SHTYNDA**
*FSUE State Scientific Center of the Russian Federation – Institute for Physics and Power Engineering, Obninsk
Email: sorokin@ippe.ru
**JSC State Scientific Center – Research Institute of Atomic Reactors, Dmitrovgrad
Russian Federation
Abstract
In the paper presented are results of comparative analysis and the choice of liquid metal coolants for fast reactors, the current status of studies on the physical chemistry and technology of sodium coolants for fast neutron reactors and heavy liquid metal coolants, namely, lead–bismuth and lead for fast reactors and accelerator driven systems. There are descriptions of devices designed for control of the impurities in sodium coolants and their removal as well as methods of heavy liquid metal coolant quality control, removal of impurities from heavy liquid metal coolants and the steel surface of components of nuclear power plants (NPPs) and relevant equipment. Attention is given to the issues of modelling of impurity mass transfer in liquid metal coolants and designing new liquid metal coolants for NPPs. Results of the analysis of NPP abnormal operating conditions are presented. The adopted design approaches assure reliable protection against accidents. Up to now, about 200 reactor-years of sodium cooled fast reactor operation and about 80 reactor-years of submarine reactor operation have been gained.
The new goals for sodium and heavy liquid metal coolant technology have been formulated as applied to the new generation fast reactors.
1. INTRODUCTION
Studies on the liquid metal coolants of nuclear power plants (NPPs) as a special trend in NPP design were initiated in the 1950s at the IPPE (former USSR) under the leadership of A.I. Leypunsky. A wide range of liquid metals having adequate nuclear, thermal, physical and chemical properties and low vapour pressure at high temperatures were considered as coolant candidates. The
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possibility of creating NPP designs that eliminate the failure of vessels and coolant loss was studied.
At that time, requirements were formulated for the coolants of various NPPs, namely, sodium for fast reactors, and lead–bismuth for nuclear submarine reactors and spacecraft (sodium–potassium and lithium). The effects of coolant on reactor neutronics, technological, corrosion and thermohydraulic character-istics, as well as on toxicity and cost, were taken into account. These issues were considered in close connection with the general safety of NPPs (including issues of reactor physics, fire safety, technological safety and protection against toxicity) [1–5].
1.1. Alkali metals
Sodium was chosen as a coolant for NPPs with fast reactors in the former USSR and in all other countries where studies on fast reactors were carried out (France, Germany, Japan, United Kingdom, United States of America, etc.). This choice was made because of sodium thermal physics and the simplicity of techno-logical procedures for maintaining specified coolant quality in the repair stage [5–7]. Sodium drawbacks (i.e. high chemical activity in oxidizing media and its intensive interaction with water, accompanied by the production of hydrogen gas, and induced radioactivity of 24Na and 22Na with half-lives of 15 h and 2.6 h, respectively) led to the introduction of a secondary (intermediate) circuit in the reactor heat removal system. This intermediate circuit prevents the penetration of water–sodium interaction products into the reactor core and the primary circuit exposure to high pressure in the case of water–steam circuit failure.
The properties of lithium are quite unique [8]. For instance, its density, which is the lowest compared with that of the other liquid metals, assures a low mass of coolant loaded into the spacecraft reactor. Its specific heat capacity, which is the highest compared with that of all other metals, allows for a decrease in coolant temperature rise in the core and an increase in the average temperature of the thermodynamic cycle and, hence, its efficiency. Besides, the lowest pressure of lithium vapour (compared with other alkali metals) makes it possible to decrease the mechanical load on the components and pipelines of NPPs at high temperatures. On the other hand, liquid lithium has significant drawbacks, namely a high melting point (~180°С) and corrosiveness caused by dissolved nitrogen. Lithium interacts with water and burns in air at high temperatures [9].
Use of sodium–potassium eutectic, owing to its low melting temperature (–12.3°С), allows for the simplification of NPP design and facilitates its operation [10]. Unfortunately, Na–K has poorer thermal physics properties than those of Na, higher vapour pressure of the eutectic components, high chemical activity causing its spontaneous ignition in air at moderate temperatures, and a
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185 tendency to form potassium peroxidates in air as a result of potassium vapour condensation in the low temperature areas. Their interaction with potassium may cause explosions.
1.2. Heavy liquid metal coolants
Mercury was the first liquid metal used as a coolant in fast reactors (Clementine and BR-2). Because of its high toxicity, limited primary resources, high vapour pressure and adverse nuclear and thermal properties, mercury is no longer considered as a possible coolant for fast reactors. Nevertheless, in the 1960s, it was widely used as a modelling fluid in the experimental studies on heat transfer in the NPP components.
The attractiveness of lead–bismuth eutectic as a coolant is due to its moderate melting point (125°С), high boiling temperature (1638°С), eliminating the possibility of its boiling onset in the high temperature areas, and low chemical activity with respect to air, water and steam, thereby preventing explosions and fires. Low working pressure in the circuit increases the reliability and safety of the components, simplifies the design and manufacturing technology and signifi-cantly facilitates operation of the primary system components. In the stage of designing NPPs for nuclear submarines [11–14], the properties of lead–bismuth TABLE 1. CHARACTERISTICS OF LIQUID METALS CONSIDERED AS CANDIDATE COOLANTS FOR NPPs
Li 0.005 60–100 Lower than
that of Na
Pb 0.016 ~1 Low High High
Bi ~10-5 40–50 Low High High
Eutectics:
Na–K 3–5 High Low Low
Pb–Bi 25–30 Low High High
a Cost in roubles in 1980.
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eutectic outweighed its drawbacks, such as corrosion and erosion activity with respect to structural materials, high density and viscosity, low heat capacity and conductivity, as well as polonium accumulation under radiation.
The interest of designers of the larger size NPPs with fast reactors using liquid lead is due to its low chemical activity with respect to water and air, its high boiling point (1745°С) and the insignificant rate of polonium accumulation under radiation. In addition to the drawbacks of lead–bismuth eutectic, the lead melting temperature is high (327°С), and its interaction with water may cause explosions.
Analysis has shown that a reactor core cooled by lead should be ‘loose’
because the lead flow cross-section should be higher by an order of magnitude than that for sodium. It means that liquid lead velocity is lower by an order of magnitude compared with that of sodium. It is notable that sodium was chosen as the coolant for a high power density, compact fast reactor core in the early stages of nuclear power, when the main goal was to achieve a high 238U–239Pu breeding ratio in fast reactors.
Solid experimental and analytical task oriented studies [1–26] led designers to consider the liquid metal system of the NPP as a complicated, heterogeneous, multicomponent system, and the technology of liquid metals (sodium and lead–bismuth) was developed taking into account issues of liquid metal interaction with other media and structural materials.
This paper presents the current status of studies on the physical chemistry and technology of sodium and heavy liquid metal coolants (lead–bismuth and lead).
2. PHYSICAL AND CHEMICAL PROPERTIES OF LIQUID METALS AND THEIR TECHNOLOGY FUNDAMENTALS
Studies carried out on the ‘coolant–structural materials–cover gas’ system included many stages, starting with determination of constants characterizing solubility of impurities in liquid metal coolants to formation of models of mass transfer in the liquid metal circuits, taking into account thermohydraulic modes.
The results of studies are used as the basis for designing computer codes to forecast system behaviour in all operation modes of the NPP. This requires knowledge of both equilibrium and kinetic constants. Considerable information has been gained about the solubility of various impurities in liquid metals and their mutual influence on solubility, kinetics of reaction in the coolant, diffusion constants, liquid metal structure and the form of impurities. Yet, this information is insufficient for a complete description of the system. The basic factors influencing mass transfer in the liquid metal circuits have been determined [2, 4,
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187 15–18]. One of these factors determining the steel corrosion rate is total solubility of its components in liquid metals. These generalized data for 316 stainless steel at 440–950°С and liquid lithium, sodium, lead, and sodium–potassium and lead–bismuth eutectics are presented [16].
On the basis of the results of analysis of the data given in Ref. [15], the following important conclusion can be made: the observed spread of data on the solubility of 316 stainless steel elements in a liquid metal coolant is most probably caused by colloid type oxide admixtures. These oxides increase the corrosiveness of liquid metals [17]. Hence, in order to decrease the steel corrosion rate in liquid metal coolants, technological impurities should be removed from liquid metals. This technology proved to be effective as applied to sodium and it has been successfully used in the operating reactors.
It should be noted that steel corrosion in sodium is affected not only by oxygen but also by carbon, hydrogen and even nitrogen cover gases, with the corrosion rate being increased in the case of the presence of both oxygen and hydrogen in sodium. Features of behaviour of the above impurities were studied and special systems and devices were designed to control impurities in the coolant and minimize corrosion of structural materials in the sodium [2, 7].
Corrosion of structural materials in heavy liquid metal coolants can be decreased by formation of the oxide protective coating on the steel surface.
Significant efforts were made to choose structural materials and determine the conditions for their reliable operation. In order to assure these conditions, the technology for lead–bismuth coolant handling was developed, as well as methods and devices for its purification, quality control and maintenance.
However, diffusion of steel elements through the oxide coating is not the only way of steel corrosion product transfer to the coolant flow. It is required that the Sherwood number characterizing the mass transfer from the fuel element cladding subject to front corrosion is much lower than the Biot number evaluating the rate of mass transfer through the oxide coating. Analysis has shown that Fe3O4 does not greatly suppress corrosion of the fuel element cladding in heavy liquid metal coolants. Some other protective film is required in order to reduce significantly penetration of corrosion products into the coolant. Even in the sodium circuits of the BN-600 reactor, corrosion products are accumulated instead of constant operation of cold traps, although the amount of corrosion products penetrating sodium is much lower compared with the amount in the lead–bismuth system at the same temperature. Estimates have shown that in order to eliminate the possibility of formation of oversaturated solutions of corrosion products in the BN-600 sodium circuits, the cold trap capacity should be increased by several orders of magnitude.
As applied to facilities with a lead based coolant, methods of slag oxide reduction using a special gas mixture, including hydrogen, were developed. A
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detailed order of these procedures can be determined in the course of special studies, taking into account the design features of the facilities and their operating modes. Indeed, however perfect the coolant purification system is, suspensions will be formed in the non-isothermal liquid metal circuit under normal operating conditions of the NPP. It is entirely possible that the presence of some suspensions is necessary for decreasing the corrosion rate. It is clarification of all these aspects that forces the designers carrying out such studies to take into account the parameters of advanced NPPs.
Therefore, there are a variety of processes taking place in a coolant–struc-tural materials–cover gas system. Under normal operating conditions, the coolant does not only carry impurities (both dissolved and suspended) along the circuit, but also significantly facilitates their interaction with structural materials and suspensions. Impurities enter coolants in the areas where their chemical potential in the coolant is lower than in structural materials and suspensions in contact with it (reactor core and high temperature downstream core, including inlet section of the intermediate heat exchanger (IHX)). In the circuit areas, where the chemical potential of the impurities in the coolant is higher than in structural materials and suspensions, impurities leave the coolant to enter structural materials and suspen-sions. In the components where coolant temperature decreases (IHX and steam generator (SG)) and critical oversaturation is reached, spontaneous formation of suspensions may occur, followed by their coagulation and precipitation on the steel surface. A detailed description of the processes depends on components of such a system and their shares, which are determined, in turn, by the ratio of intensities of the impurities’ sources and sinks.
Under abnormal conditions (e.g. sodium leak into the atmosphere or water leak into the sodium in the SGs), the range of phenomena expands. The burning process is accompanied by the increase in the local temperature and formation of aerosols. In the case of water leakage into the sodium, the rate of steel corrosion in this area increases by several orders of magnitude. As a result, a small leak grows into a large one and a hot flame is formed that strongly affects the adjacent tubes of the SG, causing their failure [19, 20].
3. SODIUM COOLANT TECHNOLOGY
The most substantial impurities in sodium coolant are oxygen, hydrogen, carbon and their compounds, including products of sodium interaction with water, air and hydrocarbons (lubricant), products of steel corrosion taking place in the course of long term reactor operation (Fe, Cr, Ni, Mn and Mg) and radio-nuclides (including tritium) [2, 7]. Studies were undertaken on the sources of impurities, their intensity and the possible negative effects caused by the
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189 impurities in the course of NPP operation. On the basis of these data, justification of permissible content of impurities in coolant and cover gas was made and the OST State Standard for sodium was developed as applied to the stages of supply and NPP operation [21]. Industrial technology of large scale production and supply of sodium meeting these requirements was adopted.
Oxygen is the most hazardous impurity from the standpoint of corrosion of structural steel. The oxygen content in sodium supplied by the manufacturer should not exceed 50 ppmand in the stage of operation, taking into account possible oxygen penetration into the circuit during repairs and in the case of circuit integrity loss, its content is limited to a 10 ppmvalue. Apart from oxygen, corrosion of structural materials in sodium is caused by carbon, nitrogen and hydrogen.
Proceeding from a 10% permissible decrease in steel strength over the course of reactor operation, 20 ppmand 30 ppmstandards of carbon content are recommended, respectively, for the primary and the secondary sodium. The carbon content in sodium supplied to the NPP from the manufacturer should not exceed 30 ppm.
The influence of hydrogen on steel corrosion is less than that of oxygen, although the combined effect of these impurities increases steel corrosion. On the basis of the approach used for oxygen, 0.5 ppm of permissible hydrogen content in sodium was adopted.
In the early stage of studies on sodium coolant, it was considered that nitrogen did not have any strong effect on the mechanical properties of steel.
However, it was revealed later that steel nitride hardening occurred because of nitrogen present in the cover gas. In this view, the nitrogen content in the cover gas at a temperature of over 300°С was limited by a ~0.3 vol. % value.
In order to effect control of impurities in sodium and cover gas, sampling devices were designed with considerable attention paid to assurance of represent-ativeness of samples and required sensitivity and accuracy of analysis. The following three designs were chosen for use from the large number of sampling devices under study: (i) tubular sampling device, (ii) distilling sampling device and (iii) semi-automatic device for radioactive sodium sampling. The minimum detectable contents are as follows: oxygen (oxide, hydroxide and carbonate forms) – 2 ppm, carbon (non-volatile forms) – 4 ppm, nitrogen (nitride forms) – 1.6 ppm and fluorides – 2 ppm.
Control of activity of radionuclides in the circuit showed that the represent-ativeness and repeatability of analysis results increase with the use of tubular flow sampling devices installed in the primary circuit bypass line and an activity measurement technique without sodium melting.