Passive core cooling system for low power research reactors
The systems of passive removal of the residual heat of nuclear reactors due to the natural convection of the coolant are widely known, which makes it possible to ensure the reliability and safety of nuclear installations when they are cooled down even in difficult situations, for example, with a complete loss of power supply. So why not use this principle as a normal operation system for small research reactors that belong to the so-called 3rd group in terms of potential hazard and do not require forced emergency core cooling systems?
Building new research reactors now takes years and is expensive. And this is due, first of all, to the ever-increasing requirements for the safety of reactor installations, causing the complexity of the design and equipment of the reactor installation. However, the initial concept for the creation and commercial development of research nuclear reactors proceeded from the goals of creating a «safe reactor» TRIGA (Training, Research, Isotopes, General Atomics), so safe that it «could be given to a group of high school students to play with them, without fear of injury.» This safety was built on the negative effects of reactivity with an uncontrolled increase in reactor power and the simplicity of the core cooling system. According to the classification, the proposed reactor, like TRIGA, refers to a pool type reactor that can be installed without a containment and is intended for research and testing used by scientific institutions and universities for such purposes as undergraduate and graduate student education, private commercial research, non-destructive testing and production of isotopes.
The proposed concept of a pool reactor with a barometric cooling circuit is a development of the approach to simplicity and safety of the first TRIGA reactors and is based on minimizing equipment and using passive principles of system operation, primarily a heat removal system that allows cooling the core with a neutron flux level of more than 1.0×1014 cm-2s-1 due to the intensification of the natural circulation of the coolant when it boils in vacuum and the use of natural air convection to transfer the thermal energy of the reactor to the ultimate sink — atmospheric air.
Demand for innovative research reactor designs
Despite the good technical characteristics of TRIGA reactors and their high safety, in order to improve the neutron-physical parameters while improving the design of installations, it was necessary to switch from natural circulation in the core to forced circulation, which somewhat complicated the heat removal system, and, consequently, increased the cost of the installation, the cost of operation and requirements for operating and maintenance personnel. However, if the system of heat removal from the core uses not only natural circulation directly in the reactor tank, but also such a physical phenomenon as the boiling of a heated coolant with a decrease in pressure, the efficiency of coolant circulation through the core can be increased many times. The experiments performed to increase the natural circulation driving head in the circuit with the coolant boiling channel with a decrease in pressure in the upper part of the circulation pipeline showed a high efficiency of using this effect when cooling the core . The use of this effect in pool reactors makes it possible to abandon the use of shut-off and control valves and circulation pumps in the primary circuit while ensuring high efficiency of heat removal from the core, which makes it possible to obtain neutron fluxes at the level neutron flux density over 1×1014 cm-2s-1. With such significant parameters of the neutron flux, the possibilities of using pool reactor plants with natural circulation increase sharply, and, consequently, the scope of their potential application expands.
The concept is based on the following premises:
- the reactor is designed as a neutron source suitable for use in a wide range of scientific and technological areas, fundamental and applied research;
- the design of a reactor facility is built on the priority of «internal safety» rather than «engineering safety»;
- in heat removal systems there is no complex and expensive heat engineering equipment that requires periodic repairs and maintenance;
- low enriched fuel is used;
- in the primary cooling circuit, the minimum level of release of radioactive products from the water surface of the reactor pool is ensured;
- the upward direction of movement of the coolant in the core ensures the stability of natural circulation during power fluctuations;
- the simplicity of design and the minimum cost of engineering systems and structures determine the low cost of the reactor plant and minimal costs when decommissioning the plant;
- the absence of pumping equipment in the cooling circuit reduces dependence on electricity suppliers and increases the safety of the reactor plant;
- compliance with global trends to increase the use of reactors while maintaining all the requirements of nuclear and radiation safety.
The concept is based on the fact that low costs for the construction of a reactor plant and a minimum amount of thermal equipment will allow such reactors to be built, for example, in medical radiological complexes or in industrial complexes for transmutation doping of silicon.
Ensuring optimal circulation conditions in the core
In the case of normal natural circulation in the core of a pool reactor (for example, as in TRIGA reactors), even with a sufficiently deep immersion of the core, a relatively small difference in the densities of the heated and cold coolant in the pool does not make it possible to provide a high pressure of natural circulation, sufficient to provide an acceptable coolant velocity in fuel assemblies for transition to the turbulent regime in order to increase the heat transfer coefficient. This is necessary to reduce the temperature difference between the fuel rod cladding and the coolant (increase the heat transfer coefficient from the fuel rods), which ensures that the fuel cladding temperature drops below the boiling point, and for the core of the pool reactor under low pressure, the boiling point is low (~ 115 … 130 ° C) which is usually limits the power of this type of reactor. In addition, the rise of the coolant heated in the core to the surface of the pool creates problems with the release of radioactive gases into the reactor room. Therefore, most often these problems are solved by switching to forced circulation through the core, by extracting the coolant from the subzone space, thus creating a downward flow in the FA. With a sufficiently high flow rate of the coolant, a turbulent mode of motion in the inter-fuel space is provided, which makes it possible to increase the power of the core without the risk of boiling on the surface of the fuel elements. However, when forced flow is stopped (stop flow), the conditions for heat removal from the core deteriorate sharply, which creates risks of overheating and destruction of fuel elements. the rise of the coolant heated in the core to the surface of the pool creates problems with the release of radioactive gases into the reactor room. Therefore, most often these problems are solved by switching to forced circulation through the core, by extracting the coolant from the subzone space, thus creating a downward flow in the FA. With a sufficiently high flow rate of the coolant, a turbulent mode of motion in the inter-fuel space is provided, which makes it possible to increase the power of the core without the risk of boiling on the surface of the fuel elements. However, when forced flow is stopped (stop flow), the conditions for heat removal from the core deteriorate sharply, which creates risks of overheating and destruction of fuel elements.
It is possible to completely eliminate the risks of fuel overheating in almost any regime by using a special coolant circulation scheme, which ensures unimpeded lifting movement of the coolant in the core in all modes, however, the heated coolant does not enter the reactor pool, but is discharged through the side branch of the vertical pipe above the core ( «chimney») and then goes to the heat exchanger. This makes it possible to strictly limit the natural circulation path to channels in pipelines, which makes it possible to increase the driving pressure due to the boiling of the coolant and reduces uncertainties in the calculation analysis and sharply limits the release of radioactive gas products from the surface of the pool.
It should be noted that the potential for increasing the specific power of the core increases significantly when using fuel rods, the operating conditions of which allow surface boiling. In this case, the allowable density of the heat flux from the surface of the fuel elements can be increased, and an excessive increase in the temperature of the coolant heated in the fuel assembly, if necessary, can be corrected by increasing the «cold» bypass flow. Thus, the potential application of the proposed scheme of heat removal through the barometric circuit makes it possible to estimate the achievable values of the neutron flux density in the core at the level of 1×1014 cm-2 s-1 and higher.
Barometric cooling circuit
The fundamental difference between the natural circulation barometric circuit in a pool reactor and conventional natural circulation circuits used, for example, in TRIGA reactors, is that a significant increase in the coolant flow through the core is ensured by using an additional and strong physical effect of boiling under reduced pressure to increase the difference between the average coolant densities in the up-flow and down-flow sections of the natural circulation circuit, which determines the generated driving head.
To ensure the effect of boiling of the cooled coolant in the rising section of the circulation circuit, it is enough to reduce the pressure (create a vacuum). This is easy to implement by raising the circulation loop above the water level in the reactor pool using circulation pipelines and using a vacuum system.
The filling of the natural circulation barometric circuit is carried out under the influence of atmospheric pressure when a vacuum is created in the upper part of the circulation circuit connected to the vacuum system. In a filled circuit, when the coolant heated in the core rises above the water level in the reactor pool, its pressure in the pipeline decreases. With an increase in the rise height of the coolant in the pipeline and with a corresponding increase in the vacuum to the saturation pressure of the coolant heated in the core, it boils over the entire volume in the upper part of the pipeline and therefore the density of the coolant flow there sharply decreases, since the main share in this flow (over 80% ) occupies the vapor volume. At the top of the barometric circuit is a heat exchanger, which condenses all the steam formed in the lifting section and partially cools the coolant, therefore, in the down-flow section, the coolant density is maximum.
Because of this, the difference in hydrostatic pressures in the upper lifting and lowering sections of the circuit increases, which causes a high driving head of natural circulation.
A feature of the barometric circuit is that it is rigidly tied to the circulation circuit through the core, and an increase in the driving pressure due to more efficient boiling leads to an increase in the flow through the core. This makes it possible to provide a positive feedback between the heating in the core and an increase in the natural circulation flow rate, and this relationship, through an increase in the driving pressure, makes it possible to increase the coolant flow rate until the change in the flow regime in the core — the transition from laminar to turbulent. In this mode, due to the increase in the heat transfer coefficient, it is possible to significantly increase the energy release in the core without switching to surface boiling on the fuel elements. This is especially important in the case of using, for example, dispersion-type fuel elements with an aluminum alloy cladding.
When implementing a barometric circuit, a scheme can be implemented with both lifting and lowering movement in the core (Figure 1).
Figure 1 — Schemes of organization through the core when constructing a barometric contour: a — a scheme with lifting movement in the core and the organization of a «chimney» with a side outlet; b — scheme with downward movement of the coolant in the core
Each of these options has its own advantages and disadvantages, which are discussed below.
The combined use of the “chimney” design with the outlet pipeline and the boiling section in the upper part of the pipeline with the upward flow of the coolant provide the barometric circuit with the utmost simplicity and reliability.
To organize ascending circulation through the fuel assembly, a cavity is created under the core, into which a cooled coolant is supplied. This cavity is directly connected to the core, so almost all incoming coolant is forced to pass through the core, with the exception of a small fraction that enters through the bypass hole from the cavity directly into the reactor pool. By adjusting the ratio of the hydraulic resistances of the core and this hole, the corresponding ratio of coolant flow rates is also set. The coolant that has entered the reactor pool through the hole is also directed through the upper part of the chimney to the side outlet and then to the rising section of the barometric circuit. Since the coolant entering through the upper part of the chimney is not heated, when mixed with the heated coolant from the core, it reduces the temperature of the mixed flow, and this can be used to control the optimal temperature of the mixed flow by changing the hydraulic parameters of the bypass hole.
With this design, even when switching to the surface boiling mode on fuel elements, the system remains stable, since boiling in the core only intensifies the circulation through the fuel assemblies. Therefore, for this variant, the achievable specific power in the core, and, consequently, the neutron flux density, can be significantly higher than in the second variant with downward movement of the coolant in the core. However, a significant disadvantage of such a circulation scheme is that it is difficult to quickly calculate the power according to thermal parameters: the flow through the core and the heating of the coolant in it. The determination of the exact coolant flow through the core is affected by the uncertainty in the value of the bypass flow, and the determination of the exact value of heating in the core is affected by the uneven energy release across the fuel assemblies and the impossibility of determining the average coolant temperature at the core exit.
The scheme of the barometric circuit with a downward flow of coolant in the core seems to be simpler and does not contain the uncertainties associated with the flow bypassing the core, as in the first variant. Thermal power can be quickly controlled by flow in the circulation loop and heating in the core, and there are no problems associated with transport and handling operations that the chimney design creates. However, this simple variant of the barometric circuit has a major drawback that negates its merits. This shortcoming is associated with the instability of operation during a power surge, which causes boiling in the core and a circulation reversal from downward to upward movement. IIn this case, the circulation of the coolant in the barometric circuit stops, and the circulation between the cells in the core may be insufficient for safe heat removal, even if the emergency protection worked normally and the reactor went into a subcritical state.
In addition, for this option, the mode of starting natural circulation is much more complicated than for the first option, therefore, despite certain technical problems associated with the presence of a “chimney”, a barometric circuit with an upward movement of the coolant in the core is certainly preferable.
Physical basis of the process of increasing the efficiency of natural circulation
The essence of the applied effect of volumetric boiling of the coolant in the circulation circuit is based on the dependence of the boiling point of water on pressure. In this case, when organizing a circulation loop through the reactor pool through pipelines (Dy ~ 300 mm) and heat exchangers that are located above the water level in the reactor pool and connected to a vacuum system, when the coolant is heated in the core and when it rises in the circulation pipe above the water level in the reactor tank, it begins to boil intensively, reaching a certain height. This height practically corresponds to the saturation pressure at the temperature of the heated coolant, measured in meters of water column. Due to volumetric boiling in the circulation pipeline, the density of the steam-water flow is sharply reduced. After the coolant rises in the circulation pipeline to a height of ~ 9 … 10 meters, the steam-water flow enters the heat exchanger, in which the vapor is effectively condensed and the condensate is cooled below the saturation temperature in the receiving chamber, into which the cooled condensate is drained and which is connected to a vacuum system that maintains the vacuum in the receiving chamber at the required level (Figure 2). In the lower section, the cooled condensate no longer boils, as it does at the same height of the ascending section. This ensures a large difference in coolant densities in the upper section of the natural circulation circuit, due to which the natural circulation driving head increases sharply, providing high flow rates and speeds in the core. into which the cooled condensate is drained and which is connected to a vacuum system that maintains the vacuum in the receiving chamber at the required level (Figure 2). In the lower section, the cooled condensate no longer boils, as it does at the same height of the ascending section. This ensures a large difference in coolant densities in the upper section of the natural circulation circuit, due to which the natural circulation driving head increases sharply, providing high flow rates and speeds in the core. into which the cooled condensate is drained and which is connected to a vacuum system that maintains the vacuum in the receiving chamber at the required level (Figure 2). In the down-flow section of the circulation circuit, the cooled condensate no longer boils as it does at the same height in the upstream section. This ensures a large difference in coolant densities in the upper section of the natural circulation circuit, due to which the natural circulation driving head increases sharply, providing high flow rates and speeds in the core.
Limiting simplification of the heat removal system of a reactor plant
Despite the fact that the primary circuit does not contain equipment that depends on the power supply and requires periodic maintenance and control, the presence of the second circuit can also complicate the operation of the reactor plant, especially if heat is removed to the final sink through evaporative cooling (cooling towers, spray ponds, etc.). First of all, this is due to the water treatment of the secondary circuit and the accumulation of hardness salts in the coolant, which impede the operation of the heat exchanger.
To solve the problems of water treatment and the problems of accumulation of hardness salts due to the evaporation of distillate, the second circuit can be performed using dry coolers (drycooler), however, in this case, it requires pumps, valves, reliable power supply, as well as qualified maintenance of a rather complex secondary circuit equipment.
Due to the relatively small power of the reactor plant, the problem of heat removal from the primary circuit can be easily solved if atmospheric air is chosen as the coolant of the secondary circuit and, at the same time, the ultimate heat sink. It is proposed to use a large ventilation pipe as a source of driving force for the circulation of atmospheric air through the air heat exchanger. With this design of the reactor cooling system, practically the only element of heat removal control remains the regulation of the flow of cooling air through the heat exchanger in order to maintain the desired temperature of the cooled coolant. This regulation can be carried out, for example, using unified air valves (UAV) with an electric drive.
The entire heat removal system from such a reactor plant is shown in Figure 3. It includes the reactor pool (1) with the active zone located in it, surrounded by a reflector (2). A vertical pipe (“chimney”) with a side outlet of the heated coolant (3) is placed above the active zone, providing organized circulation through the lifting pipeline (4). In the upper part of this pipeline there is a section of volumetric boiling up of the heated coolant (5). Further, the steam-water flow is distributed over 4 air heat exchangers (6), and the cooled condensate from these heat exchangers is drained into a receiving tank (7) connected to a vacuum system (8). The cooled condensate is sent through the downcomer pipeline (9) to the closed space under the core (10), from which most of the cooled coolant is sent to the fuel assembly, and a small part through a small hole enters directly into the reactor pool, creating a bypass (relative to the core) flow. This flow from the reactor pool enters the vertical pipe (3) from above and, after mixing with the heated coolant from the core, is directed through the side outlet to the lifting pipeline (4).
Figure 3 — Heat removal system of a pool reactor with a barometric circulation system in the primary circuit and an exhaust ventilation stack
The air cooling circuit is a key element that ensures the simplicity of the heat removal system of the considered reactor plant. With the proposed design, this circuit removes heat from the primary coolant in 4 air horizontal heat exchangers with overall dimensions of 5.0×5.0×0.22m, located in a two-story building with a ventilation pipe (Figure 4).
Figure 4 — Air cooling circuit with exhaust vent pipe
The coolant heated in the core is supplied to the building of the air cooling system through the lifting pipeline of the primary circuit, and the cooled coolant is discharged back to the pool through the downcomer pipeline. The heated coolant begins to boil at a height of 7…8 m relative to the water level in the reactor pool due to the vacuum created in the pipeline by the vacuum system, the coolant flow density in the pipeline decreases sharply by ~ 80% due to volumetric boiling, which ensures an increase in the driving pressure of natural circulation .
The steam-water flow is distributed over 4 air heat exchangers and enters their inlet collectors with a flow area of 0.23 × 0.23 m and a length of 5.2 m (item 1, Figure 5). Perpendicular to the flat surface of the collectors, from them, towards the outlet collector, 100 flat tubes with a flow section of 0.22 × 0.05 m and a length of 5.0 m are diverted (pos. 2, Figure 5). The pitch between these tubes is 50 mm, and the space between the side planes of the tubes is filled with a 0.8 mm thick zigzag aluminum strip with a 4 mm pitch between the ribs (pos.3, Figure 5).
Figure 5 — 3D model of an air heat exchanger 5.0 × 5.0 × 0.22 m
1 — input (output) collector; 2 – rectangular heat exchange tubes; 3 — finning in the form of a curved plate
The curved plates (pos.3, Figure 6) have good thermal contact with the heat exchange tubes (pos.2, Fig. 6), which makes it possible to effectively remove heat from them even at a low heat transfer coefficient to air. The heat transfer coefficient from the steam-water mixture to the inner surface of the heat exchange tube during steam condensation is very high, but it decreases sharply when switching to heat transfer from condensate moving in the tubes in a laminar mode. Therefore, the degree of condensate cooling is easily regulated by increasing or decreasing the vacuum in the heat exchanger, which sets the desired temperature level of the steam-water mixture. The degree of condensate cooling is also regulated by the intensity of the cooling air flow, which is selected depending on the ambient air temperature — the lower the temperature, the less air flow is required.
The air flow rate is regulated by the signals of maintaining the set temperature of the cooled coolant, which is carried out by an electric drive by opening or closing louver-type air dampers installed on the first floor of the air cooling system building.
The efficiency of heat transfer in heat exchangers to the final heat sink — atmospheric air is ensured by a large contact area due to the finning of the heat exchange tubes with zigzag aluminum strips, passing along which the air is heated to the water temperature in the heat exchange tubes in a wide range of air circulation speeds through the heat exchanger.
Figure 6 — Geometric parameters of the air heat exchanger 5 × 5 × 0.22 m
1 — inlet (outlet) collector 220 mm high; 2 — heat exchange tubes of rectangular shape 210×10mm with a wall thickness of 2.5 mm; 3 — fins in the form of a curved plate 0.8 mm thick, 210 mm high and 4 mm apart, closely adjacent to the side surfaces of the heat exchange tubes, located at a distance of 40 mm.
For an example of determining the calculated values of the required air flow rate for cooling the primary coolant, a thermal calculation of a typical heat exchanger cell is given, carried out using SolidWorks / FlowSimulation  when removing a heat power of 2 MW from 4 heat exchangers at a temperature of the steam-water mixture inside the tubes of 40˚С, an atmospheric air temperature of 25 ˚С and air consumption 142 m3/s. The design parameters for this case are shown in Figure 7.
Figure 7 — Changes in air parameters (temperature, velocity and pressure) when passing through a typical cell of an air heat exchanger with a heat output of 2 MW: the temperature of the steam-water mixture inside the tubes is 40˚С, the ambient air temperature is 25˚С, the air flow rate is 142 m3/s
A large air passage area (with the considered parameters, 4 heat exchangers occupy an area of 10 m2) ensures the passage of a large mass air flow, which in the cold season leads to too much cooling, or even freezing of the coolant. In this case, it is necessary to regulate the air flow using air valves, for example, louver type. Rotation of the blinds by a control signal from the control panel changes the hydraulic resistance of the air flow circulating through the exhaust pipe, providing regulation of its flow and the degree of cooling of the primary coolant. In fact, this regulation of the air flow is the only mechanism for regulating the heat removal from the reactor plant.
The flow diagram of the primary and secondary coolant flows in the air cooling system building with a chimney is shown in Figure 8.
Figure 8 — Parameters of air flow circulation through the ventilation pipe with a power output of 2 MW and air heating in heat exchangers from 20 to 42˚С
In the lower part of the building, louver-type air dampers are shown, which regulate the air flow to ensure the specified cooling of the primary coolant.
A fan can be used to reduce the capital cost of building a large ventilation stack to circulate outside air (Figure 9).
Figure 9 — Heat removal system of a pool reactor with a barometric circulation system in the primary circuit and a fan exhaust air
The use of a fan to create forced air circulation increases the dependence on power sources, but at the same time provides additional opportunities for regulating the air flow and, accordingly, regulating the temperature of the cooled coolant.
Particular attention should be paid to how the problem of removing non-condensable gases from the heat exchanger, which significantly impair heat removal, is solved. This is especially true of the starting mode, when at the initial moment the heat exchanger is filled with air. To remove gases from the outlet manifold, it has a branch in the form of a tube connecting the upper part of this manifold with a receiving tank connected to the vacuum system.
More detailed Figures 10 and 11 show the units for discharging the cooled coolant from the heat exchanger to the receiving tank, as well as the units for blowing off non-condensable gases from the heat exchangers.
Figure 10 — Scheme of flow through air heat exchangers
For stable operation of the natural circulation circuit, it is necessary to ensure equality between the driving pressure (determined by the boiling channel) and the total hydraulic resistance of the natural circulation circuit (determined mainly by pressure losses in the core).
With a large diameter of the circulation pipelines (~ 300 mm), a low circulation speed in them (~ 0.25 m/s) and the absence of fittings, the hydraulic losses in them are very small (several tens of Pascal), and taking into account the large flow sections of the heat exchangers, they also do not make a significant contribution to the hydraulic resistance of the natural circulation circuit, to compensate for the excess driving pressure, a curved tube with a narrowing at the drain from the outlet manifold to the receiving tank is used as additional hydraulic resistance.
This tube simultaneously acts as a hydraulic seal and local hydraulic resistance, the passage of the jet through which provides additional pressure loss and equalizes the driving pressure and the total hydraulic resistance of the natural circulation circuit.
Figure 11 — Scheme of discharge of the cooled coolant and purge of non-condensable gases from the air heat exchanger to the receiving tank
In this case, an increase in pressure in the outlet manifold of the heat exchanger leads to the fact that the water level in the tube designed to remove the gas increases. Since this tube is located at the top of the heat exchanger, if non-condensable gases accumulate in the heat exchanger, they accumulate in the area of this tube and periodically enter the receiving tank. Particularly important is the role of the gas outlet tube in the start-up mode, when the steam-water mixture has already partially filled the heat exchanger, the water seal is blocked for the passage of gas by the condensate that filled it. In this case, the exhaust pipe becomes the only channel for the removal of non-condensable gases and ensuring the normal operation of the heat exchanger.
Transition mode of reactor output to rated power
One of the main problems in the operation of a reactor plant with a barometric circuit is the transitional mode of bringing the reactor to its nominal power. Unlike conventional pool reactors with natural circulation, where it was enough to turn on the pool water cooling circuit and reach the nominal power level, or unlike reactors with forced circulation through the core, where it was enough to turn on the primary circuit circulation pump, in the heat removal system barometric reactor, there are no circulation pumps. Therefore, in order to start natural circulation, it is necessary to go through two stages.
At the first stage, the reactor is brought to a power of 100…200 kW to warm up the reactor pool to 60…65°C through the normal natural circulation mechanism.
At the second stage, after the coolant in the subzone space is warmed up to ~60°C, water with a temperature of ~35..40°C is supplied to the descending pipeline, and the upper part of the circulation circuit with the heat exchanger located there is connected to the vacuum system. In this case, the circulation pipelines will be filled with water from the pool, as a result of which the level in it will slightly decrease. When the circulation pipelines are filled, the supply of the downcomer pipeline with relatively cold water stops.
When heated water rises from the pool in the lifting pipeline to a level of ~7 meters above the water surface in the pool, the pressure in the upper part of the pipeline will drop to a pressure corresponding to a saturation temperature of ~60°C, due to which volumetric boiling will begin in the coolant. When the coolant with a lower temperature (~40°C) rises in the descending pipeline, it will not boil, since this temperature is lower than the saturation temperature in the created vacuum.
When the vacuum in the heat exchanger reaches ~8.5 kPa (which corresponds to a saturation temperature of 42.7°C), the steam-water mixture enters the air heat exchanger, in which the steam is intensively condensed, and the condensate is cooled to a temperature of ~ 40..41°C and drained through the water seal into a receiving chamber connected to a vacuum system. When the receiving chamber is filled with cooled coolant, the level in it rises, and the excess cooled coolant is automatically drained back into the reactor pool. In order to reduce fluctuations in the flow rate of the coolant in the up and down sections of the pipelines (Dy ~ 300mm) of the circulation circuit, they have extended horizontal sections that increase the mass of the coolant and increase the inertia of the coolant flow. In addition, the length of the lifting section of the pipeline and the duration of transportation provide the function of an oxygen activity quencher due to an increase in the delivery time of the heated coolant to the air heat exchanger, so the radiation background near the heat exchangers is significantly reduced.
Improving the thermal reliability of the reactor plant
The principles of operation of the heat removal system incorporated in the concept of a barometric pool reactor make it possible not to consider most of the initiating events recommended by regulatory documents associated with failures of heat engineering equipment due to their absence. Apart from the control and protection system (CPS), the regulation of the heat removal mode is carried out by changing the flow rate of atmospheric air passing through the air heat exchanger using electrified air valves, as well as changing the vacuum in the heat exchanger and in the receiving tank.
Optimization of the heat removal system of a research pool reactor due to the utmost simplification of the design of the cooling circuits in combination with inherent internal safety due to the lack of pressure in the systems, negative reactivity effects, to a dangerous decrease in heat removal from the core, no need for a reactor plant containment, low enrichment (less than 20% U-235) and a relatively small amount of fuel in the core — provide a high level of safety at a low cost of a reactor plant. At the same time, the simplicity and compactness of installation, as well as a low level of radiation emissions due to a special coolant circulation scheme, they allow you to quickly assemble and, if necessary, quickly dismantle. In this case, the main source of radiation contamination will not be the reactor plant itself, but the irradiated materials.
In case of violation of normal operation conditions, emergency protection is automatically triggered, and after the input of absorbing working bodies, residual heat is removed by natural circulation of the coolant directly to the pool with a sharp decrease in temperature on the fuel elements. In this case, the lifting movement of the coolant in the core becomes less intense, but does not stop.
The considered heat removal system is quite versatile and can be applied to various core configurations with different types and number of fuel assemblies, however, it is preferable that it be a compact core, above which there is a short vertical tube with a side outlet, and the irradiation volumes are located mainly in the neutron reflector , which can be made, for example, of beryllium, lined with graphite or in the form of a heavy water tank. The maximum neutron flux can be provided in the central neutron trap located in the center of the active zone. In it, for example, it will be possible to obtain activation (radiation-capture) Mo-99 (technetium generator), and if the reactor plant is built near a large medical center, the production of isotopes (especially short-lived ones) will be most profitable.
The concept of the reactor facility under consideration assumes that the main capital costs will fall not on the facility, but on technological and experimental equipment that ensures the fulfillment of the assigned tasks, which can be of the most diverse nature (nuclear physics, solid state physics, radiation materials science, neutron activation analysis of matter, neutron radiography of various products, radiation doping of silicon, production of isotopes for medical industrial purposes, etc.).
Calculation estimates for a reactor plant with a barometric circuit
To carry out the computational analysis, the design of a pool-type research reactor with a barometric cooling circuit was chosen, using demineralized “light” water as a coolant and moderator, and metallic beryllium as a reflector (Figure 12). This core is chosen as an example and is made up of 72 fuel assemblies of the VVR-M2 type (Figure 13), in the center of the core there is a neutron trap with the maximum neutron flux density. In the core, there are also operating elements of the control and protection system: reactivity compensation, automatic control and emergency protection. The beryllium reflector contains vertical channels for irradiation.
Figure 12 — Considered in the analysis of the cartogram of the core of the reactor with a beryllium reflector of the reactor
It should be emphasized that the core and the reflector can have any configuration required by the customer, based on the tasks set for such a neutron source, as well as the availability of the selected fuel assembly type and reflector materials. The presented analysis concerns, first of all, not the neutron-physical characteristics, but the possibility of implementing the barometric scheme of the cooling circuit in the pool reactor.
Figure 13 — External view of fuel assemblies of type VVR-M2
An approximate hydraulic characteristic of the core of 72 VVR-M2 fuel assemblies is shown in Figure 14.
Figure 14 — Hydraulic characteristic of the core with 72 fuel assemblies
The main technical parameters of the considered reactor plant are given in Table 1.
The analyzed reactor plant with a power of up to 3000 kW has a completely passive two-loop heat removal system, the ultimate heat sink for which is atmospheric air. The direction of circulation of the coolant through the core is lifting, the use of a «chimney» with a side outlet.
To cool the coolant and transfer heat to the final absorber, 4 air heat exchangers 5.0 × 5.0 × 0.22 m, described above, are used. Computational studies of the efficiency of such heat exchangers were carried out using SolidWorks/FlowSimulation. The average temperature of the air heated in the heat exchanger is determined by the output power and air flow:
For normal operation of the reactor plant, the air flow must be provided by natural air convection through the exhaust ventilation pipe. For the selected parameters of the (ventilation system with 12 rectangular openings 2.3 × 1.3 m with adjustable louvre valves for supplying atmospheric air, 4 heat exchangers 5.0 × 5.0 × 0.22 m, a pipe 74 m high, a diameter of the passage section 6 3 m in the lower part and 5.2 m in diameter in the upper part) using SolidWorks/FlowSimulation, estimates of the achievable air flow rates during its heating were carried out. To do this, at various values of the atmospheric air temperature and the given temperature of the heating surfaces of the heat exchangers, the parameters of natural circulation were calculated for the selected geometry of the air circulation circuit.
After processing the calculated data on the achievable flow rates of cooling atmospheric air at its various temperatures, to assess the efficiency of air cooling, graphs of the parameters of the cooling circuit for a reactor power of 1.0 MW (Figure 15) and 1.5 MW (Figure 16) were plotted, indicating the required and achievable air flow rate (marked with circle markers in the graphs), coolant temperature at the outlet of the reactor tank (pool), as well as the maximum temperature at the fuel rods and the saturation temperature in the core (to assess the margin to surface boiling).
As can be seen from the graphs, the selected parameters of the air cooling circuit with 4 heat exchangers 5.0 × 5.0 × 0.22 m ensure the removal of heat power up to 1.5 MW with a margin at an ambient temperature of up to 25°C. At a higher ambient air temperature or a higher reactor power, in order to ensure heat removal, it will be necessary to maintain either an increased temperature in the reactor pool and, accordingly, at the inlet to the heat exchanger, or it will be necessary to increase the heat transfer surface in the heat exchangers by changing its dimensions.
Non-Stationary Analysis of Power Maneuvering in a Reactor Plant with a Barometric Cooling Loop
A two-loop heat removal system of a pool reactor with a natural circulation barometric loop and use of atmospheric air as the final absorber is considered, with heat transferred to the air directly through the primary loop heat exchanger.
The complete absence of circulation pumps and shut-off and control valves makes such a system independent of power supply sources, and, therefore, very reliable. The high intensity of natural circulation in the primary circuit through Dy300 pipelines makes it possible to provide a turbulent mode of coolant movement in the core, and, accordingly, a high heat transfer coefficient from fuel elements. Therefore, even at a low boiling temperature of the coolant in the core at a depth of 10 m, which is 120°C, the condition for the absence of surface boiling on the aluminum cladding of tubular dispersion-type fuel elements is ensured.
Despite the fact that the reactor is a pool reactor, the design of the reactor cooling loop is convenient for modeling in the thermal-hydraulic code of the improved RELAP5/MOD3.2 estimation. This code is based on a one-dimensional, two-fluid model of a steam-water mixture. The model considers the phases of steam and water. The concept of a two-fluid model implies that for each phase, as for a separate liquid, the continuity equation, the momentum conservation equation, and the energy conservation equation are written. Each phase has its own speed and temperature, i.e., in general, vapor and liquid are not in mechanical and thermal equilibrium with each other.
The nodalization scheme of the reactor cooling circuit is shown in Figure 17.
Figure 17—Nodalization diagram of a reactor plant with a barometric cooling circuit
The core channels are modeled by 7 groups of fuel assemblies (Pipe 111…Pipe 117) with thermal structures and differing in energy release. The confined space in the pool below the core is represented by component BR-001, and the space directly above the core is represented by component BR-002. From BR-002, the total coolant flow from the core is fed into the so-called «chimney» (Pipe — 022), which is a vertical pipe open from above with a side branch to the Pipe-160 lifting pipeline. The rest of the basin includes the Pipe-103 components (water at the level of the core and the “chimney”, connected by a hole to the subzone space BR-001), the middle part of the basin (component BR-133), connecting the lower part of the basin (BR-103) , the top of the pool (Pipe-134) and the top of the «chimney» (Pipe-022).
The coolant heated in the core is fed through the Pipe-160 pipeline in the form of a steam-water mixture (due to volumetric boiling at low pressure) into the pipe space (Pipe-165) of a horizontal air heat exchanger located at a height of ~9 meters above the water level in the pool. The upper part of the downcomer channel of the coolant (Pipe-190) has an extension and is a receiving tank into which the coolant cooled in the heat exchanger is drained. The specified vacuum in the receiving tank is maintained through the Pipe-650 pipeline from the TV-700 vacuum system. Through the downcomer pipeline, the cooled coolant enters the closed subzone space BR-001. From which the main part of the coolant is sent to the core, and the rest — to the lower part of the reactor pool.
The second circuit, which is a cooling air flow circulating under the influence of the driving pressure of natural convection, is described by the time-dependent components TV-201 and TV-230, as well as the channels Pipe-210 and Pipe-220.
To simulate the natural circulation processes under consideration with volumetric boiling of the coolant due to a decrease in pressure in the upper part of the lifting pipeline, it is very important that the maps of two-phase flow regimes in vertical channels are programmed in the RELAP (dispersed, stratified, annular, annular-dispersed, slug, wavy-slug, mist-annular). This allows you to simulate dynamic processes with a decrease or increase in the temperature of the heated coolant.
The change in the thermal-hydraulic circulation parameters in the barometric circuit was studied using the RELAP5/MOD3.2 code for two modes:
- reactor output to a power of 3 MW and long-term operation at this power;
- reaching the reactor at a power of 1 MW and maneuvering the power from 1 to 2 MW within ~5.5 hours of the estimated time.
Figures 18.1–18.18 on the left side show graphs of the results of calculations for the first mode, and on the right side — for the second mode.
But before bringing the reactor to a high power level (Figures 18.1, 18.2), it is necessary to carry out preparatory work to heat the coolant in the pool to a temperature of ~60°C, which is achieved by operating the reactor at a low power level (for example, 200 kW) with natural circulation of the coolant in pool. This is done in order to ensure the volumetric boiling of the coolant during discharge in the upper part of the lifting pipeline and start circulation. A vacuum of up to 9 kPa (Figures 18.3, 18.4) is created by connecting a vacuum system to the upper part of the lifting section of the barometric circulation circuit, which creates a predetermined vacuum for 200 s, which contributes to filling the circuit with pool water. At the same time, in order to exclude the process of boiling of the coolant when filling the downcomer pipeline, cold water is supplied to it, temporarily reducing the temperature of the coolant there to ~35°C.
In the initial period of launching the barometric circulation circuit, when during the first ~ 400s the process of filling the circulation pipelines occurs, the coolant flow in them reaches 100 kg/s in the downcomer pipeline and over 100 kg/s in the lifting pipeline (Figures 18.5, 18.6). Also during this period, large coolant flow rates are recorded through the core and the upper part of the “chimney”, and the coolant flow through the upper part of the “chimney” is significantly higher than through the core, since the hydraulic resistance there is minimal (Figures 18.7, 18.8).
After the reactor is brought to a high power level, which ensures intensive circulation of the coolant due to boiling in the upper part of the circuit, the coolant cooled in the heat exchanger enters the core and the reactor pool to compensate for the removal of the coolant to the upper part of the «chimney». Therefore, the temperature of the water in the pool gradually decreases and within about an hour reaches an equilibrium state with the temperature of the water leaving the heat exchanger (Figures 18.9, 18.10, solid line). A decrease in the temperature in the pool leads to the fact that the temperature of the flow discharged into the circulation loop (Figures 18.9, 18.10, dash-dotted line) is noticeably lower than the temperature of the coolant at the core outlet (Figures 18.9, 18.10, dotted line).
Figures 18.11 and 18.12, in addition to core inlet and outlet temperatures, also show the corresponding maximum fuel element temperatures in fuel assemblies (Figures 18.11 and 18.12, dotted line). Despite the fact that these temperatures can exceed the saturation temperature in the core, in the considered regimes, surface boiling of fuel elements does not occur, since the calculated heat transfer coefficient is high due to the turbulent regime in the FA.
The turbulent regime in the fuel assemblies is provided by a high coolant flow through the core, which, in turn, is associated with a high driving head of natural circulation, defined as the difference in hydrostatic pressures in the ascending and descending sections of the barometric contour (Figures 18.13 and 18.14, solid and dash-dotted lines ).
The change in the considered modes of heat flux density along the high-rise sections of the maximum heat-stressed fuel element is shown in Figures 18.15 and 18.16. As can be seen from these graphs, the maximum heat flux density in the first mode at a power level of 3 MW reaches 340 kW/m2, and in the mode with power maneuvering it reaches 225 kW/m2. The corresponding fuel temperatures for the high-altitude sections of the maximum thermally stressed fuel element are shown in Figures 18.17 and 18.18.
The analysis carried out using the RELAP5/MOD3.2 code shows the operability and efficiency of the barometric circuit, which makes it possible to simplify the cooling system of the pool reactor as much as possible and minimize the amount of heat engineering equipment that requires periodic maintenance, which increases dose loads on personnel. Sufficiently high neutron fluxes allow effective irradiation in channels at minimal operating costs.
Examples of using a 3 MW reactor plant
As an example of the use of a reactor plant, the production of isotopes for medical purposes is considered — obtaining activation 99Mo according to patent RU 2703994 C1 . In the proposed method, the target is irradiated at a temperature of 20°C to 100°C. The method is carried out as follows:
Molybdenum hexafluoride of a natural isotopic composition or enriched in the Mo-98 isotope is placed in a metal ampoule by condensation, the ampoule is welded, placed in a protective metal container and irradiated in a neutron flux of 1⋅10 8 — 1⋅10 15 n / (cm2⋅s) for from 1 to 15 days. The irradiated target is transferred to a hot chamber for 1 day, opened, connected to a vacuum system, and gaseous molybdenum hexafluoride is condensed. The target is disconnected from the vacuum system and filled with a calculated amount of an alkali solution of NaOH with a concentration of 0.2 — 0.3 M. Upon irradiation as a result of neutron capture by 98 MoF6 target nuclei, the 99Mo nuclei formed are initially in an excited state. When the excitation is removed by emitting instant gamma rays, some of the recoil atoms (99Mo) receive an impulse sufficient to break chemical bonds with the removal of the fluorine atom and the formation of lower molybdenum fluoride, which is deposited on the walls of the target. After the irradiation is completed and the bulk of the molybdenum hexafluoride is removed, the non-volatile components resulting from the activation of 98MoF6 dissolve in alkali to form sodium molybdate Na298MoO4 . The resulting solution is used to charge generators. The specific activity of Mo-99 in the resulting solution at the time of manufacture is from 10 to 5000 Ci / g, depending on the magnitude of the neutron flux and the exposure time.
At a neutron flux density above 1×1015 n/(cm2s), a high efficiency of 99Mo accumulation is expected, and what is especially important, with this technology it will be possible to avoid a high consumption of 235U to obtain fragmented 99Mo and the associated severe technological problems in its extraction and handling with high-level waste generated during the decay process.
The basis of radiation doped silicon technology is the nuclear transformation of atoms under the influence of thermal neutrons, namely, the neutron transmutation transformation of ³⁰Si into phosphorus as a result of irradiation with thermal neutrons in a nuclear reactor. The fundamental point of neutron transmutation doping is that dopants are not introduced into the source material from the outside, but are formed during irradiation directly from the atoms of the doped material. The method is based on nuclear reactions that take place in a silicon crystal. Under the influence of a thermal neutron flux, the formation of a radioactive isotope ³¹Si and its subsequent decay with the formation of stable phosphorus ³¹P occurs. The resulting ³¹P creates n-type conduction.
Radiation-doped silicon is used to manufacture power semiconductor electronics and special-purpose devices of increased reliability and quality. The scope of radiation-doped silicon is extremely wide: power semiconductor devices, DC inserts for converting AC to DC, power photoelectronic converters for solar power plants, powerful diodes and thyristors for electrified rail and road transport, high-voltage and high-current semiconductor devices for nuclear physics and electronics, in electronic measuring instrumentation; in photoelectronic energy converters, in optical engineering systems, etc. Thus, the most relevant use of the semiconductor properties of silicon in various electrical devices and devices that play a crucial role in all areas of electrical engineering, electronics and communications.
The extensive world experience accumulated in the operation of facilities for the production of radiation doped silicon, including in pool reactors , allows us to hope for the creation of an efficient facility in the reactor under consideration, especially since the low cost of such a reactor facility makes it possible to make it specialized and pay special attention to technology automation. silicon irradiation.
The discussion of the results
- The concept of a pool research reactor with a barometric circuit using natural circulation is presented. The advantages of creating a simple and reliable passive system for heat removal from the core of a research reactor, using a boiling channel in the upstream section of the circulation circuit during evacuation, are substantiated;
- The use of air cooling of the coolant of the primary circuit of the reactor plant with the use of a ventilation pipe eliminates the problem of water treatment of cooling towers, which is carried out to prevent the formation of salt deposits on heat exchange tubes and on the surface of sprinklers in cooling towers;
- The design of a reactor plant is based on the priority of «internal safety» and not «engineering safety»;
- Heat removal systems do not contain complex and expensive heat engineering equipment that requires periodic repairs and maintenance (pumps, shut-off and control valves, etc.), which, in addition to reducing operating costs, allows reducing dose loads on personnel;
- In the primary cooling circuit, due to its design, a minimum level of release of radioactive products from the water surface of the reactor pool is ensured;
- The upward direction of coolant movement in the core ensures the stability of natural circulation during power fluctuations;
- The absence of pumping equipment in the cooling circuit reduces dependence on electricity suppliers and increases the safety of the reactor plant;
- The use of a circulation scheme with a lifting movement of the coolant in the core in combination with a “chimney” scheme (a vertical pipe above the core with a side branch) allows optimizing the natural circulation loop by enclosing it in rigid boundaries, which simplifies the thermal-hydraulic analysis using the RELAP5/ code MOD3.2;
- The use of manufactured types of fuel assemblies with low U-235 enrichment satisfies the IAEA requirements for newly designed reactors;
- The non-stationary analysis carried out using the RELAP5/MOD3.2 code showed that the intensification of natural circulation due to the boiling channel provides an increase in the reactor power until the neutron flux density is reached.1×1014 cm-2s-1 and higher;
- In the considered design modes, there is no boiling even on the maximum heat-stressed fuel element;
- Aluminum alloys are used in fuel elements of reactor cores at temperatures not exceeding 250-270°C . The range of maximum operating temperatures of fuel elements in the considered reactor plant does not exceed 130°C;
- In the case of an uncontrolled increase in power, the coolant boils in the core and negative reactivity is introduced due to the density effect. In this case, the natural circulation through the core is enhanced, which, taking into account the operation of emergency protection, immediately transfers the reactor to a safe state in several parameters;
- In the event of an emergency shutdown of the reactor after the emergency protection has been triggered, the reactor is cooled down in the “soft” mode of residual energy removal with a sharp decrease in the temperature of the fuel elements;
- Using three-dimensional modeling, the calculation of the main parameters of the equipment of the air cooling circuit with natural air convection, the air heat exchanger and the ventilation pipe was made. The presented results of the thermohydraulic calculation of heat transfer from the reactor core to the final recipient — atmospheric air allow us to speak about the sufficient efficiency of the proposed heat removal system;
- The simplicity of design and the minimum cost of engineering systems and structures determine the low cost of the reactor plant and minimal costs when decommissioning the plant;
- Depressurization of the pipelines of the primary circuit or heat exchanger does not lead to a significant loss of coolant from the reactor pool;
- Low operating costs with sufficiently high parameters of neutron fluxes ensure the competitiveness of the proposed reactor plants.
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- Alyamovsky, A. A. SolidWorks Simulation. How to solve practical problems [Text] / — BHV-Petersburg, 2012, — 488 p.
- RELAP5/MOD3 Code Manual Volume 2: User’s Guide and Input Requirements. INEL-95/0174, NUREG/CR-5535. 1995.
- Method for obtaining the radioisotope molybdenum-99, patent RU2703994C1
- Varlachev, V.A. Neutron transmutation doping of silicon in a pool research nuclear reactor: Abstract of the thesis. … Doctor of Technical Sciences: 04/01/07 / Varlachev Valery Aleksandrovich; [Place of protection: Nat. research Volume. polytechnic university]. — Tomsk, 2015. — 48 p.
- Device and method for coating fuel element cladding, patent RU2561975C1