Three-dimensional integrated thermal-hydraulic calculation method for lead-based cold microreactor developed based on UDF

Abstract

The application belongs to the technical field of nuclear thermal hydraulic technology, and particularly relates to a three-dimensional lead-based cold micro-heap integrated thermal hydraulic calculation method based on UDF development. The method comprises the following steps: developing a thermal physical property module of the density, constant-pressure specific heat capacity, thermal conductivity and viscosity of the coolant lead / lead bismuth; determining the value of one item of turbulent Prandtl number Prt in the turbulent flow model; obtaining the viscous resistance coefficient and inertial resistance coefficient of the single-box fuel assembly porous medium model; obtaining the viscous resistance coefficient and inertial resistance coefficient of the main heat exchanger porous medium model; developing CFD numerical simulation calculation; and performing integrated modeling on the lead-based cold micro-heap, simplifying the core, pump channel, main heat exchanger and coolant flow channel. The application has the beneficial effect that through the UDF development and calling of the physical property module and the turbulent Prandtl number calculation module, the change of the physical property parameters of the liquid metal lead in the flow process and the turbulent heat conductivity in the turbulent flow model can be simulated more accurately.

Description

Technical Field

[0001] This invention belongs to the field of nuclear thermal hydraulic technology, specifically involving a three-dimensional lead-based cold microreactor integrated thermal hydraulic calculation method based on UDF. Background Technology

[0002] A fast reactor is a nuclear reactor that does not require a moderator and directly uses high-energy fast neutrons to maintain a chain reaction. Compared to thermal neutron reactors, fast reactors have higher neutron economy, giving them unique advantages in fuel breeding, resource utilization, and nuclear waste disposal. A lead-based cold fast reactor is a type of fast reactor that uses lead or lead-bismuth alloy as a coolant. It has advantages such as high temperature resistance, corrosion resistance, and high density, making it suitable for long-life, closed-loop operation designs and representing one of the future fourth-generation nuclear reactor technologies. However, due to the high density and high specific heat capacity of lead / lead-bismuth as a coolant, its fluid dynamics and heat transfer characteristics are relatively complex, especially in the compact structure of microreactors. Furthermore, the internal structure of a lead-based cold microreactor is quite complex. Taking a megawatt-class compact lead-cooled fast reactor microreactor design as an example, the microreactor adopts an integrated layout. The steam generator, main pump, etc., are all located inside the pressure vessel. The core is located at the bottom of the reactor pressure vessel, and the main pump is located at the center directly above the core, surrounded by six helical tube-type main heat exchangers. As liquid lead coolant flows through the reactor core, it absorbs heat generated by nuclear fuel fission, causing its temperature to rise. The coolant then flows upwards into the main pump, where it is pressurized and flows into the main heat exchanger. Within the main heat exchanger cavity, the coolant flows downwards, transferring heat with the secondary cooling fluid inside the tubes and decreasing its temperature. After leaving the main heat exchanger cavity, the coolant continues to flow upwards, then flows through the pressure vessel wall vents into the descending channel, eventually returning to the reactor core, completing one cycle. The complex geometry (such as narrow channels and small fuel assemblies) requires fine mesh generation, significantly increasing computational costs. Therefore, directly performing three-dimensional, full-core integrated numerical simulations of lead-based cold microreactors presents a significant challenge.

[0003] Currently, both domestically and internationally, widely used thermal hydraulic programs for lead-based cold fast reactor internal thermal and hydraulic calculations include system programs and computational fluid dynamics (CFD) programs. In system programs, the reactor core model often consists of single or multiple flow channels, providing only average values ​​of some thermal and hydraulic parameters within the reactor. It can only serve as a part of integrated thermal and hydraulic calculations and cannot meet the precise calculation requirements of three-dimensional lead-based cold microreactors. While common CFD methods can accurately simulate the thermal and hydraulic parameters of individual components within the reactor, calculating integrated or complex models requires a large number of meshes and places high demands on computing power. Currently, thermal and hydraulic calculations for lead-based cold fast reactors often only focus on individual components such as fuel assemblies and heat exchangers. For example, Chai et al. used CFD software to simulate the three-dimensional flow and temperature distribution of the LBE (Low-Beam Embedded Element) for a 19-bar bundle, evaluating the effect of mass flow rate on the temperature change of the fuel element surface. Yang Yupeng et al. used FLUENT software to perform detailed modeling and numerical simulation of the primary and secondary sides of a small lead reactor heat exchanger and analyzed its flow and heat transfer properties, verifying the correctness of the numerical simulation method. For coupled calculations of the primary coolant system in lead-cooled fast reactors, Zhao Pengcheng established a system program suitable for steady-state thermal-hydraulic analysis of the primary coolant system in small natural circulation lead-cooled fast reactors. He also developed auxiliary models such as a steady-state fuel rod heat transfer model and a shell-and-tube heat exchanger, obtaining analysis results of the whole-core steady-state natural circulation flow distribution characteristics. Transient thermal safety characteristic analysis was conducted using the system safety analysis program ATHLET. Currently, there is a lack of integrated thermal-hydraulic calculations both domestically and internationally for full-stack, detailed three-dimensional numerical simulations of lead-based cold microreactors. Summary of the Invention

[0004] The purpose of this invention is to provide a three-dimensional integrated thermal-hydraulic calculation method for lead-based cold microreactors based on UDF (Unified Data Form). This method utilizes UDF to develop a temperature-dependent property module for lead / lead-bismuth and a turbulent Prandtl number model suitable for liquid metal coolants. A porous media model simplifies the complex fuel assembly structure and helical tube heat exchanger. Furthermore, to address the issues of large mesh count and computational load in integrated modeling, different components of the primary loop are simplified and partitioned into separate meshes. Interfaces are used for connection and coupling to achieve fluid and heat exchange between partitions. This method enables efficient and accurate numerical simulation of the reactor core, helical tube heat exchanger, pump channels, core flow distribution, and the entire reactor core, thereby reducing the workload of model modification and numerical calculations.

[0005] The technical solution of this invention is as follows: A three-dimensional lead-based cold microreactor integrated thermal-hydraulic calculation method based on UDF, comprising the following steps:

[0006] Step 1: Develop modules for the thermal properties of lead / lead-bismuth coolant, including density, specific heat capacity at constant pressure, thermal conductivity, and viscosity.

[0007] Step 2: Determine the value of the Prandtl number Prt in the turbulence model;

[0008] Step 3: Perform CFD numerical simulation calculations on a 1:1 model for a single fuel assembly to obtain flow velocity and pressure drop parameters. By fitting the fv relationship, perform pressure drop coupling of the core fuel assembly to obtain the viscous drag coefficient and inertial drag coefficient of the porous medium model of the single fuel assembly.

[0009] Step 4: Obtain the viscous drag coefficient and inertial drag coefficient of the porous medium model of the main heat exchanger;

[0010] Step 5: Conduct CFD numerical simulation calculations on the flux distribution in the core region of the lead-based cold microreactor;

[0011] Step 6: Perform integrated modeling for lead-based cold microreactors, simplifying the reactor core, pump channels, main heat exchangers, and coolant flow channels.

[0012] The empirical formula for determining Prt in step 2 is as follows:

[0013]

[0014] Step 3 involves performing a 1:1 CFD numerical simulation of a single fuel assembly using Fluent software to obtain flow velocity and pressure drop parameters. By fitting the fv relationship, pressure drop coupling of the fuel assembly is performed to obtain the viscous drag coefficient and inertial drag coefficient of the porous medium model of the single fuel assembly. Based on the formula... The Reynolds number range is calculated by switching flow models using the functions DEFINE_EXECUTE_AT_END(calculate_Re_and_switch_model), RP_Set_String("models / viscous","laminar"), and RP_Set_String("models / viscous","k-epsilon"), thus realizing a combined calculation method for "inter-cell flow + intra-cell flow" within the reactor core.

[0015] Step 4 involves performing a 1:1 CFD numerical simulation of the spiral tube heat exchanger using Fluent software to obtain flow velocity and pressure drop parameters. By fitting the fv relationship, pressure drop coupling of the main heat exchanger is performed to obtain the viscous resistance coefficient and inertial resistance coefficient of the porous medium model of the main heat exchanger.

[0016] In step 6, the reactor core fuel assembly and the helical tube main heat exchanger are simplified using a porous medium model, and the mesh is divided into zones. The coolant thermal properties and Prandtl number UDF are compiled, and the thermal simulation results of the single-box fuel assembly and the main heat exchanger are coupled to realize the integrated thermal calculation of the reactor core, the main heat exchanger and other coolant channels.

[0017] The beneficial effects of this invention are as follows: Taking a small-scale lead-cooled fast reactor as an example, by developing and calling UDFs for the physical property module and the turbulent Prandtl number calculation module, it is possible to more accurately simulate the changes in physical property parameters of liquid metallic lead during flow and the turbulent thermal conductivity in the turbulence model. By conducting 1:1 numerical simulation calculations on single-cell fuel assemblies and helical tube heat exchangers, the porous media model parameters of the corresponding regions are obtained, thereby simplifying the complex fuel assembly structure and helical tube heat exchanger, greatly reducing the computational cost. In addition, to address the problem of a large number of integrated meshes, different components of the primary loop are divided into zones for meshing, and the various components are coupled and connected through interfaces, thereby realizing the core flow distribution of the lead-based cold microreactor and the thermal-hydraulic numerical simulation calculation of the integrated model. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the circuit;

[0019] Figure 2 The flowchart of the three-dimensional lead-based cold microreactor integrated thermal-hydraulic calculation method based on UDF provided by the present invention is shown below.

[0020] Figure 3 A simplified integrated model for small-scale lead-cooled fast reactors. Detailed Implementation

[0021] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0022] This invention provides a three-dimensional integrated thermal-hydraulic calculation method for lead-based cold microreactors based on User Defined Function (UDF) technology. It aims to solve the challenges of large differences in structural dimensions and difficult mesh generation within the complex superimposed space when performing three-dimensional thermal-hydraulic calculations on lead-based cold microreactors. Applying this method simplifies the complex internal structure and couples the computational structures of various components through UDF technology, thereby achieving integrated three-dimensional thermal-hydraulic calculations across the entire reactor core.

[0023] like Figure 2 As shown, the three-dimensional lead-based cold microreactor integrated thermal-hydraulic calculation method developed based on UDF includes the following steps:

[0024] Step 1: Develop a module for the thermal properties of the coolant lead / lead-bismuth. This module includes functions representing the thermal properties of liquid lead / bismuth, such as density, specific heat capacity at constant pressure, thermal conductivity, and dynamic viscosity, as a function of temperature. To improve the accuracy of the numerical simulation results, considering the impact of temperature rise on parameters such as thermal conductivity, specific heat capacity at constant pressure, and density during heat transfer, the physical properties of liquid lead are expressed as functions of temperature in the calculations, as shown in Table 1 below. The dynamic changes of these properties with temperature are implemented by writing a UDF program.

[0025] Table 1

[0026]

[0027] Wherein, λ, c p ρ and η are the thermal conductivity, specific heat capacity at constant pressure, density, and dynamic viscosity of liquid lead, respectively.

[0028] Step 2: In numerical simulation calculations, the quality of the simulation depends to a certain extent on the chosen turbulence model. Selecting the correct turbulence model is crucial. One important factor in the turbulence model is the Prandtl number, Pr. t The value of Pr for conventional fluids t =0.85 is no longer applicable to liquid metals, therefore a UDF program needs to be developed to modify the turbulence model applicable to liquid metals. The modified empirical formula is:

[0029]

[0030] Where Pe is the Bekele number, which characterizes the relative magnitude of convection and diffusion.

[0031] Step 3: Using Fluent commercial software, perform 1:1 CFD numerical simulations of a single fuel assembly. Key thermal parameters such as flow velocity and pressure drop within the fuel assembly are extracted through post-processing. Pressure drop coupling of the core fuel assembly is performed by fitting the fv relationship, based on the Ergun equation:

[0032]

[0033] Where D p denoted as average particle diameter, μ as fluid viscosity coefficient, ΔP as pressure drop, L as porous medium thickness, and v as flow velocity.

[0034] The viscous drag coefficient and inertial drag coefficient of the porous medium model of a single fuel cell assembly were obtained; a UDF program was written to calculate the viscous drag coefficient based on the formula. The Reynolds number range is calculated. When the Reynolds number is less than 2300, the fluid flow is laminar; when the Reynolds number is greater than 2300, the fluid flow is turbulent. Different flow models need to be selected according to different flow states. Therefore, the functions DEFINE_EXECUTE_AT_END(calculate_Re_and_switch_model), RP_Set_String("models / viscous","laminar"), and RP_Set_String("models / viscous","k-epsilon") are used to switch the flow model. This implements a combined calculation method for "inter-cell flow + intra-cell flow" in the reactor core.

[0035] Step 4: Using Fluent commercial software, a 1:1 CFD numerical simulation of the spiral tube heat exchanger is performed. Key thermal parameters such as flow velocity and pressure drop are extracted through post-processing. By fitting the fv relationship, the pressure drop of the main heat exchanger is coupled to obtain the viscous drag coefficient and inertial drag coefficient of the porous medium model of the main heat exchanger. The calculation formula is the Ergun equation:

[0036]

[0037] Where D p denoted as average particle diameter, μ as fluid viscosity coefficient, P as pressure drop, L as porous medium thickness, and v as flow velocity.

[0038] Step 5: Conduct CFD numerical simulation calculations on the flow distribution in the core region of the lead-based cold microreactor and extract the mass flow rate Q at the inlet of each fuel assembly. i The average mass flow rate of the reactor core fuel assembly was calculated. To provide a quantitative and intuitive understanding of the uniformity of flow distribution at the reactor core inlet, a normalized flow distribution factor q is defined. i The flow rate Q at the inlet of a certain fuel assembly box i Average flow rate at the entire core inlet The ratio:

[0039]

[0040] Where n is the number of fuel assembly boxes.

[0041] By normalizing the flow allocation factor q i The deviation can reflect the effect of core flow distribution, and can be used as a basis to optimize and improve the flow distribution device, thereby improving the uniformity of fuel assembly mass flow distribution.

[0042] Step 6: Perform integrated 3D modeling for the lead-based cold microreactor, reasonably simplifying components such as the core, pump channels, main heat exchanger, and coolant flow channels. The core fuel assemblies and helical tube main heat exchanger are simplified using a porous media model, omitting detailed modeling of these parts in the 3D model. During mesh generation, the overall model is divided into different regions, including the core fuel assemblies, lower chamber, upper core space, and descending channels between the inner and outer containers, with different mesh sizes assigned. This partitioned meshing significantly reduces the number of meshes, and when the model is modified, only specific regions need to be replaced and re-meshed, greatly reducing the workload of mesh generation.

[0043] Step 7: In the Fluent numerical simulation, compile the coolant thermal properties and Prandtl number UDFs; determine the key boundary condition settings: set the inlet mass flow rate and temperature, the main heat exchanger as a bulk cold source, the fuel assembly as a bulk heat source, and the outlet as a pressure outlet boundary condition, etc.; couple the thermal simulation results of the single-cell fuel assembly and the main heat exchanger, i.e., the porous medium parameters; set the mesh interfaces of different regions as surfaces of the interface to realize the transmission of data between regions. Through the above operations, the integrated thermal calculation of the reactor core, the main heat exchanger, and other coolant channels is achieved.

[0044] This invention enables integrated three-dimensional full-core numerical simulation, suitable for small-scale lead-based cold fast reactors with complex internal structures. Through the development and invocation of UDFs for the physical property module and the turbulent Prandtl number calculation module, it can more accurately simulate the changes in physical properties of liquid lead during flow and the turbulent thermal conductivity in the turbulence model. In three-dimensional geometric modeling, fuel assemblies with narrow channels, helical tube heat exchangers with multiple tube bundles, and main pump channels can be simplified. Their internal structures can be replaced by porous media models obtained through 1:1 calculations, significantly reducing the workload of modeling and the computational cost of mesh generation. Simultaneously, the model structure of the partitioned mesh can be coupled through an interface, further reducing the workload of model modification and numerical calculations.

[0045] Example:

[0046] A three-dimensional integrated thermal-hydraulic calculation method for lead-based cold microreactors based on UDF includes:

[0047] Step 1: Selecting a UDF function

[0048] Through the Ergun equation, a formula for porous media:

[0049]

[0050] Prandtl number formula for turbulence: 1000 < Pe ≤ 6000

[0051] Physical property formulas (e.g., liquid lead metal)

[0052] C P Pb = 176.2 - 4.923 × 10⁻⁴ -2 ·T+1.544·10 -5 ·T 2 -1.524·T -2 J / (Kg·K),

[0053] ρ Pb =11441-1.2795·T Kg / m 3 ,

[0054] λ Pb =9.2 + 0.11·TW / (m·K)

[0055]

[0056] Reynolds number formula: Select the corresponding code.

[0057] (2) Develop a module for the thermal properties of lead / lead-bismuth coolant, including density, specific heat capacity at constant pressure, thermal conductivity, and viscosity. To accurately simulate the effect of temperature changes on the thermal properties of the material during heat exchange, it is necessary to consider the dynamic impact of temperature rise on parameters such as thermal conductivity, specific heat capacity at constant pressure, and density of liquid lead or lead-bismuth coolant. Based on this, by consulting the "Lead-Bismuth Thermal Properties Handbook," the physical properties of liquid lead / lead-bismuth were expressed as functions of temperature, and a UDF program was written accordingly.

[0058] Through this UDF module, Fluent can call the function expressions of various physical property parameters during the solution process, and update the physical property parameters such as thermal conductivity, specific heat capacity and density of the coolant in real time according to the current temperature of the computing unit, thereby improving the accuracy of the calculation and the reliability of the simulation.

[0059] (3) A module for calculating the Prandtl number of turbulence was developed. For the value of the Prandtl number Prt in the turbulence model, Prt = 0.85 for conventional fluids is no longer applicable to liquid metals. In existing turbulence models, only turbulent viscosity, i.e., the turbulent momentum diffusivity, is usually established, without directly defining turbulent thermal conductivity. To model turbulent thermal conductivity, Prt is defined as the ratio of turbulent momentum diffusivity to turbulent thermal conductivity, and a UDF program is written. Through this UDF module, Fluent can automatically call the relevant turbulent Prandtl number function, thereby achieving more accurate thermal fluid simulation.

[0060] (4) A 1:1 CFD numerical simulation was performed on a single fuel assembly. During the simulation, a fine meshing strategy was adopted to obtain key thermal parameters such as core coolant inlet and outlet temperatures and coolant flow rate, as well as drag characteristics such as pressure drop, which can meet the design requirements for oxide corrosion and fuel rod bundle melting point of lead-based reactors. By fitting the fv relationship, the pressure drop of the core fuel assembly was coupled to obtain the viscous drag coefficient and inertial drag coefficient of the porous medium model of the single fuel assembly. In subsequent calculations, the core fuel assembly can be simplified by the porous medium model to improve the calculation efficiency.

[0061] A method for simultaneously calculating inter-cell flow and intra-cell flow in the reactor core is implemented by writing a User-Defined Function (UDF) program. In this step, the UDF needs to perform the following two functions: ① extract key thermal parameters for 1:1 simulation calculations of a single cell assembly; ② calculate the Reynolds number (Re) using the extracted thermal parameters and determine whether to use a laminar or turbulent flow model. This UDF can achieve simultaneous calculations of inter-cell and intra-cell flow under different flow conditions.

[0062] (5) A full-scale CFD numerical simulation of the spiral tube heat exchanger was performed using a 1:1 model. During the simulation, a fine mesh generation strategy was also adopted. The obtained temperature distribution and flow pressure drop data can be used to further derive the viscous drag coefficient and inertial drag coefficient of the porous medium model applicable to the main heat exchanger region. The spiral tube heat exchanger region can be simplified using a porous medium model to effectively reduce the computational complexity in the integrated simulation.

[0063] (6) Calculations were performed on the flow distribution of the lead-based cold microreactor to obtain the flow distribution characteristics and normalized flow distribution factor in the pool, thereby evaluating the effect of the core flow distribution. Based on the calculation results of the flow distribution, an optimization scheme for the core flow distribution can be proposed to achieve a more uniform flow distribution, improve the flow uniformity of coolant in various regions of the core, avoid local overheating, and improve the overall safety and operational stability of the microreactor.

[0064] (7) Integrated 3D modeling was performed for lead-based cold microreactors. Lead-based cold microreactors face challenges such as large differences in structural dimensions within complex superimposed spaces and difficulties in mesh generation. Therefore, the core, pump channels, main heat exchanger, and coolant flow channels were reasonably simplified. The core fuel assemblies and helical tube main heat exchanger were simplified using a porous medium model. Detailed modeling of these parts was omitted in the 3D model. The overall model was divided into different regions, such as the core fuel assemblies, lower chamber, upper core space, and descending channels between the inner and outer containers, with different mesh sizes for mesh generation.

[0065] (8) In the integrated numerical calculation, the meshes of each region are connected by defining the wall boundary as an interface to realize the fluid and heat exchange between the partitions, ensuring that the calculation results between the regions can be smoothly transferred. The coolant thermal properties and Prandtl number UDF are compiled, and the key boundary conditions are determined: the inlet is set with mass flow rate and temperature, the main heat exchanger is set as a bulk cold source, the fuel assembly is set as a bulk heat source, and the outlet is set as a pressure outlet boundary condition, etc. The thermal calculation results of the single-box fuel assembly and the main heat exchanger are coupled. The above components are simplified by the porous medium parameters to realize the integrated thermal calculation of the core, the main heat exchanger and other coolant channels.

Claims

1. A three-dimensional integrated thermal-hydraulic calculation method for lead-based cold microreactors based on UDF, characterized in that, Includes the following steps: Step 1: Develop modules for the thermal properties of lead / lead-bismuth coolant, including density, specific heat capacity at constant pressure, thermal conductivity, and viscosity. Step 2: Determine the value of the Prandtl number Prt in the turbulence model; Step 3: Perform CFD numerical simulation calculations on a 1:1 model for a single fuel assembly to obtain flow velocity and pressure drop parameters. By fitting the fv relationship, perform pressure drop coupling of the core fuel assembly to obtain the viscous drag coefficient and inertial drag coefficient of the porous medium model of the single fuel assembly. Step 4: Obtain the viscous drag coefficient and inertial drag coefficient of the porous medium model of the main heat exchanger; Step 5: Conduct CFD numerical simulation calculations on the flux distribution in the core region of the lead-based cold microreactor; Step 6: Perform integrated modeling for lead-based cold microreactors, simplifying the reactor core, pump channels, main heat exchangers, and coolant flow channels.

2. The three-dimensional lead-based cold microreactor integrated thermal-hydraulic calculation method based on UDF as described in claim 1, characterized in that: In step 1, the physical properties of the coolant liquid lead / lead-bismuth are expressed as a function of temperature in the calculation, as shown in the table below.

3. The three-dimensional lead-based cold microreactor integrated thermal-hydraulic calculation method based on UDF as described in claim 1, characterized in that: The empirical formula for determining Prt in step 2 is as follows: Pe is the Berkeley number.

4. The three-dimensional lead-based cold microreactor integrated thermal-hydraulic calculation method based on UDF as described in claim 1, characterized in that: Step 3 involves performing a 1:1 CFD numerical simulation of a single fuel assembly using Fluent software to obtain flow velocity and pressure drop parameters. By fitting the fv relationship, pressure drop coupling of the fuel assembly is performed to obtain the viscous drag coefficient and inertial drag coefficient of the porous medium model of the single fuel assembly. Based on the formula... The Reynolds number range is calculated by switching the flow model using the functions DEFINE_EXECUTE_AT_END(calculate_Re_and_switch_model), RP_Set_String("models / viscous","laminar"), and RP_Set_String("models / viscous","k-epsilon"), thus realizing the combined calculation method of "inter-cell flow + intra-cell flow" in the reactor core.

5. The three-dimensional lead-based cold microreactor integrated thermal-hydraulic calculation method based on UDF as described in claim 1, characterized in that: Step 4 involves performing a 1:1 CFD numerical simulation of the spiral tube heat exchanger using Fluent software to obtain flow velocity and pressure drop parameters. By fitting the fv relationship, pressure drop coupling of the main heat exchanger is performed to obtain the viscous resistance coefficient and inertial resistance coefficient of the porous medium model of the main heat exchanger.

6. The three-dimensional lead-based cold microreactor integrated thermal-hydraulic calculation method based on UDF as described in claim 1, characterized in that: In step 6, the reactor core fuel assembly and the helical tube main heat exchanger are simplified using a porous medium model, and the mesh is divided into zones. The thermal properties of the coolant and the Prandtl number Prt of the turbulence are compiled, and the thermal simulation calculation results of the single-box fuel assembly and the main heat exchanger are coupled to realize the integrated thermal calculation of the reactor core, the main heat exchanger and other coolant channels.

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