Introduction

Nuclear energy, as one of the clean energy sources, has more potential for future development. The canned motor of the main pump is an essential part of the primary circuit system of the island in nuclear power plants. The main pump canned motor is composed of cover, shaft, stator and rotor components, bearings, housing, etc. The insulating material in the stator winding is prone to failure when overheated, and is considered one of the most common failure modes. To avoid this overheating, the canned motor is water cooled, including primary water and secondary water. Therefore, the accurate prediction of the temperature distribution and the hotspot temperature under different conditions is crucial in the design phase of motors and generators.

Nowadays, reports are mainly focused on analyzing the winding temperature field1,2 and its influence factors3. The methods adopted are mainly the thermal equivalent network methods or finite volume methods. Most of studies are for steady-state conditions. If the secondary water supply in the motor heat exchanger fails, the speed of the secondary water quickly drops to 0 m/s in the short time, resulting in poor cooling performance. However, the motor and power supply of the main pump remain in the working condition. Due to electromagnetic losses, the components still generate large amounts of heat, so there is a risk of the motor overheating. In addition, the primary water is also used to lubricate the bearings in the rotor components. The poor cooling performance of the second water causes the primary water temperature to rise. To avoid water lubrication failure, automotive industry standards specify that the highest primary water temperature near the bearings must not exceed 95 °C within the power failure response time of 5 min. Therefore, predicting the transient temperature pattern and temperature rise of the cooling water and major components is crucial for the thermal safety and qualification of the motor product compared to the steady-state rated conditions.

In general, the temperature increase is mainly due to electromagnetic losses in the main components. Currently, there are extensive studies on electromagnetic losses for the canned motor of main core pump4,5, mainly on the harmonic losses of the shielding sleeve, the eddy current losses, and the influencing factors using the finite element method6.

In recent decades, the researches concerning with transient thermal problems related to variable operating conditions and motor failures have flourished. A 25-node equivalent network modeling approach has been proposed to study the cooling of a synchronous motor stator under intermittent operating conditions7. Tang YM performed a simulation of the transient temperature rise of the motor for electromechanical braking based on magneto-thermal coupling8. Yang XF predicted the performance of heat transfer and temperature distribution in transient state by using the conjugate heat transfer method for DC motors and using three types of cooling stator shells, using a periodic load cycle of 8.5 kW at 35 s continuous power and 19 kW at 5 s peak power9. An infrared monitoring test method has been proposed for the manufacture of quality control of DC motor defects such as inter-turn short circuits, etc10. In contrast, research on transient thermal temperatures focused mainly on permanent magnet synchronous motors (PMSM), for example, applying the coupled electromagnetic thermal analysis method in transients to prevent the winding from exceeding its peak temperature11,12, optimization design of waterway structure parameters13, a transient temperature field test and simulation study for PMSM with silicon gelatin potting between the end windings and the casing gap under peak load conditions using the finite element method14, the study of the steady state and transient fault temperature fields for the winding short circuit problem of PMSM using a three-dimensional lumped parameter thermal model15 and a finite element simulation model16, respectively. Similarly, Li F established a multi-physics field analysis method, ambient air as a fluid zone, and CFD simulation to predict steady-state and transient temperature distributions, etc17.

In recent years there has been an increasing trend to investigate transient flow characteristics within the coolant pump, such as: in accidents with loss of coolant during small interruptions18, hydraulic properties in accidents with station failure19, start-up transients20, the shutdown coasting transition process21 and a rotor seizing accident22 as well as the transient temperature distribution of the high temperature direct drive permanent magnet synchronous motor with electrically operated valve under fault conditions23. In general, most of the research on transient thermal analysis of motors mainly involves methods including the finite element method, the lumped parameter method or network, and the finite element/volume method.

This paper focuses primarily on thermal modeling and analysis of a small nuclear main pump in the SWSB faults. The main nuclear pump under study consists of a shielded motor with a spirally wound heat exchanger and a reactor coolant pump under the base. The conjugate numerical simulation method based on the finite volume method has been established to predict the temperature evolution of the cooling water and the main components when the secondary water supply fails for 310 s. In addition, some of these results can also be transferred to other types of canned motors.

Geometrical model and cooling waterway

Table 1 shows the main parameters of the motor. Taking into account the characteristics of the motor waterway and the geometry structure, the physical model is selected as the quarter of the main nuclear pump, as shown in Fig. 1, and the position of the cover, shaft, housing, stator, and rotor components, etc. can be observed. A double-layer spiral tube is wound outside the motor casing where the core segment is located. In the zone between the baffle and the pipes, spiral channels are formed through which secondary water can flow from top to bottom. The main pump is located at the bottom of the motor.

Table 1 Main performance and geometric parameters of the canned motor.
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Fig. 1
figure 1

Geometry model of the simulation domain.

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Figure 2 shows the structure of the cooling waterway in the domain. Primary cooling water circulation is based on the driving pressure head created by the rotation of the auxiliary impeller. Primary water firstly enters the upper end-cover and the center hole of the shaft. When it flows to the outlet of the auxiliary impeller, part of it is forcibly rotated upward by the thrust bearing and the guide bearing gap and returns to the cover to form an upper water lubrication circulation circuit. The other part of the primary water firstly flows down into the stator and rotor gap, lubricating the lower guide bearing, etc. Then it enters the stator head bore and flows up the spiral tubes of the heat exchanger, where the primary water is cooled by the secondary water in the spiral channel. Finally, it enters the end cover again to start the next main circulation circuit. Therefore, the structure of the cooling water path in the canned motor for small nuclear main pumps is different from the structure of the large motor before1,2. The established geometry model is simplified to create a high-quality mesh. Therefore, the overall flow resistance of the primary cooling water pathway is different from that of the prototype. The length of the inlet pipe before entering the end cover is cut off by a small part. Therefore, the boundary of the primary water inlet or outlet can be set as shown in Fig. 1. The advantage of this treatment is that when predicting the motor flow and temperature field, the volume flow rate of the primary water circulation at the inlet boundary can match the actual measured values3.

Fig. 2
figure 2

Cooling water path in the simulation domain.

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Mathematical model and solution conditions

To simplify the solution process, the following assumptions are adopted:

  1. (1)

    Primary and secondary water are treated as incompressible fluids because the Mach number in this project is much smaller than 0.7.

  2. (2)

    It is assumed that the electromagnetic loss is evenly distributed among the corresponding components.

  3. (3)

    The stator winding model is simplified, accordingly, the actual physical process and thermal properties of the equivalent insulation materials do not change24.

  4. (4)

    Most of the stator windings in the nitrogen end cavity are filled with a bonded rope binding ring, the value of the thermal conductivity of the nitrogen gas in the cavity is very low. Therefore, the isolation of the stator windings from the nitrogen gas environment can be considered adiabatic.

Mathematical model

To predict the performance of transient flow and heat transfer, a 3D model is constructed contains the equations of mass, momentum and energy conservation (1)-(3), as well as the relationship Eq. (4) between the absolute velocity vector and the relative velocity vector. Primary water in the inner pore of the shaft and the auxiliary impeller rotates with the shaft, which is in the rotating reference coordinate system. Except those mentioned, all other liquids in the computational domain are in the fixed coordinate system and the angular velocity vector \({{\varvec{\Omega}}}\) = 0.

$$\frac{\partial \rho }{{\partial t}} + \nabla (\rho {\text{u}}_{{\text{r}}} ) = 0$$
(1)
$$\frac{{\partial \left( {\rho {\mathbf{u}}_{{\text{r}}} } \right)}}{\partial t} + \nabla (\rho {\mathbf{u}}_{{\text{r}}} \times {\mathbf{u}}_{{\text{r}}} ) + \rho (2{\varvec{\varOmega}}\times {\mathbf{u}}_{{\text{r}}} +{\varvec{\varOmega}}\times{\varvec{\varOmega}}\times {\mathbf{r}}) = - \nabla p + \nabla \tau + {\mathbf{F}}$$
(2)
$$\frac{\partial }{\partial t}\left( {\rho E{}_{{\text{r}}}} \right) + \nabla (\rho {\mathbf{u}}_{{\text{r}}} \times H_{{\text{r}}} ) = \nabla \left( {\frac{\lambda }{{c_{p} }}\nabla T + \tau_{{\text{r}}} {\mathbf{u}}_{{\text{r}}} } \right) + S_{h}$$
(3)
$${\mathbf{u}} = {\mathbf{u}}_{{\text{r}}} + \, {{\varvec{\Omega}}} \times {\mathbf{r}}$$
(4)

where ρ, t, \(\nabla\), T is density, time, divergence and temperature, respectively, r is the position vector between microelements in the rotating coordinate system, \(\rho (2{\varvec{\varOmega}}\times {\mathbf{u}}_{{\text{r}}} +{\varvec{\varOmega}}\times{\varvec{\varOmega}}\times {\mathbf{r}})\) is the Coriolis force, F, \(\tau\) is the volumetric force and surface viscous stress, respectively, Er is the relative internal energy, Hr is the relative total enthalpy, and \(S_{h}\) is the internal heat source. For a solid microelement in the computational domain, \({\mathbf{u}}_{{\text{r}}}\) = 0, Eq. (3) becomes the differential formula of transient conduction.

In steady state t = 0 s, the Reynolds number (Re) at the spiral tube inlet is 5.3 × 104, the Re at the spiral channel inlet is 4.3 × 104, and the Re at the cover inlet is 4.8 × 104, indicating that the primary and secondary water in the channel is in a turbulent state. The thickness of primary water gaps in all types of bearing, shield sleeve between stator and rotor, and the channels of the spiral waterway are relatively small, and the viscous shear stress at the above boundary layers is relatively large. Therefore, the two-equation SST k-ω model is selected for turbulent transport Eq. 3.

Initial, solving boundary conditions, and method

The convergence results 3 for the steady state flow and thermal field are used as the initial field (t = 0) for the transient calculation of the motor with the CAP in the SWSB fault, as shown in Fig. 3.

Fig. 3
figure 3

Motor temperature contour with steady state CAP conditions (t = 0 s).

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For comparison purposes, the initial and boundary conditions in steady state and transient conditions with CAP/EHAP in SWSB fault are listed in Table 2, where the magnitude of the volumetric flow rate test value for primary inlet, secondary water, and the temperature is set according to the average value of the measurement data.

Table 2 Initial and inlet boundary conditions at steady/transient SWSB condition under rated operations (t = 0).
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It should be noted that the wall boundary condition is set as the initial conditions in the transient state of the SWSB fault instead of the volume flow rate boundary condition at the secondary water inlet. A gauge pressure is set at each outlet remains \(p^{\prime\prime} = {\text{0 Pa}}\).

For both steady state and transient state simulations, periodic boundary conditions are established on the corresponding left and right sides of the fluid and solid in the domain, respectively, as shown in Fig. 1 and 2. Universal control variable \(\phi\) should satisfy the following approximate formula.

Left-hand to right-hand side:

$$\phi (\theta ,t) = \phi \left( {\theta + \frac{3\pi }{2},t} \right)$$
(5)

Right-hand to left-hand side:

$$\phi (\theta ,t) = \phi \left( {\theta + \frac{\pi }{2},t} \right)$$
(6)

The heat conducted along the circumferential direction to external surfaces such as the housing, heat exchanger, and coolant pump is dissipated by a combination of radiation and convection.

The calculation formula for the total heat flow rate is as follows:

$$Q = A\left( {h_{{\text{c}}} + h_{{\text{r}}} } \right)\Delta t = Ah_{{{\text{tot}}}} \left( {t_{{{\text{surface}}}} - t_{{{\text{ambient}}}} } \right)$$
(7)

where hc and hr are the convective and radiant surface heat transfer coefficients, respectively, which can be set to appropriate values. htot is the total surface heat transfer coefficient.

In this model, the primary waterway is cut off before the motor inlet. To ensure the continuity of physical quantities such as temperature and flow rate, etc., the magnitudes at the outlet surface of the primary waterway are assigned to the inlet surface in real-time at each iteration, as shown in Eq. (8). It achieves a closed and continuous flow cycle for primary water.

$$\left\{ \begin{gathered} \overline{{T^{\prime}_{{\text{pri}}} }} = \sum {\overline{{T^{\prime\prime}_{{\text{pri}}} }} \Delta A^{\prime\prime}/\sum {\Delta A^{\prime}} } \hfill \\ \overline{{u^{\prime}}}_{pri} \Delta A^{\prime}{ = }\overline{{u^{\prime\prime}}}_{pri} \Delta A^{\prime\prime} \hfill \\ \end{gathered} \right.$$
(8)

where \(A^{\prime }\), \(A^{\prime\prime}\) is the area of the primary water inlet or outlet and the size of the area is the same. \(\overline{{T^{\prime}_{{\text{pri}}} }}\), \(\overline{{T^{\prime\prime}_{{\text{pri}}} }}\), \(\overline{{u^{\prime}}}\), \(\overline{{u^{\prime\prime}}}\) represent the average area temperature and average area velocity at the primary water inlet and outlet, respectively.

The UDF programming in C language for Eq. (8) was performed in Fluent 18 software in the transient iterative calculation process by loading the relevant settings and compilation links of the UDF C language programmed file at the primary water inlet location in the SWSB fault.

To reduce the error caused by the inaccuracy of the motor material property parameters, we developed customized data files that document temperature-dependent variations in thermal properties (including density, specific heat capacity, and thermal conductivity) for both circuits. These parameters were systematically integrated into Fluent’s material property database to minimize numerical errors caused by variations in material properties, as detailed in Reference25.

Kinetic viscosity of primary water and secondary water, the thermal conductivity used in the CFD models mentioned is temperature-dependent under corresponding pressure 15 MPa and 0.41 MPa. Table 3 shows the values and equations of the density ρ, the specific heat cp , and thermal conductivity λ in the main components. This updates the motor material and coolant parameters after each iteration according to the updated component temperatures.

Table 3 Values of the thermal- physical property parameters and losses in the main components.
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In addition, electromagnetic loss is the key to the rise in temperature of the main components of the motor. The values of the heat sources in Eq. (3) are assigned using the volume average method. Table 3 shows the values of electrical loss and eddy current loss in the main components determined using the finite element method (FEM) at rated speed (2923 rpm)26.

Verification of grid independence is carried out in detail. Considering time-consuming, the total number of grids in the selected domain is about 3.54 million to ensure that the number of fluid grids in the narrow gap of the motor is sufficient3. In the solid parts with greater thermal conductivity, the grids are correspondingly sparsely distributed. The value of dimensionless distance y + near the wall in model is within 21.06.

The relevant differential equations for describing flow and heat transfer are discretized into algebraic equations. The second-order upwind scheme is adopted for the convection term. The central difference scheme is used for the diffusion term. The SIMPLE scheme is used for the pressure–velocity coupling. An implicit and separation solution scheme based on a pressure solver is applied.

When predicting transient heat transfer, the size of the time step (\(\Delta t\)) has a decisive influence on the stability of the simulation results and the overall calculation time. At the beginning of the transition, the adaptive method is selected to determine the size, because the total calculation time in the adaptive method is very long, so a fixed value of 0.001 s, 0.01 s, 0.05 s, 0.1 s is tried and is set as a replacement for the adaptive after t = 50 s and the maximum number of iterations is 100 within the same time layer. Finally, the results are verified to be more stable when the value is set to 0.05 s. The performance parameters of the workstation used are Inter(R) Xeon(R) Gold 5118 CPU, which uses 38 of its cores for parallel calculations. It took more than 10 days to complete the transition prediction for 310 s in the SWSB fault with CAP and obtain the convergent solution.

Results and discussion

Temperature results and experimental validation

The comprehensive performance test of the prototype is carried out in winter with the CAP. The upper lubrication water temperature test points (referred to as upper test points) are mounted in the end cover near the upper wall surface of the upper thrust bearing. The water temperature test points of the lower guide bearing (referred to as lower test points) are arranged at 90 °intervals along the circumference in the horizontal circular channel around the stator head. A test point is located at each center outer surface of the nose end for the three winding insulations in the lower cavity of the stator using a PT100 B-class four-wire platinum RTD temperature sensor. The position can be seen in Fig. 1, 2 and 3. All measuring devices are calibrated prior to the experiment. The maximum uncertainty of the PT100 temperature test is ± 0.55 °C, which is small and can be neglected in a later analysis.

To prove the accuracy of the proposed methodology, the predicted transient temperature magnitudes at the location of all temperature test points are compared with the corresponding test data, as shown in Fig. 4. It can be seen that the predicted and measured water temperature values agree well, and most of the predicted temperature values are higher than the values measured at the upper and lower measurement points in the SWSB fault. The predicted values at the upper measurement point have an average error of + 0.847% during the 310 s period. For the lower test point, the error in the initial time from 0 to 25 s gradually increases with time. The error of the predicted value is + 5.97% at 310 s, which is within the acceptable industrial range, indicating that the simulation results are relatively more accurate. Meanwhile, the predicted peak temperature of the stator winding insulation in the bottom cavity is 121.05 °C, and the maximum temperature rise of the winding insulation at the test point is 1.24 °C and 1.66 °C, respectively, according to measurement and simulation. The rate of temperature rise of the stator winding insulation is not significant and is only 0.00473 °C/s.

Fig. 4
figure 4

Temperature variation curves of measuring points versus time.

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Comparison of the temperature evolution for the main components and waters

Figure 5 shows the temperature contours of the main components, including the housing, stator winding insulation, stator core and shield sleeve. It can be seen that the temperature of the stator shielding sleeve and the housing increases significantly with time. The temperature of the contact surface between the outer casing and the secondary water becomes higher than that of the inner zone of the casing, where the temperature of the secondary water has increases and transfers heat energy to the casing along the radial decrease direction, compared to the temperature at t = 0 s in the steady state. In contrast, the stator winding insulation and the temperature increase of the stator core with time are not obvious in the SWSB fault.

Fig. 5
figure 5

Transient temperature evolution contour of the main components in the SWSB fault.

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Figure 6 shows the volume-averaged temperature profile for the main components. To simplify the analysis and comparison, primary water, secondary water in the heat exchanger, and gap water between the stator and rotor are also shown in Fig. 6.

Fig. 6
figure 6

Transient volume average temperature profile of the main components and the cooling water.

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It can be observed that the temperatures of each component begin to increase gradually over time SWSB fault 5 min, and the speed of temperature is different. The temperature rise of the stator core is 3.2 °C, slightly higher than that of the stator winding insulation. Due to the smaller volume and mass, the temperature rise rate of the rotor core is greater than that of the stator core, causing the temperature difference between the stator and rotor core to become smaller and smaller as time increases. The temperature rise rate of the stator and rotor shield sleeve is the largest among the main components, and the value of the temperature rise rate is almost consistent with the gap water and primary water in the spiral tubes of the heat exchanger. The temperature difference among the primary water in the spiral tubes, the gap water, the stator and rotor shield sleeve change slightly. The volume-averaged temperature of the primary water in spiral tubes is higher than that of the stator shield sleeve, and gap water were at the same height. This is because the primary water absorbs the heat generated in the main components and the heat from the lower main pump until it reaches the spiral tubes of the heat exchanger. At t = 310 s, the temperature value between them is in the range of 69.62 ~ 72.33 °C, which means no more than 3 °C, and the temperature difference between them becomes smaller.

But in contrast, the casing is different from most other components. The casing temperature rise rate is relatively lower, except for the stator core and the stator winding insulations. The casing is adjacent to the secondary water in the heat exchanger and the yoke portion of the stator core. It is interesting to observe that the temperature magnitude of the casing is between the range of secondary water and the stator core during the 0–18 s period. The casing absorbs the amount of heat transferred from the stator core and then transfers it to the secondary water. After t = 18 s in the SWSB fault, the casing is heated by high-temperature secondary water, which means that the direction of heat transfer changes to the opposite direction. This is mainly because that the magnitude of the natural convection heat transfer coefficient on the outer surface of the casing decreases sharply compared to forced convection heat transfer, and the temperature of the secondary water whose speed is almost 0 m/s increases rapidly in the spiral channels of the heat exchanger. The temperature rise of the casing is relatively smaller than other solids in Fig. 6 over 310 s because the thickness is larger than that of the thin stator/rotor shield sleeve, although they are all close to primary water or secondary water with low temperatures. The temperature rising rate of the stator/rotor shield sleeve is 0.1043 °C/s and 0.1034 °C/s, respectively, which is higher than other solid parts.

Evolution of the transient temperature rise of primary water

This part mainly focuses on the evolution characteristics of the temperature rise of primary waters. Figure 7 shows the temperature contour of the primary water evolving with time at t = 0, 30, 180, 240, and 310 s from the upper primary water inlet, the end cover to the lower flywheel water, and in the spiral tubes the specific position names are shown in Fig. 2. The bottom of the flywheel is connected to the high-temperature coolant (287.3 °C) of the reactor with a total pressure of 15 MPa. From Fig. 7, it can be seen that all the primary water temperature over the lower flywheel gradually increases with time, for example, at the position of the upper or lower measuring points (as shown in Fig. 4), the spiral tubes after the moment of t = 30 s.

Fig. 7
figure 7

Evolution profile of the primary water temperature contour versus time.

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Taking into account the primary water in the motor with different cooling functions in different positions, the primary water path in the motor is divided into three sections. Section “Introduction” extends from the primary water inlet to the water inlet in the stator rotor gap (below the outlet of the auxiliary impeller), where the upper bearing lubricating water circulation takes place. The location between the water gap outlet and the lower annulus space where the set of lubrication water measuring points is defined as Sect. “Mathematical model and solution conditions”. The middle gap zone, where the water was in a of Taylor-Coulter-Poisson turbulent flow state, is defined as Sect. “Geometrical model and cooling waterway”. The definition of the percentage of sectional temperature increase is as follows:

$$\eta_{i} \left( t \right) = \frac{{\Delta T_{i} \left( t \right)}}{{\Delta T_{tot} \left( t \right)}},i = {1,2,3 }$$
(9)

where ΔTtot is the total temperature rise of the primary water, and \(\Delta T_{{{\text{tot}}}} = T_{{\text{bot - test}}} - T_{{\text{pri - inlet}}}\). \(\Delta T_{i}\) is the temperature rise of the primary water in section i.

Figure 8 shows the evolution of primary water temperature T, \(\Delta T_{i}\), and \(\eta_{{\text{i}}}\) over time in these three sections. It can be seen that the average temperature of area T at each monitoring inlet and outlet of the primary water path increased with the SWSB fault time. This is because the mode of heat transfer between the secondary and primary water tubes changes from forced convection to natural convection. The surface heat transfer coefficient dropped dramatically4. It causes the heat transfer performance of the heat exchanger to deteriorate. However, the temperature rise rate values ΔT1, ΔT2, ΔT3, and ΔTtot all decrease because the heat transfer temperature difference between the high-temperature components and the low-temperature water decreases with increasing SWSB failure time, as shown in Fig. 6.

Fig. 8
figure 8

Temperature variation curves of primary water versus time.

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The η1 for the upper water circulation does not change significantly over time. The η2 for Sect. “Geometrical model and cooling waterway” gradually decreases over time. The η3 at 310 s is about 4% higher than at the initial time 0 s. The η3 is higher than η2, the main reason is that the primary water in Sect. “Mathematical model and solution conditions” is mixed with the high-temperature water on the upper clearance of the flywheel through eight radial holes, as shown in Fig. 7. The primary water below the flywheel is connected to the coolant of the nuclear pump, where the coolant temperature is maintained at 287.30 °C. It also highlights that the effect of the high temperature of the nuclear pump coolant on the lubricating water temperature at the lower position of the motor under transient SWSB fault conditions gradually increases with time.

Given that the amount of heat transferred from the canned motor to the secondary water would be less in summer with extremely hot ambient parameters (EHAP) than in winter at the laboratory, the motor temperature field is predicted to operate with EHAP under steady-state and transient-rated conditions. The initial and inlet boundary conditions in the SWSB fault are given in Table 2. The maximum temperatures for the lubricating water and the insulation of the stator winding are all below the permitted temperature, and the characteristics of temperature evolution are the same.

Conclusions

This paper presents a methodology and predicted the characteristics of temperature rise evolution of the primary/secondary cooling water and main components within 310 s for the canned motor with 0.24 MW capacity in the SWSB fault. The predicted and measured temperatures agree well with a maximum error of + 5.97%. The main conclusions are as follows.

  1. (1)

    The temperature rise of the stator winding insulation with time is not significant and the temperature rise rate is 0.00473 °C/s according to the test. The temperature rise rate of the stator/rotor shield sleeve is 0.1043 °C/s and 0.1034 °C/s, respectively, which is higher than other main components.

  2. (2)

    The temperature difference between the main components and the primary water becomes smaller, the magnitude is not more than 3 °C at 310 s, except for the housing. The temperature distribution tends to be uniform.

  3. (3)

    The temperature difference between the stator and rotor shield sleeve changes slightly with time, and the rate of temperature rise with time is consistent with that of the gap water.

  4. (4)

    The temperature of the primary and secondary water and the percentage of temperature rise in the lower lubricating water section increase with time. The effect of the high temperature of the lower coolant in the nuclear main pump on the temperature of the primary lubricating water in the lower position of the motor gradually increases over time during the transient SWSB fault.