JPS6279397A - Fast reactor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a flow rate side landing gear device and a control method in the event of an accident in a fast reactor.

[Background of the invention]

Generally, the coolant temperature in the fast reactor core is determined by the ratio of the coolant flow rate supplied by the circulation pump to the core power. That is, the coolant flow IIkF (kt/see
) f Thorium specific heat Cp (cal/kg c'l When core power Q ('''sec) is given, the coolant temperature difference ΔT at the core entrance and exit is:

F−Cp·ΔT=Q (1) is given. Here, ΔT is ΔT=(core outlet temperature−core inlet temperature). The flow rate of the coolant can be controlled by controlling the rotation speed of an electric motor (hereinafter referred to as pump motor) that rotates the pump.

The power output of the reactor core can be controlled by controlling the control rods. When the coolant temperature exceeds the boiling point (930C), the coolant boils.

This may cause damage such as melting of the fuel rods.

Coolant temperature is not allowed to exceed the boiling point.

Further, if the power/flow rate ratio (Q/F) changes suddenly, the temperature of the coolant at the exit of the reactor core changes rapidly, which is undesirable because it gives thermal stress to the structural members of the reactor.

Conventionally, the pump system for fast reactors was as follows. The external power source consists of a motor, fluid coupling, and generator called an MFG set. It is supplied to a series of power generation systems. The electric motor of the MFG set is rotated by an external power source.
The generator is rotated, thereby generating electricity.The fluid coupling that connects the '1 motor and the generator allows the torque transmitted from the motor to the generator to be controlled, and the rotational speed f1 of the generator to be controlled. can. As a result, the frequency of the power generated by the M FG set becomes f. The power generated by this MFG set is supplied to the induction motor, which is the pump motor, to rotate the pump. Since the number of rotations of the pump motor is equal to the frequency f of the MFG set power, the fluid coupling eventually causes the pump to rotate. The flow rate will be controlled.

On the other hand, when a nuclear reactor is scrammed in the event of an accident, the core output drops to about 7% of its rated power in 2 to 3 seconds.

Therefore, conventionally, at the same time as the scram, the power to the MFG set was turned off and the MFG set, pump motor, and pump were stopped to prevent sudden changes in output/flow rate.

When the power to the pump motor is cut off, the pump rotates only by inertia and the flow rate decreases exponentially, so the thermal shock of the structural material ＼, in addition to the slow-flow coupling, controls the rotation speed of the pop motor. A direct control device is installed to dampen thermal shock by applying braking to the pump motor and stopping the pop motor suddenly during scram, bringing the rate of decrease in output closer to the rate of decrease in flow rate.

However, conventional pump systems only stop the pump in the event of an accident, and do not have a mechanism to appropriately control the flow rate in the event of an accident. Therefore, in the unlikely event that the atomic scram fails, the reactor will not shut down and only #, turtle will decrease despite transient fluctuations in output and coolant temperature. . As a result, the coolant temperature rises, causing boiling, which ultimately leads to damage to the fuel and core.

[Purpose of the invention]

The purpose of the present invention is to control the transient behavior of a nuclear reactor when a scram fails until the reactor is stopped by one manual scram.
The object of the present invention is to provide a nuclear reactor having a control system (IIa) that can be controlled to prevent coolant boiling and fuel damage.

[Summary of the invention]

In a reactor that fails a scram, control rods cannot be used to control reactor power until a manual scram is performed. Therefore, only the flow rate (or the rotational speed of the pump) can be controlled from the outside. At the same time, in fast reactors, when the temperature of the coolant in the core increases, the leakage of the inner shield increases due to thermal expansion of the core, which causes negative reactivity. can be used to increase the output of a nuclear reactor.

In order to achieve the object of the present invention, firstly, the power of the reactor and the temperature of the coolant at the entrance and exit of the reactor core are determined.

A detector system that detects the flow rate, etc., and a calculator that inputs these signals and calculates the bacteriostatic flow rate are provided.

A frequency converter is then provided to vary the power frequency supplied to the pump motor in order to control the pump flow rate. This is because in the event of an external power loss accident, the MFG set itself will stop, making it impossible to use the flow rate leg using the fluid coupling.

In addition, when external power is lost, the power to turn the pump itself is lost, so it can be used as an energy storage element.

A flywheel is attached to the rotating motion of the MFG set so that the MFG set continues to rotate for a while even when external power is lost.

The above device is used to control the flow rate of the coolant, but in the event of an accident such as loss of external power, the rotational energy of the flywheel as an energy storage element will be used to rotate the pump. The rotational energy of is, in principle, proportional to the mass of coolant that the pump can flow in the event of a loss of external power.

Since the amount of stored energy is finite, the pump should be operated with as little flow rate as possible in order to keep the pump running for as long as possible.

To achieve this, the flow rate is reduced to control the core coolant temperature to be as high as possible.

Due to the negative reactivity effects of the reactor, the power of the reactor tends to decrease, so the flow rate can be further reduced, and overall the energy to drive the pumps can be saved considerably. However, as already mentioned, it is not allowed that the coolant boils or the fuel melts, so there is an upper limit to the coolant temperature, and it is optimal to control the coolant flow rate as low as possible below the upper limit. There are certain controls. The arithmetic unit of the present invention includes a function to calculate such an optimal value. Optimal bactericidal control, such as water supply, has the advantage of reducing the installed capacity of energy storage elements such as flywheels, or increasing the time available for manual scram treatment. There is.

[Examples of the Invention] The present invention will be described below using Examples. Examples of the present invention are shown as the first factor. The external power source 9 turns the electric motor 3 of the MFG set, and this torque is transmitted to the generator 6 via the fluid coupling 4. G! The machine 6 is provided with a flywheel 5 as an energy storage element. The electric power generated by generator 6 is.

It is supplied to the pump motor 2 to turn the pump 1 and supply coolant to the reactor. The flow of coolant is 1 ton.

A loop is configured that circulates between the nuclear reactor 12 and the heat exchanger 20. Between generator 6 and pump motor 2.

A frequency converter 10 is provided. Normally, the power frequency of the generator 6 is controlled by the fluid coupling to control the rotation speed of the pump motor 2, but during a scram, the power frequency of the 1 generator 6 is controlled by the frequency converter 10 and the pump motor 2 is controlled by the frequency converter 10.
Change the rotation speed.

From the reactor, there are detectors and signal lines 12, 17, 14, and 16 that detect core output Q, coolant flow rate F, core inlet coolant temperature 'pin, and core outlet coolant 1M degrees Tout. It is provided. Furthermore, a computing unit 18 is provided, and signal lines 12.17° and 14.16 are connected to the computing unit 18.

The computing unit 18 has the following functions.

Upper limit of coolant temperature to prevent coolant from boiling 'p c
, max and the upper limit value Tf, max of the fuel temperature at which the fuel does not melt are given in advance. The realistic values are: TC, max is about 880C, Tf, max
is about 2200 tll'.

The computing unit performs the following operations.

(1) Calculate the coolant temperature Tc, f when the fuel temperature becomes Tf, max. 'l'cf is T(maXI
Q.

Tcf = f (Tf, max, Q, Tin) as a function of Tin
...Calculated in (2). The function f is determined by design calculation or experiment, and is stored in the arithmetic unit as an analog or digital program.

(2) The smaller of Tcf, Tc, and max is T.
Defined as m.

Tm = Min (Tcf, Tc, max)
-(3) (3) Deliver a flow rate Fm such that the core outlet thermal insulation becomes Tm.

Ftn = g (Tm, Q, Tin)
(4) Here, the function g is given by 1 if formula (1) is used.

(4) Calculate the rotation speed f1 of the pump that provides the flow iFm.

f−”h (Fm) =・(5)
Here, the function is determined by the torque percentage curve of the pump.

(5)° Let fIIIgl be the minimum number of revolutions of the pump required for decay heat removal. This is used as an auxiliary cooling system.
It is equal to the rotation speed of the provided pony motor 7. f
The larger of m and f is defined as f.

f, = MaX (r,,, fM,]
-(a) The rotational speeds calculated by the calculation a18 as described above are transmitted to the river wave number converter 10 via the signal line 19. Frequency conversion atort.

The frequency of the electric power from the generator 6 is converted to f.

As a result, the rotation speed of the pump 1 becomes f.

The operation according to this embodiment will be described with respect to the case where the scram at the time of an accident is successful and the case where it is unsuccessful.

If the scram is successful, the core power Q will decrease rapidly,
reaches decay heat level. Therefore, equation (4) and (5
) is given by the formula. Coolant flow rate Fm Pump rotation speed f1
also becomes smaller, and the flow n is rapidly narrowed down. However, (6
) The flow rate according to the formula will never fall below the decay heat removal level. With this control method, the pump is effectively braked.

If the scram fails, the core power Q does not necessarily decrease, and the reactor undergoes a transient change.

We will discuss typical accidents such as flow loss accidents and overpower accidents.

Figure 2 shows transient changes in the case of an external power loss accident (flow loss accident). In the case of an external power loss accident, Tm given by equation (3) is the rated core exit temperature (530C')
higher than Because of this, in the early days.

The side control is performed so that the flow rate is throttled and the coolant temperature rises.
'L reactor's reaction flaw coefficient is approximately -2X10-'Δ
/C, so as the coolant temperature increases,
The output decreases as shown in FIG. The flow rate is controlled to decrease as the output decreases.

The shaded area in FIG. 2, which is the time-integrated curve of the pump flow rate, is proportional to the work done by the energy stored in the flywheel. By ramping up the coolant temperature quickly in the beginning, negative reactivity effects of the reactor can be actively exploited, thus helping to save energy in the flywheel.

The accident continued for about 20 minutes without the coolant boiling, and during this time it was possible to shut down the reactor with one manual scram.

In addition, in the process of rapidly reducing the pump flow rate at the beginning of an accident,
Since the rotational speed of the pump turns the induction kJJ machine 2, the induction motor operates as a generator and stores energy in the MF'G set with a flywheel. In the end, the energy loss in the single-operation method consists of only one near-air loss of electrical equipment and one major loss of rotating bodies and fluids, which are sufficiently small compared to the amount of stored energy.

The reduction in core outgassing due to this operating method is due to the reactivity effect due to the increase in coolant &[.

In order to completely and safely shut down the reactor, it is necessary to take measures to inject some kind of negative reactivity, such as a manual scram. By using this operating method, there will be sufficient time to take the above measures. After consuming the energy stored in the flywheel, the pony motor 7 is used.

It is sufficient to maintain the flow rate at a decay heat removal level.

The conventional method does not include the concept of feedback control using an arithmetic unit as in the present invention, so it is not possible to achieve the optimum flow rate after a scram occurs. In the conventional technology,
In case of external power loss accident.

This results in a transient change as shown in Figure 3. Here, the pump flow rate only decreases exponentially. In order to reduce the flow rate slowly, a flywheel with a large moment of inertia is required. ! @ In the case of figure 3, half the flow rate rs
N, the time is about 40 seconds. As the flow rate decreases, the coolant hardness increases, but as negative reactivity enters the reactor, the output decreases slightly.

Coolant never boils. When the half-life time of the pump flow rate is set to 40 seconds or less (that is, the moment of inertia of the flywheel is reduced), the output/flow rate ratio increases and the core exit temperature exceeds the boiling point. The shaded area in Fig. 4 shows the case where the embodiment of the present invention (Fig. 2) is used and the case where the conventional technology (Fig. 2) is used.
This is the difference in pump flow rate compared to when using the pump (Fig. 3). According to the present invention, the flow rate driven by the pump can be reduced to about 172 ml.

This means that the energy stored in the flywheel is approximately 1/2
It will be fine.

Next, we will discuss the transient changes in the case of an overpower accident in which scram fails. FIG. 5 shows a transient change at the time of an overpower accident according to the embodiment t+lj (FIG. 41) of the present invention.

Originally, in the event of an overpower accident, loss of the external power source 9 is not assumed, so it is considered that the energy to drive the pump can always be supplied. In an overpower accident, the output exceeds the rated output, so Tm in equation (3) is.

Often, an upper limit value T c f is specified to ensure that the fuel does not melt. The flow rate can be controlled so that the core outlet 4K reaches Tm of equation (3). As the coolant temperature increases, negative reactivity enters the reactor. If this negative reactivity is greater than the positive reaction that caused the overpower accident, the output will gradually decrease, but if the positive reactivity effect is greater,
Output continues to increase. If the output continues to increase, the fuel will eventually melt and the coolant will boil. Even in that case, by implementing the present invention, it is possible to melt the fuel,
The time it takes for the coolant to reach boiling can be extended to the maximum. In the prior art, since the flow rate feedback sub-system using the calculation method is not possible, the flow rate only decreases exponentially, and in the event of an overpower accident due to a failure in scram, the coolant immediately boils.

〔Effect of the invention〕

As described above, according to the present invention, even if a scram fails in the event of a nuclear reactor accident, it is possible to prevent boiling of coolant and damage to fuel until the reactor is shut down by manual scram. , nuclear reactors can be controlled without using control rods.

If the pump is a main cooling system pump or an MHD type pump, the same control as in the present invention can be performed by controlling the DC voltage applied to the pump. In this case, the fluid coupling of the lVF'G set is no longer necessary.

The frequency converter can be replaced with a rectifier with a simpler structure, and the amount of equipment waste can be further reduced.

[Brief explanation of drawings]

Figure 1 is a configuration diagram of an equipment system showing an embodiment of the present invention, and Figure 2 is a diagram showing changes in output, flow rate, and core exit temperature per mouth during a flow loss accident due to scram failure when the present invention is implemented. Figure 4 is a diagram showing changes in output, flow rate, and core exit temperature over time during a flow loss accident caused by failure of scram in the conventional technology. An explanatory diagram of the difference in pump flow rate in the technology (Figure 3), and Figure 5 is a diagram showing changes over time in output, flow rate, and core outlet temperature during an overpower accident in which scram failed when the present invention was implemented. be. 1... Pump, 2... Pump motor, 3... Electric motor, 4... Fluid coupling, 5... Flywheel, 6
River generator, 7... Pony motor - 18... Auxiliary power supply, 9... External power supply, 10... Wave number converter, 11.
...Reactor core, 12...Reactor vessel, 13...Control rod drive mechanism.

Claims (1)

[Claims] 1. In a sodium-cooled fast reactor that has a reactor vessel containing a reactor core, a cooling system that has a circulation pump that supplies coolant to the core, and a control rod that controls the core output, the core power, core outlet coolant temperature, core inlet It has a detector that detects each of the coolant temperature and core flow rate, and a computing unit that uses the output signals of these detectors to calculate the center temperature of the fuel rods in the core. controlled by
A rotation speed controller that changes the rotation speed of the electric motor by changing the power frequency of the electric motor that directly drives the circulation pump, and a rotation speed controller that is connected to a generator that generates power for the electric motor and uses a part of the generated power. A fast reactor characterized by being equipped with an energy storage element that stores energy.