JPS5827879B2 - Nuclear reactor critical proximity method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for bringing a nuclear reactor to criticality, and more particularly to a method for bringing a nuclear reactor to criticality by extracting a plurality of neutron absorbing elements arranged in a reactor core.

Here, the neutron absorption element is located in the reactor core and has the ability to absorb neutrons generated by a nuclear reaction and control the output of the reactor.

Control rods and poison tubes (or poison solutions in poison tubes) in heavy water reactors correspond to this.

The present invention can be widely applied to nuclear reactors in which a plurality of neutron absorption elements are arranged in the core.

The scope of application is light water reactors with control rods, heavy water reactors with poison tubes, and others.

However, in the following explanation, for the sake of simplicity, a heavy water nuclear reactor having a poison tube will be used as an example.

Poison tubes have conventionally been installed in heavy water nuclear reactors for the purpose of rapidly shutting down the reactor by removing poison solution therein.

FIG. 1 shows a cross-sectional view of the core of a heavy water-moderated nuclear reactor with a poison tube.

In FIG. 1, a core tank 1 is filled with heavy water 9.

Inside the reactor core tank 1 are a fuel assembly 2, a poison tube 3, a neutron source 8 that emits neutrons to induce nuclear fission in the reactor,
Start-up area neutron instrumentation A4, neutron instrumentation B5, neutron instrumentation C6, neutron instrumentation D that measures neutron flux in each region of the reactor core
I is provided.

In this figure, the fuel assembly is only schematically shown as a circle, but in reality, it is as shown in Figure 2 (vertical cross-sectional view of the core, showing one embodiment of the present invention). Fuel assembly 2
is housed in the pressure tube 11.

The pressure tube 11 is housed in a calandria tube 12.

A coolant (light water) 10 flows through the pressure pipe 11 to remove reaction heat generated in the fuel assembly.

In this type of nuclear reactor, a poison solution is injected into the poison tube 3 when the reactor is shut down.

Since the poison solution absorbs neutrons that contribute to nuclear reactions, the reactor can be stopped by injecting the poison solution.

The present invention provides a method for restarting this shut down nuclear reactor to achieve criticality (a state in which the nuclear fission chain reaction is maintained at a constant rate in the reactor), that is, a near-criticality method.

The present invention devises a method for drawing out poison solution from a poison tube to provide a good near-criticality method.

Conventionally, there have been three approaches to criticality for heavy water-moderated nuclear reactors having this type of poison tube:

The first method is to sufficiently lower the heavy water level to remove the poison liquid in the poison tube, and then raise the heavy water level to bring it near criticality.

Second, after injecting enough liquid poison into the heavy water moderator,
In this method, the poison liquid in the poison tube is removed, and then the poison in the moderator is removed to achieve near-criticality.

The third method is to lower the heavy water level or inject poison into the heavy water moderator, remove the poison liquid in the poison tube, and then insert a booster rod to achieve near criticality.

The booster rod is made of a material that has high fission performance and emits large amounts of neutrons.

The material is usually highly enriched uranium.

In the above three methods, do not inject liquid poison into the heavy water or draw out the poison liquid from the poison pipe while leaving the heavy water level as it is because the output may suddenly increase and there is a risk of a startup accident. be.

Next, the drawbacks of the above three methods will be described.

In the first method, a heavy water 4-level control system must be installed for startup, making the system complicated.

Furthermore, there is a problem in that it is difficult to cool the calandria tube when the heavy water cylinder is low.

In the second method, a large amount of poison must be injected into the moderator and removed.

Therefore, the frequency of replacing the replacement resin for removing poison increases.

Another drawback is that it takes several minutes for the poison concentration in the heavy water in the reactor core to reach a predetermined value after the ion exchange resin begins to remove the poison, resulting in a slow response.

In the case of the third method, a booster rod must be prepared for activation.

Boosf rods are usually highly enriched uranium and are expensive, making the reactor uneconomical.

Furthermore, there are drawbacks such as the need for a drive system for the booster rod.

By devising a method for removing the poison solution in the poison tube, it becomes possible to perform a critical approach method by drawing out the poison solution from the poison tube, which has been considered difficult in the past.

Previously, the method of making a nuclear reactor critical by drawing poison solution out of a poison tube without manipulating the level of heavy water was not used because of the fear of a start-up accident, as mentioned above.

The startup accident referred to here is due to an operator mistakenly removing the poison solution in the poison tube in the center of the core, which has a large reactivity value, in the vicinity of criticality, resulting in supercriticality, a sudden increase in output, and the release of fuel into the core. This is an accident that causes melting.

Start-up accidents are particularly likely if the poison solution is removed rapidly.

An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art and to provide a near-criticality method for a nuclear reactor that can accurately predict in advance the neutron absorption element to be extracted at the critical point.

A feature of the present invention is that the extraction numbers of the plurality of neutron absorption elements that have been extracted and the respective readings of the neutron instrumentation at the time of their extraction are stored, and The purpose is to predict the neutron absorbing element to be extracted near the critical point based on the relationship with each reading of the instrumentation.

A preferred embodiment of the present invention will be described below.

FIG. 1 is a sectional view of the core of a heavy water-moderated nuclear reactor in which the near-criticality method of the present invention is implemented, as partially described above.

Poison tube 4 is installed in order to ensure reactor shutdown ability.
One fuel per fuel assembly is placed in the heavy water moderator.

Suppose now that poison solution has been injected into all poison tubes in the reactor and the reactor is in a stopped state.

Let us assume that the reactor in this state is restarted to achieve criticality.

First, the order of drawing out the poison solution in the poison tube is set in advance.

This drawing order is, for example, as shown in FIG.

Each block in FIG. 3 indicates the position of a poison tube in the core cross section.

The numbers written in the blocks are the order of withdrawal of the poison solution.

A method for determining the drawing order will be briefly described.

The extraction order is determined with the following objectives: 1. If a startup accident occurs, the scope of the accident will be minimized, and 2. startup and operation are easy.

To achieve this, the following conditions must be satisfied.

“Minimize the withdrawal reactivity at the critical point.

'' As a method for achieving the above conditions, first, among those whose reactivity is less than a certain value (reactivity for easy operation), the one with the highest extraction reactivity is first extracted.

After completing the extraction of the four 90° rotation targets, select the one with the next highest degree of reactivity and pull it out in the same manner.

This calculation is performed many times to find the drawing order that minimizes the critical point, and then sets it.

That's all. FIG. 3 shows an example of the drawing order determined as described above.

However, this order is not fixed, and changes as the reactor conditions such as fuel burnup change.

Figure 4 shows the change in the extraction reactivity value per poison tube when the poison solution is extracted in the order shown in Figure 3.

From FIG. 4, it can be seen that the 24th poison tube becomes critical.

This is because the withdrawal reactivity is minimized.

However, the fact that the 24th point will be the critical point is just a predicted value calculated before starting the reactor, and it does not necessarily mean that criticality will actually be reached at this number. It is quite possible to reach that goal.

Assume that the optimum withdrawal order of the poison solution for a given state of the reactor is as shown in FIG.

After storing the extraction order shown in FIG. 3 in the storage device 29, the poison solution is extracted according to the instructions from the storage device 29.
Perform critical proximity.

The critical proximity method will be explained with reference to FIG.

First, we will start by explaining how to shut down a nuclear reactor.

During operation of the nuclear reactor, the power range neutron meter device 3 constantly detects the power and sends it to the neutron flux-power converter 19.

When the output of the reactor becomes larger than the output set by the output setting device 20, the poison injection valve 22 and the heavy water position control valve 32 are activated by the command from the poison rapid injection indicator 21.
Let's open.

By opening the two types of valves, poison is injected into the poison pipe 3 from the poison dissolving tank 15.

The poison solution in the poison solution tank 16 is injected into the poison solution tank 15 in advance by the poison solution circulation pump 24 .

The high-pressure gas in the high-pressure gas tank 14 applies pressure to the poison liquid level in the poison dissolving tank 15, helping to inject the poison liquid.

At this time, valves 17 and 18 are closed to prevent the poison solution from flowing out from the poison pipe 3.

Furthermore, since the heavy water position control valve 32 is open, the heavy water 9 flows into the heavy water dump tank 31, reducing the amount of moderator, thereby assisting in shutting down the reactor.

A heavy water constant level (pressure gauge) 33 is installed in the system of the reactor core tank 1, heavy water level control valve 32, and heavy water dump tank 31. It has a function of closing the position control valve 32.

After the above operation I, the reactor is shut down.

This shut down nuclear reactor is restarted using a poison tube as follows.

First, inside the core tank 1 is a neutron moderator.

Re-inject heavy water.

Heavy water in the heavy water dump tank 31 can be injected into the core tank 1 by the pump 34 .

In order to increase the neutron flux within the reactor core, the poison solution in the poison tube 3 is drawn out.

The command from the poison tube drawing order storage device 29 is transmitted to the poison tube drawing instruction device 30.

From the instructions ζ of the device 30, first turn on the poison water level coarse control valve 1.
7 is opened, and the poison solution is drawn out in accordance with the order stored in the poison tube drawing order storage device 29.

The extracted poison solution is transferred to the poison solution tank 16.
flows into.

In the case of a poison tube from which poison solution is drawn out at the initial stage of reactor startup, the poison water level fine adjustment valve 18 is closed.

A poison water level gauge (pressure gauge) 23 is installed in each poison tube, and the detected value of 23 is manually inputted to a poison tube extraction position calculator 28 to determine the position of the poison tube that has been or is being extracted. can be known.

In parallel with the extraction of the poison solution, the neutron flux is constantly measured by the startup region neutron instrumentation devices 4 to 7.

FIG. 5 shows a computer simulation of the correspondence between this measured value (reading N of the starting region neutron instrumentation) and the poison tube extraction order number.

In FIG. 5, No is the value of N when the poison tube drawing order number is O.

When number O, N=N. becomes.

Furthermore, since N becomes infinite at the critical point, -=0.

Since the number 24 is -O, it is considered that the criticality is achieved at the number 24.

From the graph of FIG. 5, it is considered that the relationship between the poison tube extraction order number M and the inverse ratio of neutron counts can be approximated by:

Here aK is a coefficient and t is an integer.

'''0 Thoughts 7 direction also 6°0.1zu゜ into reverse J'''? Shape 7・
It is stored in the neutron inverse ratio storage device 25.

The value of N is sent to the least squares fitting calculator 26, made to correspond to the drawing number M of the Poyns tube, and fitted to equation O.

Assuming approximation using a cubic equation, t=4, al=a, a2=b
If 1a3=C1a4=d, then P=A, B, C, D, corresponding to the activation region neutron instrumentation A, B, C, D.

The coefficients aP, bP, CP, and dP can be obtained by substituting the 6-output number M and the corresponding K into the above equation.

For example, coefficients aA and bA corresponding to neutron instrumentation A.

When calculating cA and dA, use the four values of the drawing number M,
Substitute the value of - corresponding to each extraction number into equation (1) to create four equations.

The number of unknowns is 4 in total from aA to dA, so %
All values of aA"'-dA are found.

Similarly, another neutron instrumentation B. Corresponding to C and D (1
) unknowns aB and bB in the equation.

CB t dB +°””””t aDtbDPCDt
dD can also be determined.

The least squares fitting calculator 26 calculates the coefficient aPybP,
After calculating CP and dP, the critical point predictor 27 calculates (1
) is used to predict the poison tube withdrawal number at the critical point as follows:

Since (-g>, -0 at the critical point, equation (I) is ap+bpM+cpM2+d M" = 0 ・"・
-・(2).

Solve equation (2) for M, and put the root M in the square symbol []
Convert it to an integer [M] by

hi0 is the drawn poison tube number at the critical point.

Since P=A, B, C, D, [M] neutron instrumentation A, B,
It is obtained for each of C and D.

Let these be CMPIP=A, B, C, and D.

The obtained four values [MP'JP- Compare A, B, C2D, (MI)] If you find the value where p = A, B, C, D match, it will be the drawn poison tube at the critical point. will be the number.

Even if all four values do not match, it does not matter in practice if at least two values match, that is, [:'MA:l(MB)] is critical. .

The predicted value of the above critical point is always
It flows to the poison extraction instruction device 30.

Until the poison pipe number reaches the critical point, a signal from 30 is sent to the revolution water level rough adjustment valve 17, and the poison solution flows down into the poison solution tank 16 at a high speed.

However, when the extraction of the poison solution from the poison tube stops near the critical point, the poison extraction instruction device 3
The signal from 0 is sent to the poison fine adjustment valve 18, which causes the poison solution to flow down at a slow rate and intermittently.

By using the poison fine adjustment valve 18, it is possible to prevent a sudden increase in output due to excessive removal of the poison solution.

In the above embodiment, the order of drawing out the poison tubes is set in advance, but for example, if the poison tube bow drawing order storage device 29 malfunctions and the operator mistakenly removes the solution from the poison tube with a large reaction value. Even if a startup accident occurs because the scheduled extraction order is not followed due to pulling out the neutron If the fuel can be pulled out late, major accidents such as fuel melting can be prevented.

Figure 6 shows what happens when such a startup accident occurs.

The maximum fuel temperature is only 1,600°C, and the fuel does not melt.

In FIG. 6, A is the maximum local output, the opening is the maximum temperature of the fuel pellet, and C is the maximum temperature of the fuel cladding.

In the embodiment shown in Fig. 2, heavy water dump devices 31 and 32 are provided as a backup safety system, but in order to rapidly reduce the negative reactivity injection effect by the heavy water dump, the heavy water level near the critical level is set to 7. As shown in the figure, it is desirable to lower it to the fuel level.

According to this embodiment, it is possible to predict which poison tubes will be pulled out near the critical point based on the relationship between the extraction numbers of the multiple poison tubes that have already been pulled out and multiple readings of the neutron instrumentation corresponding to those operations. Therefore, the poison tube to be drawn near the critical point can be accurately predicted well before the critical point O is reached.

Therefore, the number of the poison tube to be extracted near the critical point can be notified to the operator in advance, which facilitates the operation of the nuclear reactor.

Additionally, it is possible to safely bring a nuclear reactor to criticality.

This embodiment can further produce the following effects.

(1) The reactor can be started by drawing out the poison liquid in the poison pipe for shutting down the rapid reactor, making the system simpler and easier to operate than the conventional control system.

It also reduces plant construction costs.

(2) The safety of the furnace is improved as a result of being able to prevent fuel melting accidents due to a sudden increase in output that may occur when starting up the furnace.

(3) As a result of reducing the frequency of operation of the liquid poison injected into the heavy water moderator, the frequency of replacing the poison resin and the loss of heavy water are reduced, and the performance of the plant is improved.

According to the present invention, the neutron absorption elements to be extracted near the critical point are predicted based on the relationship between the extraction numbers of the plurality of neutron absorption elements that have already been extracted and the plurality of readings of the neutron instrumentation corresponding to their operations. , it is possible to accurately predict the neutron absorption element that will be extracted near the critical point.

[Brief explanation of the drawing]

Fig. 1 is a horizontal front view of the core of a heavy water reactor, Fig. 2 is a route diagram illustrating the reactor criticality proximity device and its operating principle, which shows one embodiment of the present invention, and Fig. 3 An example of the order of drawing out the poison liquid in the poison pipe according to the method, Fig. 3
The drawing reactivity value when the solution is drawn out one by one in the drawing order shown in the figure, and Figure 5 shows the criticality value from the change in the inverse ratio read by the neutron instrumentation when near-criticality is carried out using the method of the present invention. Figure 6 shows the maximum local output and fuel pellets when a part of the device of the present invention (drawing order memory device) fails and the operator pulls out the solution from the poison tube with the maximum reactivity. Time variation of maximum temperature and fuel cladding temperature, 7th
The figure is a diagram showing the relationship between the zero position of heavy water and the input reactivity of a heavy water dump device with a backup stop system. 4.5, 6.7...Starting region neutron instrumentation, 17
...Poison water level rough adjustment valve, 18...
Poison water level fine adjustment valve, 25...Neutron inverse ratio storage device, 26...Least squares fitting calculator, 27...Critical point predictor, 29...・
Poison tube extraction order storage device, 30...Poison extraction instruction device.

Claims (1)

[Claims] 1. A near-criticality method for a nuclear reactor, in which a plurality of neutron absorption elements arranged in the core of a nuclear reactor are sequentially removed from the core in a preset order to bring the reactor to criticality. , the extraction number of the neutron absorption element is Ml, the reading of the neutron instrumentation is N, and the coefficient is a. Neutron instrumentation reading N
, predict the extraction number M of the neutron absorption element to be extracted near the critical point from the root, and select the neutron absorption element extracted near the critical point from the neutron absorption element extracted before Near-criticality method for nuclear reactors that also pulls out at a slow speed.