JPH0390895A - Detection of failed fuel assembly - Google Patents

Abstract

PURPOSE:To detect the presence or absence of a failure by fluctuating the output level of the used fuel assembly during in-pile storage by the withdrawing operation of shielding bodies. CONSTITUTION:The shielding bodies 2, 4 are disposed on the outer periphery of the reactor core and further, the used fuel 3, 5 are stored on the outer side thereof. The core output is kept constant and the level of the radiation detection signal of a failed fuel detecting system rises to about 10 times the level before the withdrawing and the presence or absence of the failure is easily known, for example, the shielding body 4 is withdrawn and if the fuel assembly 5 enclosing the failed fuel element is adjacent thereto. The detection is executed by a method of waiting until the signal level of a radiation detector is held constant by a cover gas method or delay neutron method, then withdrawing one piece of the shielding bodies 2, 4 adjacent to the reactor core side and comparing the level of the radiation detection signal before and after the withdrawing of the shielding bodies 2, 4 while maintaining the reactor output constant. This procedure is repeated and if the signal level attains the specified value or above before and after the withdrawing of the shielding body 4 where there is a change in the signal level during the course of this repetition, the fuel adjacent to this shielding body 4 is regarded to be failed and this position is determined.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention provides a method for identifying (confirming) damage to a fuel assembly during storage in a fast breeder reactor by pulling out a shield. Concerning a method for detecting aggregates.

(Prior art) A fuel assembly for a fast breeder reactor has a large number of fuel elements (also called fuel pins) arranged in a wrapper tube at equal intervals with spacers interposed therebetween.During normal operation of the reactor, the fuel elements are coated. The fissile material in the fuel pellets loaded into the tube, e.g.
u reacts with neutrons and undergoes nuclear fission, producing a large amount of fission products. The fuel element is generally Pup, and fuel pellets obtained by compacting and sintering UO□ powder into a cylindrical shape are placed on a cylindrical cladding tube made of stainless steel, for example, and on the cladding tube.

It consists of an end plug that closes the lower end. The structure is such that the fission products produced by nuclear fission are contained within the cladding and are not released to the outside.

(Problem to be Solved by the Invention) However, during operation of a nuclear reactor, there is a possibility that the cladding tube may be damaged due to some unpredictable cause, and fission products to be contained may be released outside the fuel element.

Operating a nuclear reactor with a damaged fuel element is not desirable from the standpoint of safety as well as operation. Therefore, fast breeder reactors are generally equipped with a detection device that can quickly detect damaged fuel.

Conventionally, two types of this detection device are known. One method is to sample the cover gas sealed above the coolant in the reactor vessel and measure whether the cover gas contains gaseous components of fission products.
It is called the G (cover gas) method. The other method is to install a neutron detector in the primary cooling system connected to the reactor core and detect fuel assembly damage by measuring delayed neutrons from fission products contained in the coolant. It is called the (day-ray neutron) method. Conventionally, by using the above two methods and combining them with the operation of control rods, it was possible to confirm which position of the fuel assembly in the core was damaged. However, with regard to spent fuel assemblies stored in the reactor, there is a problem in that it is not possible to identify the location of the damaged fuel assembly.

The present invention was made in order to solve the above problems, and it is possible to remove fuel assemblies containing damaged fuel elements using the CG method or the D method.
It is an object of the present invention to provide a method for detecting a damaged fuel assembly that can easily and quickly identify which fuel assembly in the fuel stored in a reactor is damaged using the N method.

[Structure of the Invention] (Means for Solving the Problems) The present invention provides a method for detecting a damaged fuel assembly in a nuclear reactor in which spent fuel is stored in the reactor on the outer periphery of a shield. It is characterized by identifying fuel assemblies.

(Function) By pulling out the shield, the output level of each spent fuel assembly stored in the reactor is varied, and the presence or absence of damage is detected and identified by comparing the signal levels before and after pulling out. do.

(Example) An embodiment of the present invention will be described with reference to the drawings.

Note that the present invention is not limited to this example,
It can also be applied to embodiments having a similar meaning.

The figure is a conceptual plan view of the core of a fast breeder reactor with an electrical output of 10,100 O class, viewed from above. As shown in the figure, a large number of fuel assemblies 1 each containing a large number of fuel elements housed in a trumpet tube are arranged hexagonally in a honeycomb shape to constitute a reactor core. A shield 2 for shielding neutrons is installed around the outer circumference of the core.
Furthermore, spent fuel 3 is stored in the reactor outside of the reactor.

Here, it is assumed that fission products are detected by the CG method or the DN method during normal operation.

For example, after a certain period of time has elapsed after a fuel element is damaged, the release of fission products from the damaged hole becomes constant and becomes a value proportional to the production rate of fission products, that is, the output level. The signal level of the radiation detector is constant.

However, in this state, the correlation between the output level of each fuel assembly or the output level of the fuel element and the signal level of the radiation detector is not clear, and it is difficult to identify which fuel element in which fuel assembly is damaged. Can not.

Therefore, in the present invention, in this state, the output level of each spent fuel assembly stored in the reactor is varied by pulling out the shield, and the radiation detector in the CG or DN method is proportional to the output level. The fuel assembly containing the failed fuel element is identified by utilizing the signal level of the fuel element.

In this method, the shields adjacent to the core side of the spent fuel 3 in the figure are pulled out one by one.

Here, regarding a certain shield 4, the output level of the spent fuel assembly 5 adjacent to the shield 4 after extraction is approximately 10 times the output level before extraction, assuming the total power of the core is constant. becomes.

Therefore, if the fuel assembly surrounding the damaged fuel element is adjacent to the pulled-out shield, the signal level of the radiation detector of the damaged fuel detection system will be lower than before the pulled-out shield, assuming the core output is constant. After that, it will be about 10 times more. Therefore, the presence or absence of damage can be easily detected.

That is, specific examples of the method for detecting a damaged fuel assembly according to the present invention are listed below.

(1) Wait until the signal level of the radiation detector using the CG method or the DN method becomes constant.

Hereinafter, the same applies to the operation of pulling out each shield.

(2) One of the shields adjacent to the core side of the spent fuel stored in the reactor is pulled out.

(3) Compare the signal level of the radiation detector before and after the shield removal operation, assuming the reactor output is constant.

(4) Repeat steps (2) and (3) above.

(5) If the signal level of the radiation detector changes during the steps (1) to (4) above and exceeds a certain value before and after pulling out the shield, the fuel adjacent to the shield will be damaged. Determine the location of the damaged fuel.

It should be noted that the shield does not necessarily have to be completely withdrawn; if the shield can be removed from the axial range where fissile material is installed (usually referred to as the "core height"), the same effect can be obtained even if the shield is partially withdrawn. can be obtained.

Furthermore, by combining the changes in the signal levels of the radiation detectors during extraction for a plurality of shields, the identification accuracy of positioning (range of damaged fuel assemblies) can be further improved.

Furthermore, the spent fuel assembly may be pulled out instead of the shield. In this case, if the pulled out assembly is damaged,
Although the signal level will conversely decrease, the damaged fuel assembly can be identified in exactly the same manner as in the example described above.

[Effects of the Invention] According to the present invention, a damaged fuel assembly can be identified or specified at an early stage by a simple method of pulling out a shield or a fuel assembly, thereby improving the operability, availability, and safety of a nuclear reactor. It has the effect of improving.

[Brief explanation of drawings]

The figure is a conceptual plan view of the reactor core viewed from above to explain an embodiment of the method for identifying damaged fuel assemblies according to the present invention. 1... Fuel assembly 2... Shielding body 3... Spent fuel assembly 4... Shielding body adjacent to the spent fuel assembly 5... Spent fuel assembly adjacent to the shielding body (8733)

Claims (1)

[Claims] (1) A method for detecting a damaged fuel assembly in a nuclear reactor in which spent fuel is stored in the reactor on the outer periphery of a shield, characterized in that a damaged fuel assembly in the spent fuel is identified by a pulling operation of the shield. A method for detecting damaged fuel assemblies.