**2. Neutron flux**

It is convenient to consider the number of neutrons existing in one cubic centimeter at any one instant and the total distance they travel each second while in that cubic centimeter. The number of neutrons existing in a cm3 of material at any instant is called neutron density and is represented by the symbol n with units of neutrons/cm3. The total distance these neutrons can travel each second will be determined by their velocity.

A good way of defining neutron flux (߶) is to consider it to be the total path length covered by all neutrons in one cubic centimeter during one second. Mathematically, this is the equation below.

$$
\phi = n \,\, v \tag{1}
$$

<sup>\*</sup> Corresponding Author

Improving the Performance of the Power Monitoring Channel 233

control the detector drive system and measure the flux signal sensed by the detectors. The flux mapping data are used to verify the reactor core design parameters, and to determine the fission power distribution in the core. The new designed path selector for a guide the neutron

Fig. 1. The new designed path selector for KEPRI unit 1 reactor (Cho et al., 2006)

**4. Neutron flux monitoring and measurement in nuclear reactors** 

Kori nuclear units 1–4.

has gradually evolved.

**4.1 Neutron detectors and instruments** 

The path selector system is composed of four inner path selectors and an outer path selector. With the benefit of the double indexing path selector mechanism, the reliability of the detector drive system has been improved five times higher than that of a conventional system. Currently, the developed in-core flux mapping systems have been deployed at the

In thermal nuclear reactors, most of the power is generated through fission induced by slow neutrons. Therefore, nuclear sensors those are to be part of reactor control or safety systems are generally based on detectors that respond primarily to slow neutrons. In principle, many of detector types can be adapted for application to reactor measurements. However, the extreme conditions associated with reactor operation often lead to substantial design changes, and a category of slow neutron detectors designed specifically for this application

It is conventional to subdivide reactor instruments into two categories: in-core and out-ofcore. In-core sensors are those that are located within narrow coolant channels in the reactor

detectors through the reactor core are shown in Figure 1.

where: � = neutron flux (neutrons cm-2 s-1), � = neutron density (neutrons cm-3), and v = neutron velocity (cm s-1). The term neutron flux in some applications (for example, cross section measurement) is used as parallel beams of neutrons travelling in a single direction. The intensity of a neutron beam is the product of the neutron density times the average neutron velocity. The directional beam intensity is equal to the number of neutrons per unit area and time (neutrons cm-2 s-1) falling on a surface perpendicular to the direction of the beam. One can think of the neutron flux in a reactor as being comprised of many neutron beams travelling in various directions. Then, the neutron flux becomes the scalar sum of these directional flux intensities. Macroscopic cross sections for neutron reactions with materials determine the probability of one neutron undergoing a specific reaction per centimeter of travel through that material. If one wants to determine how many reactions will actually occur, it is necessary to know how many neutrons are travelling through the material and how many centimeters they travel each second. Since the atoms in a reactor do not interact preferentially with neutrons from any particular direction, all of these directional beams contribute to the total rate of reaction. In reality, at a given point within a reactor, neutrons will be travelling in all directions (DOE, 1993).

### **3. Power monitoring in nuclear reactors**

In order to ensure predictable temperatures and uniform depletion of the fuel installed in a reactor, numerous measures are taken to provide an even distribution of flux throughout the power producing section of the reactor. This shaping, or flattening, of the neutron flux is normally achieved through the use of reflectors that affect the flux profile across the core, or by the installation of poisons to suppress the neutron flux where desired. The last method, although effective at shaping the flux, is the least desirable since it reduces the neutron economy by absorbing the neutrons (DOE, 1993).

In recent years, power monitoring systems are under developing in research centers. Sakai et al. (Sakai et al., 2010) invented a power monitoring system for boiling water reactors (BWRs). In the BWR, the output power alternately falls and rises due to the generation and disappearance of voids, respectively, which may possibly generate power oscillation whereby the output power of the nuclear reactor oscillates and is amplified. The power monitoring system has a local power range monitor (LPRM) unit that has a plurality of local power channels to obtain local neutron distribution in a nuclear reactor core; an averaged power range monitor (APRM) unit that receives power output signals from the LPRM unit and obtains averaged output power signal of the reactor core as a whole; and an oscillation power range monitor (OPRM) unit that receives the power output signals from the LPRM unit and monitors power oscillation of the reactor core. The output signals from the LPRM unit to the APRM unit and the output signals from the LPRM unit to the OPRM unit are independent. A new flux mapping system (FMS) in Korea Electric Power Research Institute (KEPRI) was installed in Kori's unit 1 nuclear power plant. An in-core neutron FMS in a pressurized water reactor (PWR) yields information on the neutron flux distribution in the reactor core at selected core locations by means of movable detectors. The FMS having movable neutron detectors is equipped with detector cable drive units and path selectors located inside the reactor containment vessel. The drive units push and pull their detector cables, which run through guide tubes, and the path selectors route the detector cables into the predetermined guide tubes. Typically, 36–58 guide tubes (thimbles) are allocated in the reactor depending on the number of fuel assemblies. A control system of FMS is located at the main control room to

where: � = neutron flux (neutrons cm-2 s-1), � = neutron density (neutrons cm-3), and v = neutron velocity (cm s-1). The term neutron flux in some applications (for example, cross section measurement) is used as parallel beams of neutrons travelling in a single direction. The intensity of a neutron beam is the product of the neutron density times the average neutron velocity. The directional beam intensity is equal to the number of neutrons per unit area and time (neutrons cm-2 s-1) falling on a surface perpendicular to the direction of the beam. One can think of the neutron flux in a reactor as being comprised of many neutron beams travelling in various directions. Then, the neutron flux becomes the scalar sum of these directional flux intensities. Macroscopic cross sections for neutron reactions with materials determine the probability of one neutron undergoing a specific reaction per centimeter of travel through that material. If one wants to determine how many reactions will actually occur, it is necessary to know how many neutrons are travelling through the material and how many centimeters they travel each second. Since the atoms in a reactor do not interact preferentially with neutrons from any particular direction, all of these directional beams contribute to the total rate of reaction. In reality, at a given point within a reactor, neutrons will be travelling in all directions (DOE, 1993).

In order to ensure predictable temperatures and uniform depletion of the fuel installed in a reactor, numerous measures are taken to provide an even distribution of flux throughout the power producing section of the reactor. This shaping, or flattening, of the neutron flux is normally achieved through the use of reflectors that affect the flux profile across the core, or by the installation of poisons to suppress the neutron flux where desired. The last method, although effective at shaping the flux, is the least desirable since it reduces the neutron

In recent years, power monitoring systems are under developing in research centers. Sakai et al. (Sakai et al., 2010) invented a power monitoring system for boiling water reactors (BWRs). In the BWR, the output power alternately falls and rises due to the generation and disappearance of voids, respectively, which may possibly generate power oscillation whereby the output power of the nuclear reactor oscillates and is amplified. The power monitoring system has a local power range monitor (LPRM) unit that has a plurality of local power channels to obtain local neutron distribution in a nuclear reactor core; an averaged power range monitor (APRM) unit that receives power output signals from the LPRM unit and obtains averaged output power signal of the reactor core as a whole; and an oscillation power range monitor (OPRM) unit that receives the power output signals from the LPRM unit and monitors power oscillation of the reactor core. The output signals from the LPRM unit to the APRM unit and the output signals from the LPRM unit to the OPRM unit are independent. A new flux mapping system (FMS) in Korea Electric Power Research Institute (KEPRI) was installed in Kori's unit 1 nuclear power plant. An in-core neutron FMS in a pressurized water reactor (PWR) yields information on the neutron flux distribution in the reactor core at selected core locations by means of movable detectors. The FMS having movable neutron detectors is equipped with detector cable drive units and path selectors located inside the reactor containment vessel. The drive units push and pull their detector cables, which run through guide tubes, and the path selectors route the detector cables into the predetermined guide tubes. Typically, 36–58 guide tubes (thimbles) are allocated in the reactor depending on the number of fuel assemblies. A control system of FMS is located at the main control room to

**3. Power monitoring in nuclear reactors** 

economy by absorbing the neutrons (DOE, 1993).

control the detector drive system and measure the flux signal sensed by the detectors. The flux mapping data are used to verify the reactor core design parameters, and to determine the fission power distribution in the core. The new designed path selector for a guide the neutron detectors through the reactor core are shown in Figure 1.

Fig. 1. The new designed path selector for KEPRI unit 1 reactor (Cho et al., 2006)

The path selector system is composed of four inner path selectors and an outer path selector. With the benefit of the double indexing path selector mechanism, the reliability of the detector drive system has been improved five times higher than that of a conventional system. Currently, the developed in-core flux mapping systems have been deployed at the Kori nuclear units 1–4.
