**8. Thermal methods for power monitoring of nuclear reactor**

Power monitoring using thermal power produced by reactor core is a method that is used in many reactors. To explain how the method is used for reactor power measurement, a research reactor is studied in this section. In IPR-R1, a TRIGA Mark I Research Reactor, the power is measured by four nuclear channels. The departure channel consists of a fission counter with a pulse amplifier that a logarithmic count rate circuit. The logarithmic channel consists of a compensated ion chamber, whose signal is the input to a logarithmic amplifier, which gives a logarithmic power indication from less than 0.1 W to full power. The linear channel consists of a compensated ion chamber, whose signal is the input to a sensitive amplifier and recorder with a range switch, which gives accurate power information from source level to full power on a linear recorder. The percent channel consists of an uncompensated ion chamber, whose signal is the input to a power level monitor circuit and meter, which is calibrated in percentage of full power. The ionization chamber neutron detector measures the flux of neutrons thermalized in the vicinity of the detector. In the present research, three new processes for reactor power measurement by thermal ways were developed as a result of the experiments. One method uses the temperature difference between an instrumented fuel element and the pool water below the reactor core. The other two methods consist in the steady-state energy balance of the primary and secondary reactor cooling loops. A stainless steel-clad fuel element is instrumented with three thermocouples along its centerline in order to evaluate the reactor thermal hydraulic performance. These processes make it possible on-line or off-line evaluation of the reactor power and the analysis of its behavior.

Improving the Performance of the Power Monitoring Channel 255

Fig. 20. Reactor power measured by neutron channel and by fuel element temperature

There is a good agreement between the two results, although the temperature difference method presents a delay in its response, and it is useful for steady-state or very slow transient. It is notable that the thermal balance method presented in this report is now the standard methodology used for the IPR-R1 TRIGA Reactor power calibration. The heat balance and fuel temperature methods are accurate, but impractical methods for monitoring the instantaneous reactor power level, particularly during transients. For transients the power is monitored by the nuclear detectors, which are calibrated by the thermal balance

The new developed on-line monitoring method which is based on a temperature difference between an instrumented fuel element and the pool water below a research reactor in practice, as known power measuring by thermal balance is as following. The reactor core is cooled by natural convection of demineralized light water in the reactor pool. Heat is removed from the reactor pool and released into the atmosphere through the primary cooling loop, the secondary cooling loop and the cooling tower. Pool temperature depends on reactor power, as well as external temperature, because the latter affects heat dissipation in the cooling tower. The total power is determined by the thermal balance of cooling water flowing through the primary and secondary loops added to the calculated heat losses. These losses represent a very small fraction of the total power (about 1.5% of total). The inlet and outlet temperatures are measured by four platinum resistance thermometers (PT-100) positioned at the inlet and at the outlet pipes of the primary and secondary cooling loops. The flow rate in the primary loop is measured by an orifice plate and a differential pressure transmitter. The flow in the secondary loop is measured by a flow-meter. The pressure transmitter and the temperature measuring lines were calibrated and an adjusted equation was added to the data acquisition system. The steady-state is reached after some hours of reactor operation, so that the power dissipated in the cooling system added with the losses should be equal to the core power. The thermal power dissipated in the primary and

where ����� is the thermal power dissipated in each loop (kW), �� is the flow rate of the coolant water in the loop (kg. s-1), �� is the specific heat of the coolant (kJ kg-1 °C-1), and ∆T is

����� � �� � ��� ∆� (10)

(Zacarias Mesquita and Cesar Rezende, 2007).

method (Zacarias Mesquita and Cesar Rezende, 2007).

**8.2 Power measuring channel by thermal balance** 

secondary loops were given by:

#### **8.1 Power measuring channel by fuel and pool temperature**

To evaluate the thermal hydraulic performance of the IPR-R1 reactor one instrumented fuel element was put in the core for the experiments. The instrumented fuel is identical to standard fuel elements but it is equipped with three chromel-alumel thermocouples, embedded in the zirconium pin centerline. The sensitive tips of the thermocouples are located one at the center of the fuel section and the other two 25.4 mm above, and 25.4 mm below the center. Figure 18 shows the diagram and design of the instrumented fuel element (Zacarias Mesquita and Cesar Rezende, 2010).

Fig. 18. Diagram of the instrumented fuel element (Zacarias Mesquita and Cesar Rezende, 2010)

The instrumented fuel element which is placed in proper thimble (B6 position) is obvious in Figure 19, a core upper view.

Fig. 19. Core upper view with the instrumented fuel element (Zacarias Mesquita and Cesar Rezende, 2010)

During the experiments it was observed that the temperature difference between fuel element and the pool water below the reactor core (primary loop inlet temperature) do not change for the same power value. Figure 20 compares the reactor power measuring results using the linear neutron channel and the temperature difference channel method (Zacarias Mesquita and Cesar Rezende, 2007).

To evaluate the thermal hydraulic performance of the IPR-R1 reactor one instrumented fuel element was put in the core for the experiments. The instrumented fuel is identical to standard fuel elements but it is equipped with three chromel-alumel thermocouples, embedded in the zirconium pin centerline. The sensitive tips of the thermocouples are located one at the center of the fuel section and the other two 25.4 mm above, and 25.4 mm below the center. Figure 18 shows the diagram and design of the instrumented fuel element

Fig. 18. Diagram of the instrumented fuel element (Zacarias Mesquita and Cesar Rezende,

The instrumented fuel element which is placed in proper thimble (B6 position) is obvious in

Fig. 19. Core upper view with the instrumented fuel element (Zacarias Mesquita and Cesar

During the experiments it was observed that the temperature difference between fuel element and the pool water below the reactor core (primary loop inlet temperature) do not change for the same power value. Figure 20 compares the reactor power measuring results using the linear neutron channel and the temperature difference channel method (Zacarias

**8.1 Power measuring channel by fuel and pool temperature** 

(Zacarias Mesquita and Cesar Rezende, 2010).

2010)

Rezende, 2010)

Mesquita and Cesar Rezende, 2007).

Figure 19, a core upper view.

Fig. 20. Reactor power measured by neutron channel and by fuel element temperature (Zacarias Mesquita and Cesar Rezende, 2007).

There is a good agreement between the two results, although the temperature difference method presents a delay in its response, and it is useful for steady-state or very slow transient. It is notable that the thermal balance method presented in this report is now the standard methodology used for the IPR-R1 TRIGA Reactor power calibration. The heat balance and fuel temperature methods are accurate, but impractical methods for monitoring the instantaneous reactor power level, particularly during transients. For transients the power is monitored by the nuclear detectors, which are calibrated by the thermal balance method (Zacarias Mesquita and Cesar Rezende, 2007).

#### **8.2 Power measuring channel by thermal balance**

The new developed on-line monitoring method which is based on a temperature difference between an instrumented fuel element and the pool water below a research reactor in practice, as known power measuring by thermal balance is as following. The reactor core is cooled by natural convection of demineralized light water in the reactor pool. Heat is removed from the reactor pool and released into the atmosphere through the primary cooling loop, the secondary cooling loop and the cooling tower. Pool temperature depends on reactor power, as well as external temperature, because the latter affects heat dissipation in the cooling tower. The total power is determined by the thermal balance of cooling water flowing through the primary and secondary loops added to the calculated heat losses. These losses represent a very small fraction of the total power (about 1.5% of total). The inlet and outlet temperatures are measured by four platinum resistance thermometers (PT-100) positioned at the inlet and at the outlet pipes of the primary and secondary cooling loops. The flow rate in the primary loop is measured by an orifice plate and a differential pressure transmitter. The flow in the secondary loop is measured by a flow-meter. The pressure transmitter and the temperature measuring lines were calibrated and an adjusted equation was added to the data acquisition system. The steady-state is reached after some hours of reactor operation, so that the power dissipated in the cooling system added with the losses should be equal to the core power. The thermal power dissipated in the primary and secondary loops were given by:

$$q\_{cool} = \dot{m}.c\_P.\Delta T\tag{10}$$

where ����� is the thermal power dissipated in each loop (kW), �� is the flow rate of the coolant water in the loop (kg. s-1), �� is the specific heat of the coolant (kJ kg-1 °C-1), and ∆T is

Improving the Performance of the Power Monitoring Channel 257

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the difference between the temperatures at loop the inlet and outlet (°C). Figure 21 shows the power evolution in the primary and secondary loops during one reactor operation.

Fig. 21. Thermal power evolution in the cooling system (Zacarias Mesquita and Cesar Rezende, 2007).
