**5. Several advanced power measuring and monitoring systems**

The power range channels of nuclear reactors are linear, which cover only one decade, so they do not show any response during the startup and intermediate range of the reactor operation. So, there is no prior indication of the channels during startup and intermediate operating ranges in case of failure of the detectors or any other electronic fault in the channel. Some new reliable instrument channels for power measurement will be studied in this section.

#### **5.1 A wide-range reactor power measuring channel**

The power range channels of nuclear reactors are linear, which cover only one decade, so they do not show any response during the startup and intermediate range of the reactor operation. So, there is no prior indication of the channels during startup and intermediate operating ranges in case of failure of the detectors or any other electronic fault in the channel. A new reliable instrument channel for power measurement will be studied in this section. The device could be programmed to work in the logarithmic, linear, and log-linear

Improving the Performance of the Power Monitoring Channel 239

The channel shows an excellent linearity. A very important check was the response of the test channel at the operating mode switching level, and it was found that the channel smoothly switched from log to linear operating mode. The designed channel has shown good performance throughout the operation and on applying different tests. The self-

16N is one of the radioactive isotopes of nitrogen, which is produced in reactor coolant (water) emitting a Gamma ray with energy about 6 MeV and is detectable by out-core instruments. In this section, a 16N instrument channel in relation to reactor power measurement will be studied. The reactor power and the rate of production of 16N have a linear relation with good approximation. A research type of 16N power monitoring channel subjected to use in Tehran Research Reactor (TRR). Tehran Research Reactor is a 5 MW pool-type reactor which use a 20% enriched MTR plate type fuel. When a reactor is operating, a fission neutron interacts with oxygen atom (16O) present in the water around the reactor core, and convert the oxygen atom into radioactive isotope 16N according to the following (n, p) reaction. Also another possible reaction is production of 19O by

Of course, water has to be rich of �O �� for at least 22% to have a significant role in 19O

�� → P�

In addition to �O �� (99.76%) and �O �� (0.2%), other isotope of oxygen is also exist naturally in water, including �O �� (0.04%). �N�� (0.037%) produced from �O �� by the (n, p) reaction which

� → N�

���� �O �� + β ��

Since activity ratio of 16N to 17N is 257/1, thus activity of 17N does not count much and is negligible. Primary water containing this radioactive 16N is passed through the hold-up tank (with capacity of 384.8 m3, maximum amount of water that can pour to the hold-up tank is 172 m3 and reactor core flow is 500 m3 h-1), which is placed under the reactor core and water flow from core down to this tank by gravity force. The hold-up tank delays the water for about 20.7 min. During this period activity of the short lived 16N (T1/2 = 7.4 s) decays down to low level. The decay tank and the piping connection to the reactor pool are covered with heavy concrete shielding in order to attenuate the energy of gamma emitted by the 16N nuclei. To investigate the amount of 16N in Tehran Research Reactor by direct measurements

16N\* is produced and radiate gamma rays (6MeV) and β particles during its decay chain.

� + N<sup>∗</sup> �

�� + H�

� + n�

�� + γ(2.8 MeV) (2)

� + γ(6.13 MeV) (4)

�� (3)

� (5)

� (6)

�� → O�

�n� + O�

��� �O �� + β ��

�O �� + n�

�N�� �.�� �

monitoring capabilities of the channel will improve the availability of the system.

**5.2 A new developed monitoring channel using 16N detector** 

�n� + O�

N<sup>∗</sup> � �� �.� �

O (n, γ) O� ��

�� reaction.

producing, but 18O is exist naturally (0.2%)

will decayed through beta emission.

�

modes during different operation time of the reactor life cycle. A new reliable nuclear channel has been developed for reactor power measurement, which can be programmed to work in the logarithmic mode during startup and intermediate range of operation, and as the reactor enters into the power range, the channel automatically switches to the linear mode of operation. The log-linear mode operation of the channel provides wide-range monitoring, which improves the self-monitoring capabilities and the availability of the reactor. The channel can be programmed for logarithmic, linear, or log-linear mode of operation. In the log-linear mode, the channel operates partially in log mode and automatically switches to linear mode at any preset point. The channel was tested at Pakistan Research Reactor-1 (PARR-1), and the results were found in very good agreement with the designed specifications. A wide range nuclear channel is designed to measure the reactor power in the full operating range from the startup region to 150% of full power. In the new channel, the status of the channels may be monitored before their actual operating range. The channel provides both logarithmic and linear mode of operation by automatic operating mode selection. The channel can be programmed for operation in any mode, log, linear or log-linear, in any range. In the log-linear mode, the logarithmic mode of operation is used for monitoring the operational status of the channel from reactor startup to little kilowatt reactor power where the mode of operation is automatically changed to linear mode for measurement of the reactor power. At the low power operation, the channel will provide monitoring of the proper functioning of the channel, which includes connection of the electronics with the chamber and functioning of the chamber, amplifier, high-voltage supply of the chamber, and auxiliary power supply of the channel. The channel has been developed using reliable components, and design has been verified under recommended reliability test procedures. The channel consists of different electronic circuits in modular form including programmable log-linear amplifier, isolation amplifier, alarm unit, fault monitor, high-voltage supply, dc-dc converter, and indicator. The channel is tested at PARR-1 from reactor startup to full reactor power. Before testing at the reactor, the channel was calibrated and tested in the lab by using a standard current source. The channel has been designed and developed for use in PARR-1 for reactor power measurement. The response of the channel was continuously compared with 16N channel of PARR-1, and the test channel was calibrated according to the 16N channel at 1 MW. After calibration, it was noticed that the test channel gave the same output as the 16N channel. The channel response with Reactor Power is shown in Figure 5.

Fig. 5. Response of the channel at different reactor power at PARR-1 (Tahir Khaleeq et al. 2003)

modes during different operation time of the reactor life cycle. A new reliable nuclear channel has been developed for reactor power measurement, which can be programmed to work in the logarithmic mode during startup and intermediate range of operation, and as the reactor enters into the power range, the channel automatically switches to the linear mode of operation. The log-linear mode operation of the channel provides wide-range monitoring, which improves the self-monitoring capabilities and the availability of the reactor. The channel can be programmed for logarithmic, linear, or log-linear mode of operation. In the log-linear mode, the channel operates partially in log mode and automatically switches to linear mode at any preset point. The channel was tested at Pakistan Research Reactor-1 (PARR-1), and the results were found in very good agreement with the designed specifications. A wide range nuclear channel is designed to measure the reactor power in the full operating range from the startup region to 150% of full power. In the new channel, the status of the channels may be monitored before their actual operating range. The channel provides both logarithmic and linear mode of operation by automatic operating mode selection. The channel can be programmed for operation in any mode, log, linear or log-linear, in any range. In the log-linear mode, the logarithmic mode of operation is used for monitoring the operational status of the channel from reactor startup to little kilowatt reactor power where the mode of operation is automatically changed to linear mode for measurement of the reactor power. At the low power operation, the channel will provide monitoring of the proper functioning of the channel, which includes connection of the electronics with the chamber and functioning of the chamber, amplifier, high-voltage supply of the chamber, and auxiliary power supply of the channel. The channel has been developed using reliable components, and design has been verified under recommended reliability test procedures. The channel consists of different electronic circuits in modular form including programmable log-linear amplifier, isolation amplifier, alarm unit, fault monitor, high-voltage supply, dc-dc converter, and indicator. The channel is tested at PARR-1 from reactor startup to full reactor power. Before testing at the reactor, the channel was calibrated and tested in the lab by using a standard current source. The channel has been designed and developed for use in PARR-1 for reactor power measurement. The response of the channel was continuously compared with 16N channel of PARR-1, and the test channel was calibrated according to the 16N channel at 1 MW. After calibration, it was noticed that the test channel gave the same output as the 16N channel. The channel response

Fig. 5. Response of the channel at different reactor power at PARR-1 (Tahir Khaleeq et al.

with Reactor Power is shown in Figure 5.

2003)

The channel shows an excellent linearity. A very important check was the response of the test channel at the operating mode switching level, and it was found that the channel smoothly switched from log to linear operating mode. The designed channel has shown good performance throughout the operation and on applying different tests. The selfmonitoring capabilities of the channel will improve the availability of the system.

#### **5.2 A new developed monitoring channel using 16N detector**

16N is one of the radioactive isotopes of nitrogen, which is produced in reactor coolant (water) emitting a Gamma ray with energy about 6 MeV and is detectable by out-core instruments. In this section, a 16N instrument channel in relation to reactor power measurement will be studied. The reactor power and the rate of production of 16N have a linear relation with good approximation. A research type of 16N power monitoring channel subjected to use in Tehran Research Reactor (TRR). Tehran Research Reactor is a 5 MW pool-type reactor which use a 20% enriched MTR plate type fuel. When a reactor is operating, a fission neutron interacts with oxygen atom (16O) present in the water around the reactor core, and convert the oxygen atom into radioactive isotope 16N according to the following (n, p) reaction. Also another possible reaction is production of 19O by O (n, γ) O� �� � �� reaction.

Of course, water has to be rich of �O �� for at least 22% to have a significant role in 19O producing, but 18O is exist naturally (0.2%)

$$\text{Mg}^{1} + \text{}^{18}\_{8}\text{O} \rightarrow \text{}^{19}\_{8}\text{O} + \text{}^{\gamma}\_{8}\text{O MeV} \tag{2}$$

$$\rm \rm {^1\_0n} + \rm {^{16}\_8O} \rightarrow \rm {^1\_1P} + \rm {^{16}\_7N^\*} \tag{3}$$

16N\* is produced and radiate gamma rays (6MeV) and β particles during its decay chain.

$$\text{M}^{16}\_{7}\text{N}^{\*} \xrightarrow{72\text{ s}} \text{16} + \text{ }^{0}\_{-4}\text{β} + \text{\textchi} \text{(6.13 MeV)}\tag{4}$$

In addition to �O �� (99.76%) and �O �� (0.2%), other isotope of oxygen is also exist naturally in water, including �O �� (0.04%). �N�� (0.037%) produced from �O �� by the (n, p) reaction which will decayed through beta emission.

$$\rm N\_{8}^{17}O + \rm {^1\_{0}n} \rightarrow \rm {^{17}\_{7}N} + \rm {^1\_{1}H} \tag{5}$$

$$\text{H}\_{7}^{17}\text{N} \xrightarrow{4.14\text{ s}} \text{I}\_{8}^{16}\text{O} + \text{I}\_{-1}^{0}\text{S} + \text{I}\_{0}^{1}\text{n} \tag{6}$$

Since activity ratio of 16N to 17N is 257/1, thus activity of 17N does not count much and is negligible. Primary water containing this radioactive 16N is passed through the hold-up tank (with capacity of 384.8 m3, maximum amount of water that can pour to the hold-up tank is 172 m3 and reactor core flow is 500 m3 h-1), which is placed under the reactor core and water flow from core down to this tank by gravity force. The hold-up tank delays the water for about 20.7 min. During this period activity of the short lived 16N (T1/2 = 7.4 s) decays down to low level. The decay tank and the piping connection to the reactor pool are covered with heavy concrete shielding in order to attenuate the energy of gamma emitted by the 16N nuclei. To investigate the amount of 16N in Tehran Research Reactor by direct measurements

Improving the Performance of the Power Monitoring Channel 241

gamma and beta radiations. The dose received in these areas (except near the hold-up tank charcoal filter box which is shielded) are below the recommended dose limits for the radiation workers (0.05 Sv/year), therefore it can be seen that the radiation risk of 16N is reduced due to design of the piping system and hold-up tank which is distanced from the core to overlap the decay time. Thus, 16N decay through the piping and hold-up tank is reduced to a safe working level. It could be seen that 16N system is able to measure the reactor power enough accurately to be used as a channel of information. For the pool type research reactor which has only one shut down system also could be used to increase the

**6. Power monitoring by some developed detectors and new methods** 

**6.1 Micro-pocket fission detectors (MPFD) for in-core neutron flux monitoring** 

There is a need for neutron radiation detectors capable of withstanding intense radiation fields, capable of performing ''in-core'' reactor measurements, capable of pulse mode and current mode operation, capable of discriminating neutron signals from background gamma ray signals, and that are tiny enough to be inserted directly into a nuclear reactor without significantly perturbing the neutron flux. A device that has the above features is the subject of a Nuclear Engineering Research Initiative (NERI) research project, in which miniaturized fission chambers are being developed and deployed in the Kansas State University (K-State) TRIGA Mark-II research reactor (McGregor, 2005). The unique miniaturized neutron detectors are to be used for three specific purposes (1) as reactor power-level monitors, (2) power transient monitors, and (3) real-time monitoring of the thermal and fast neutron flux profiles in the core. The third application has the unique benefit of providing information that, with mathematical inversion techniques, can be used to infer the three-dimensional (3D) distribution of fission neutron production in the core. Micro-pocket fission detectors (MPFD) are capable of performing near-core and in-core reactor power measurements. The basic design utilizes neutron reactive material confined within a micro-sized gas pocket, thus forming a miniature fission chamber. The housing of the chamber is fabricated from inexpensive ceramic materials, the detectors can be placed throughout the core to enable the 3D mapping of the neutron flux profile in ''real-time''. Initial tests have shown these devices to be radiation hard and potentially capable of operating in a neutron fluence exceeding 1019 cm-2 without noticeable degradation. Figure 7 shows a cutaway view of the basic detector concept. It consists of a small ceramic structure, within which is a miniature gas-filled

In this section, several neutron detectors and power monitoring systems are reviewed.

Application of a micro-pocket fission detector for in-core flux measurements is described in section 6.1. SIC neutron monitoring system is examined experimentally and theoretically. Development of an inconel self-powered neutron detector (SPND) for in-core power monitoring will be reviewed in section 6.3. Furthermore, a prototype cubic meter antineutrino detector which is used as a new device for measuring the thermal power as an out-core detection system, will be discussed. Finally, two passive approaches for power

reactor safety (Sadeghi, 2010).

measurement are discussed.

pocket.

of gamma radiation and examine the changes with reactor power, the existing detectors in the reactor control room used and experiment was performed. To assess gamma spectrum for the evaluation of 16N in reactor pool a portable gamma spectroscopy system which includes a sodium-iodide detector is used. The sodium-iodide (NaI) detector which is installed at reactor outlet water side is used for counting Gamma rays due to decay of 16N which depends directly on the amount of 16N. Some advantages of the power measurement using 16N system:


It is expected that the amount of 16N which is produced in reactor water has linear relation with the reactor power. Comparison of theory and experience is shown in Figure 6.

Fig. 6. Comparison of theory and experimental data from 16N channel (Sadeghi, 2010)

Based on graph which resulted from experimental data and the straight line equation using least squire fit, it is appear that the experimental line deviated from what it expected; it means that the line is not completely straight. It seems this small deviation is due to the increasing water temperature around the core in higher power, density reduction and outlet water flow reduction which cause 16O reduction and so 16N. At the same time the amount of 16N production decreases and thus decreasing gamma radiations, this will reduce the number of counting, but on the other hand, since the number of fast neutron production in reactor can increase according to reactor power and moderator density became less, the possibility of neutron interaction with water would increased. During past years, linearity of the curve as the experimental condition and the measurements were improved. Now that this linearity is achieved, by referring to the graph, it could conclude that 16N system is suitable to measure the reactor power. Safety object of the new channel is evaluated by the radiation risk of 16N, dose measurement performed in the area close to the hold-up tank for

of gamma radiation and examine the changes with reactor power, the existing detectors in the reactor control room used and experiment was performed. To assess gamma spectrum for the evaluation of 16N in reactor pool a portable gamma spectroscopy system which includes a sodium-iodide detector is used. The sodium-iodide (NaI) detector which is installed at reactor outlet water side is used for counting Gamma rays due to decay of 16N which depends directly on the amount of 16N. Some advantages of the power measurement




other gammas from impurities do not intervened the measurements.

with the reactor power. Comparison of theory and experience is shown in Figure 6.

Fig. 6. Comparison of theory and experimental data from 16N channel (Sadeghi, 2010)

Based on graph which resulted from experimental data and the straight line equation using least squire fit, it is appear that the experimental line deviated from what it expected; it means that the line is not completely straight. It seems this small deviation is due to the increasing water temperature around the core in higher power, density reduction and outlet water flow reduction which cause 16O reduction and so 16N. At the same time the amount of 16N production decreases and thus decreasing gamma radiations, this will reduce the number of counting, but on the other hand, since the number of fast neutron production in reactor can increase according to reactor power and moderator density became less, the possibility of neutron interaction with water would increased. During past years, linearity of the curve as the experimental condition and the measurements were improved. Now that this linearity is achieved, by referring to the graph, it could conclude that 16N system is suitable to measure the reactor power. Safety object of the new channel is evaluated by the radiation risk of 16N, dose measurement performed in the area close to the hold-up tank for

not have any effects on the measurements.

using 16N system:

gamma and beta radiations. The dose received in these areas (except near the hold-up tank charcoal filter box which is shielded) are below the recommended dose limits for the radiation workers (0.05 Sv/year), therefore it can be seen that the radiation risk of 16N is reduced due to design of the piping system and hold-up tank which is distanced from the core to overlap the decay time. Thus, 16N decay through the piping and hold-up tank is reduced to a safe working level. It could be seen that 16N system is able to measure the reactor power enough accurately to be used as a channel of information. For the pool type research reactor which has only one shut down system also could be used to increase the reactor safety (Sadeghi, 2010).
