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 1|O (n, y) 1gO reaction.

Of course, water has to be rich of 1|O for at least 22% to have a significant role in 19O producing, but 18O is exist naturally (0.2%)

Jn + *|0 ^ ^0 + y(2.8 MeV) (2)

£n + “0 ^P + “N* (3)

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

r6N* igo + _op + y(6.13 MeV) (4)

In addition to “0 (99.76%) and ^0 (0.2%), other isotope of oxygen is also exist naturally in water, including 1g0 (0.04%). 1yN (0.037%) produced from 1g0 by the (n, p) reaction which will decayed through beta emission.

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 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:

— Power measurement by 16N system uses the gamma from decay of 16N isotope only, so other gammas from impurities do not intervened the measurements.

— Since 16N system installed far from the core, fission products and its gamma rays would not have any effects on the measurements.

— Energy dissipation of heat exchanged with surroundings would not intervene, because water temperature would not use in this system for reactor power measurements.

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.

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 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).