RRFM 2009 Transactions - European Nuclear Society

compared with that of the delayed neutron measurements in the primary cooling circuit. The
delayed neutron measurement system is used as the radiation monitor for the primary cooling
system (PCS).
Fig. 2 shows the variations of the delayed neutron measurements of the PCS radiation monitor
and the activity concentrations of Na-24 and Ar-41 in the coolant for the last two years. As
shown in the figure, Ar-41 activity concentration has hardly changed during 2 years. It means
that the quantity of the dissolved air in the coolant is almost constant. The variation of the
Na-24 concentration for one period of the reactor operation shows no significant change. But,
the change between each period is relatively large since the total reaction rate of the
aluminium inside the reactor core is changed for each period. At any rate, it is confirmed that
the concentrations of Na-24 and Ar-41 are independent of the delayed neutron measurements
of the PCS radiation monitor.
3x10 6
2x10 6
1x10 6
4x10 6 0
Operation in 2007-2008
1500
Specific activity [Bq/liter]
: Ar-41
: Na-24
: PCS
1200
900
600
300
PCS radiation [cps]
0
1-1 5-1 9-1 1-1 5-1 9-1
Date [m-d]
Fig 2. Variations of the delayed neutron measurements of the PCS radiation monitor and the
activity concentrations of Na-24 and Ar-41 in the coolant for last two years.
As shown in Table 1, there are a lot of fission products in the coolant water. Among them, in
order to choose the proper nuclide for detecting the fuel defect, the half-life, decay scheme,
peak area and interference of adjacent peaks were considered. And then, the variation of the
concentration of each nuclide was compared with that of the delayed neutron measurements in
the primary cooling circuit. From the comparison, the proper nuclides and their gamma-ray
peaks for the fuel defect detection were determined, and they were 166 keV peak from Ba-139,
250 keV peak from Xe-135, 307 keV from Tc-101 and 1436 keV from Cs-138. The half-lives of
the selected nuclides are from 14 min. to 9 hr. Even though these peaks were located on a
large Compton continuum of the gamma-ray spectrum of the coolant, the peak areas were
determined with relatively small uncertainties.
Fig. 3 shows the variations of the delayed neutron measurements of the PCS radiation monitor
and the activity concentrations of the selected nuclides. In April, 2008, a fuel rod with a very
small defect was found during the reactor operation. Due to the defective fuel, the delayed
neutron measurements of the primary cooling system (PCS) were increased slightly as shown
in Fig. 2 and 3. But, this value was much smaller than that to trigger the fuel failure detection
system (FFDS). As shown in Fig. 3, the concentration of the selected nuclide shows very
drastic change when the delayed neutron value is increased slightly. The concentrations are
increased by over 5-10 times in comparison with the case of no defect. Thus, it is confirmed
that the very sensitive and exact detection of the fuel defect would be possible by this method
even if the defect is very small.
322 of 455