Neutron bubble detector

The neutron bubble detector (trade name BD-IOOR) is a reusable, passive integrating dosimeter that allows instant, visible detection of neutron dose. The bubble detector consists of a glass tube filled with thousands of superheated liquid drops in a stabilizing matrix. When exposed to neutrons, these droplets vaporize, forming visible permanent bubbles in an elastic polymer. The total number of bubbles formed is proportional to the neutron dose equivalent H. The bubbles can be counted manually or by a machine. Figure 16.15 shows the response of the bubble detector as a function of neutron energy. 

I said No, no. I ll have a go, 1 just haven t thought about that before . Pause. I don t think you would detect any significant difference in reactor coolant outlet temperature. The instrument response time will be too slow . Pause. However, 1 know the sodium coolant absorbs some neutrons, because that is how radioactive sodium-24 forms in the primary coolant. So, if bubbles of argon were passing up through the reactor core, the absence of sodium would mean that the neutron flux would increase a bit and we would see some noise on the signals from the neutron detectors . 

At 20 min, the steam bubbles in the reactor coolant caused the out-of-core source-range neutron detector to read higher than expected. Normally, water in the downcomer annulus, outside the core but inside the reactor vessel, shields these detectors. But, because the water was now frothy, it was not shielding the detectors as well as usual. Not realizing that the apparent increase in neutrons reaching the detectors was caused by steam bubbles in the reactor coolant, the operators feared the possibility of a reactor restart. Although it is now known that their fears were unfounded, at the time they were one more source of distraction. 

A sample of reactor coolant analyzed a few minutes after the loop B pumps were shut off indicated a low boron concentration. This finding, coupled with apparently increasing neutron levels, increased the operators fears of a reactor restart. As explained earlier, the source range neutron detector count rate was increasing because steam bubbles in the downcomer allowed more neutrons to reach the detector. There was no actual danger of recriticality. It is now believed the sample was diluted by condensed steam, causing the indication of low boron concentration. 

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