Views
5 years ago

Applying the pulsed ion chamber methodology to full range reactor ...

Applying the pulsed ion chamber methodology to full range reactor ...

the I

the I from chamber 1. R--4 is closed as well as R-1 and R-3 when measuring I , from chamber 2 ss The timing sequence for one data recording cycle in the PIC mode begins when the cathode impedance circuitry is tripped to its high state by IC- 4. The positive going pulse of IC-4 is fed into IC-8. The negative output of IC-8 is then inverted by Q-21 and nhen fed to I C - 9 and IC-10. The pulse width of IC-8 is set such that it is slightly greater in width than that of those used in the T-H triggering circuitry from IC-6 and 7. This is to ensure that the A-D converters are commanded to read their inputs only after the T-H has the data sample ready for them. IC-9 and IC-10 provide the pulses necessary to activate tne A-D's. Since the A-D con- verters require 250 msec to sample, the output pulses of IC-9 and IC-10 are set accordingly. l'C-11, which is triggered by IC-10 at; the end of its pulse, indicates to the computer, through the interface, that the data is ready to be read. The IC-'M pulse width is set so as to allow the HP9S21 2->0 msec to record the data from both A-D converters. When the system is in the steady state mode, only A-Dl is read by the computer, as described earlier. The codes used by the computer to control the system and output the resulting data are given in the Appendix. The system operated flawlessly over the 3 months of evaluation. No major deficiencies were found. The systems response remained both linear and free of gain drift with the calibrating accuracy of +2%. Detector Sel ection The fundamental part of any nuclear radiation measuring system is the detector. The capability of a system, even with the most sophisticated electronics, is ultimately determined by the sensor. The only detector

proven capable of approaching the limit? set forth in the beginning of this chapter is the U fission chambpr, It gives the maximum possible gamma discrimination, with relatively low burnup and has been proven to operate under the most adverse conditions, all of which are necessary characteristics fur the PIC application. For these reasons two matched pairs of RSN-34A-M1 fission chambers were obtained. A scale diagram is shown in Figure 3-7. Of each pair, one had a coating of 93% ""U. Thus one chamber of each set was sensitive to both neutrons and gammas, while the other was sensitive only to gammas. As a consequence they formed a gamma compensating pair. One set was filled at the manufacturing facility with 1 atmosphere (STP) of a high purity Ar-5%N« gas mixture. The other set was ordered with fill tubes attached. They were pumped down to 2 x 10~ 8 "err at 300°C, cooled, and then filled to 2 atmospheres with research grade high purity neon. The argon-nitrogen cas mixture was used because of its proven characteristics in conventional fissiun chamber operation. Neon on the other hand, was reported to have a volume recombination coefficient that vas independent of oernperature; a dosired gas charac- teristic of the PIC reactor power measurement system. These chambers, along with the previously described PIC system, were then evaluated. Chapter IV describes the experimental procedures used to accomplish this, as well as the results obtained. 4!