Multipurpose Current Amplifier for Nuclear Reactor Power Measurements

The SPERT reactors at the National Reactor Testing Station near Idaho Falls, Idaho, are routinely subjected to rapid power excursions to investigate their inherent safety characteristics. During these rapid excursions, the power varies from 10-2 to 1010 watts within milliseconds. Current amplifiers used in conjunction with ion and rad chambers to monitor this power change require a bandwidth of 20 kHz with a signal to noise ratio of 40 db or better at current levels of 10-6 amperes and higher. This measurement is further complicated by electrical fields from plant operation as well as gamma fields with strengths of up to 1000 R/hr at the amplifier location. A current amplifier put into use at the SPERT facilities, and which lends itself to reactor pulse applications, is a modified version of a commercially available solid state charge amplifier. The modification consists of replacing the capacitive feedback network in the charge amplifier with a resistive network. The modified amplifier has a bandwidth of 35 Hz at 10-9 amperes, 350 Hz at 10-8 amperes, 3.5 kHz at 10-7 amperes, and 20 kHz at 10-6 or greater amperes. These bandwidths will hold with up to 100 feet of RG-58 cable on the input. The amplifier has signal to noise ratios of 34 db, 40 db, 46 db on the corresponding current ranges of 10-9, 10-8, 10-7 amperes. For current ranges of 10-6 amperes and higher, the signal to noise ratio is that inherent to the amplifier, 52 db.