The hydrogen-monitoring system for ast coolant

Abstract

The loop in the AST-500 reactor system circulates water containing a dissolved heliumhydrogen mixture, which shifts (i) to the left. The hydrogen content in the coolant is standardized at 0~ and 0.i MPa as 2 ml'kg -l and should be monitored reliably during operation. This requires sensitive, fast, and fault-free monitoring systems. This paper deals with the development and testing of such a system, which consists of a gas probe (loop with hydrogenpermeable selective membrane) placed in the coolant, a semiconductor sensor based on a Pd-Si02-Si structure, a gas flow switching unit, and an electronic measuring unit. The system is controlled by an Iskra 1256 minicomputer. Semiconductor Sensor. There is a detailed discussion [4] of the scope for making a sensor based on an MOS structure, as well as of the interaction between hydrogen and a catalytically active palladium electrode. It has been found that the rate of change in the flat-band voltage AU when the gas mixture is applied is related to the hydrogen partial pressure PH by 8AU/Stlr=o =a(l--8)2PH2 in which a is a constant factor governed by the rate constants fo~ hydrogen adsorption and desorption on palladium, and e is the active-center filling at the Pd-SiO 2 boundary. The sensor consists of the Pd-Si02-Si structure and a temperature sensor (p-i-n diode), which are placed at the center of the temperature pattern on a thin sapphire substrate, on the opposite side of which there is a heater. This assembly is mounted on a porous thermal insulator in a microcircuit body. The basic technical parameters are: working temperature 170~ measurement range for hydrogen in inert gas (helium) 10-5-10-2%, and time for one measurement a few seconds, The sensitivity is appropriate to the system. Gas Probe. There are high neutron and ~-ray levels, together with high pressures (about 2 MPa) and temperature instability, so the sensor cannot operate in the coolant. A gas probe may be operated as a loop containing a hydrogen-permeable membrane placed in the medium to eliminate the effects of those factors on the sensor. Particular interest attaches to metal membranes (nickel ones, and also palladium-base alloys) with high radiation resistance and capable of withstanding high temperatures and pressures. We used a membrane made of Pd + 6% Ru, which was a tube 3 mm in diameter with wall thickness 0.i mm and total surface area about 70 cm 2. Helium passed through the probe at 2 cm3"sec -l. The hydrogen diffusing through the membrane entered the carrier gas and was transported to the measurement chamber, which contained the sensor. The partial pressure of the hydrogen entering the helium is uniquely related to the hydrogen concentration in the coolant for the temperature range 70 to 200~ for hydrogen contents in the water from 0.2 to 20 ml'kg -I. The relaxation time in the membrane in response to temperature change is not more than I00 sec.