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Sensors for sodium coolant monitoring in Fast Reactors

G.Periaswami
Materials Chemistry Division, Indira Gandhi Centre for Atomic Research

Fast Reactors provide the best means of utilizing the vast thorium reserves of India to meet its future energy needs. The greater neutron economy of Fast Reactors contributes more neutrons for the conversion of Thorium-232 into Uranium-233 and helps maximise the utilization of thorium. The need to maintain a hard neutron spectrum precludes the use of moderating materials such as water as coolant in fast reactors. Liquid sodium metal is the coolant of choice in fast reactors as it is non-moderating. In addition, pure sodium is highly compatible with austenitic stainless steel, which is the structural material for fast reactors. But presence of impurities can affect the compatibility. Oxygen in sodium can bring about enhanced corrosion and activity transport and complicate reactor operation and maintenance. Carbon in sodium can lead to carburization of stainless steel.

Since there are many sources of impurities in an operating fast reactor, it is essential to monitor the coolant continuously using reliable sensors. Monitoring will also help detect malfunctions such as steam generator leak, oil leak through shaft seal of pump and air leak into cover gas. Sensors to meet the reactor requirements of a fast reactor have been developed at IGCAR.

Hydrogen Sensor
In the steam generator of a fast reactor, the high pressure steam and hot sodium are separated by a single steel wall. Any defect in the tube walls can cause steam to leak into sodium. Since the sodium water reaction is highly exothermic and caustic producing, the leaks can expand rapidly and lead to explosions. The best means of detecting these leaks at the very inception is to monitor the sodium at the steam generator outlet for hydrogen. The device currently in use in fast reactors, does this by measuring the hydrogen flux through a nickel membrane kept exposed to sodium at 450oC on one side and vacuum on the other. The hydrogen flux is measured by using a mass spectrometer and related to hydrogen in sodium. Such a device is complex, costly and difficult to operate. A simple electrochemical hydrogen sensor, which can operate truly on-line, has been developed at IGCAR as an alternative. This sensor uses an electrode concentration cell where sodium forms one electrode and a slurry of Li/ LiH the other electrode. A hydride ion conducting solid electrolyte, CaCl2-LiCl-CaH2, separates the two electrodes. Since the electrodes and the electrolyte are not chemically compatible, they are separated by a thin iron membrane, which is highly permeable to hydrogen. At 450oC, this cell gives an emf output which is proportional to the log of hydrogen concentration in sodium. These sensors are capable of detecting a change of 4 ppb at a level of 70 ppb of Hydrogen in Sodium which is normally encountered in fast reactors. The composition of the electrolyte is carefully optimized to obtain good ionic conductivity as otherwise the electronic conduction introduced by impurities, such as Iron, can lead to lower and unstable signals. Two such meters have been incorporated in FBTR.

Cover gas hydrogen meter
When the sodium temperatures are low (<300oC), as is the case during reactor startup, the hydrogen gas formed in sodium/water reaction does not react with sodium quickly enough for the in-sodium meters to respond well. Good amount of the hydrogen gas bubbles reach the cover gas. Monitoring the cover gas for hydrogen is helpful under these conditions. A katharometer based system has been designed and developed at IGCAR for this purpose. The front end of this system is a thin walled nickel tube of 3 mm in diameter and 7 meter in length kept exposed to the cover gas at 500oC. The hydrogen diffusing into the coil is flushed out with argon carrier gas and measured using the thermal conductivity detector. This device has been constructed and put to use at IGCAR for monitoring the FBTR cover gas. Its performance was validated in sodium loops before incorporating it in the reactor. It could sense a change of 50 ppm of hydrogen in argon cover gas very reliably.

Polymer Sensor for Hydrogen in Argon
A polymer sensor, based on a proton conducting polymer membrane (Nafion, PVA / H3PO4), has been developed for measuring hydrogen in argon. One side of the membrane, which is exposed to hydrogen sample, is coated with Palladium electrode and the other side exposed to air is coated with Platinum electrode. The current obtained by short-circuiting the electrodes, is found to be proportional to the hydrogen level in the sample gas. The response is linear over the range of 1 to 200 ppm of Hydrogen in argon. Being a current signal, it can be transmitted without any interference. The types of membrane and electrodes used have been optimized to get good stability and lifetime.

This sensor is compact and reliable for use in the cover gas hydrogen meter in place of the katharometer. The polymer hydrogen sensor has also been put to use for other applications. These include corrosion rate measurements in dilute chemical decontamination of PHWR components and hydrogen measurement in zircalloy.

Carbon Sensor
Carbon in sodium coolant can lead to carburization and embrittlement of structural materials. Monitoring the carbon activity in sodium can help detect oil leaks into sodium through the pump shaft seals in FBRs. For this purpose, an electrochemical sensor based on a molten salt electrolyte (Li2CO3-Na2CO3) has been developed at IGCAR. This again is an electrode concentration cell with sodium having dissolved carbon forming one electrode and pure graphite the other electrode. The housing is similar to that of the hydrogen meter where the outer iron thimble, which holds the electrolyte, is exposed to sodium. The graphite reference electrode is kept inserted into the molten electrolyte. The sensor gives an emf response proportional to the logarithm of carbon activity in sodium. The optimum operating temperature for this sensor is 600oC.

These sensors have performed well in sodium loops at IGCAR in the range of 5 ppb to 5 ppm of carbon in sodium. One such sensor has been incorporated in the secondary circuit of FBTR

Oxygen Sensor
Oxygen in sodium can lead to enhanced corrosion and activity transport. Air leak into reactor during fuel handling is a possible source for oxygen in sodium. Monitoring the sodium for oxygen thus becomes important. The most commonly used device for measuring oxygen level in sodium is the plugging indicator, which measures the temperature at which the oxide precipitates and plugs a cooled orifice. However, this is not sufficiently selective and prone to interference from hydride. The widely investigated electrochemical sensor for oxygen in sodium uses an oxide-ion conducting solid electrolyte, Yttria Doped Thoria (YDT). YDT is compatible with sodium and can serve as a pure ionic conductor at very low oxygen pressures encountered in sodium systems. But YDT is very expensive and highly sensitive to thermal shock. In IGCAR, work on the basic properties such as electrolytic domain boundaries of the other well known oxide ion conductor namely calcia stabilized zirconia (CSZ) showed that, if used at low temperatures, it can perform well in oxygen sensors for sodium monitoring. CSZ based sensor, which uses K/K2O as reference electrode, was developed at IGCAR and tested in sodium loops. The results indicate that CSZ sensor can give good performance if operated around 250oC. This sensor works well in the range of 1 ppm to 20 ppm of oxygen in sodium. However, the average useful life of these sensors was found to be not more than six months. Attempts to improve the lifetime by coating the CSZ with a ternary zirconate have met with qualified success. Effort is underway to identify suitable coating materials that can withstand sodium attack better.

Oxygen risk monitors
Fast reactors use inert gases as cover gas (argon) and blanket gas (nitrogen). Any leak of these gases into operating areas can reduce oxygen levels. A zirconia based sensor that can detect depletion of oxygen in air has been developed in IGCAR. This uses a CSZ electrolyte coated with platinum electrodes on both the sides operating at 600oC. Standard air is used as the reference gas. The emf signal given by this sensor varies with logarithm of oxygen pressure. A change of ± 0.1% of oxygen in air can be reliably detected.

A more rugged version of this sensor has also been developed for measuring oxygen level in the breathing air of the combat aircraft pilot. The emphasis here is on lightweight, compactness and speedy response when started from room temperature. A miniature solid electrolyte thimble equipped with a micro heater suspended using Nilo diaphragms was developed to meet the above requirements. The sensor suitably housed to withstand the harsh conditions, has been handed over to DRDO for evaluation.