The use of liquid sodium as a coolant in fast breeder reactors has been made safer, thanks to a sensor — electrochemical hydrogen meter — developed by scientists at the Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam, off Chennai. The sensor has been thoroughly tested at IGCAR; it was also tested at the Phenix fast breeder reactor in France.
“It was first tested in Phenix in 2009 for one year,” said T. Gnanasekaran, Raja Ramanna Fellow at the Chemistry Group, IGCAR. “Now another sensor has been installed a few days ago in one of the experimental sodium loops in Cadarache, France.”
Liquid sodium metal, not water, is used for extracting heat from the extremely hot core (where nuclear fission takes place) of a breeder reactor. Aside from other properties, liquid sodium has excellent heat transfer properties compared with water.
The liquid metal at about 550 degree C transfers the heat to water in the secondary circuit to generate steam; the steam eventually runs the turbine. Any large-scale mixing of sodium and steam should be prevented as it can lead to explosive events.
The pressure on the sodium side is low (1 bar) as the liquid sodium is at an operating temperature of 550 degree C, well below the 883 degree C boiling point. However, at about 160 bar, the pressure on the steam side is very high. But all that separates sodium and steam is a thin (4-5 mm) ferretic steel tube through which steam flows.
There is a possibility, even if remote, of tube failure. Steam, which is at a higher pressure than sodium, tends to leak into the coolant when the tube develops a leak. On reaction with sodium, hydrogen and sodium hydroxide are formed. Sodium hydroxide, which is a caustic material, further aggravates the problem. Due to its low melting point, sodium hydroxide turns into a molten material at the site of the crack causing further corrosion of the tube.
“Continuous monitoring for any steam leak even at its inception is therefore extremely important,” he pointed out. Since the operating temperature of sodium is high, hydrogen and other reaction products get dissolved in it. Hence the presence of dissolved hydrogen in sodium is continuously monitored to detect the initiation of a leak. “If undetected at the micro and small leak stages, steam leaks can develop into a large leak and lead to explosive events,” Dr. Gnanasekaran pointed out.
“Hydrogen level in sodium will shoot up even if a small amount of steam leaks into the coolant. Our sensor can measure dissolved hydrogen down to 70 parts per billion (ppb) in sodium,” he said.
The sensor is able to detect dissolved hydrogen at extremely low levels as it uses a high temperature hydride ion-conducting solid electrolyte. “Solid electrolytes conduct by ions,” he said. Only a few solid electrolytes are known. “But a hydride ion-conducting solid electrolyte is needed for measuring dissolved hydrogen concentration in sodium at elevated temperatures,” Dr. Gnanasekaran explained. “Such an electrolyte was not available.”
This forced the IGCAR scientists to develop a hydride ion-conducting solid electrolyte. They used a reference electrode and the solid electrolyte to construct the sensor — electrochemical galvanic cell. “The cell [sensor] gives an electrical output depending on the dissolved hydrogen concentration in sodium,” Dr. Gnanasekaran explained. “The critical component is the solid electrolyte.”
The sensor kept at the outlet of the steam generator is a lot more robust and inexpensive than the sensor used earlier. Several sensors can be introduced at different locations of the steam generator to increase the sensitivity.