In high temperature helium cooled nuclear reactors the presence of small amounts of gaseous impurities, such as oxygen or oxygen containing compounds, is an important factor in corrosion and mass transfer that affects the operating lifetime and safety of the reactor, as is well known in the art. Graphite and certain metals considered to be otherwise quite suitable for specific applications in a nuclear reactor, such as stainless steel and nickel alloys, for example, are readily reacted with, corroded by, or otherwise degraded in water, carbon dioxide or oxygen, even in very small amounts, such as a few parts per million (p.p.m) of the oxygen containing impurities.
While considerable precautions are generally taken to insure that the presence of oxygen containing impurities do not exceed the barest possible minimum, it is quite apparent that during normal operation of the reactor, regular tests for the presence of oxygen should be made to minimize the degradation of materials, e.g., by oxidation or carburization of metals, hydrolysis of fuel, and pitting of structural graphite components, and to minimize the movement of fission products, carbon transfer and the formation of particles in the primary reactor system. Furthermore, a sudden, though slight, increase in the oxygen impurity level present might be the first detectable indication of a failure or an otherwise undetected deviation from normal operation, and it would be exceedingly helpful if the increase were to be detected immediately so that corrective action could be taken before extensive damage occurs.
Among the various techniques and devices developed to measure the presence of oxygen containing impurities in fluid streams is the voltaic cell, such as the "Self-sealing Electro-chemical Oxygen Meter" described in U.S. Pat. No. 3,791,953, which issued on Feb. 12, 1974; U.S. Pat. No. 3,378,478, which issued on Apr. 16, 1968; and U.S. Pat. No. 3,711,394, which issued on Jan. 16, 1973.
In the aforementioned U.S. Pat. No. 3,711,394 an elongated tube in the form of a solid electrolyte is utilized. The long tube shape is dictated by the necessity for making a gas tight seal at low temperatures. Since elastomers are used, the seal is made at the top end where temperatures are low. However, the resulting cell assembly has had a temperature gradient from top-to bottom that has affected the performance of the cell assembly. Furthermore, in a reactor environment, the presence of radiation can lead to rapid deterioration of the elastomer seal.
The above described ceramic tubes have had the additional difficulty that they have been fabricated by slip-casting or isostatic pressing and then sintering. Thus, the dimensions of the finished tubes have been difficult to control accurately, and the slip-cast tubes in particular contained impurities that could deleteriously affect the electrochemical cell behavior. The isostatically pressed and sintered tubes have also been inordinately expensive and time consuming to manufacture, and they still have contained undesirable impurities.
To overcome many of the problems associated with the preparation and use of tubular shaped ceramic elements, special deformable metal seal rings have been used. Also, thin discs of electrolyte have been used to perform the function of the tubes, as described in U.S. Pat. No. 3,791,953 which is incorporated by reference herein. However, it was difficult or impossible with these cells to effectively and efficiently measure a substantially broad range of oxygen concentrations. This is because the electrolytes suffer from limitations due to the presence of electronic conduction at low oxygen concentrations, since the electronic conduction reduces the sensitivity of the measurements.