ABSTRACT

Hydrazine as an oxygen scavenger has been widely used for the feed water treatment of PWR secondary side and fossil power plants in the world. However, there is some concern over health and safety issues related to use of hydrazine. Carbohydrazide is listed up as one of alternative oxygen scavengers. In this study, laboratory tests were performed to examine the thermal decomposition rate of carbohydmzide and its reducing effect on carbon steel corrosion in comparison with hydrazine.

Test results revealed that carbohydrazide was stable below 373K. The thermal decomposition ratios of carbohydrazide were less than 50% after 20 minutes exposure in the range of 373K to 428K. Its thermal decomposition rate constant was derived from experimental test data. The measurement of dissolved oxygen concentration also showed that carbohydrazide possessed a fairly good oxygen scavenger property.

Judging from test results mentioned above, it was concluded that carbohydmzide is applicable as an alternative oxygen scavenger for feedwater treatment in PWR and fossil power plant.

INTRODUCTION

It is well known that hydrazine possesses excellent reducing action and oxygen scavenging property. Hydrazine as an oxygen scavenger has been widely used for the feedwater treatment of PWR secondary side and fossil power plants in the world. Recently, PWRS have operated with All Volatile Treatment (AVT) employing high hydrazine content water chemistry. It is reported that the use of hydrazine as a scavenger of trace quantities of dissolved oxygen reduce the corrosion of balance-of-plant materials in the recommended pH range in PWRsl). High hydrazine water chemistry is expected to yield a reduction in the corrosion product transport to the steam generators.

However, there is some concern over health and safety issues related to use of hydrazine. Carbohydrazide is listed up as one of safety substitute of hydrazine2). Carbohydrazide has replaced hydmzine in 20% of the United States electric utility industry3). Moreover, it is reported that the reducing action and oxygen scavenging property of carbohydrazide is comparable with that of hydrazine, and that in particular, at low temperature, its oxygen scavenging property is superior to that of hydrazine4),5),6). In USA, some good results concerning the corrosion restriction effect of carbohydrazide has been reported 4),7).On the other hand, the result of electrochemical measurements at Kewanuee Nuclear Power Plant shows carbohydrazide has no deleterious effect2).

However, the fundamental property of carbohydrazide, such as thermal decomposition rate, reducing action and oxygen scavenging property, has not been adequately clarified.

In this study, the thermal decomposition rate of carbohydrazide was determined by using the autoclave apparatus, absorption spectrophotometry and so on. The effect of carbohydmzide on the lowering of dissolved oxygen concentration and the corrosion of carbon steel is also mentioned by comparing with the reducing effect of hydrazine.

EXPERIMENTAL PROCEDURES

Chemical component of scavenger and pretreatment of test specimen

The ELIMIN-OX(1) consisting of diluted carbohydrazide ((N2H)2CO), the trade name of chemical agent patented by Nalco Chemical Company, was used as an oxygen scavenger. 60% hydrated hydmzine is also used as an oxygen scavenger.

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