Thermal Decomposition of 3-Methoxypropylamine as an Alternative Amine in PWR Secondary Systems

The secondary coolant of pressurized water reactors is buffered to slightly alkaline pH by ammonia or amines in order to suppress corrosion. 3-Methoxypropylamine (MPA) is one of the promising alternative amines. The thermal decomposition of MPA was studied under two conditions: (i) a dissolved oxygen (DO) concentration of less than 5 ppb at 280°C for 1.5 h and (ii) a DO concentration of 20 ppb at 70°C for 2 h. The initial MPA concentration was 10 ppm. After the tests, concentrations of MPA and carboxylic acids were measured. Approximately 9 to 15% of MPA was decomposed after the tests. Carboxylic acid concentrations were as follows: (i) formate 110 ppb, acetate 260 ppb and propionate 400 ppb at 280°C, and (ii) formate less than 2 ppb, acetate 60 ppb and propionate 1270 ppb at 70°C. The reaction mechanism of MPA decomposition was estimated from the present experimental results. At 280°C, the hydrolysis of the ether bond initiates the decomposition, and the subsequent bond cleavage of C-N and/or C-C occurs. At 70°C, hydrogen abstraction by an oxygen molecule is the initiation reaction. MPA radicals and HO2 or C1 compounds propagate a chain reaction and result in a relatively high yield of propionate.