Radiolytic gas production from concrete containing Savannah River Plant waste

Abstract

To determine the extent of gas production from radiolysis of concrete containing radioactive Savannah River Plant waste, samples of concrete and simulated waste were irradiated by /sup 60/Co gamma rays and /sup 244/Cm alpha particles. Gamma radiolysis simulated radiolysis by beta particles from fission products in the waste. Alpha radiolysis indicated the effect of alpha particles from transuranic isotopes in the waste. With gamma radiolysis, hydrogen was the only significant product; hydrogen reached a steady-state pressure that increased with increasing radiation intensity. Hydrogen was produced faster, and a higher steady-state pressure resulted when an organic set retarder was present. Oxygen that was sealed with the wastes was depleted. Gamma radiolysis also produced nitrous oxide gas when nitrate or nitrite was present in the concrete. With alpha radiolysis, hydrogen and oxygen were produced. Hydrogen did not reach a steady-state pressure at <140 psi. From these results, estimates of pressure in conceptual containers (cylinders 2 feet ID by 10 feet tall, 90% full) of SRP waste concrete were made. During the first 300 years of storage when radiolysis will mainly be from beta-gamma radiation (from /sup 137/Cs and /sup 90/Sr), hydrogen will reach a steady-state pressure of 8 to 28 psi, and oxygen will be partially consumed. These predictions were confirmed by measurement of gas produced over a short time in a container of concrete and actual SRP waste. The tests with simulated waste also indicated that nitrous oxide may form, but because of the low nitrate or nitrite content of the waste, the maximum pressure of nitrous oxide after 300 years will be <60 psi. After decay of these fission products, alpha radiolysis from /sup 238/Pu and /sup 239/Pu will predominate; the hydrogen and oxygen pressures will increase to >200 psi.