SAFETY AND ENVIRONMENTAL ASPECTS OF ORGANIC COOLANTS FOR FUSION FACILITIES

Organic coolants, such as OS-84, offer unique advantages for fusion re actor applications. These advantages are with respect to both reactor operation and safety. The key operational advantage is a coolant that can provide high temperature (350-400-degrees-C) at modest pressure (2 -4 MPa). These temperatures are needed for conditioning the plasma-fac ing components and, in reactors, for achieving high thermodynamic conv ersion efficiencies (> 40%). The key safety advantage of organic coola nts is the low vapor pressure, which significantly reduces the contain ment pressurization transient (relative to water) following a loss of coolant event. Also, from an occupational dose viewpoint, organic cool ants significantly reduce corrosion and erosion inside the cooling sys tem and consequently reduce the quantity of activation products deposi ted in cooling system equipment. On the negative side, organic coolant s undergo both pyrolytic and radiolytic decomposition, and are flammab le. While the decomposition rate can be minimized by coolant system de sign (by reducing coolant inventories exposed to neutron flux and to h igh temperatures), decomposition products are formed and these degrade the coolant properties. Both heavy compounds and light gases are prod uced from the decomposition process, and both must be removed to maint ain adequate coolant properties. As these hydrocarbons may become trit iated by permeation, or activated through impurities, their disposal c ould create an environmental concern. Because of this potential waste disposal problem, consideration has been given to the recycling of bot h the light and heavy products, thereby reducing the quantity of waste to be disposed. Preliminary assessments made for various fusion react or designs, including ITER, suggest that it is feasible to use organic coolants for several applications. These applications range from firs t wall and blanket coolant (the most demanding with respect to decompo sition), to shield and vacuum vessel cooling, to an intermediate cooli ng loop removing heat from a liquid metal loop and transferring it to a steam generator or heat exchanger.