On the Application in Nuclear MHD Cycles of Non-Equilibrium Ionization from Metastable Molecular States Excited by Fission Fragments

Non-equilibrium ionization by the Kerrebrock effect (ohmic heating) has the disadvantage that energy input and conductivity are dependent on current density. This may lead to serious inhomogeneities and short circuits in the plasma. An alternative method of non-equilibrium ionization is proposed, whereby fission fragments emitted from thin uranium-bearing foils in the nuclear reactor excite long-lived states in the plasma. If a mixture of nitrogen, inert gas, and caesium vapour isused, the excitation and ionization will persist till the gas leaves the MHD generator. Three modes of excitation are examined in detail: monatomic nitrogen, metastable electronic, and metastable vibrational states of nitrogen. In the conditions of interest all have lifetimes of a few milliseconds or greater. The system of equations is solved for concentration and conductivity in a variety of conditions, using experimental rate and equilibrium constants. Typically, if 5% of the reactor power is deposited in the gas by fission fragments, excited species concentrations are 1016 cm-3. Conductivities are on the order of 10 mho/m and persist for 5-10 msec in a gas at 2 atm and 1500°K. The non-equilibrium heating rate is several watts per cubic centimeter, comparable to the ohmic heating method; Analyses of MHD generators with simultaneous ohmic and metastable nonequilibrium heating are presented. Because of variations in gas density operating conditions should be chosen to maximize the energy in vibrational metastables, otherwise conductivity variations become excessive along the generator. Some experiments relating to lifetime of the metastable electronic species in nitrogen are described. Afterglow plasmas of several hundred milliseconds duration at 1 to 2 atm have been obtained. (author)