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oxygen leaves little doubt that tritium will be oxidized. The behavior of KNOB and NaN02 is being investigated by experiments now in progress. In subsequent experiments we expect to observe the vapors over HITEC salt with and without H2. 7.4 DISSOCIATINGGAS HEAT TRANSFER SCHEME AND TRITIUM CONTROL IN MOLTENSALT POWER SYSTEMS

E. L.Compere The use of a dissociating-gasheat transfer systemusing nitrogen dioxide (NzO4 NO2 +NO t 02)(? -* N2 t 02),under development in the U.S.S.R., appears to offer some interesting possibilities to the Moltensalt Breeder in terms of mitigation of tritium losses from the power system. Under dissociating conditions the effective heat capacity and effective thermal conductivity of the gas are increased several fold, resulting in increased efficiency of heat transport and the possibility of lower equipment costs. In the Soviet fast reactor propasal,5 pressurized nitrogen dioxide is the primary fluid, removing heat from fuel elements and driving a gas turbine. Thermal and radiation decompositions are by implication not significant, and I4C production was not mentioned. Similar usage in molten-salt systems, though replacing both a secondary salt and a steam system with the dissociatinggas system, would involve the possibility of leakage of nitrogen oxides and oxygen into the primary salt circuit and subsequent reaction with graphite. Although such reactions would doubtless not be extremely violent probably the gases would behave about like pressyrized air - the use of nitrogen dioxide to transfer heat directly from molten-salt fuel will not be further considered here. For a Moltensalt Breeder Reactor, the dissociating gas system could drive a gas turbine,ieplacing the steam turbine system; the primary coolant presumably would be lithium beryllium fluoride or some other such salt. Heat would be transferred to nitrogen dioxide at 100 atm or more, possibly reaching temperatures of 600 to 700째C. At temperatures above about SOO"C, appreciable (and rapidly reversible) dissociation into NO and O2 occurs (absorbing about 14 kcal/mole). As the gases pass through the turbine and cool, exothermic recombi-

*

5. A. K. Krasin et al., Atomnaya Energiya 30, 180-85 (February 1971); translated by F. Kertesz, The BRC-30, Experimental Power Plant with a Gas-Cooled, Fast Reactor and a DirsociatingHeat Zkans$er Agent, OWL-tr-2500.

nation occurs at a rapid rate until most is recombined. The reaction of NO and O2 is kinetically third order; the rate increases slightly as temperature is decreased. The nitrogen dioxide associates (rapidly and reversibly) to dinitrogen tetroxide at temperatures of 100 t o 200"C, with release of an additional 6 to 7 kcal/mole NO2. The dinitrogen tetroxide can be condensed with a fairly low heat of vaporization. (The heat of vaporization of N z 0 4 is 9.1 kcaVmole at the atmospheric boiling point of 2 l0C, and of course diminishes to zero at the critical temperature of 158째C and 99 atm.) The ease of vaporization at low temperatures and the relatively high heat of dissociation at high temperature may facilitate the recovery of thermal energy from the molten-salt system. Though both NO2 and NO are thermodynamically susceptible to decomposition into the elements, rates appear acceptably slow. Any decomposition of NO2 appears first to produce NO, which may decompose further, so NO is the critical substance. The thermal decomposition of nitric oxide as reported by Yuan, Slaughter, Koerner, and F. Daniels6 and Fraser and Daniels7 proceeds by both a second-order homogeneous mechanism for which the rate at 600째C is indicated to be -4 X X moles liter-' hr-' and a heterogeneous, zero-order reaction for which the rate of 600째C is indicated to be less than about 3 X 10" moles/m" hr-' . Though not trivial, such rates may well be tolerable, depending on processing and makeup requirements in particular systems. In any event the heat of decomposition of NO,-20 kcaymole, is of the same order as heat transferred, and there is no increase in molecules on decomposition. Consequently any decomposition into N2 and O2 would require release of these gases and makeup, but little hazard should ensue. The materials of construction should be stainless steels, or chromium containing alloys such as Inconel (or Hastelloy N) which can be heated in pressurized air to the temperatures in question. Oxide films will build up on the surface in the usual way and inhiiit corrosion processes. Tritium entry into the power system would be inhibited by the films. It will become diluted by the entire gas phase, reducing its activity. Furthermore, any tritium entering the system should be oxidized and re-

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6. E. L. Yuan et al., "Kinetics of the Decompositionof Nitric Oxide in the Range 700-1800"," J. Phys Chem 63,952-56 (1959). 7. J. M. Fraser and F. Daniels, 'The HeterogeneousDecomposition of Nitric Oxide with Oxide Catalysts," J. Phys Chem 62,215-20 (1958).

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ORNL-4728  

http://www.energyfromthorium.com/pdf/ORNL-4728.pdf

ORNL-4728  

http://www.energyfromthorium.com/pdf/ORNL-4728.pdf

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