An assessment of helium gas behavior in SP-100 class reactors

Abstract

A theoretical investigation into the behavior of helium gas in the primary heat transport system (PHTS) of the SP-100 space nuclear power system was performed. Results indicate that helium gas dissolved in the primary coolant will most likely diffuse out of solution directly into existing bubbles i n the system accumulators/gas separators before reaching a concentration sufficient to drive a nucleation process elsewhere in the loop. Differences in individual loop flow rates of only a f e w percent were demonstrated to have a significant impact on the relative gas diffusion rates in the loop accumulators. Small bubbles (<15 pm radius) which m a y e s c a p e the gas separators will not expand from temperature and pressure changes as they circulate in the PHTS. Bubbles smaller than =40 pm in radius will eventually collapse by mass diffusion. NOMENCLATURE 2 As WA NuAB = mass transfer surface area (m ) -mass transfer rate (kg of He/sec) = dimensionless mass transfer Nusselt number (also known as the Sherwood Number) CO = analytic He concentration in 3 bulk Li (kg/m ) Cs = saturation concentration of He in 3 bulk Li (kg/m ) * Cs = saturation concentration of He in 3 Li at low flow accumulator (kg/m ) 3 AC, = CO-C = supersaturation (kg/m ) AC, = CO-C = supersaturation at low flow k * 3 accumulator (kg/m ) AC,/C = relative supersaturation (dimensionless) DAB = diffusion coefficient of He 2 in Li (m /sec) Dh D. = annulus inner diameter (m) = channel hydraulic diameter (m) DO K L n Pr pL pV P g R r r C r r' eq Re sc T V "b wA a r P P U $ = annulus outer diameter (m) = Henry's Law Constant (m /s ) = length of annulus (m) = amount of inert gas (moles) = Prandtl Number (dimensionless) = liquid pressure at operating 2 2 conditions (Pa) = liquid vapor pressure at temperature T (Pa) = inert gas partial pressure inside bubble (Pa) (J/mole -K ) = Universal Gas Constant = 8 . 3 1 4 4 = bubble radius (m) = vapor bubble radius at incipient nucleation (m) = bubble radius at mass and mechanical equilibrium (m) = radius of liquid meniscus in the equivalent cavity at surface pre -conditioning temperature and pressure (m). = Reynolds Number = pDhV/p (dimensionless) = Schmidt Number = p/pDAB (dimensionless) = liquid temperature (K) = liquid velocity in the channel (m/s) = gas bubble volume (m ) 3 = mass transfer rate (kg/s) = rc/r' (dimensionless) = ratio of gas removal rate at low flow accumulator to gas removal rate at full flow accumulator -AC:/AC, (dimensionless) = Li dynamic viscosity (kg/m-s) = Li density (kg/m ) = Li surface tension ( N , " ) = a measure of inert gas concentration in the nucleation cavity, a function of cavity geometry, contact angle, and moles of gas in the cavity (m-N/K) 3