Sodium Boiling Experiment in 37-Pin Bundle under Loss-of-Flow Conditions

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Haga, K.*

Haga, K.*

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A series of loss-of-flow experiments was conducted in an electrically heated 37-pin bundle which simulated a MONJU fuel subassembly. The flow was reduced exponentially by a pump power control system. The radial temperature profile at boiling inception showed a trapezoidal shape independent of the flow decay curve and heat flux. Boiling initiated at the top end of the heated section, then the bubble expanded mainly to the upstream central subchannels and to the downstream unheated section according to the expansion of the saturated temperature region. When the voided zone covered the whole flow cross-section, the growth and collapse of a piston-like bubble repeated and the inlet flow decreased rapidly keeping the oscillation expanding. Dryout occurred when the flow oscillation became large enough to cause temporary flow reversal. Although reentry of the liquid slug rewetted the dried pin surfaces, the subsequent flow reversal caused dryout again.