This paper is based on the PhD thesis investigation by H. Lurie [1] on the transient heat transfer from a vertical submerged metallic ribbon undergoing a step in Joule heating leading to boiling on its surface. The tests were made in deaerated distilled water at atmospheric pressure with pool temperatures at saturation and 112 deg F subcooled, and with heat generation rates per unit of ribbon surface area from nonboiling to 1.6 × 106 Btu/ft2hr. Although the heat capacity of the ribbon is low, the surface temperature overshoot compared to the steady-state temperature is minor with values of less than 10 deg F. The time required to reach this overshoot, or the time required to reach steady state, is very short and decreases with increasing heat flux. These values are short compared to Goldstein and Eckert [2] and Siegel’s [3] estimates of the time required to develop the hydro-dynamic and thermal boundary layers in natural convection, and indicate that nucleate boiling heat transfer is probably a weak function of the fluid circulation. Some further support for this is evidenced by calculated transient temperatures based on steady nucleate boiling heat transfer which are in reasonable agreement with the measured performance.

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Parametric analyses were performed to determine the optimum heat source configuration relative to previously determined selection criteria. Heat source sizes, weights, volumes, material interaction depths, and thermal expansion differentials between the various components were calculated for all candidate designs. These analyses included single-layer, two-layer, and three-layer designs of flat-ended and hemispherically-ended configurations of all material combinations under consideration. Selective and nonselective vent systems were designed to vent the fuel-generated helium from the heat source to retain and/or decay the Radon-220 to levels below the maximum allowable body burdens. The capsule wall thickness required to survive bullet impact, granite impact, and crushing was determined for the various configurations, and the thickness of a selectively-vented capsule required to survive creep due to the internal helium pressure was also determined. Based on these analyses, minimum required wall thicknesses were determined and an optimum design of each material configuration was generated. Forty-three candidate designs were scored against the selection criteria. The recommended system, which is currently being fabricated, is a rightcircular cylinder consisting of a Pt-20Rh liner, a T-111 strength member, a Pt-20Rh clad and a nonselective pressure relief device/capillary tube assembly. The liner and clad are interconnected to seal off the strength member from external contamination.

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