Radioactively induced sublimation in solid tritium

Abstract

A horizontal cylindrical cavity bounded by isothermal walls was partially filled with liquid tritium which was then frozen by reduction of the temperature to 1.0 K below the triple point. Visual observations revealed that the solid subsequently redistributed itself into a layer of uniform thickness covering the complete interior of the cavity. The time constant for this effect depends on the age (or 3 He content) of the tritium and not on the initial filling fraction. Time constants of 14.9, 92, 219, and 234 min were measured for tritium 0. 04, 7, 16, and 17 days old, respectively. PACS numbers: 52.55.Pi, 05.70.Fh, 23.20.Nx, 64.70.Hz Radioactive isotopes are classic examples of materials which exhibit internal self-heating. Tritium decays to 3 He, emitting a p particle and an antineutrino. Because the /3*s are reabsorbed within approximately 10 ^m, condensed tritium samples have a nearly uniform selfheating rate q and a quadratically increasing temperature profile in the direction away from the containing boundary. The interior surface of a thick layer of tritium can thus be warmer than the interior surface of a nearby, thinner layer, as long as the exterior surfaces of these layers are equal in temperature or are radiating with equal emissivities to an infinite thermal sink. Being warmer, the interior surface of the thicker layer has a higher vapor pressure than the interior of the thinner layer, and a preferential sublimation-condensation effect can occur, tending to make the layers uniform in thickness. This effect, dubbed the "/? heating effect," was first proposed by Martin, Simms, and Musinski 1 as a method of preparing the "ideal" inertial-confinement fusion target, namely a uniform spherical shell of DT.Martin and Simms 2 have constructed a one-dimensional model of the p heating effect which predicts that layer-thickness equilibration proceeds exponentially in time with a minimum time constant T m { n =H s /q, where H s is the heat of sublimation of the solid. For pure T2 at 19.6 K, T m in = 14.4 min, representing the rate constant for the hypothetical case where there is no impedance to the flow of vapor in the cavity. Without repeating the arguments used in Ref. 2, we can easily derive the above expression for one-dimensional slabs of material bounded by semi-infinite isothermal plane surfaces. Imagine a layer of radioactively heated solid completely filling a finite space between two such semi-infinite surfaces, both at the same temperature. At steady-state conditions, the solid will have a parabolic temperature profile with a maximum at the exact center. Take a thin sliver of the solid starting at a distance 8 from the center and expand it many times its original width, turning the solid into vapor and simultaneously creating a space between two layers of solid of unequal thickness, as shown in Fig. 1. We now assume that the impedance to the flow of vapor is so small that the process of sublimation and condensation can transport heat just as effectively as did the original solid sliver. In other ...

X-ray imaging of cryogenic deuterium-tritium layers in a beryllium shell

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“…4, give time constants of 27.0 and 29.7 minutes, in good agreement with previous measurements for D-T of 30 minutes. [19,20,32] The time constant is expected to change with the sample age, due to buildup of 3 He, and will be measured in the future.…”

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Core science and technology development plan for indirect-drive ICF ignition. Revision 1

“…Once a process has been selected for a given [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] L-17588-3 optic, the optimum equipment required for that process will need to be designed to minimize project costs. Depending on the finishing approach, this will include devices such as continuous polishers with laps greater than 150 in., edging and generating tools customized to square and rectangular optics, small-tool polishers, and ion-&am mills.…”

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