Design of the capsule for irradiating coated particles in the HRB-15b experiment

The purpose of this work is the design of a sample capsule for the HRB-l5b experiment. This capsule is to be used to test seven batches of fissile fuel particles and six batches of fertile fuel particles under simulated reactor operating conditions and to evaluate these particles for possible use as fuels in future High Temperature Gas Cooled Reactors (HTGR). The specific purpose of this design is to maintain the test particles at a temperature of l000°c (l832°F) for about six months or seven 23-day cycles with as little temperature variation as possible. The capsule will be irradiated in the High Flux Isotope Reactor (HFIR). This duration is needed to attain the desired maximum particle exposure of 8.0 x 10²¹ n/cm²(E>0.18Mev).

The capsule is cylindrical in shape, 3.556 cm (1.4 in.) outer diameter and 48.26 cm (19.0 in.) length. The capsule has a central graphite rod with nine longitudinal holes (eight for thermocouples and one for gas intake). The central graphite rod also includes a central region for inert-particle testing. . Surrounding the central rod are 184 vertically stacked disks (washer shaped), 0.2032 cm (0.08 in.) thick, with one hundred and sixteen 0.10922 cm (0.043 in.) diameter holes drilled in one side of each disk. Either fissile or fertile fuel particles of the same batch are individually loaded in a predetermined number of holes in each disk. The outside of the capsule is a cylindrical double-walled stainless steel containment vessel.

To obtain an approximately uniform power generation rate along the length of the capsule, the disks were stacked in an alternating manner (fissile, fertile, fissile, etc.). For analysis, the power produced by both fissile and fertile particles was smeared over each two-disk region.

To ensure that the l000°c (l832°F) design criterion would be met by this smeared particle power, a one-dimensional code, GENGTC, was applied to each two-disk region to fix the gas gap between outside disk diameter and inner stainless steel containment and the number of particles loaded into each disk. The temperature during capsule operation is controlled by varying the sweep gas mixture. The outside diameter of the disks varied from 1.115 cm (0.439 in.) to 1.179 cm (0.464 in.); particles loaded into each disk varied from 54 to 116; and the sweep gases used were helium, neon, and argon.

A generalized heat transfer computer code, HEATING V, was used for a two-dimensional analysis of a typical 800 µm (0.0315 in.) diameter particle in a graphite hole. An empirical equation was developed to relate the measured thermocouple temperature to the particle temperature. This equation allows the calculation of the maximum thermocouple setpoint for any given gas mixture and particle power.

Due to the capsules close proximity to the control plates and the high flux gradients over its length, the fuel particle temperature below the design temperature. Drilling l45 holes per disk would decrease temperature fluctuations during cycles and increase the capsule loading by as much as 3000 particles.

The experiment was a success. It maintained the test particles at or below the design temperature and fluence. The methods and assumptions used in obtaining this capsule design worked well and are recommended for similar capsule design.