Production of (14±2) MeV neutrons in a reactor with an enriched 3LiD irradiation capsule

Abstract

A small double-walled zirconium irradiation capsule containing 95.56% enriched 6LiD was tested in the Georgia Tech Research Reactor in an incident neutron flux of 4.65 × 10 13 n/cm 2 sec at 1 MW power. The 6Li(n, t) 4He- 2H(t, n) 4He reaction sequence produced a (14±2) MeV flux of 4.48 × 10 10 n/cm 2 sec inside the approximately 1.3 cm 3 irradiation volume, indicating for this design an effective conversion ratio 9.6 × 10 −4, equivalent to a thermal conversion ratio of 2.46 × 10 −3. The fast neutron flux present inside the irradiation volume was monitored by various (n, 2n), (n, α) and (n, p) reactions having thresholds from 1 to 14 MeV, and the incident flux of thermal and epithermal neutrons was monitored at various locations over the exterior surface of the capsule by the 59Co(n, γ) 60Co reaction. In addition, the In+Cd and Cd ratios were measured without the capsule in the beamport, in order to determine the approximate energy profile of the thermal and epithermal flux of reactor neutrons. The temperature rise in the capsule as a function of reactor power level due to heat generated in the 6LiD layer was measured in a non-water-cooled location, indicating a rise to 350°C at a power level of only 300 kW in a few minutes time, so that the maximum flux (at 1–5 MW power) can be produced only with water cooling of the capsule. The presence of a large flux of epithermal neutrons above the Cd cutoff (0.4 eV) gives rise to appreciable (n, γ) reactions; to remove most of this epithermal flux requires a relatively thick shield of boron nitride inside the 6LiD layer. Some design characteristics of a larger capsule which can accommodate both the thicker boron nitride shielding and water cooling channels are discussed.