Solvent Performance in THTR Nuclear Fuel Reprocessing. Part I: Calculation of Doses Received by TBP-n-Paraffin Extractant in Reprocessing THTR Fuels Applying a THOREX Flowsheet

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Abstract
The radiolytic load of the 30 vol% tributylphosphate-n-paraffin extractant to be used in the Jülich Pilot Plant for Thorium Element Reprocessing facility for reprocessing thorium high-temperature reactor (THTR) fuel elements with high burn-up values (85 000 MWd/MT of heavy-metal atoms) was calculated. At a radioactivity level of ∼2000 Ci/ℓ, the effective beta-particle power density of the feed solution ranges up to 15 W · ℓ−1. Most of the energy absorbed by the extractant is due to beta radiation (99%). About 1% originates from gamma radiation; contributions from alpha-particle emitters are negligible. The calculations consider the geometric parameters of the applied mixer-settler and the operational parameters of the flowsheet. The highest exposure expected will be ∼0.2 Wh · ℓ−1 · pass−1 when reprocessing fuel with 85 000 MWd/MT burnup after a cooling time of 100 days. For an easier comparison of the calculated value with other reported values, a coefficient is introduced describing the specific exposure of the extractant in terms of energy absorption per hour of passing through the contactor at a power density of 1 W· ℓ−1 in the feed solution. This coefficient is independent of such individual flowsheet conditions as heavy-metal concentration or power density in the feed solution. Comparison of calculated data with other reported data for THOREX and PUREX reprocessing runs exhibits only about a four-fold specific load of the extractant in case of reprocessing high-burned-up THTR fuel with respect to low-enriched low-burned-up light water reactor fuel. This underproportional increase is due to the specific fission-product spectrum of the investigated THTR fuel arising in the course of its reactor residence time.