In-situ Tritium Measurements of the Tokamak Fusion Test Reactor Bumper Limiter Tiles Post D-T Operations

Abstract

-The Princeton Plasma Physics Laboratory (PPPL) Engineering and Research Staff in collaboration with members of the Japan Atomic Energy Research Institute (JAERI), TritiumEngineering Laboratory have commenced in-situ tritium measurements of the TFTR bumper limiter. The Tokamak Fusion Test Reactor (TFTR) operated with tritium from 1993 to 1997. During this time ~ 53,000 Ci of tritium was injected into the TFTR vacuum vessel. After the cessation of TFTR plasma operations in April 1997 an aggressive tritium cleanup campaign lasting ~ 3 months was initiated. The TFTR vacuum vessel was subjected to a regimen of glow discharge cleaning (GDC) and dry nitrogen and "moist air" purges. Currently ~ 7,500 Ci of tritium remains in the vacuum vessel largely contained in the limiter tiles. The TFTR limiter is composed of 1,920 carbon tiles with an average weight of ~ 600 grams each. The location and distribution of tritium on the TFTR carbon tiles are of considerable interest. Future magnetically confined fusion devices employing carbon as a limiter material may be considerably constrained due to potentially large tritium inventories being tenaciously held on the surface of the tiles.In-situ tritium measurements were conducted in TFTR bay L during August and November 1998. During the bay L measurement campaign open wall ion chambers and ultra thin thermoluminscent dosimeters (TLD) affixed to a boom and end effector were deployed into the vacuum vessel. The detectors were designed to make contact with the surface of the bumper limiter tile and to provide either real time (ion chamber) or passive (TLD) indication of the surface tritium concentration. The open wall ion chambers were positioned onto the surface of the tile in a manner which employed the surface of the tile as one of the walls of the chamber. The ion chambers, which are (electrically) gamma insensitive, were landed at four positions per tile. The geometry for landing the TLDÕs provided measurement at 24 positions per tile. The instrumentation was positioned on the tiles (96 tiles in each bay) by technicians who manipulated the boom and end effector from outside the vacuum vessel port. In addition to obtaining bay L tile measurements, 3 bumper limiter tiles were collected from the vacuum vessel for further analysis. During the bay L measurement campaign it was observed that several of the tiles in the lower third of the bumper limiter exhibited considerable surface flaking.

Plasma-material interactions in current tokamaks and their implications for next step fusion reactors

“…Flaking of co-deposits after termination of operations was observed [564,565]. Analysis of tiles that experienced DT plasmas showed relatively high concentrations of tritium at the top and bottom of the bumber limiter [566], as predicted by earlier BBQ modelling [201].…”

“…This was paralleled by a fall of ≈3500 Ci (0.35 g) in the inventory, after which the inventory was ≈14700 Ci ( 1.5 g), all of which is perhaps more than 1 µm from the surface. Even though it can only detect T close to the surface, beta decay, has potential for providing an in-situ monitor of the near-surface inventory in a tokamak, and this technique may well be useful in future machines [564].…”

Plasma-material Interactions in Current Tokamaks and their Implications for Next-step Fusion Reactors

“…Erosion/re-deposition modelling in TFTR [563] yielded results that are consistent with the above experimental measurements. Flaking of co-deposits after termination of operations was observed [564,565]. Analysis of tiles that experienced DT plasmas showed relatively high concentrations of tritium at the top and bottom of the bumber limiter [566], as predicted by earlier BBQ modelling [201].…”

Visual tritium imaging of In-Vessel surfaces

“…These include the deployment of open wall ion chambers, TLDÕs, smear surveys, and pin diodes [5,6 ]. The major advantages of the tritium imaging system described here compared to non-imaging techniques are that (a) the tritium imaging system used is non-destructive, as the surface of the item being imaged is not disturbed, (b) this imagining method provides a real time assaying technique, and (c) it is fast and relatively inexpensive [7,8,9].…”