CASL

The deposition of CRUD (Chalk River Unidentified Deposits) on fuel rods and in other areas of light water reactor (LWR) coolant loops is a serious issue, with potential safety and power reduction implications. Its in-depth understanding is sorely needed as reactors move to higher power densities or change chemistry programs. Methodologies to date to predict CRUD formation and its composition are under development with a truly predictive model still unavailable. Recently at Oak Ridge National Laboratory (ORNL) a thermochemical analysis of CRUD forming conditions predicted the formation of a chemical compound/phase that had been heretofore not considered. Shortly after that modeling prediction, a significant-sized sample of that actual phase material was discovered in the coolant system of an LWR. This instance is encouraging in that work under the CASL program is moving toward the improved description of LWR behavior for which it was designed.

Currently, the EPRI/WEC (Electric Power Research Institute/Westinghouse Electric Company) simulation capability (known as “BOA”) for CRUD analysis only considers uptake of boron (present in the coolant of pressurized water reactors) into CRUD layers via the compounds lithium metaborate (LiBO2) or lithium tetraborate (Li2B4O7). The CASL Materials Performance and Optimization (MPO) Focus Area is pursuing improved thermodynamic models for CRUD layer nucleation, growth, and impact on overall PWR performance (particularly a reactor performance issue known as axial offset anomaly or “AOA” and CRUD-induced power shift or “CIPS”). This effort is being embodied within a new CASL simulation capability known as “MAMBA”. During a recent CRUD meeting, MPO predicted the stability of an additional boron-containing phase at local CRUD conditions: metaborite (HBO2). Soon after this prediction, a sizable metaborite "rock" was discovered in the residual heat removal cooling system of a US reactor. The “rock” had formed under unique circumstances and different conditions than observed in fuel CRUD, but the discovery prompted further collaboration amongst the CASL team and it showed the metaborite phase could be stable as a separate phase in the fuel CRUD. The implication of this discovery is that more boron-containing phases than are currently considered may contribute to AOA/CIPS, and this will support both BOA and MAMBA boron concentration model mechanisms. This is an encouraging early result in high resolution thermodynamic models being pursued by the CASL MPO team.

Specific results of the analysis can be seen in the included plot. The predicted amount of thermodynamically stable material is shown as a function of “dryout” (evaporation of cooling water) of an aqueous solution of lithium and boron, represented by the ratio of boron to water (B/H2O). With increasing dryout the metaborite phase forms, and it is only for nearly complete dryout conditions that a lithium borate phase (lithium tetraborate) is predicted to appear.

Contact: Theodore M. Besmann, 865-574-6852, besmanntm@ornl.gov
Funding Source: DOE Office of Nuclear Energy