Generic thermal-hydraulic and physicochemical analysis for implementing an ATHLET module to simulate thermohydraulic consequences of zinc borate precipitation in the PWR core (joint research project AZora)

Sub-Project:

Lab scale experiments to enable modelling of crystallization and precipitation processes of zinc borates as well as heat transfer properties of zinc borate layers

Motivation:

In the case of a loss-of-coolant accident, corrosion of hot-dip galvanized inventory like tread gratings and support grids of sump strainers in boric acid-containing coolant solutions can be expected, possibly resulting in subsequent depositions of formed zinc borates as sparingly soluble corrosion products in hot zones of the PWR reactor core.

Objectives:

The main goal of the project is to generate data and correlations on the following topics based on generic experimental investigations:

• Solubility and crystallization behavior of formed zinc borates in the coolant
• Parameter dependent deposition rates of zinc borates on cladding tube surfaces as layers or as mobile particles in the heated coolant
• Dependence of the thermal conductivity of zinc borate layers on cladding tube surfaces on the conditions of formation (temperatures, zinc concentrations, flow conditions)
• Removal of zinc ions from the coolant using ion exchangers

The resulting data and correlations are to be used by the project partner GRS as a basis for the modeling and computer-aided simulation of thermal-hydraulic effects caused by zinc borate depositions in the reactor core. In such way, it should be possible in the future to get more accurate and reliable analysis of thermohydraulic consequences of zinc borate deposits in the reactor core for various loss of coolant accident scenarios. Furthermore, the comparison of the LOCA scenarios can be improved with respect to safety-related questions.

Methods and results:

© HZDR

Left:  Basic structure of the measuring cell with heated zircaloy block for the determination of parameter-dependent zinc borate deposition rates and thermal conductivities of grown zinc borate layers on hot surfaces of cladding tubes
Right: Surface of the Zr4 unit before and after a zinc borate precipitation experiment

The started ZnB deposition experiments are intended to investigate possible ZnB layer formation on fuel rod surfaces during a postulated loss of coolant accident (LOCA). In addition to camera images, important investigation methods for characterizing formed ZnB layers and their properties (e.g. thermal conductivity) are laser triangulation (layer thickness measurement) and thermal calorimetry (e.g. for determining the thermal conductivity of grown ZnB layers).

Publications:

U. Harm, M. Wiezorek, H. Kryk, U. Hampel
LOCA Scenario-Related Zinc Borate Precipitation
Studies at Lab Scale, Proceedings of the Annual Meeting on Nuclear Technology (AMNT 2019), Berlin 2019

W. Kästner, S. Alt, A. Seeliger, F. Zacharias, U. Harm, R. Illgen, U. Hampel, H. Kryk,
Modelling thermal-hydraulic effects of zinc borate deposits in the PWR core after LOCA - Experimental strategies and test facilities, International Journal for Nuclear Power, Vol. 65 (2020), Issue 6/7, 341-345