(47ao) Off-Gas Flammability Control for a High Level Nuclear Waste Glass Melter System Based on Process Modeling and Pilot Testing

Off-Gas Flammability Control for

A High Level Nuclear Waste Glass Melter System

Based on Process Modeling and Pilot Testing

Alexander S. Choi

Savannah River National Laboratory

Aiken, SC 29808

In February 1996, the Defense Waste Processing Facility (DWPF) at the U.S. Department of Energy’s Savannah River Site (SRS) commenced its first radioactive operation to convert over 300 million liters of high level nuclear waste into a stable borosilicate glass matrix for a long-term storage in a geological repository.  At the heart of the DWPF operation is the joule-heated melter which converts a 50 wt% slurry feed containing pretreated waste and glass-forming frit into glass and the off-gas treatment system which removes volatile contaminants such as cesium and mercury from the melter exhaust. One of the critical safety requirements for the melter off-gas system is to maintain the concentration of flammable gases in the off-gas downstream of the Quencher below 60% of the lower flammability limit (LFL) under all operating conditions, while keeping the entire system under moderate vacuum.

The original DWPF design for the melter off-gas flammability control was to monitor the concentration of flammable gases using the LFL analyzers and to interlock the feeding off when readings exceeded a preset limit.  However, it was shown during cold run tests that even after frequent calibration and maintenance these analyzers still were determined to be not reliable enough to function as a Safety Class instrument.  Furthermore, in the event of sudden off-gas surges that could lead to a significant spike in the flammable gas concentration, these LFL analyzers could not provide adequate protection against potential off-gas flammability, since the duration of surges is typically much shorter than the inherent time delay in the response of these analyzers.

As a result, all LFL analyzers and associated interlocks and alarms were removed, and a new strategy was adopted to mitigate potential off-gas flammability that is in compliance with the NFPA Code 69; it requires that all operating variables that can contribute to potential off-gas flammability be monitored, and necessary safety interlocks and operating procedures to control those variables be devised.  This paper describes a comprehensive method currently in place at the DWPF to monitor and control these variables using two process models; a steady state model that describes the batch-to-glass conversion chemistry and a transient model that describes the melter off-gas system dynamics and combustion kinetics of flammable gases. Some of the pilot test results that were used to develop and validate the model parameters are also covered briefly.