EPA's TSCREEN Model as a First Step Toward Characterizing Accidental Releases of Chlorine and Chlorine Dioxide, 1997 Environmental Conference Proceedings

At the Flint River market pulp mill, compressed liquid chlorine (for water treatment) and aqueous ClO2 solution (for pulp bleaching) are stored on-site. Weyerhaeuser has characterized time-dependent maximum ambient concentrations resulting from an accidental release, using EPA’s TSCREEN model. Weyerhaeuser’s consultant devised a technique to characterize the continuously declining release rate of chlorine dioxide from a hypothetical tank puncture. Similarly, the consultant calculated the time-dependent nature of the chlorine release from a storage cylinder valve breakage incident. In such an event, the release will occur in two stages, i.e., an initial short-term pressurized release, followed by a longer heat transfer-limited release. The TSCREEN model assumes a constant mass release rate until the cylinder is empty; in so doing, the model substantially overestimates time-dependent mass release rates, and underestimates the duration of the release. The consultant characterized time-dependent mass release rates, and performed sequential short-term screening model calculations to develop plots of time-dependent ambient concentrations at various receptors. The model selects worst-case meteorological conditions for each receptor, including the assumption that each receptor is downwind of the source. Calculated concentrations are also highly sensitive to assumed duration of release; this sensitivity makes the incremental (sequential fixed-rate release calculations) approach to developing time-dependent release estimates highly uncertain, especially during the rapidly-changing initial release period. Accurate time-dependent release characterization will require a more sophisticated model. Weyerhaeuser’s consultant also calculated time-dependent chlorine concentrations at hypothetical indoor locations. The calculations should be useful in devising risk management practices as required under Section 112(r) of the Clean Air Act.