CSB: Inspection Found High Risk of Tank Failure 10 Months Before Fatal Incident at Paper Mill

An inspection conducted by Nippon Dynawave Packaging Company LLC found that a tank holding caustic “white liquor” was no longer fit for service at the NDP pulp and paper mill in Longview, Washington, about 10 months before the tank’s catastrophic failure resulted in the deaths of eleven workers, according to the Chemical Safety and Hazard Investigation Board. In July 2025, an inspection contractor hired by NDP reported that parts of the tank’s metal shell had been corroded below the minimum safe thickness and warned of a “[h]igh likelihood/consequence of failure” if the tank were not repaired, CSB states. Additional inspections in October 2025 and February 2026 also determined that significant portions of the shell were too thin to safely contain its contents. But the company did not conduct an internal inspection of the tank, perform repairs, remove it from service, or reduce its operating conditions before it failed on May 26, 2026. 

In an update of its investigation (PDF), CSB explains that the substance known as white liquor consists mostly of sodium hydroxide and sodium sulfide, both of which are corrosive to metals. On the morning of May 26, a process upset required NDP to stop sending white liquor from the storage tank into a vessel known as the digestor, where the liquid would be mixed with wood chips for processing into paper products. White liquor continued to enter the storage tank, filling it to at least 90 percent capacity before it failed. About 900,000 gallons of scalding hot, corrosive liquid were released into the facility’s courtyard and entered the lower floors of the surrounding buildings. Eleven NDP employees died as a result of the incident, and three were seriously injured. Five other employees and a firefighter received injuries that did not require hospitalization. 

CSB’s investigation is still in progress. The agency’s Aug. 28 news release and August investigation update (PDF) cover the most recent findings. Full conclusions, analyses, and recommendations will be included in the final report.