In July 2025, 10 months before the incident, NDP had G Tank inspected by performing an external visual inspection and external ultrasonic thickness testing. The results of the inspection showed significant thinning below the minimum safe thickness in the lower regions of the tank, according to an investigation update from the Chemical Safety and Hazard Investigation Board (CSB).
NDP scheduled additional inspections while continuing to operate G Tank normally. NDP had the same inspection contractor inspect the tank two more times, in October 2025 and February 2026. Both inspections were external thickness inspections and both identified large portions of the tank remained thinner than the calculated minimum safe wall thickness.
G Tank Inspection
NDP did not internally inspect G Tank or make any repairs to the tank’s shell between the July 2025 inspection and the May 2026 incident. The company also did not remove the tank from operation or derate the tank for reduced service, according to the report.
On May 26, 2026, the 1.2-million-gallon atmospheric storage tank catastrophically failed at NDP releasing 900,000 gallons of a caustic liquid known as white liquor, killing 11 NDP employees and seriously injuring three others. Five additional NDP employees and one firefighter sustained injuries that did not require inpatient hospitalization.
“The minimum safe thickness is the amount of material necessary for an atmospheric storage tank to safely contain its contents,” said CSB Investigator-in-Charge Drew Sahli. “G Tank’s shell had areas thinner than the minimum thickness required for continued service at its typical operating conditions.”
“The inspection identified significant tank-wall thinning below the minimum safe thickness months before the failure,” said CSB Supervisory Investigator Mark Wingard. “Our investigation is examining why the tank remained in service without (the company preparing it), or operated it under reduced conditions.”
The mill uses the Kraft process to produce pulp from wood chips. In the Kraft process, wood chips end up sent to a vessel called a digestor, where they mix with a caustic liquid referred to as “white liquor,” which contains sodium hydroxide and sodium sulfide. Under high temperatures and pressures in the digestor, the white liquor breaks down the structural polymers in wood (“lignin”), that hold another component of wood, called cellulose, together. After they remove the lignin, cellulose fibers, or pulp, are further processed into paper products.
White Liquor Details
White liquor is a slightly yellow, liquid mixture of sodium hydroxide (8 to 15 weight percent), sodium sulfide (3 to 10 weight percent), disodium carbonate (3 to 10 weight percent) and water. Meanwhile, white liquor is a basic liquid, having a pH of about 13. Both sodium hydroxide and sodium sulfide have a National Fire Protection Association (NFPA) health rating of 3, meaning they can cause serious or permanent injury. Specifically, white liquor causes serious burns to the skin, eyes, and respiratory system and has an irritating odor. Sodium hydroxide and sodium sulfide are corrosive to metals.
An image of the courtyard where G tank failed. This shows the locations of the buildings and the tank and the direction in which the white liquor flowed. All of the workers depicted in the figure were on the ground floors.
Source: Cowlitz County, annotated by CSB
Meanwhile, G Tank, the tank involved in the incident, ended up constructed in 1987. The tank consisted of 40 carbon steel plates welded together in five rings, or “courses,” that made up the outer walls. Each course was made from eight, 26-foot-long plates. The tank had no secondary containment to capture spills, according to the CSB update.
In a timeline to the disaster in the CSB report, on Tuesday, May 26 at 1:55 a.m., a process upset occurred in NDP’s digestor unit.
The upset required workers to remove the digestor from service until they could conduct repair. As a result, white liquor flow to the digestor slowed down, and at 4:08 a.m., white liquor flow from G Tank to the digestor completely stopped.
Flow Slowed
Although the flow of white liquor from G tank to the digestor stopped, white liquor from the recausticizers continued to flow to G tank at a reduced rate. As a result, G Tank’s level began to increase. By 6:22 a.m., G Tank was 90 percent full.
At 7 a.m., employees began arriving at work for their upcoming shift. These employees went to the electrical and mechanical maintenance shops and were waiting for their morning meetings to start.
At 7:09 a.m., G Tank catastrophically failed, releasing approximately 900,000 gallons of white liquor.
Meanwhile, at the time of the tank failure, 17 employees were present on the ground level. Seven maintenance employees and one foreman were in the mechanical maintenance shop, eight maintenance employees were in the electrical maintenance shop, and one operator was in the courtyard.
Most of the tank’s contents released when it failed, creating a massive “wave” of white liquor that rapidly flooded into the courtyard and toward the buildings. As the white liquor deluged the courtyard, the force of the moving liquid crushed vehicles and destroyed one of the surrounding buildings’ roll-up doors and one of its walls.
A pre-incident satellite image of the courtyard where G tank was built.
Source: Google Earth, annotated by CSB
Entered Lower Floors
White liquor entered the lower floors of the buildings through door seals and the destroyed roll-up doors, where the morning meetings were occurring.
After the tank failed, the eight employees in the mechanical maintenance shop tried to evacuate. Five of the maintenance employees and the foreman escaped, but the other two employees were fatally injured from exposure to the white liquor.
Additionally, all eight employees in the electrical maintenance shop and the worker in the courtyard died due to exposure to the white liquor. Of the six workers who escaped from the mechanical maintenance shop, two suffered serious injuries due to chemical burns.
NDP sent out a radio call for its medical emergency response (MER) team to assist with the initial response to the incident, however, the radio call did not specify white liquor released. Subsequently, one of the MER members attempted to rescue an NDP employee from a pool of white liquor in the courtyard. The responder waded into the liquid, believing that it was water, and ended up seriously injured due to chemical burns.
The Cowlitz County Fire Department and Sheriff’s Office also responded to the incident. During the emergency response, three additional NDP employees and one Cowlitz County firefighter sustained injuries that did not require hospitalization.
At the same time as the emergency response, most of the workers who were in the control room above the electrical maintenance shop ended up evacuated by NDP management. Three operators remained in the control room to shut down the mill. They were not injured.
Unstable Building
After the initial response ended up completed, recovery efforts began at NDP. However, due to concerns about the remaining volume of the highly corrosive white liquor both in the structurally compromised tank and in the flooded courtyard, the last fatally injured person could not end up recovered until four days after the incident.
CSB is continuing its investigation and is gathering facts and analyzing several key areas, including:
- Inspecting G Tank and determining the failure mode and cause(s) of the tank failure
- NDP’s mechanical integrity policies and practices
- NDP’s actions to inspect, repair, and/or replace G Tank and other atmospheric storage tanks
- The company’s policies and practices to manage change
- NDP’s capital project management policies and practices
- NDP’s emergency response policies and procedures
- NDP hazard evaluation policies and procedures
- Regulatory requirements and industry guidance for paper mills
Click here for more detail on the incident update.