as fuels ran short, causing the mill to run out of fibers. The mills were run down by 1945, and production did not reach pre-war levels until 1950.

PM4 and PM5
A major upgrade to the plant took place in the period after 1950, following General Director O. T. Jarlsby's ten-year plan. Production was eased throughout the period by increased access to lumber and higher demand. The Anker Mill was rebuilt to produce paperboard. A new lumber cleaning mill opened in 1951, followed by a new central furnace the following year, a pyrite roaster and lumber chopper in 1953 and a new beater in 1954. Tistedalsfoss II Power Station and a spirit factory with an annual production of  opened in 1955. A hydrocyclone facility for the sulfite pulp mill,plenum. The zirconium liners had a higher melting point than stainless steel and thus were intended to spread out a molten mass to aid in its eventual cooling.

Two segments of the zirconium liner broke free from the conical flow guide. They had been screwed in place. It was theorized afterward that pressure from the sodium flow broke the segments away from the screws holding them in place.

Fig. 4: A damaged zirconium liner

The freed segments were carried upward by the sodium. One segment apparently lodged in outer edge of the reactor core support structure. The other segment wound up against the bottom side of the reactor core. During reactor operation in June 1966, workers noticed that the temperature of the sodium exiting some fuel assemblies was 20 to 25