According to the safety report, it consists of a dome welded on to a cylinder, all made from 60mm-thick steel. Every weld was X-rayed at the time of construction, and the entire structure was then sheathed in extra-tough stainless steel.
In its first years of operation, every test known to science was carried out on the vessel's integrity and as testing methods have improved so has the regime at Halden. Ultrasonic probes of all descriptions regularly examine every millimetre of the reactor for flaws and, each time, the results are the same.
"There has been no defect development since the first in-service inspection," said the 1997 report. After almost 40 years of service, the Halden reactor was squeaky clean.
But while the authorities were obsessively checking the casing against the possibility of radiation leaks, they failed to notice that a steady stream of contaminated cooling water was being released into the sewers of Halden, thanks to an elementary plumbing blunder.
The mistake was made in 1991, but it wasn't until 1999 that it came to light. Radioactive caesium had ended up in the sewage works. And that's not all. From 1994, sludge from the waste treatment plant was sold as fertiliser to local farmers, who then spread it on fields around the town.
The Institute for Energy Technology, which operates the Halden reactor, interpreted the scale of the leak as presenting an insignificant risk to human health - and blamed the piping error on the municipality.
But environmental groups were quick to point out that for nuclear engineers to have no idea where their cooling water was going hardly inspired confidence in a plant that was supposedly dedicated to improving safety.