It is not widely known that high levels of radionuclide emissions are released from shutdown nuclear reactor buildings, especially gaseous H-3 (tritium) and C-14.
For example, UK emissions data from the Environment Agency’s annual RIFE reports (see Appendix 2) reveal that UK’s Winfrith nuclear reactor buildings (which were closed in 1995) still emitted 2 x 1012 Bq per year of tritium in 2016 ie more than 20 years later. Similar emissions exist at the long-closed reactors at Trawsfynyedd, Dounreay, Chapelcross and indeed all closed Magnox stations.
In Canada, the small experimental reactors at Whiteshell in Manitoba and Rolphton in Ontario (both closed more than 30 years ago) are still emitting GBq/a quantities of tritium. In these cases, the reactors fuels have also long been removed as well.
The question arises – where do these nuclides come from? And the answer, apparently, is that they ooze out of the massive concrete structures of the buildings housing nuclear reactors and from their concrete containment structures. During their operational years, very large amounts of radioactive tritium (H-3) in particular are created and travel into these structures. Why? Because very large amounts of tritium is are created during nuclear fission. Why again? Because tritium is both an activation product and a tertiary fission product of nuclear fission. This is a problem for all reactors but it is a serious problem in light water reactors and a very serious one in heavy water reactors.
As discussed in a previous post, this oozing out of reactor buildings is the main source of the tritium which continues to emanate from the destroyed reactor buildings at Fukushima eight years after the accident. It needs to be recalled that the Fukushima reactors were very old: they had been operating for about 40 years. The tritium created during these 40 years of nuclear fission is the source of Fukushima’s contaminated water.
Some may ask how does concrete retain all this radioactive hydrogen? My hypothesis is – in the hydrates of the complex mineral salts which constitute cement, including calcium–aluminium-silicate–hydrates. A common example is calcium choride whose chemical formula is CaCl2.6H2O and which has 6 water molecules attached to its structure.
Over time the hydrogen atoms in these hydrated salts become radioactive via the process of molecular exchange (whereby the stable H atoms in hydrated salts swap places with radioactive 3H atoms) mainly at the interior and exterior surfaces of reactor buildings. Eventually after decades of reactor operation it is theorised that most or all of these hydrates become tritiated. After the reactor ceases operation, the reverse occurs and radioactive tritium gradually oozes out of the reactor buildings.
There may be an additional process whereby tritium adheres to metal atoms in the reactor in the form of hydrides. It is not known to what extent this occurs in the stainless steel machinery of nuclear reactors.