Heroes of telemark

Last week one of the last surviving heroes of Telemark, Birger Stromsheim died at the age of 101. Living in Norway, I feel I should always make a point of mentioning the story of the Norwegian heavy water production and its sabotage when dealing with Nuclear fission in class.  Norway has an abundance of hydroelectric power which is used in the production of aluminium and also Nitrogen fertilisers. The fertiliser is made by combining Nitrogen and Hydrogen which is produced by the electrolysis of water (hence the need for electrical energy). All water contains a fraction of heavy water which is water made of the hydrogen isotope Deuterium (²H,  1 proton + 1 neutron).  When water is electrolysed, the light water splits up faster than the heavy water so the water left has more heavy water than light. If the left over water is put through the process again and again the concentration of heavy water is increased.

Heavy water is used to moderate neutrons, it is good at this because it has a high probability for an interaction and when a collision does take place the neutron loses a lot of energy, however it was not used in the atom bomb but in the process used to make the Plutonium used in the bomb. Naturally occurring Uranium contains a high percentage of ²³⁸U which absorbs neutrons to become ²³⁹U.

This ²³⁹U then decays into ²³⁹Np which further decays into ²³⁹Pu by beta decay. The Plutonium is a nucleus that undergoes fission if initiated by the addition of adding a neutron. The reason this is done is because it is easier to turn the abundant ²³⁸U into Pu than enrich the ²³⁵U present in the natural occurring Uranium.

The early atom bomb did not actually contain a moderator at all; the problem with using a moderator is that slowing the neutrons down makes the time between fissions too long so an explosion wouldn’t happen.  Fast neutrons can cause fissions, it’s just less likely. If you have a high enough percentage of fissile material in the fuel then a chain reaction will happen without a moderator. The next problem they had was to keep the fuel together; if the explosion goes off to quickly the fissile material is blown apart before it has time to split. Anyway I don’t want to give away too many details or the heroes of Telemark will be visiting me.