I prefer to keep seed in the fridge at between 3-5C, if you keep them in that situation you will be able to keep them for some years...
If you freeze seed you will not be able to refreeze it so be prepared to use it after this one time storage method. Breaking larger batches up into smaller ones is a good option for this method also.
Heat and direct sunlight are just two methods that reduce the germination levels by basically cooking the embryo inside the seed capsule. You can tell nothing from the seed as it looks fine but inside has been killed. Irradiation is just one way the food industry treat things to preserve them longer...it is for me a very bad option but it does give shelf life a little longer. The agricultural industry has used it in Holland to erradicate exporting bugs and eggs via flower/plant export...in a limited way. If this irradiation occurs to seed it is killed, again without visible signs. The use of Kirilian photography is something we at MNS have dabbled with a little.It seems to be one of the safest ways to tell if the embryo inside the seed case is alive or dead. So maybe in the future we will impliment this to scan the seeds for viability.Till then you will need to use the fridge or freezer....all the best Sb