Cask containing method for spent fuel assembly and subcriticality measuring device for a cask containing system

An area for a spent fuel storage pool is sectioned into an ordinary rack area for disposing spent fuel assemblies taken out from a reactor core and a preliminary storage rack area having the same constitution as a cask for containing spent fuel assemblies. Preceding to cask-containment, the spent fuel assemblies are temporarily transferred once in the preliminary storing rack area from the ordinary rack area to ensure subcriticality and then contained in casks. In addition, those fuels having a higher burn-up degree are disposed coaxially to the central portion and those having not higher burn-up degree are disposed at the outer circumferential portion. The spent fuel assemblies can surely be contained in the casks, or the process of containing the spent fuel assemblies to the casks or the subcriticality after the containment can be evaluated thereby capable of further ensuring the subcriticality. The spent fuel assemblies can be transferred or stored safely and reliably at a good efficiency. (N.H.)