21. Cauchois systematically studied the x-ray spectra of heavy elements and actinides . 22. Like the lanthanides, the actinides form a family of elements with similar properties. 23. All actinides form AnH 3 hydrides of black color with salt-like properties. 24. A key need is application of actinide solid solutions in durable crystalline host phases. 25. Silicon-34 also occurs naturally as the result of radioactive decay of actinides . 26. A series of different matrices exists which can control this production of heavy actinides . 27. The two forms of IFR waste produced, contain no plutonium or other actinides . 28. All plutonium isotopes and other actinides , however, are fissionable with fast neutrons. 29. Plutonium like other actinides readily forms a dioxide plutonyl core ( PuO 2 ). 30. Uranium like other actinides readily forms a dioxide uranyl core ( UO 2 ).