Nope, not even close, for a Boiling Water Reactor. The fuel elements are clad with Beryllium, chosen for its VERY low neutron capture cross section. BUT, unfortunately, it is a HORRIBLE metal, expecially in contact with water at 300+ C.
No, they use a zirconium alloy, beryllium is used in moderator rods. It would be an absolutely horrific choice for a pressure vessel as it is extremely brittle, reacts with oxygen (you have a lot of dissolved oxygen in your water, especially in open spent fuel pools) and it absorbs alpha radiation-> gets converted to carbon, you really don't want that.
Btw, those rods are quite sturdy, and because they always have an extremely high outside pressure due to the superheated water and quite a bit of expansion room for gases (they have springs to push the pellets together->also a good way of dealing with the gases) they are probably rather good at containing gases, especially really fat ones like Xe, Kr and I.
Well, not in a pool of COLD water, anyway. But, reactor loop water is not usually cold, and a major steam leak definitely can release the cesium and other low-volatility nuclides.
Well, if there really was a steam leak like you described why didn't we register a spike for iodine and the noble gases? They are a notorious PITA to filter out, so we would have seen a lot of them and not so much stuff that can be filtered quite easily.
And i heavily doubt that you would get a lot of caesium leakage from a steam leak as the hydroxide that forms when it comes into contact with water is one of the most soluble hydroxides out there (3kg per litre of water @20°C, let that sink in), you would get a few grams of CsOH in hundreds of tons of water->i doubt that the release of a few kg of water in the form of steam would lead to a spike like that.
Let's consider where the Ruthenium and Caesium might have come from:
-could they be the decay products of some gaseous fission products that escaped? Well, Caesium can't come from that: Cs can be created via a beta minus decay of Xenon, but neither Cs-134 nor 137 form. What about Ruthenium? It sits between a lot of metals, so no.
-could they come directly from U-235 fission? Probably, the products of a fission reaction are roughly the same weight, and since both Cs and Ru are roughly half as heavy as uranium they might have formed in two different fission reactions.