A lead apron, when exposed to a strong enough source of gamma rays, will be unable to sufficiently attenuate the radioactive flux. It will absorb gamma photons, and then emit a bunch of of them right into you. Radiation shielding has to take into account the total flux, and then it has to be built in layers to a given thickness and overall density. It doesn’t do you any good if the incident radiation is all absorbed, only for 99.9% of the original flux to be re-emitted by the shield!
You can think of it as roughly analogous to the excitation of atoms leading to emission of a photon in the visible range. It doesn’t help to wear a pair of goggles that will absorb all of the light you’re hoping to block, only to have your goggles glow brightly as a result. You’d rightly conclude that you need much thicker goggles, and the same is true here.
No holes needed, and a lot of the original gamma radiation is being absorbed and subsequently emitted, which defeats the point. There are also complications of a substance being exposed to some forms of radiation (primarily neutron) becoming radioactive themselves. It wouldn’t be visible, but it wouldn’t be good either.
A note that shielding, in addition to considering total flux, has to consider the type of radiation. Gamma (very high energy photon) is very penetrating, beta (high energy electron) is less so, and alpha (a helium nucleus) hardly penetrates at all. You also have to consider, in the case of a molten core, other contaminants and radionuclides actually getting on you or in you. Alpha emitters are pretty harmless outside of you, where your skin stops them cold. If you eat or breath them in however, they’re deveststing. So your concern with alpha emitters tends to be covering yourself and wearing a respirator, while that won’t even help a little with gamma.
It would not have holes in it. The main reason why the gamma radiation makes it through is because it never interacted with any of the lead. And even if a (unlucky) gamma photon interacts inside the lead sheet it would not do much damage there. The only thing that really do damage to the material is neutron radiation and even there the damage is on the microscopic level (atoms getting knocked out of place) than anything macroscopic.
From the point of view of the radiation, there's a lot of empty space between lead atoms and a lot of it will just slip through and not interact with the lead at all. A fraction will slam into lead atoms, a fraction will slam into your atoms, a fraction will keep going.
More like a sheet of paper held up to the light. Some light gets through, and if you have thicker paper it will allow less light through, but it doesn't noticeably damage the paper.
Depends how long you leave the paper exposed to light. if you have a sheet of paper half exposed and half shielded for a year of sunlight the effect is marked.
The equivalent in lead would probably manifest in some way, perhaps a visible patina on the surface? or, as a differential in decay products. you could call that 'damage' if you wanted to.
You can think of it as roughly analogous to the excitation of atoms leading to emission of a photon in the visible range. It doesn’t help to wear a pair of goggles that will absorb all of the light you’re hoping to block, only to have your goggles glow brightly as a result. You’d rightly conclude that you need much thicker goggles, and the same is true here.
No holes needed, and a lot of the original gamma radiation is being absorbed and subsequently emitted, which defeats the point. There are also complications of a substance being exposed to some forms of radiation (primarily neutron) becoming radioactive themselves. It wouldn’t be visible, but it wouldn’t be good either.
A note that shielding, in addition to considering total flux, has to consider the type of radiation. Gamma (very high energy photon) is very penetrating, beta (high energy electron) is less so, and alpha (a helium nucleus) hardly penetrates at all. You also have to consider, in the case of a molten core, other contaminants and radionuclides actually getting on you or in you. Alpha emitters are pretty harmless outside of you, where your skin stops them cold. If you eat or breath them in however, they’re deveststing. So your concern with alpha emitters tends to be covering yourself and wearing a respirator, while that won’t even help a little with gamma.