In Gensokyo, the sarcophagus is divine white roses.
The White Roses are representative of the color of concrete, kind of, but all the people who died trying to contain the Chernobyl disaster, especially since the highest concentration of roses is around Utusho herself, kind of similar to how most of the crew working at the PP itself died later. Despite her lying on a bed of vines, it is a rather shallow concentration, one that is unlikely to keep her contained. Again similar to how the Sarcophagus is crumbling and leaking radiation. She isn't dead, shes merely resting before she wakes again. The white space works to the advantage because around her, there is nothing. A complete absence of life.
A scary thought is how only five to ten percent of the radiation actually escapes, and its predicted that when the containment block fails (which its going to, it was only built for fifteen years at the most, and even then it was supposed to be improved and maintained) it will release radiation in a manner akin to a wine cork popping. Most of the radiation didn't actually remain in Chernobyl, but traveled to neighboring provinces.