This is a video of Youtuber Edward Sarkissian exploding an M67 frag grenade inside an 87-pound steel SentrySafe fire/waterproof home safe to see what will happen. What happens? It gets BLOWN UP. So, if you were wondering if "inside a home safe" would be a secure place to dispose of your extra grenades, the answer is not even for the first one, and definitely none after that. So while this safe may be fireproof, it certainly isn't *lobs dog toy like hand grenade, bounces off wall and back at my feet* fire-in-the-holeproof.
Keep going for the video (everything up to 4:15 is prepping the grenade inside the safe), as well as a video of opening a safe with grenades taped to the outside (with $10,000 and a gold bar inside).
Thanks to el major, who agrees where the hell did he get those grenades?