I could be wrong, I'm not an expert on the matter by any means, but I think the reason traditional nuclear reactors are so heavily fortified against attack is specifically because of the threat of a catastrophic meltdown if their cooling and control systems are destroyed. If these nuclear batteries can't melt down and rely on passive safety measures then the need for fortification is drastically reduced. Placing them in a fairly standard concrete building either underground or bermed with earth on all sides would likely be sufficient. You'd probably still want security around to make sure no one tried to manually tamper with the equipment.