Insurers issuing the standard policy pursuant to section 20-1503, are authorized to affix thereto a written statement that the policy does not cover loss or damage caused by nuclear reaction, nuclear radiation or radio-active contamination, all whether directly or indirectly resulting from an insured peril under said policy, provided, that nothing here contained shall be construed to prohibit the attachment to any such policy of an endorsement or endorsements specifically assuming coverage for loss or damage caused by nuclear reaction, nuclear radiation or radio-active contamination; however, subject to the foregoing and all provisions of the said policy, direct loss by fire resulting from nuclear reaction or nuclear radiation or radio-active contamination is insured against by such policy.