The Standard Fire Policy prescribed by this chapter is not intended to cover loss or damage caused by nuclear reaction or nuclear radiation or radioactive contamination, whether or not directly or indirectly resulting from an insured peril under said policy. Insurers issuing the Standard Policy pursuant to this chapter are authorized to affix thereto or include therein a written statement that the policy does not cover loss or damage caused by nuclear reaction or nuclear radiation or radioactive contamination, all whether directly or indirectly resulting from an insured peril under said policy. Provided, however, that nothing herein 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 or nuclear radiation or radioactive contamination.