Method for decommissioning heavy water reactor facility

A method for decommissioning a heavy water reactor facility that includes a calandria, a calandria vault accommodating the calandria, a reactivity mechanism deck supported by the calandria vault to be located at an upper portion of the calandria and including a plurality of through-holes, and a plurality of guide tubes connected to the calandria through the plurality of through-holes, comprising:

• removing the plurality of guide tubes from the plurality of through-holes;
• installing a plurality of shielding stoppers in the plurality of through-holes;
• removing the shielding stopper installed in one through-hole of the plurality of through-holes, and inserting a cutting device into a lower portion of the reactivity mechanism deck through the one through-hole to cut a connection portion between the reactivity mechanism deck and the calandria vault by using the cutting device;
• and
• separating the reactivity mechanism deck from the calandria vault.