Reducing neutron activation to support early dismantling of light water reactors

A method is proposed for new nuclear power plants (NPP) with light water reactors (LWR) to reduce the amount of neutron activated radionuclides by many orders of magnitude for the sake of accelerated decommissioning. This proposal allows the remaining structures of the biological shield (BioS) and the reactor pressure vessel (RPV) after the reduction to stay with radiation doses in or below the range of occupational dose limits of ICRP 60, and with the infrastructure of the plant still intact. Adjusting the plant to the proposal will not interfere with major design parameters, and time frame and costs of dismantling will substantially be reduced. It is a generic feature of these NPPs that the thermal neutron current, caused by the build-up of thermalized fast neutrons in the BioS, is directed from the BioS into the RPV, but not in the sense of a reflector. The importance of any original reflector feature of the BioS is insignificant due to the attenuation in the crossings of thermal neutrons through the RPV-wall. The method proposed here consists of two steps: In step A the content of neutron activated radionuclides in the outer regions of the activated zones, i.e. in the BioS and in the peripheral regions of the RPV is curbed during operation by an appropriate insertion of absorbers in and at the BioS. By that no thermal neutron current into the RPV and no ensuing build-up of activation will occur there. Subsequently, after the shut-down, in step B dismantling by milling goes on exclusively from the inside to the outside of the RPV because of the now steady outward decline of activation in the RPV-wall. Milling is done in the opened, water-filled RPV with the flooding area attached to it, by taking advantage of the infrastructure available, in a way so that contour, water-tightness and structural stability of the RPV be maintained. The reductions hold true for all activation products, cobalt 60Co and long lived ones. The detached activated material is to be shipped to a waste deposit site. The final dismantling of the remaining primary system and the BioS, both with minimized content of radionuclides might then be enabled in short time frame in an imploding way, with less expenditures for radiation protection, by taking advantage of the still intact infrastructure of the NPP, within the protective umbrella of the intact reactor containment. Numerical analyses for the most important steps are provided. Attention was given to comparisons, wherever possible, with data in the open literature that helped to gain confidence into the method of this proposal.

► Reduction of neutron activation in reactor pressure vessel and biological shield. ► Removal of neutron activation from reactor pressure vessel exclusively from inside. ► Intact infrastructure and low surface doses favourable to early decommissioning. ► No Infringement of main Plant parameters by new dismantling proposal. ► Early decommissioning of nuclear power plants reduces dismantling costs.