Exposure Budgets for Locating Reactor Fuel Debris by Muon Attenuation at Fukushima Daiichi

This paper computes the muon attenuation, exposure and resolution budget for imaging the Fukushima Daiichi reactor buildings with cosmic-ray muons. Sight lines through 8 metres of concrete and steel shielding at a density of 3.2 grams per cubic centimetre present an opacity of 2560 grams per square centimetre (25.6 metres of water equivalent), which sets a minimum muon energy of 5.143 GeV and a transmitted vertical intensity of 5.7 per square metre per second per steradian. For a detector of 1 square metres with the stated acceptance and efficiency, divided among 900 image elements, the stated feature produces a flux contrast of 0.3921 and reaches five standard deviations in 0.4127 days of counting, while the target precision of 0.05 per image element requires 1.015 days. A deployment of 90 days delivers a per-element precision of 0.00531, corresponding to a thickness sensitivity of 0.04248 metres. The combined angular resolution is 30.6 milliradians, of which tracking contributes 2.357 milliradians and multiple scattering at the threshold momentum 30.5 milliradians. The budget is compared with published muography of the same class of target, for which muon imaging at Fukushima Daiichi reported the absence of large fuel masses in the Unit 1 reactor core region and located dense material in containment, using detectors of order one square metre over months. Every number is computed by the accompanying script from the stated inputs. Computed, not measured.