THICKNESS SPECIFYING METHOD OF LOWER PART OF REACTOR PRESSURE VESSEL

PROBLEM TO BE SOLVED: To provide a thickness specifying method of a lower part of a reactor pressure vessel capable of accurately monitoring a thickness of the lower part of the reactor pressure vessel at a time of a severe accident.SOLUTION: A thickness specifying method of a lower part of a reactor pressure vessel includes: a setting step of a step S1 for setting a γ-ray source for radiating a monochromatic γ-ray and a radiation detector between the lower part of the reactor pressure vessel and the reactor pressure vessel; a scattering angle determination step of a step S2 for determining an angle of a scattering angle θ; a window setting step of a step S3 for setting first and second windows to a pulse height discriminator; and a measuring step of a step S4 for measuring the monochronic γ-ray and the scattering γ-ray detected by the first and second windows. The setting step of the step S1 arranges a vertex of the scattering angle θ, the γ-ray source, and the radiation detector so as to represent the vertex of a triangle, respectively, in a side view. The first and second windows are set so that the lower limit discrimination level is at first and second levels lower than the energies of the monochromatic γ-ray and the scattering γ-ray.SELECTED DRAWING: Figure 2 【課題】過酷事故時に、原子炉圧力容器の下部の厚みを精度良く監視可能な原子炉圧力容器下部の厚み特定方法を提供する。【解決手段】単色γ線を放射するγ線源と放射線検出器を、原子炉圧力容器の下部と原子炉格納容器の間に設置するステップＳ１の設置工程と、散乱角θの角度を決定するステップＳ２の散乱角決定工程と、波高弁別器に対し第１及び第２のウィンドウを設定するステップＳ３のウィンドウ設定工程と、第１及び第２のウィンドウで検出される単色γ線及び散乱γ線を測定するステップＳ４の測定工程と、を備え、ステップＳ１の設置工程は、側面視で、散乱角θの頂点と、γ線源と、放射線検出器がそれぞれ三角形の頂点をなすように配置され、第１及び第２のウィンドウは、その下限ディスクリミネーションレベルが、それぞれ単色γ線及び散乱γ線のエネルギーよりも低い第１及び第２のレベルに設定される。【選択図】図２