中国锦屏地下实验室二期（CJPL-II）建设中关键科学和技术问题研究

It is very important for one nation to build an underground laboratory which is a key platform for some basic researches such as dark matter search, double beta decay and so on. Tsinghua university, collaborating with Yalong River Hydropower Development Company, Ltd., had built China Jinping underground Laboratory phase-I (CJPL-I) in the middle part of Jinping mountain tunnel which has a rock overburden of more than 2400m since 2010. The muon flux of CJPL-I has been measured and is about 60 per year per day, which is the lowest muon flux in the world. Together with the relatively low radioactive rock and convenient transportation, CJPL-I has been one of the best underground laboratory in the world. CDEX collaboration and PandaX collaboration has started to run their prototype dark matter detectors at CJPL-I. The scale of construction of CJPL-I is only 240㎡, which is not enough for the experiment demand. There are many applications for the use of CJPL space from China and aboard. CJPL decides to build CJPL phase-II (CJPL-II) underground laboratory with larger space to accommodate more experiments. This project will focus on the studies of the key scientific problems in the construction of CJPL-II including: 1) the measurement of key physical parameters such as muon flux, gamma and neutron background, and so on; 2) the stability of the rock structure near the site of CJPL-II;3) the quality control of low background environment during the construction of Phase-II;4)The possible technology of the ventilation system for CJPL-II with large underground space and long-distance air transportion will also be studyed. These study can be very useful for CJPL-II construction.

地下实验室是一个国家关键性的重大基础科学研究设施，建设地下实验室意义重大。清华大学与雅砻江流域水电开发有限公司于2010年底建设完成中国锦屏地下实验室一期（CJPL-I），垂直岩石覆盖约2400m，我国自主的CDEX合作组和PandaX合作组已在CJPL-I中开展前期的暗物质实验。CJPL-I建设空间小，仅能同时开展2个实验项目。基于国内外研究组对CJPL可用空间的巨大需求，CJPL决定建设空间更大的中国锦屏地下实验室二期（CJPL-II）。本项目通过对极深地下大洞径情况下围岩构造、深部工程灾害防控与稳定性的研究，筛选CJPL-II建设地点；通过宇宙线通量及环境中子通量关键物理参数测量，评价CJPL-II建设地点；研究极深地下实验室低本底辐射环境建设实施方法，开发地下大型空间超长距大口径高换气率主动通风技术，为建设一流地下实验室提供支撑。

本项目针对CJPL-II建设过程中科学与技术问题，开展了系统研究。主要研究内容是：.1. 通过建立深部岩体力学与工程安全分析方法，结合现场勘察、工程类比和数值分析，确定了CJPL-II工程选址以及四洞八室、交错布置的洞群开挖方案；在洞群开挖过程中，采用了多种测试技术，通过岩体破裂、变形和应力等参数的综合原位监测，有效监测并预警可能发生的工程灾害，为CJPL-II开挖提供了全方位、多尺度的安全监测；.2.开展了CJPL-II宇宙线通量水平及环境中子本底研究，宇宙线缪子通量水平为(8.4±5.9)×10-7/m2/s，中子总注量率为3.11×10-5/cm2/s，为后续入驻的实验组提供了重要的物理数据；.3.建立了一套CJPL材料放射性控制流程，全过程监测，保障了环境低本底；研制了一套基于低温活性炭吸附除氡技术的装置，降低氡浓度约100倍，为进一步降低实验室内部的氡浓度提供了技术保障；对CJPL-II进行了就地伽马谱测量，为进一步分析环境本底提供了数据；.4.根据CJPL-II现场环境工程实际，结合物理实验要求，确定了极深地下大型空间长距离大流量高换气率主动通风系统方案和大容量、高稳定性的供电系统方案，为建设一流地下实验室提供必要的支撑和保障。.本项目为CJPL-II建设成为世界一流地下实验室提供了基础。