Risk analysis and reliability of the GERDA Experiment extraction and ventilation plant at Gran Sasso mountain underground laboratory of Italian National Institute for Nuclear Physics

Abstract The aim of this study is the risk analysis evaluation about argon release from the GERDA experiment in the Gran Sasso underground National Laboratories (LNGS) of the Italian National Institute for Nuclear Physics (INFN). The GERDA apparatus, located in Hall A of the LNGS, is a facility with germanium detectors located in a wide tank filled with about 70 m3 of cold liquefied argon. This cryo-tank sits in another water-filled tank (700 m3) at atmospheric pressure. In such cryogenic processes, the main cause of an accidental scenario is lacking insulation of the cryo-tank. A preliminary HazOp analysis has been carried out on the whole system. The risk assessment identified two possible top-events: explosion due to a Rapid Phase Transition - RPT and argon runaway evaporation. Risk analysis highlighted a higher probability of occurrence of the latter top event. To avoid emission in Hall A, the HazOp, Fault Tree and Event tree analyses of the cryogenic gas extraction and ventilation plant have been made. The failures related to the ventilation system are the main cause responsible for the occurrence. To improve the system reliability some corrective actions were proposed: the use of UPS and the upgrade of damper opening devices. Furthermore, the Human Reliability Analysis identified some operating and management improvements: action procedure optimization, alert warnings and staff training. The proposed model integrates the existing analysis techniques by applying the results to an atypical work environment and there are useful suggestions for improving the system reliability.

摘要 本研究旨在针对意大利国家核物理研究所（INFN）格兰萨索地下国家实验室（LNGS）的GERDA实验开展氩气泄漏风险分析评估。位于LNGS A大厅的GERDA装置是一套搭载锗探测器的实验设施，安置于容积约70立方米的低温液化氩气大型储罐中。该低温储罐又置于另一个常压充水的700立方米水箱内。在此类低温工艺场景中，意外事故的主要诱因是低温储罐保温失效。研究团队已针对整个系统开展初步的危险与可操作性分析（HazOp）。风险评估确定了两类潜在顶事件：快速相变（Rapid Phase Transition, RPT）引发的爆炸，以及氩气失控蒸发。风险分析结果显示，后者的发生概率更高。为避免氩气泄漏至A大厅，研究团队针对低温气体抽取与通风系统开展了危险与可操作性分析、故障树分析及事件树分析。通风系统相关故障是引发此类事故的主要诱因。为提升系统可靠性，研究提出了两项改进措施：采用不间断电源（UPS），并升级风门开启装置。此外，人员可靠性分析还明确了若干操作与管理优化方向：优化作业流程、增设预警警报以及开展人员培训。本研究提出的模型整合了现有分析技术，并将其应用于非典型工作环境，同时为提升系统可靠性提供了切实可行的改进建议。