NHPC Ltd had identified the risk of methane gas at the under-construction Teesta VI hydropower project in Sikkim months before a methane-related explosion killed 25 workers on July 20, according to documents available on the company’s website.
The blast occurred inside the head-race tunnel of the 500-MW project in Namchi district after a rockfall trapped workers inside. NHPC had attributed the incident to a sudden burst of suspected methane gas trapped in the surrounding rocks, which triggered an explosion and released dense fumes and toxic gases. A detailed investigation into the accident is still underway.
Documents show that a methane leak had already been reported at the site on January 14 during excavation at HRT Face 4A. Several workers reportedly suffered burn injuries, following which excavation work was suspended.
A subsequent safety audit by the CSIR-Central Institute of Mining and Fuel Research recommended strict safety protocols, including effective ventilation and continuous methane monitoring. It also called for the use of rugged, handheld methanometers approved by the Directorate General of Mines Safety for real-time gas detection.
However, the available documents do not establish whether all the recommended safety measures were fully implemented before excavation resumed.
NHPC records also show that the Ministry of Power issued a tender on June 17 for the quarterly calibration of three methanometer-cum-toximeter units at the project over a two-year period. It remains unclear when the equipment was procured or whether all three units were operational when the fatal explosion occurred.
Further concerns have emerged from an expression of interest issued by NHPC on July 10, just 10 days before the blast. The document sought expert agencies to address excavation-related challenges, specifically including the presence of methane gas, along with water seepage, slush, cavity formation and face collapses.
Safety experts said the documents raise questions about the level of preparedness at the project despite the reported awareness of methane risks. Himanshu Thakkar of the South Asia Network on Dams, Rivers & People called for an independent and transparent investigation to determine whether adequate monitoring and safety measures were in place.
NHPC was contacted for clarification on whether the recommended safety measures had been implemented and whether methane-monitoring equipment was operational at the time of the accident. The company had not provided an immediate response.
Following the tragedy, NHPC said a detailed investigation would be conducted to determine the cause of the incident and that it would continue supporting rescue agencies and affected families.
