Paradox of the Сrystal peaceful underground nuclear explosion site (Yakutia, 1974): Evidence from electrical resistivity tomography and water runoff chemistry

• The block-rubble artificial cover at the nuclear explosion epicenter is unfrozen and waterlogged. • Evidence shows brine ourflow and technogenic fill materials from the hole. • Conditions allow radionuclide migration, but their concentrations in runoff remain low. This paper comprehensively discusses data from electrical resistivity tomography, land geomagnetic surveys, radionuclides, and the major-ion chemistry of water flowing at the epicenter of the Crystal peaceful underground nuclear explosion (it was carried out in 1974) in 1990. The integrity of the seal at the emplacement hole cap was compromised during the explosion, the soils at the epicenter were contaminated by radionuclides. To immobilize these radionuclides cryogenically and prevent migration of radionuclides from the central explosion area (at a depth of 98 meters), the head of the emplacement hole and its surroundings were buried in 1992 with block-rubble stone material composed of pure limestone and dolomite fragments. Additionally, the block-rubble artificial cover was further reinforced in 2006. The artificial cover is revealed to remain unfrozen and waterlogged nowadays. Snowmelt and rainwater are observed to seep through this cover, while air convection within it likely leads to the condensation of atmospheric moisture. The bicarbonate magnesium-calcium waters that originate from the percolation of rain, melted snow, and atmospheric condensate through the cover are enriched with technogenic ions such as SO₄²⁻, Mg²⁺, Ca²⁺, Li⁺, K⁺, and Sr, alongside natural components like Cl⁻, Mg²⁺, Ca²⁺, K⁺, Na⁺, Br, Sr, I, and Li⁺, which are derived from underground natural gas-brine fluids that rise along the open emplacement hole and the damage zone. It is hypothesized that Li⁺ could be a component of the nuclear explosive device. Given the low current activity of radionuclides in water flowing from beneath the artificial cover (measured in Bq/dm³): ³H – 4–12, ⁹⁰Sr – 0.004–0.4, and diminishing levels of ²³⁹,²⁴⁰Pu (<10⁻⁶), the current migration of technogenic radionuclides with water is assessed as insignificant. Therefore, there is no immediate need for additional measures to strengthen or modify the block-rubble artificial cover.