Japan has restarted the Kashiwazaki-Kariwa nuclear power plant, the largest in the world, as the country seeks to reduce its dependence on oil imports disrupted by the ongoing conflict involving Iran and the Strait of Hormuz. However, the decision has intensified scrutiny of a problem that Japanese authorities have failed to resolve for decades: the nation is rapidly running out of space to store its growing stockpile of highly radioactive spent nuclear fuel, and no credible plan for permanent disposal exists.
As of December 2025, Japan's 17 operational nuclear plants collectively held more than 17,000 tons of spent fuel, utilizing approximately 80 percent of their total cooling pool capacity. Kashiwazaki-Kariwa is one of three plants whose cooling pools are projected to reach full capacity within just five years at current rates of fuel consumption and storage. Once those pools are full, the reactors would be forced to shut down regardless of energy demand, creating what nuclear policy experts have described as a ticking clock hanging over the entire industry.
Japan has long insisted that its strategy for managing spent fuel centers on recycling and reprocessing — extracting usable plutonium and uranium from spent fuel rods for reuse in nuclear reactors. However, this strategy has encountered repeated and fundamental failures. The Monju prototype fast-breeder reactor, which was designed specifically to burn reprocessed plutonium fuel, suffered a catastrophic sodium leak in 1995 and never operated again, eventually being permanently decommissioned. Without this key component, the reprocessing cycle cannot function as envisioned, and the Rokkasho reprocessing facility has faced decades of delays and cost overruns.
The failure of the reprocessing strategy has left Japan with one of the world's largest civilian stockpiles of separated plutonium — enough, according to nuclear security analysts, to arm thousands of atomic bombs. This stockpile has drawn international concern, particularly from neighbors in East Asia, and has complicated diplomatic discussions about nuclear nonproliferation. Despite these concerns, the Japanese government has continued to pursue reprocessing as its official policy, even as the practical obstacles to its implementation have grown more severe with each passing year.
Facing the imminent saturation of existing storage facilities, the government has begun exploring the possibility of using Minamitorishima, a remote Pacific island located south of Tokyo, as a potential storage site for spent nuclear fuel. However, environmental groups and local fishing communities have raised strong objections, citing the ecological sensitivity of the surrounding marine environment and the risks of transporting highly radioactive materials across open ocean. The proposal remains in its earliest stages with no formal approval or timeline for development.
The challenge of permanent disposal extends far beyond finding a storage location. Experts estimate that selecting a final disposal site, constructing the necessary deep underground facilities, and completing the regulatory approval process would require more than 100 years from start to finish. Once operational, such a facility would need to be monitored for tens of thousands of years to ensure that the buried radioactive waste does not contaminate groundwater or escape into the surrounding environment — a timeframe that exceeds the entire recorded history of human civilization.
The decision to restart nuclear power amid the Iran war-driven oil crisis and the persistent disruptions to shipping through the Strait of Hormuz reflects the difficult energy tradeoffs facing Japan, a nation with virtually no domestic fossil fuel resources. While nuclear power provides a reliable alternative to imported oil and gas, the unresolved spent fuel crisis means that Japan is effectively generating more radioactive waste for which it has no permanent solution. Anti-nuclear activists have called the situation unsustainable and have demanded that the government invest more heavily in renewable energy sources rather than expanding a nuclear fleet whose waste problem remains fundamentally unresolved.
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