ON March 11th 2011 a tsunami engulfed Japan’s north-east coast, flooding towns, farmland and severely damaging the Fukushima Dai-ichi nuclear power plant. Almost 19,000 people were killed by the tsunami, and a further 160,000 were evacuated after the power plant’s core leaked radiation into the sea and surrounding area in the worst nuclear disaster since Chernobyl in 1986. Five years on, Fukushima is still an exclusion zone and tens of thousands are stuck in temporary shelter. Japan shut down all but two of its 43 reactors over safety concerns. And other countries have recently sped up efforts to replace nuclear power with greener renewables. Germany plans to close all 17 of its nuclear plants by 2022, a process that has already begun. France passed a bill last year mandating that the share of nuclear power in the country’s electricity mix be cut from 75% to 50% within ten years. Meanwhile commodity prices have slumped, resulting in a cut to wholesale electricity prices. That makes it harder for many nuclear plants to cover their running costs, also leading to closures. Where markets are freer, it is harder for nuclear-power operators to make money, and too risky for them to build costly plants from scratch.

INTERACTIVE MAP: Our guide to the world's nuclear-power producers Yet as countries try to cut carbon emissions, such closures often lead to the burning of more fossil fuels. Adding renewable-energy capacity does not solve the problem: when the wind doesn’t blow and the sun doesn’t shine, nuclear energy still provides the best low-carbon source of reliable “baseload” electricity. Thus in Germany the proportion of electricity produced by coal-fired plants has increased since reactors were closed down, so pushing up carbon emissions. Governments should shape energy policies to keep existing reactors running: they provide low-carbon energy, and decommissioning them is as likely to create safety problems as leaving them running. It is reckoned that decommissioning the Fukushima plant will cost $20 billion, while the clean-up could come to $100 billion.