Marine biogenic materials such as corals, shells, or seaweed have long been recognized as recorders of environmental conditions. Here, the bivalve Cerastoderma edule is used for the first time as a recorder of past seawater contamination with anthropogenic uranium, specifically ²³⁶U. Several studies have employed the authorized radioactive releases, including ²³⁶U, from nuclear reprocessing plants in La Hague, France, into the English Channel, and Sellafield, England, into the Irish Sea, to trace Atlantic waters and to understand recent climate induced circulation changes in the Arctic Ocean. Anthropogenic ²³⁶U has emerged over recent years as a new transient tracer to track these changes, but its application has been challenged owing to paucity of fundamental data on the input (timing and amount) of ²³⁶U from Sellafield. Here, we present ²³⁶U/²³⁸U data from bivalve shells collected close to La Hague and Sellafield from two unique shell collections that allow the reconstruction of the historical ²³⁶U contamination of seawater since the 1960s, mostly with bi-annual resolution. The novel archive is first validated by comparison with well-documented ²³⁶U discharges from La Hague. Then, shells from the Irish Sea are used to reconstruct the regional ²³⁶U contamination. Apart from defining new, observationally based ²³⁶U input functions that will allow more precise tracer studies in the Arctic Ocean, we find an unexpected peak of ²³⁶U releases to the Irish Sea in the 1970s. Using this peak, we provide evidence for a small, but significant recirculation of Irish Sea water into the English Channel. Tracing the 1970s peak should allow extending ²³⁶U tracer studies into the South Atlantic Ocean.