ET.317

ET.317 was a thermonuclear weapon of the British Royal Navy, developed for the UK version of the UGM-27 Polaris missile.

The US Polaris A-3T warhead was the US W-58. Britain considered but never used the W-58 because the British safety authorities considered it unsafe in several respects. Instead they fitted a hybrid of a US W-59 fusion secondary, triggered by a new British primary based on a Cleo boosted-fission device tested in Nevada as PAMPAS and TENDRAC. Variants of this basic design were used or intended for several other delivery systems, including the WE.177 bomb, UK Skybolt ALBM, and the Blue Water SSM (a tactical battlefield missile), and several others.

Documents declassified and released into the public domain in 2012 disclose that ET.317 was a warhead that used the fission-fusion-fission process, where a boosted-fission device codenamed Jennie triggered a fusion secondary codenamed Reggie which in turn was encased in U-238 depleted uranium. In the conditions created in the fusion process, vast quantities of free neutrons are liberated, and the normally unfissionable U-238 casing fissions. This results in a smaller, lighter, cheaper, dirtier weapon than one using only a fission-fusion process. Fission of the depleted uranium casing can generate a significant proportion of total weapon yield, and compared to other fissile material U-238 is cheap and plentiful as a waste product from HEU manufacture. Jennie was based on the Cleo device tested at PAMPAS and TENDRAC. Reggie was based on the US W-59 fusion secondary.