Originally Romeo test was the 6th in the Operation Castle schedule. However, extremely successful
Bravo test changed the schedule, and Romeo has been moved to second position.
The device used in the Romeo test, Runt I, was a larger version of the device used in Bravo test. It measured 224.9
inches in length, and had a diameter of 61.4 vs. 179.5x53.9 in Bravo device. The difference was in the fuel. Runt I used
inexpensive, unenriched Lithium (7.5% Li6 isotope) deuteride. Original yield projection was 4 mgt, with the
range of 1.5Mgt-7Mgt. As you can see no one was certain how this device would work. Mainly it was an experiment to see if
the cheap thermonuclear fuel would work.
After Bravo test the projected yield has been doubled, 8Mgt, and the maximum yield increased to 15 Mgt. The explosion yielded 11 Mgt. Hence, it was almost 3 times higher than the original prediction.
Romeo was the first US nuclear test conducted on the barge. Considering that multi-megaton explosions were practically costing an island per explosion, there were no other choices. For the reference, the Romeo barge has been anchored in the Bravo crater.
Runt was later deployed as Mark 17 bomb.