Talk:Japanese nuclear weapon program/@comment-78.149.197.99-20160509104656

                                   From the Making of the Atomic Bomb.

His calculations, he told Nobuuji, indicated that 10 Kilograms of U235 of at least 50 percent purity should make a bomb, although cyclotron experiments would be necessary to determine  whether 10 Kg will be sufficient, or whether it will require 20 Kg, or even 50 Kg.



                      Translation from the TONIZO report for 17th Nov 1944

The results of the cyclotron measurements will reveal whether 10 Kg is enough or even 20 Kg or 50 Kg may be necessary. There is not much difference in the quantity between 0.7% and 10 % of U235 content but it becomes vastly different if it is over 50 %.(a) Nevertheless it is very difficult to achieve 50 % and impossible to make 100 %. Whether uranium becomes a bomb or not, we don’t know unless we look into the speed of the reaction. The speed is so high that we cannot be measured practically so just have to do by calculation. (b)

(a)  In the region of 10 % or less enrichment the critical mass is in tons. ( Perrin 1939, UO bare sphere 40 tons, with iron reflector 17 tons. At 50 % U235 the critical mass with reflector is in tens of kilo-grams.

(b)  For an average fission neutron of 2 Mev, the mean time between fissions is about 10^-8 seconds (10 nano-seconds).