Talk:Japanese nuclear weapon program/@comment-92.2.199.21-20160321121703

                          The Race for the Atomic Bomb ?



                     Translation of the Japanese Tonizo Dossier.



 Meeting on Uranium research at the Nishina laboratory. 6th July 1943.



 Attendance:      Dr. Nishina,   Gen. Nobu-uji,     Ishida ( gishi / engineer ?)

 Dr. Nishina; The minimum mass of uranium 235 is about 10Kg and is determined by the balance between the neutrons generated by fission and those lost though the surface. If too many are lost then there will be no sustained chain reaction. Though this critical mass is about 10 Kg, it will not make a bomb, there needs to be extra, assume an extra 10 Kg (1).

 Gen. Nobu-uji asks if this extra will also under go fission. Nishina says, only a portion will undergo fission, the rest will be lost in the explosion (2). At present it is not within our capability to implement such a device. There are other reasons why a bomb is not practical and therefore not recommended ( fu-tokusaku. In order to achieve the highest temperature and largest possible explosion, the bomb needs to be held ( hoji suru) together for 1/30 to 1/20 [micro]* second and to achieve this it requires a large and heavy tamper or reflector (‘bombe’ in katakana ) (3). The weight would be enormous ( jindai ), therefore it is considered impractical and ‘as a bomb’ not suitable ( tekito narazaru ).

 Footnote; (1)Modern nuclear parameters yield a critical mass for U235 of about 17 Kg with a substantial reflector. The Hiroshima bomb used 80% U235 and was about 2.8 critical masses. (2) Only about 700 grams of U235 out of 64 Kg underwent fission at Hiroshima., the rest being lost in the explosion. (3) Due to the exponential increase of the fission process, 99.5% of the energy is released in the last 5 or so fission periods. At about 10 nano-seconds per period this is 50 nano-seconds i.e.. 1/25 micro-second. During this period the energy released must overcome the inertia of the tamper holding ( hoji suru ) the device together.
 * The word micro is missing from the text. Greek or scientific notation is absent from the text, micro-second = haku man bun no ichi byo.

  Ishida would appears to be the scribe at this meeting and with somewhat limited knowledge of the subject.