Talk:Japanese nuclear weapon program/@comment-88.106.122.183-20160623082311

Formalities concerning administration and expenses.

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

      Attending ;  Dr. Nishina, Lt. Gen. Nobu-uji,   Ishida (gishi)

     On the 2nd July, general manager of the Army Aeronautic Technical Research, Yasuda (Takeo) assigns the uranium research to Dr. Nishina and the director of Riken has received a confirmation of this. It is noted that the Navy has been informed of these matters but as yet have not responded. This may have been a genuine delay, though there may also have been  an element of Satsuma- Choshu animosity.

 The three projects under consideration are, atomic bomb, nuclear reactor and nuclear power generator. The first two are the ones under consideration at this meeting.

These projects are allocated 2000 Yen for one year to which two ‘officer’ are to be assigned. The officers are probably recently qualified PhD students, conscripted into the army and seconded to the project. At that time a visit to a bathhouse cost 12 sen (100 sen = 1 yen) and an o-bento ( lunch box) about 40 sen. Today that would be about 450 yen and 800-1000 yen respectively. The sum of 2000 yen is estimated to be in the region of  £35,000 / 24,000 $ today. By present days standards this is not a sum to be sniffed at but considering future implications, no more than a warm up fee.

In the question and answers session following, Nishina points out that the initial research is fundamental for both projects. In connection with the bomb, he points that a large factory that would consume a vast amount of electricity will be necessary and outlines the general technique of uranium isotope separation (Clusius-Dickel).

He then explains the difference between a uranium bomb and a nuclear reactor with reference to fast neutrons and the use of water to create thermal neutrons respectively.

<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"TimesNewRoman",serif">On the subject of the bomb, he estimates a critical mass with reflector of 10 Kg. A good estimate given the vagueness of the nuclear parameters available at the time. He points out that for a bomb, extra U235 is necessary and that only a small portion will actually undergo fission. At this point Nishina seems to express some ambivalence about the project, ‘there are other reasons why it is not advisable’ and for technical reasons the weight of the reflector’ would be so great that it is judged not be suitable as a weapon’.

<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"TimesNewRoman",serif">  From diffusion theory and experimentation, Nishina would have known that neutron reflection is close to 90% for thickness of about two or so diffusion lengths, which for the heaviest material is in the region of 10 to 15 cm, As a result the reflector would be no more than a ton (Serber, Los Alamos Primer). Interestingly, Nishina’s old fellow student Heisenberg, used the same argument of weight as a hindrance to the building of a bomb (critical mass = 10 tons).

<span style="font-size:12.0pt;font-family:"TimesNewRoman",serif;mso-fareast-font-family: "MSMincho";mso-ansi-language:EN-GB;mso-fareast-language:JA;mso-bidi-language: AR-SA">Nishina then outlines the use of a Polonium-Beryllium neutron source to initiate the chain reaction. No mention is made of an assembly mechanism but the ‘gun barrel’ approach is fairly obvious. Finally, Nishina mentions that his own 1.5 metre cyclotron is not yet working due to the shortage of materials as a result of military priorities but that when it is complete,” the vital work can commence”. He then adds as a rider, ‘that America is planning a cyclotron 10 times more powerful’