gained only by a proportionate increase in the mass of the scarce and cxpensive }
or.
fe
oN
ent
.
better to usc the extra quantities to make extra bombs.
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It would be much
vant op
fissionable matcrial within it would be very poor economy,
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1
‘
ey
re~
It so happens, however, that in atomic bombs the total anguntof/enerey
Oya
leased per kilogram of fissionable material (i.c., the efficiency of energy
release) increases with the size of the bomb 4?
This factor, weighed against
\
those mentioned in the previous paragraph, indicates that there is a theoretical |
.
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optinun size for the bomb which has perhaps not yet been determined and which
nay verywell be appreciably or even considerably larger than the Nagasaki bomb.
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But it should be observed that considcrations of military cconomy are not the
only factors which hold dowm the optimmm size.
Onc factor, already noted, is the
Steeply ascending difficulty as the number of subcritical masses increases of
‘» securing sinuitancous and perfect wmiion among them,
the envelope or tamper.
Another is the probiem of
If the inercase of weight of ths tamper is at all pro
portionate either to the increase in the amount of fissionable material used or
to the amount of energy rcleased, the gross weicht of the bomb might quickly
press against the technically usablc linits,
In short, the fact that ean enormous
increase in the power of the bomb is thcorctically conccivable docs not zican that
it is Likcly to occur, cither soon or lator.
It has always been theoretically
possible to pour 20,000 tons of TNT together in one case and detonate it as a
45.
Smyth Report, paragraph 2:18. This phonorcnon is no doubt cue to the fact
that the greatcr the margin above the critical mass linits, the faster the re-
action and honce the greater the proportion of matcrial which undergoes fission
before the heat generated expands and disrupts the bomb, It might be noted also
that even if there wore no cxpension or bursting to hait it, the reaction would
cease at about the tine the fissionable matorial remaining fell below critical
mass conditions, which would also tend to put a premium on having a large margin
above critical nass limits.
At any rate, anything like 100 per centdetonation
of the explosive contents of the atomic bomb is totally out of the question.
In this respect ato:ztic explosives differ markedly fron ordinary "high explosives"
Like TNT or torpex, where there is no difficulty in getting a 100 per cent re-
action and where the energy relcased is thereforc directly proportionate to the
amount of explosive filler in the bomb.
ee)