Now, every ton of pure uranium metal contains just fourteen pounds of the fissionable element uranium 235. The latter, when split, releases the neutrons that create plutonium out of nonfissionable uranium 238. On the basis of one hundred per cent efficiency, impossible in this operation, the yield of plutonium would thus be fourteen pounds per ton. Since the plutonium must be extracted long before all the U-235 atoms have been split, however, the likelihood is that the yield would be no more than two to four pounds per ton. Russia would thus need tens of thousands of tons of uranium ore to build up a sizable stockpile of A-bombs, and while she may be able to process low-grade ores, it would take her much longer to produce a given quantity of plutonium than it takes us to produce it from our much richer ores. For example, an ore containing fifty per cent uranium would yield a given quantity of plutonium ten times faster than an ore containing only five per cent, unless a refining plant ten times the size is built at ten times the cost of construction and of operation. If we take Professor Oliphant’s published estimate that the critical mass (that is, the minimum amount) needed for an A-bomb is between 10 and 30 kilograms (22 and 66 pounds), we get a clear picture of the enormous difference there is between rich ores and poor ores for the building up of an A-bomb stockpile, and a further concept of the difficulties that Russia will face in trying to produce an H-bomb. According to the best available prewar information, the pitchblende of the Belgian Congo has a uranium content as high as 60 to 80 per cent; the Canadian ore yields from 30 to 40 per cent. A conservative estimate would thus place the average uranium content of Belgian and Canadian ores at somewhere around 50 per cent. This contrasts sharply with a prewar figure of around 3 per cent uranium for the pitchblende of Czechoslovakia, and the ore in the mountains of Saxony is of even lower grade.