Energy and Climate: Vision for the Future by Michael B. McElroy

Author:Michael B. McElroy
Language: eng
Format: epub
Publisher: Oxford University Press
Published: 2016-08-15T00:00:00+00:00

The combined mass of the products of this reaction is less than the mass of the initial reactants. The difference, according to the Poincaré-Einstein relation, is reflected in a surplus of energy in the products. The bulk of this energy, approximately 96%, is communicated to the product nuclei in the form of kinetic energy with the balance taken up by the accompanying neutrons.3

Averaging over all possible reaction paths, fission of 235U results in emission of 2.42, so-called prompt, neutrons. Some of the nuclei formed in the initial fission process are further unstable. They decay on a variety of time scales ranging from 0.2 to 56 sec, averaging about 2 sec, leading to emission of an additional 1.58 (so-called delayed) neutrons per 100 fission events (Bodansky 2004). In a typical light water reactor, for every 10 neutrons absorbed by 235U, approximately 6 neutrons (the higher energy fraction) are absorbed by 238U, leading to production of fissile 239Pu. Capture of an additional neutron results in production of 240Pu, which is subject to further fission. The abundance of 239Pu increases with time as 238U is depleted. Under normal circumstances, fuel assemblies are removed from a reactor and replaced with fresh fuel on a schedule of approximately once every 3 years as the abundance of 235U drops below a critical threshold. Care must be taken to protect the security of the depleted fuel rods after they are removed from the reactor since the 239Pu they include could be used to provide fuel for a nuclear bomb. The bomb dropped on Hiroshima, Japan, on August 6, 1945, was triggered by a high concentration of 235U. 239Pu provided the energy source for the bomb dropped 3 days later on Nagasaki.

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