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Background Briefing Paper on the Nuclear Fuel Cycle


Hill Path Projects Ltd

Ref:  HPP/EN02/09-06

Background Briefing Paper On THE NUCLEAR FUEL CYCLE

(Issue 6, dated 22nd April 2010)

Author:   D R MacDonald BSc CEng MIET


The purpose of this paper is to present a brief understanding of the nuclear fuel cycle from the mining of uranium ore to waste disposal. The fuel processing and enrichment processes are described in outline and the viability of obtaining fuel for a major new British civil nuclear programme, built initially on current pressurised water reactor designs and leading to a fast breeder programme, is explored. A basic description is given of nuclear fission.

The Physics of Nuclear Fission

The descriptions reactor physics and design are based upon “Nuclear Power”, edited by R V Moore [1].

Nuclear fission occurs when a large atom, such as uranium, splits (fissions) into fragments; these fragments consist of two or more fission products (elements of lower atomic number), β particles, free neutrons and γ radiation. During the fission process some mass is lost and thus energy is created in accordance with Einstein’s equation E = MC2. The energy is transmitted in the form of increased kinetic energy of the resulting particles and γ radiation.  Nuclear fission may occur naturally due to the normal decay process or may be induced by the impact of a free neutron.

Consider a collection (a “pile”) containing fissile material such as U235 (Uranium 235). Some of these atoms will decay naturally releasing free neutrons and, if there are sufficient additional atoms of U235 in the immediate vicinity, one or more of these may be struck and split by a newly released neutron, releasing more free neutrons which will cause further fissions and so on. There will then be a self sustaining or an increasing nuclear reaction. This is the fundamental concept behind of both nuclear reactors and nuclear weapons. However, the world in which we live is inherently stable (it could not exist if it were otherwise!) and as the chain of fission reactions occurs so the heat produced will increase, causing the nuclear pile to expand thus moving the fissile atoms further apart and allowing more free neutrons to escape from the pile until, on average, there is less than one new fission caused by each previous fission whereupon the process is effectively halted.

In fact, the neutrons emitted by the fission of an atom have a wide spread of energies from “fast” (~ 2 MeV) to “slow” (~ 0.025 eV). The fission process may be initiated by either the impact of a “fast” or a “slow” neutron but is less likely to be caused by a neutron of intermediate energy. The “slow” fissile process can be facilitated by the presence of a “moderator”, such as water or carbon, which slows the intermediate and fast neutrons. Slow neutrons are also known as “thermal” neutrons and this process is known as a thermal nuclear reaction. A fast nuclear reaction does not require a moderator but a high concentration of fissile atoms is necessary for a self sustaining process.

There are only four fissionable isotopes, two of uranium, U233 and U235, and two of plutonium,     Pu239 and Pu241. Neither U233 nor Plutonium occur in nature (except in tiny quantities in the bowl of a naturally formed nuclear pile in West Africa) so the starting point for nuclear fission is the uranium isotope U235. U235 constitutes only 0.7% of naturally occurring uranium of which the remaining 99.3% is the non fissile isotope U238.

There is a further group of “fertile” isotopes. These are Th232 (Thorium), U238 (99.3% of naturally occurring Uranium) and Pu240 (an isotope of Plutonium). By “capturing” a high energy fast free neutron and then undergoing a rapid radioactive decay process, these isotopes become the fissile isotopes U233, Pu239 and Pu241 respectively. However, in practice, there is now no major thorium cycle nuclear reactor programme whilst Pu240 and Pu241 play only a very minor part in nuclear power. Consequently for this paper the processes described concern only the fissile atom U235, its interaction with the fertile atom U238 and the production of fissile Pu239 material.

The Nuclear Reactor Core

The core of a nuclear reactor must contain a sufficiently large and concentrated mass of fissile material to facilitate a nuclear chain reaction. The fuel is arranged in metal clad rods clustered together in assemblies over which the coolant gas or liquid flows to carry away the heat generated. In addition to the fuel there are also control rods of neutron absorbent material (usually boron); by inserting these rods the nuclear reaction may be slowed and thus the power generated reduced; conversely, by withdrawing the control rods, the power levels are increased. In addition, if the reactor is a thermal nuclear reactor, there will be moderator to slow the fast neutrons and increase the efficiency of the nuclear reaction; the moderator material is usually heavy water (a very efficient moderator), light water or carbon. In the case of water, the moderator may also act as coolant. To increase the efficiency of the nuclear reaction and reduce the size of the nuclear pile, fuel may be enriched by increasing the concentration of the fissile uranium U235 or by adding fissile plutonium Pu239.

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