2.3.1 Standard reactors

Most existing energy producing reactors are of the light-water cooled type, either pressurized water (PWR) or boiling water (BWR). Although other types of commercial reactor like the heavy-water CANDU have interesting characteristics, our discussion focuses on the light-water reactors. The power of commercial reactors ranges between 600 and 1500 electric MWatts (MWe), with thermodynamical efficiencies close to 33%. As an example, we consider a 1000 MWe reactor.

Each fission produces approximately 200 MeV (185 MeV at the moment of fission and 15 MeV produced by subsequent fl radioactivity).[5] Accord­ingly the fission of 1 kg of a fissile isotope typically produces 80 TJ (or

1900 toe). A 3GW (thermal gigawatts) reactor, yielding 1 GWe (electrical gigawatt), produces annually about 7TWhe for an availability of 80%. It burns annually about 1 ton of fissile isotopes which is equivalent to two million tons oil equivalent (toe). More precise numbers are given in table 2.15, where material inventories at loading and discharge are given [36].

In table 2.15 a burn-up of 33GWd/ton (gigawatt-day/metric ton) is assumed. The table shows the following interesting features:

• The amount of 235U which has disappeared equals 674 kg. This accounts not only for the fission of this nuclide, but also for its neutron captures, at least the 111 kg of 236U produced.

• This means that at least 383 kg of the higher isotopes, mostly 239Pu, have contributed to fission. This can also be considered as an indirect fission of the 238U isotope, which lost 673 kg corresponding essentially to the production of plutonium. Of these 673 kg only 286 kg are found in the form of plutonium isotopes and minor actinides.

• The mass balance between the initial and discharge inventories is not exact. This is due to the mass equivalence of the energy produced (about 1 kg) and to the neutrons captured in the structure elements and cooling water (2 kg corresponding to approximately 0.5 neutron per fission).

The nuclear wastes to be considered can be divided into three categories:

1. The plutonium and minor actinides, with very high radiotoxicities due to their dominant alpha-decay. They have long lifetimes, up to 25 000 years for 239Pu and more than two million years for 237Np. They would require either long-term underground disposal or transmutation. In the latter case they can only disappear by fission (this is usually called incineration). The fission of 280 kg of plutonium and minor actinides would produce

about 2 TWh of electrical energy. This means that at least one incinerating reactor for four PWRs would be needed if one wants to completely incin­erate the plutonium and the minor actinides.

2. The long-lived fission products, nuclides with lifetimes longer than 1000 years which decay by emission. The main fission fragments involved are shown in table 2.16, together with the amounts produced yearly by a 1 GWe reactor.

3. The medium-lived fission products, essentially 90Sr and 137Cs, which have very high activities at discharge and small neutron capture cross-sections. It does not seem realistic to transmute them and they would, then, set a minimum duration of around 300 years during which the wastes are radioactive and require supervised storage.

The inefficient use of uranium in current thermal reactors has con­sequences on the amount of mining required, as well as on the level of resources. In the absence of recycling, each 1 GWe reactor requires annually about 100 tons of fresh natural uranium. Typically currently used uranium ores have grades around 0.25% [37]. This means that a 1 GWe reactor requires the extraction of 40 000 tons of ore, to be compared with the two million tons of oil which would be needed to produce the same amount of energy. The rather large amount of mill tailings is associated with radioactivity due to the descendents of uranium, especially to a con­tinuous flow of radon during a long period (75400 years as defined by the half-life of the parent 230Th). This radon gas escapes more readily from the tailings than from the unmined uranium ore.

The uranium reserves are estimated around 5 million tons at costs close to the present. The present world power production is about 350 GWe, requir­ing an annual 40 000 tons of natural uranium. Thus the present known reserves are estimated to last 125 years. Again, it is not a problem as long as the present small contribution of nuclear power to the overall energy pro­duction is maintained. However, as in the wastes case, should the nuclear share increase to a 30% level, the reserves would be reduced to approximately 40 years, no more than the oil reserves. One should note, however, that there is a very large reserve of uranium in sea water, amounting to about three billion tons [37], at a concentration of 3.2 parts per billion. It seems possible to extract this uranium at a cost ten times higher than the current cost, which would increase the cost of the produced electricity by 50%.