Have fast reactors lost the Midas touch?

1 March 2002

Fast reactors appear to be no more effective than other facilities in transmuting long-lived radionuclides. More research is needed. By B R Bergelson, A S Gerasimov, G V Kiselev and V G Tikhomirov

Specialists in nuclear technologies must develop effective methods to dispose of the industry's wastes. Transmutation is often cited as an alternative to long-term storage of radionuclides in serviced and guarded buildings with stabilised temperature and pressure.

There is a general consensus that transmutation of actinides can be most effectively achieved using fast reactors. This viewpoint rests on data that suggest all even isotopes of transuranium elements, except 238Np and 242Am, and the odd isotopes 237Np, 241Am, and 243Am are fissioned in the fast spectrum more effectively than in a thermal reactor. But transmutation may also be effective in the thermal neutron spectrum, where even isotopes are easily transformed into odd ones, which are easily fissioned.

Transmutation efficiency

The efficiency of transmutation of long-lived nuclides in different reactors has been compared, using radiotoxicity R as a measure of radiation levels. The values of R for separate isotopes were defined on the basis of maximum permissible activity in water according to radiation safety standards. The long-lived radiotoxicity of spent nuclear fuel is mainly due to plutonium and minor actinides — neptunium, americium and curium. As the contribution of long-lived fission products is relatively low, only the transmutation of the minor actinides was analysed.

The processes of transformation of minor actinides, together with their daughter nuclei, were calculated.

Plutonium accumulated in the nuclear fuel during operation of power reactors was discounted, but plutonium formed during transmutation of isotopes of neptunium, americium and curium was included. The plutonium is mainly 238Pu formed during 237Np irradiation and 242Cm decay.

The software package Scale-4.3 was used for the multigroup calculations. The VVER-1000, BN-600, Superphenix (SPX)-1200, the lead-cooled BREST-1000 (under development), and the liquid fuel high flux subcritical ADS-800 were considered as reactors-transmuters. In the model, the nuclides were assumed to be under constant

neutron flux. The neutron spectra used in this model were the average core spectra in the reactors under consideration. The reactors' typical 80% capacity factor was taken into account by a 20% reduction of the neutron flux for actinide irradiation.

The transmutation process is believed to take place under continuous feed by new portions of minor actinides. The rate of feed was assumed to be equal to the average rate of accumulation of minor actinides in VVER-1000 or SPX-1200 reactors. The values and compositions of the feed were calculated for fuel burn-up in the VVER-1000 of 40GWday/ton and in the SPX-1200 of 44GWday/ton.

The calculated amount of the minor actinides extracted from the spent nuclear fuel after 10 years cooling is presented in the Table above right. It is normalised for 1GW of electric power, so the SPX-1200 data were recalculated for 1000MW power.

The results of calculations of continuous irradiation of minor actinides in different reactor-transmuters are presented in Figures 1 and 2 in the form of dependence of total radiotoxicity of minor actinides and plutonium formed from them during the irradiation time. Data is obtained from the feed from the VVER-1000 (Figure 1) or SPX-1200 (Figure 2).

The main parameters of the transmutation modes presented in the Figures 1 and 2 are shown in the Tables below. Under conditions of continuous feed, some kind of equilibrium appears in a reactor-transmuter when the rate of incineration of transmuted nuclides becomes equal to the rate of feed. Plutonium formed in the process of transmutation is also included in the equilibrium mass of minor actinides. The last lines of the Tables present the period of time t which compares accumulation of radiotoxicity in a reactor-transmuter with a storage facility without transmutation. So t is the period of time for which the radiotoxicity of minor actinides accumulated in a reactor-transmuter becomes equal to radiotoxicity of neptunium, americium, and curium accumulated in a long-term storage facility.

Reactor-transmuter effectiveness

The results are evidence of the following points.

• Accumulation of radiotoxicity in a reactor during transmutation is mainly due to transformation of the long-lived isotopes 237Np, 241Am and 243Am into comparatively short-lived alpha-emitters (238Pu, 242Cm, 244Cm):

237Np+n 238Np (b, 2.12 days), 238Pu (b, 87 years) …,

241Am+n 242Amg (b, 16 hours), 242Cm (b, 161 days), 238Pu …,

243Am+n 244Am (b, 10 hours), 244Cm (b, 18.1 years) …

During the first 10-20 years after transmutation begins, the rate of accumulation of radiotoxicity is approximately the same in all fast reactors (slightly higher in the thermal reactors), but higher than the rate of accumulation in a long-term storage facility without transmutation.

• The equilibrium radiotoxicity depends on the type of reactor-transmuter and, to a lesser degree, on the composition of feed. The lowest equilibrium radiotoxicity is reached in the ADS-800. It is 4-5 times less than in the reactor-transmuters VVER-1000, BN-600, SPX-1200, BREST-1000 using feed with minor actinides extracted from VVER-1000 reactors. In the case of feed from the SPX-1200 type, the equilibrium radiotoxicity in the ADS-800 is 6-8 times less than in other reactor-transmuters. The highest level of equilibrium radotoxicity is reached in the BREST-1000.

• Comparison of radiotoxicity formation in reactor-transmuters and of accumulation of radiotoxicity in storage facilities (time t) shows that accumulation in the storage facilities is safer during the first 60-110 years than in the VVER-1000, BN-600, SPX, and BREST- 1000 reactors. The reactor-transmuters turn out to be more favourable 60-110 years after transmutation. For the high flux facility ADS-800 this period is only 10-12 years.

• Long operation of reactor-transmuters with solid fuel is possible only with repeated refuelling and processing of the spent nuclear fuel. This explains why the time t may be longer than the data obtained from the model of continuous irradiation. At the same time, multiple chemical reprocessings increase the potential risk of contamination of the environment during the transmutation process.

• The results shown in the Tables are evidence that the process of transmutation is least effective in BREST-1000 reactors because of their relatively low fuel power density. This property is a feature of similar facilities.

• The equilibrium mass and radiotoxicity of minor actinides (including plutonium) in reactor-transmuters are of a similar order (or higher) as loaded plutonium and its radiotoxicity in the core of a 1000MWe fast reactor. Their potential risk of contamination of the environment is comparable.

• The process of transmutation is most effective in the subcritical ADS-800. Because it uses liquid fuel, high thermal neutron flux, and continuous removal of the fission products, it may operate in the mode of equilibrium with a relatively low content of long-lived radiotoxicity in the blanket.

Transmutation of long-lived radiotoxicity in fast reactors is not effective in comparison with similar processes in other facilities. A subcritical facility with liquid fuel (molten salts) with high flux of thermal neutrons and continuous mode of operation seems most effective for transmutation.

Placing the most hazardous radionuclides in reactor-transmuters (with high temperatures and pressures) is much more dangerous than storing them in specially equipped buildings. Further research should be carried out to find more efficient means of transmutation of long-lived radionuclides.

Transmutation of minor actinides extracted from the VVER-1000 type reactor
Transmutation of minor actinides extracted from the SPX-1200 type reactor

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