6. TRANSMUTATION
6.2. Fuel concepts for transmutation
There are a wide variety of solid fuel types with a large range of chemical components, for example oxides, nitrides, carbides and metal alloys. In addition, fuel ranges from low density phases (e.g. sphere packs) to high density
forms (e.g. pellets), and from homogeneous (solid solutions) to heterogeneous (cercer or cermet) materials. These can be used in critical or subcritical reactor systems for transmutation–incineration purposes. At the ITU, considerable effort has gone into the development of fabrication methods for fuels and targets. Details of the fuels and targets and fabrication methods are given in Table 2.
6.2.1.1. Oxide fuels and targets
(a) Light water reactor MOX minor actinides and fast reactor MOX minor actinides
Conventional LWR MOX fuel fabrication could produce LWR MOX 1%
neptunium without major refurbishment in the MOX fabrication plant. The incorporation of 241,243Am into homogeneous LWR MOX 1% americium would be possible if additional shielding is put around the gloveboxes or in (semi-) automatic production units. However, this option can only be taken if a separation of americium and curium has been realized. The presence of the slightest trace of 244Cm induces an important neutron and gamma background [66].
The FR MOX MA fuel fabrication can rely on the experience gained during FR MOX fuel fabrication. However, due to the type of fuel fabrication (powder mixing, pelletizing, sintering) and the use of existing facilities, curium is also excluded from handling in a conventional FR MOX fuel fabrication plant. From the reactor physics point of view, the MA content of the fuel can be increased to 2.5% neptunium or americium without a major impact on reactor safety [67]. Homogenous recycling of neptunium is the preferred option, while the preference for americium is for heterogeneous recycling, which would allow fabrication of americium pins separately from the bulk of FR MOX fuel pins.
The presence of significant concentrations of americium requires novel production methods, for example sol gel, infiltration and vibropac techniques.
These methods are under active development in the European Union and the Russian Federation [63, 68].
(b) Fertile free fuel or targets
The advantage of this IMF utilization in LWRs or FRs lies in a better plutonium consumption as compared with MOX. R&D programmes on IMF have led to the selection of YSZ doped with erbia and plutonia at the PSI [41]
and the deployment of experimental projects including material preparation
TABLE 2. PROGRAMME OF TRANSMUTATION–INCINERATION, SHOWING FUELS AND TARGETS FABRICATED, AT THE INSTITUTE FOR TRANSURANIUM ELEMENTS
Programme Reactor Fuel/target Method Status
FACT FR2 (1981) (U0.5Am0.5)O2 Sol gel PIEa complete
MTE2 KNK II NpO2; (U0.5Am0.5)O2 Sol gel PIE complete (1984–1985) (U0.73Pu0.25Np0.02)O2 Sol gel PIE complete (U0.73Pu0.25Am0.02)O2 Sol gel PIE complete
SUPERFACT1 Phenix (U0.74Pu0.24Np0.02)O2 Sol gel PIE complete (1986–1988) (U0.74Pu0.24Am0.02)O2 Sol gel PIE complete (U0.55Np0.45)O2 Sol gel PIE complete (U0.6Am0.2Np0.2)O2 Sol gel PIE complete
POMPEI HFRb Tc metal Casting PIE in progress
(1993–1994) Tc–50% Ru metal Casting PIE in progress Tc–80% Ru metal Casting PIE in progress
TRABANT1 HFR (U0.55Pu0.40Np0.05)O2 Sol gel PIE in progress (1995–1996) (Pu0.47Ce0.53)O2-x Sol gel PIE in progress
EFTTRA-T1 HFR (1994–1995)
Tc metal (three pins) Casting PIE complete
EFTTRA-T4 HFR (1996–1997)
MgAl2O4–12% Am INRAM (pellets)
PIE complete
EFTTRA-T4bis HFR (1997–1999)
MgAl2O4–12% Am INRAM (pellets)
PIE in progress
TRABANT2 HFR (U0.55Pu0.45)O2 Sol gel Irradiation to be started (U0.6Pu0.4)O2 (two pins) Mechanical
mixing
Irradiation to be started
[69], irradiation tests on inert matrices utilizing accelerators and tests on the IMF within research reactors. Several studies summarize the results obtained so far for zirconia implantation with iodine, xenon and caesium as represent-ative fission products, including reactor tests performed in the HFR at Petten [70] and in the HBR at Halden [71].
Fabrication of americium target pins based on the impregnation technique (INRAM) has been undertaken at the ITU. Porous ZrO2 has been loaded with 10–20% AmO2 for irradiation in the HFR. Higher americium enrichments, up to 40%, are being investigated [72]. The main issue to be solved is the homogeneous enrichment of the porous support.
6.2.1.2. Non-oxide fuels and targets
Non-oxide materials are fuel or target components that can be used as fertile or fertile free material in fast systems. The advantages of these materials are that they have a higher density and better thermal conductivity than oxides.
The higher density results in a decrease in the moderation and consequently a harder neutron spectrum. The higher thermal conductivity allows lower peak temperatures in the reactor with improved safety characteristics.
ANTICORP Phenix Tc metal Casting Irradiation to
be started
METAPHIX Phenix U, Pu, Zr Casting Irradiation to
be started U, Pu, Zr, MA 2%, rare
earth 2%
Casting Irradiation to be started U, Pu, Zr, MA 5%, rare
earth 5%
Casting Irradiation to be started U, Pu, Zr, MA 5% Casting Irradiation to
be started
a PIE: Post-irradiation examination.
b HFR: High Flux Reactor.
TABLE 2. (cont.) PROGRAMME OF TRANSMUTATION–INCINERATION, SHOWING FUELS AND TARGETS FABRICATED, AT THE INSTITUTE FOR TRANSURANIUM ELEMENTS
Programme Reactor Fuel/target Method Status
(a) Fast reactor metal and nitride fuels
A uranium–15% plutonium–10% zirconium alloy with a melting point of 1180°C is the preferred fuel form for the IFR reactor. It is manufactured by casting the electrochemically obtained uranium–15% plutonium–10%
zirconium alloy at 1300°C in stainless steel fuel pins. In principle, it might be possible to use the same technology to produce FR metal plutonium–MA fuel, but reactor physics criteria limit the MA concentration to 5% in sodium cooled reactors. Electrorefining is under investigation for the reprocessing of such fuels.
Nitride fuel has been investigated because of its high thermal conduc-tivity and its inertness towards liquid sodium and lead. The 15N isotope has also been considered for use in nitride fuels. Owing to its low neutron absorption cross-section, it does not lead (in contrast to 14N) to the production of environ-mentally hazardous 14C through (n,p) reactions. Nitrogen-15 is, however, very expensive to produce. The technology to produce nitride fuel is still at the laboratory stage [73]. Nitrides of TRUs can be anodically dissolved in a molten LiCl–KCl bath at 500°C. Electrorefining has been carried out in small laboratory conditions. The main issue is the use and recovery of the (expensive) enriched 15N during the fuel processing and recycling operations. From a waste management point of view it has to be stressed that, if produced with natural nitrogen, the generation of 14C in spent nitride fuel by the (n,p) reaction would have important environmental consequences during recycling. Pyrochemical processing seems to be the preferred recycling technology.
(b) Fertile free fuels and targets
The fabrication of (Pu, Zr)N fuel utilizing a chemical precipitation technique for oxyhydroxide followed by nitration has been undertaken at the PSI in the framework of the CONFIRM project. Pellets of zirconium nitride loaded with 10–20% plutonium nitride have been produced for irradiation in the Studsvik reactor.
6.2.2. Liquid fuel
Two basic types of liquid reactor fuel were investigated and tested in the early stages of nuclear power development: water solutions of uranium salts and molten uranium salts (fluorides). Molten salt fuels look attractive for transmutation, owing to the possibility of on-line pyroprocessing and continuous correction of fuel composition. Two small pilot molten salt reactors with on-line radiochemistry were constructed in the USA in the 1960s [74] and
operated successfully for a few years. The transmutation potential of molten salt systems (both critical reactors and ADSs) are under investigation in the USA [75], the European Union [76] and the Russian Federation [77].
6.3. TRANSMUTATION POTENTIAL OF VARIOUS REACTOR TYPES