Plutonium recycling in light water reactor MOX

Since natural uranium contains only 0.72% of the fissile ²³⁵U isotope, the recycling of uranium and plutonium from spent fuel through the RFC scenario (Fig. 6) has been from the beginning of the nuclear era the standard scenario of nuclear energy production. There has, however, been reduced support for this approach in many States in recent years, owing to economic factors and partic-ularly to proliferation concerns.

By processing according to this RFC scenario, the major fraction (~99.9%) of the uranium and plutonium streams is extracted and only a very minor fraction of the major actinides is transferred to the HLLW (and conse-quently to the vitrified HLW) and eventually to the geologic repository.

However, if public and/or political acceptance of very long term disposal of HLW cannot be obtained, the removal of MAs from high active residua or HLLW would be a technical solution that might reduce the residual radiotox-icity of the HLW. Moreover, with increasing burnup, the generation of MAs becomes more and more important. The addition of an MA partitioning

Enrichment 2434

Storage: two years

Losses (%)

U 100

Pu 100

MA 100

FP 100

All mass flow in kg/TW·h(e)

Losses:

U 2276.01 Pu 29.21 MA 3.59 U natural

18 424

Depleted U 15 990

Uranium oxide fabrication

Uranium oxide irradiation

in LWR

High level waste TRU = 32.82 HM = 2308.83

FIG. 5. The once through fuel cycle. FP: fission products.

module to the standard reprocessing plant would, in such a case, be the most obvious change to the current RFC. States with a reprocessing infrastructure (China, France, India, Japan, the Russian Federation and the UK) and their associated partners could in the medium term realize a partial partitioning scenario by which the HLW would be practically free from long lived TRUs.

However, the question arises of what to do with the recovered uranium, plutonium and MA fractions. Those States that chose to reprocess their spent fuel did this with the main purpose of recovering the major actinides (uranium and plutonium), to save on fresh uranium purchase (20%) and to use the

LWR

LWR MOX fabrication Purex

reprocessing Cooling

LWR uranium oxide

Fuel fabrication

Conversion/

enrichment

Uranium processing

Mining Depleted

UO₂

MOX disposal

HLW disposal

REPU disposal Spent

LWR MOX HLLW Vitrification

Pu

REPU

FIG. 6. The LWR reprocessing fuel cycle. REPU: residual plutonium.

residual fissile components of the spent fuel (c. 1% ²³⁵U, 1% Pu) corresponding to about 25% of the regular expense of the uranium enrichment step.

From the radiotoxic point of view the overall gain is rather limited, since only about 25% of the recycled plutonium is consumed and about 10% is transformed into a long term radiotoxic MA source term. Recycling of spent LWR fuel as MOX provides an overall mass reduction, with a factor of about five, but does not significantly reduce the total radiotoxicity. Double, or perhaps at the limit triple, recycling of LWR MOX is theoretically possible in LWRs if fresh plutonium is available, but in the short term the resulting radio-toxicity drastically increases throughout the subsequent recycling campaigns because of ²⁴⁴Cm buildup [23]. This avoids reprocessing or fabrication of new fuels.

4.1.3. Plutonium recycling in light water reactor MOX and fast reactor MOX The stock of plutonium accumulated at the large European reprocessing plants that was intended to be used in liquid metal fast breeder reactors (LMFBRs) became redundant in a cheap uranium market economy. As a result, the use of separated plutonium achieved industrial significance in western Europe, where increasing quantities of PuO₂ were transformed into LWR MOX fuel and irradiated in specially licensed reactors in Belgium, France, Germany and Switzerland.

The reuse of plutonium is to a certain extent a first step in a global P&T scenario that has to be brought into a broader perspective of reuse of resources and reduction of the long lived waste produced during the nuclear age.

However, if LWR MOX use is to continue for a long period of time there will be an accumulation of spent LWR MOX fuel, since there is at present little incentive to reprocess it.

This fuel offers, after two successive recyclings in an LWR, a degraded plutonium spectrum that makes it unsuitable for further thermal reactor operation. The only solution for this non-sustainable situation is the creation of a significant FR capacity, an option that was envisaged many years ago.

Reprocessing of LWR MOX fuel together with LWR UO₂ (in a ratio of 2:1) can be performed in the present large reprocessing plants. However, the reprocessing of a pure spent LWR MOX stream at an industrial throughput is beyond the capability of the present plants. A campaign for the reprocessing of 5 t of spent FR MOX fuel has been successfully carried out at the La Hague plant in France. The recovered plutonium can be transformed in FR MOX fuel in existing MOX fuel fabrication plants; Fig. 7 shows a flowsheet and mass inventories of such a cycle.

The real bottleneck of this scenario is the construction and financing of a sufficiently large FR fleet, which must be associated with an equivalent FR MOX reprocessing capacity. Based on flowsheet calculations [24] the FR fleet would gradually have to be upscaled towards a fraction of 36% FRs in order to achieve at equilibrium a zero buildup of the plutonium mass.

Multiple recycling by aqueous processing of this type of spent fuel cannot be accomplished without the introduction of fast centrifugal extractors in the first extraction cycle of the Purex process, and MAs would accumulate in the HLLW.

4.1.3.1. Associated MOX fuel fabrication and refabrication problems

A large amount of industrial experience has been gained in FR MOX fuel fabrication, since FR programmes have been undertaken in many nuclear States for several decades. The fabrication of FR MOX fuel with 15–25%

plutonium has been realized routinely and on a commercial basis. However, the plutonium quality used for these purposes is derived from low burnup UO₂ fuel with low ²³⁸Pu and ²⁴²Pu contents. The burnup of spent LWR UO₂ and LWR MOX has, however, reached 50 GW·d/t HM. The isotopic composition of

FR

FIG. 7. The LWR (UO₂ and MOX) FR fuel cycle with conventional and advanced Purex reprocessing.

plutonium resulting from the reprocessing of such fuels is seriously degraded, with higher ²³⁸Pu and ²⁴²Pu levels and lower ²³⁹Pu and ²⁴¹Pu concentrations.

Still higher plutonium concentrations are envisaged (up to 45%) in the use of advanced FBuRs (CAPRA). The recycling of fuels containing high ²³⁸Pu levels and limited amounts of MAs is still more difficult and requires the design and construction of remotely operated fuel fabrication plants.

For homogeneous recycling of MAs in FR MOX, admixtures of 2.5%

237Np and/or ²⁴¹Am are currently being studied. Neptunium-237 is a pure alpha emitter (except for the small in-growth of ²³³Pa), and there is no major handling problem involved; however, the admixture of ^241,243Am at the 2.5% level will induce a gamma field around the gloveboxes or hot cells. The major interfering radionuclide in FR MOX is ²³⁸Pu at the 3% level, which is a heat and neutron source.

FR MOX fuel fabrication with a 2.5% americium admixture will also be influenced by the degree of separation of the rare earths (strong gamma emitters) and by ²⁴⁴Cm, which will accompany the americium fraction when separated from HLLW. The presence of ~17% ²⁴⁴Cm in the americium and curium fraction of recycled FR MOX fuel will further increase the neutron emission up to ~4.4 × 10¹⁰ n/s per t HM.

The separation coefficients from rare earths and ²⁴⁴Cm required in order to permit industrial fuel fabrication operations will greatly depend on the permissible rare earth concentration in fresh FR MOX fuel and on the permissible ²⁴⁴Cm concentration in the fuel fabrication plant. The industrial processing of MAs in an extended reprocessing–partitioning option calls for the design and construction of dedicated MA fuel fabrication facilities in the vicinity of the major LWR UO₂ reprocessing plants.

It has to be remembered that specially equipped MA fuel fabrication laboratories, for example those of the ITU, are licensed to handle 100 g of

241Am and 5 g of ²⁴⁴Cm [25].

Heterogeneous recycling of MAs is a means of avoiding the dilution of troublesome radionuclides, for example ²⁴⁴Cm, throughout the fuel fabrication step and is carried out in small but dedicated and heavily shielded facilities. The waste handling resulting from the high specific activity of the MA radionuclides will strongly influence the generation of secondary waste with high alpha activity. The operation of hot cell type fuel fabrication plants is a new dimension in waste handling that will have to be assimilated to the present vitrification technology in terms of radiotoxic throughput.

The expected volumes of alpha active secondary waste call for new treatment technologies or for the integration of the secondary waste types in the mainstream of HLLW vitrification.

The development of specific MA waste treatment and conditioning methods using very sophisticated equipment as dedicated incinerators for alpha waste and production facilities for ceramic or cermet types of waste using, for example, hot isostatic pressing devices will be necessary if a full segregation between gamma active and long term alpha active waste types has to be accomplished.

4.1.4. Plutonium and minor actinide recycling in light water reactor UO₂ and