Using VISTA to evaluate MOX fuel trends

Computer tools for calculating uranium and fuel cycle service requirements are often based on sophisticated databases with information on every reactor. The International Atomic Energy Agency’s (IAEA’s) nuclear fuel cycle databases project collects and analyses information on existing and planned nuclear fuel cycle facilities worldwide, using the Nuclear Fuel Cycle Information System (NFCIS) database. Such models are suitable for estimating short-term open cycle requirements, but become complex for the closed cycle, where recycling of separated fuel material is taken into account.

The IAEA developed the Nuclear Fuel Cycle Requirements Simulation System (VISTA), to simplify long-term estimations and integrate information from other IAEA databases. VISTA is a scenario-based tool designed to calculate spent fuel arisings, actinide generation, plutonium separation and use, and other information related to the back end of the fuel cycle. It can be used to generate estimates for a closed fuel cycle that take into account separated fuel material and the use of mixed oxide (MOX) fuel.

In order to analyse MOX fuel trends the model estimates, for eight reactor types, average requirements corresponding to a given level of electricity generation and fuel cycle strategy. The eight types are PWR, BWR, PHWR, AGR, GCR, RBMK, VVER-440 and VVER-1000. The cycle is “closed” by calculating two sets of fuel loads and unloads. One set is for reactors using uranium fuel. The second is for uranium and MOX fuels. Here the fuel cycle strategy is defined, via a special scenario file, as the fraction of discharged fuel reprocessed. The amount of plutonium (and other actinides) in the spent fuel is calculated using the IAEA’s Calculation of Actinide Inventory (CAIN) model. Plutonium is then separated and MOX fuel is fabricated and loaded as a part of the core of some LWRs.

To use the database the input parameters are divided into three groups:

• Strategy parameters. Capacity variants and reprocessing–recycling strategies, reactor mixture and load factors, all on an annual basis.

• Fuel parameters. Average discharge burn-up, average initial enrichment and average tails assay on an annual basis.

• Control parameters. Share of MOX fuel used, lead and lag times for different processes and the number of spent fuel reprocessing cycles.

VISTA’s results are divided into the following groups:

• Natural uranium, conversion and enrichment service requirements.

• Fresh fuel requirements and spent fuel arisings.

• Total plutonium arisings.

• Reprocessing and MOX fabrication service requirements.

• Separated plutonium utilisation.

The model has already been used to investigate general issues such as front end requirements, new market conditions, and the effect of climate change. In this application, it compared: spent fuel arisings and storage capacity; reprocesing requirements and capacities; and MOX fuel usage and MOX fuel fabrication capacities.

Main Assumptions

VISTA estimates are based on scenarios of the future of nuclear power and its fuel cycle. For the purpose of this paper, two nuclear capacity variants coupled with three reprocessing-recycling strategies were selected.

Regarding capacity, according to the IAEA database 437 nuclear power units were operated in 32 countries, as of December 1997, with a total electric generating capacity of 351 GWe. Future trends are based on IAEA projections as shown in Table 1 (left).

The fuel cycle strategies differ in the reprocessing ratio and number of cycles used (Table 2). The S1 strategy assumes that half of the spent fuel is offered for reprocessing (50% reprocessing ratio) and that plutonium extracted from UO2 spent fuel can be recycled twice in LWRs. This strategy is used as a maximal reference strategy. The strategies S2 and S3 assume one cycle only, without reprocessing spent MOX fuel, and with an end-of-period reprocessing ratio of 50% and 35%, respectively. These strategies are considered to be more probable than S1.

Spent Fuel Arisings and Storage

VISTA calculations predict worldwide spent fuel arisings. Annual arisings reached a peak of about 11 300 tHM in 1990 and decreased to about 10 200 tHM in 1998. This reduction is due to the use of high burnup fuel, which reduces the amount of fissile material in the fuel cycle and the amount of spent fuel to be managed. However, annual fuel discharges will increase as the installed generating capacity increases. Figure 1 illustrates the balance between increased burnup and increased nuclear capacity.

Figure 2 shows a comparison between spent fuel storage capacities and requirements, calculated by the VISTA code using the scenarios mentioned above. VISTA predicts that if planned storage facilities are constructed on schedule, storage capacities will have about 100 000 tHM surplus over the total storage requirements until 2015. This comparison clearly demonstrates that serious consideration must be given to dealing with increasing amounts of spent fuel: either storage capacities must be increased or reprocessing of spent fuel must continue.

The worldwide storage capacity may be sufficient if reprocessing continues until 2015, but the same trend shows that continuous effort on spent fuel management is required. The storage situation is radically different in different countries; some countries, for example those in Western Europe, have enough capacity. Others do not.

Reprocessing

Spent fuel reprocessing is a proven technology and reprocessing services are now available on a commercial basis to reactor operators. Current and projected reprocessing capacities worldwide are shown in Table 3. The figures assume that:

• France is successfully operating its reprocessing plants at La Hague for LWR fuel and has already reprocessed 12 000 tHM.

• The UK has a capacity of 2700 tHM/y.

• In Japan, construction work on the Rokkasho-Mura reprocessing plant

(800 tHM/y) continues, but delays will cause it to be several years behind schedule.

• Russia planned, in the 1980s, to commission a second large scale reprocessing plant (RT-2 at Krasnoyarsk) in 2005, but it has been cancelled for financial reasons.

• India is commissioning Kalpakkam

(100 tHM/year) for its PHWRs.

• China plans to build a major plant by 2020.

The total projected reprocessing capacity will increase over the period 1998-2010 due to the deployment of the new plants in Japan and India. After 2010, closure of the Sellafield B205 plant is likely and a second plant may start up in China.

Figure 3 compares worldwide reprocessing capacities with the projected reprocessing requirements calculated by the VISTA code. The requirements shown here are actually a sum of LWR and non-LWR reprocessing requirements. In the scenarios mentioned above, LWR nuclear generating capacity increases. Since reprocessing requirements follow the trend of annual spent fuel discharges, VISTA estimations predict reprocessing requirements for LWR fuels of around 2000 tHM in 2000, decreasing to around 1700 tHM in 2005 and increasing to 2000 tHM in 2015. VISTA also projects a reduction in reprocessing requirements for non-LWR fuels, especially due to the end of the GCR Magnox programme in the UK in the next 10 to 15 years. It should be noted here that reprocessing requirements are very sensitive to political decisions in different countries and may change greatly.

Plutonium

Plutonium is generated (and is partly burned) during the operation of uranium-fuelled nuclear reactors and forms part of their spent fuels. The IAEA estimates that in 1998 about 76 t of plutonium was contained in discharged spent fuels worldwide. The annual plutonium production, as presumed from spent fuel arisings in Figure 1, will remain more or less the same until 2015. The cumulative amount of plutonium in spent fuels from power reactors worldwide is predicted to reach 2000 t in 2015.

Some of the plutonium contained in spent fuel has been separated and around one third has so far been used to manufacture MOX fuel for LWRs and experimental and prototype FBRs. Most of the separated plutonium is currently stored, mainly at the British, French and Russian reprocessing sites.

As one of the main aims of this work was to estimate MOX fuel trends, VISTA was used to investigate the use of separated plutonium to fabricate MOX fuel for LWRs. VISTA estimates that up to 25 t/year of plutonium will be used in MOX fuel towards 2010, assuming that MOX fuel forms 30% of LWR cores worldwide. This is much greater than 1995, for example, when about 8 t of plutonium was used in LWRs and in breeder reactor development programmes.

MOX fuel fabrication is becoming a mature industry, particularly in Belgium, France and the UK. These countries have MOX fabrication plants, as do Japan and India. Table 4 lists the status of the current and projected MOX fuel fabrication capacities worldwide.

VISTA calculations assume that the plutonium separated by reprocessing LWR fuel is used to fabricate MOX fuel loaded in LWRs. The calculation assumes that the plutonium used is from fuel reprocessing, and does not consider plutonium from stockpiles.

It is assumed that the share of MOX assemblies in the core of LWRs using this type of fuel is 30%. A comparison of MOX fuel production capacities and predicted MOX fuel requirements to 2010 is given in Figure 4.

At present, MOX fuel requirements are near the full production capacities. VISTA shows increased MOX fuel requirements in the short-term, with some reduction later. The reason is linked directly to the annual reduction in spent fuel discharges, and therefore a reduction in the spent fuel which is sent to reprocessing plants. Together with a parallel reduction in fresh fuel requirements, the result will be that less MOX fuel is required. However, this may change as it depends on the continuation of reprocessing or using of stockpiles of stored plutonium. The trend may also change if the share of MOX fuel in the core increases above 30%. As can be seen in Figure 4, in all the scenarios the supply will be in excess of demand up to 2010 because of the commissioning of the Sellafield SMP, expansion of the MELOX plant capacity and deployment of Japan’s Rokkasho plant.

The imbalance between separation and use of plutonium resulted in an estimated inventory of separated civil plutonium of about 190 t at the end of 1998. This corresponds to earlier projections and published data. Future trends on inventories of separated civil plutonium, based on the assumptions mentioned above, were calculated using VISTA and are shown in Figure 5.

VISTA inventory estimations in 1996 and 1997 agree with IAEA track records on real world inventory. Future separated plutonium inventory forecasts have significant associated uncertainties and are quite sensitive to MOX fuel fabrication and spent fuel reprocessing assumptions. The situation regarding plutonium stocks and their use differs from country to country. In some countries MOX programmes are actively implemented; in others, recycling of separated plutonium is not expected to take place in the short term. In addition, limited quantities of military stocks of weapon plutonium may soon be transferred into the civilian sector.

On the basis of the assumptions mentioned above, VISTA predicts that stockpiles of separated civil plutonium will continue to rise. This trend may change if multiple recycling of plutonium takes place, as is demonstrated using the S1 reprocessing and multiple recycling strategy, or in the case of using plutonium from stocks to fabricate MOX fuel. The results of calculations for the multiple recycling strategy (S1), demonstrate a trend of reducing separated civil plutonium inventories. VISTA estimates that in this strategy, fabricating MOX to feed 30% of LWR cores will require more plutonium than will be available from reprocessing. This may come from plutonium stockpiles and so will lead directly to reduced separated civil plutonium inventories.