Economical analysis of alternative options in Candu fuel cycle

ECONOMICAL ANALYSIS OF ALTERNATIVE OPTIONS IN CANDU

FUEL CYCLE

Serkan YILMAZ, Okan ZABUNOGLU

Nuclear Engineering Department, Hacettepe University, Beytepe, Ankara, Turkey

ABSTRACT

In this study, fuel cycle options for CANDU reactor were studied. Three main options in a CANDU fuel cycle involve use o f : (1) natural uranium (0.711 weight percent U-235) fuel, (2) slightly enriched uranium (1.2 weight percent U-235) fuel, and (3) recovered uranium (0.83 weight percent U-235) fuel from light water reactor spent fuel. ORIGEN-2 computer code was used to identify composition of the spent fuel for each option , including the standard LWR fuel (3.3 weight percent U-235). Uranium and plutonium credit calculations were performed by using ORIGEN-2 output. WIMSD-5 computer code was used to determine maximum discharge bumup values for each case. Cost estimations were carried out using specially-developed computer programs. Comparison of levelized costs for the fuel cycle options and sensitivity analysis for the cost components are also presented .

1. INTRODUCTION

Unit cost of electricity generation consists of 4 main components; capital cost, operation and maintenance cost, fuel cycle cost and decommissioning cost of nuclear power plant. Contribution of the nuclear fuel cycle costs to the unit cost of electrcity generation is about 10 - 20 % depending on the reactor type and fuel cycle strategies. Since economic competitiveness of nuclear power is a very important parameter, the nuclear fuel cycle cost should be analyzed by considering alternative fuel cycle strategies.

The standard CANDU cycle uses natural uranium(NU) as fuel; then , yellow cake is directly converted to U 0 2 and fabricated into CANDU fuel assemblies. Alternative cycles use slightly enriched uranium and recovered uranium as fuel[l,2]. In the SEU cycle, yellow cake is converted to UF6 and UF6 is enriched to 1.2 w/o U-235. Enriched UF6 is converted to U 02 and fabricated into SEU fuel assemblies. In the RU cycle, CANDU fuel contains 0.83 w/o U-235, which can be obtained from spent LWR fuel. Inclusion of reprocessing of spent LWR fuel in the cycle is also investigated. Then, for the RU cycle 2 options are to be considered: (1) Input to the CANDU cycle is RU directly. In this option, nowhere in the cycle Pu exists; value of RU only is to be taken into account. (2) Input to the CANDU cycle is spent LWR fuel which is to be reprocessed in order to obtain RU with 0.83 w/o U-235 and Pu. RU is fabricated into CANDU assemblies. In this option value of recovered Pu is also to be taken into account.

To summarize ; 4 cycles to be considered are, (1) NU fuel cycle, (2) SEU fuel cycle, (3) RU fuel cycle without reprocess of spent LWR fuel, (4) RU fuel cycle with reprocess of spent LWR fuel.

2. REACTOR AND FUEL CYCLE DATA

The reference CANDU reactor adopted for this study is assumed to be in operation in the year 2007 with a net output of 640 MWe. It is assumed that reactor lifetime is 40 years and load factor 80 %. Unit process costs and lead and lag times for front-end and back-end processes are mostly based on Canadian experience; however, when need arises, data from other sources are also used[3,4,5].

It is assumed that spent fuel will be stored on-site for 10 years and then according to present plans, disposed of without reprocessing.

For reference calculations, an escalation ratio of 0 % and a discount rate of 5 % are assumed. For the RU cycle calculations, U and Pu credits are estimated to be 50 $ per kg recovered U and 5 $ per g fissile Pu.

Table 1 exhibits all the reactor and fuel cycle data including cost data, cost parameters and U and Pu credits[3,4,5].

For each fuel cycle option, discharge burnup value and fuel residence time are calculated using non-linear reactivity model[6]. Results are given in Table 2.

Table 1. Reference data for CANDU fuel cycle cost calculations 1. Reactor data and fuel cycle data

Reactor type PHWR

Thermal output 2133 MWt

Net electrical output 640 MWe

Load factor 80 %

Commissioning year 2007

Life of plant 40 years

Lead time for uranium purchase 17 months

Lead time for Conversion to UO2 13 months

Lead time for Fabrication 10 months

Lead time for Enrichment 12 months

Spent fuel storage period 10 years

Reprocess period 1 year

2. Cost parameters, U and Pu credits

Base date of monetary unit 2000

Monetary unit US $

Natural Uranium $ 20 /kgU

Conversion to UO2 $ 3 /kgU

Enrichment $ 80 / SWU

Fabrication $ 70 /kgU

Spent fuel storage $ 5 /kgHM-year

Disposal of spent fuel $ 80 /kgHM

Reprocess Cost $ 600 /kgHM

Discount Rate 5 %

Escalation Ratio 0 %

U credit for LWR spent fuel $ 50 /kg U

Pu credit for LWR spent fuel $ 5 /g-Puf

Table 2. Discharge bumup values and fuel residence times

Fuel Type Fuel Enrichment

(weight %)

Discharge Burnup (MWD/ton U)

Fuel Residence Time (Days)

NU 0.711 7,500 365

RU 0.83 12,600 613

SEU 1.2 22,500 1094

3. CALCULATIONAL METHOD

All the component costs are discounted back to a selected base date and added together in order to arrive at a total fuel cost in present value terms.

The total discounted cost of the nuclear fuel cycle can be written as:

I

where Fi(t)= cost for the i th component at time t , to= reference date(commissioning year), L = reactor life time, T1= maximum value of lead time (in front-end), T2= maximum value of

If C is the constant levelized fuel cost per unit of electricity sent out by a reactor, the total cost of fuel cycle can also be written as

t

L

£ C x E (t )/(1 + r )t

(2)

where ; E(t) = net electrical output at time t.

From the balance of (1) and (2), levelized fuel cycle cost can be calculated by the following equation

t=to + L +T2 to + L

C= £ £ F ( t)/(1 + r )(t- o) / £ E (t)/(1 + r ) t-t° (3) i t= to -T1 t =to

4. RESULTS

For the 4 cycles selected (NU, SEU, RU without reprocess, RU with reprocess) levelized fuel cycle cost calculations were performed over a lifetime of 40 years and time valued to the year of commissioning of the reactor.

Effect of uranium unit price on fuel cycle costs is shown in Figure 1. Increase of fuel cycle costs with uranium prices could be observed clearly for NU, SEU and RU without reprocess. Since input to the RU with reprocess cycle is spent LWR fuel, fuel cycle cost for RU with reprocess cycle is insensitive to uranium prices. Although input to the RU without reprocess cycle is recovered U, since value of RU is affected by U prices; however the rate of increase is lower than those for NU and SEU cycles. Since SEU cycle has a higher uranium cost contribution to total cost than NU cycle, rate of increase of SEU fuel cycle cost with increasing U prices is higher than NU cycle.

Costs of all cycles increase with increasing escalation ratios, as shown in Fig 2. Since RU with reprocess cyle has the lowest share of back-end costs in the total cost, the rate of increase of RU with reprocess cycle cost with increasing escalation ratio is the lowest.

Reprocess cost sharply affects the cost of RU with reprocess cycle, as can be seen in Figure 3. Above a reprocess cost of nearly 150 $/kgHM, RU with reprocess cycle becomes the most expensive option.

An increase in spent fuel disposal costs slighlty increases costs of all cycles. For the NU cycle, the rate of increase of fuel cycle cost with disposal cost is slightly higher than those for others because spent fuel disposal cost in NU cycle has a higher contribution in percentage to the fuel cycle costs compared to the others. It should also be noted that the HLW disposal costs, which are incurred in the RU with reprocess cycle, are included in the cost of reprocess.

Unit cost of enrichment ($/ kg-SWU) affects cost of SEU cycle only since it is the only cycle where enrichment takes place. As shown in Fig 5, below an enrichment cost of 130 $/kgSWU, the SEU cycle is the cheapest one.

Effect of the discount rate on the fuel cycle costs are shown in Figure 6. Front-end process costs increase with increasing discount rate since front-end payments are made before electricity generation, while back-end process costs decrease as discount rate increases because they are made after electricity generation. As a result, total fuel cycle cost makes a minumum for NU, SEU and RU without reprocess cycles at discount rates of 16%, 6% and 12% respectively. However, for the RU with reprocess cycle, such a minumum cannot be observed because reprocess cost, which has a very high share (81%) in the total cost.

As will be seen from Figure 6, the overall effect on the total levelized unit fuel cost is for minima to occur at the 16 per cent discount rate for NU fuel cycle, 6 per cent discount rate for SEU fuel cycle, 12 per cent discount rate for RU fuel cycle.

Levelized fuel cycle costs calculated for the reference cases are presented in Table 3. Cost of SEU and RU fuel cycles are considerably lower than the NU cycle. Cost of RU with reprocess cycle is the highest, and affected to a great extent by reprocessing cost.

Sensitivity calculations were made to analyze the impact on the total fuel cycle cost of variations in the economical paramaters such as discount rate and escalation ratio and in the unit prices for fuel cycle components. The results of sensitivity analysis of uranium price, enrichment price, fabrication price, spent fuel disposal cost, reprocess cost, escalation ratio and discount rate for the considered cycles are summarized. Sensitivity analysis for fuel cycle costs are given in Table 4 and sensitivity analysis for variations from reference fuel cycle costs are also presented in Table 5.

It could be observed from Table 5 that SEU fuel cycle is the most sensitiv one to the variations in uranium unit price. Variation from reference cost for SEU cycle is 74 % for 100 $/kgU unit uranium price. Since, NU has highest fabrication and spent fuel disposal cost shares in total cost, increase of fabrication cost and spent fuel disposal cost effect the NU fuel cycle more than the others. Variations from reference cost for NU cycle are 18 % for 100 $/kgU unit fabrication price and 23 % for 150 $/kgHM unit spent fuel disposal cost.

Since RU reprocess cycle has the highest (90%) front-end costs share in total cost, RU reprocess cycle has 52 % the smallest variation from reference cost for 2 % escalation ratio. Because of highest front-end costs share or lowest back-end costs share, it has 122 % highest variation from reference cost for 30 % discount rate.

SEU RU RU-Reprocess

Uranium price($/kgU)

Figure 1. Effect of uranium price on fuel cycle costs

Escalation ratio(%)

SEU RU RU-reprocess

Figure 2. Effect of escalation ratio on fuel cycle costs

Reprocess Cost($/kgHM)

SEU RU RU-Reprocess

SEU RU RU-Reprocess

SF Disposal Cost($/kgHM)

Figure 4. Effect of spent fuel disposal cost on fuel cycle costs

NU SEU RU RU-Reprocess

Enrichment Cost($/kg SWU)

Figure 5. Effect of enrichment cost on fuel cycle costs

RU RU-Reprocess

Discount Rate(%)

Table 3. Levelized fuel cycle costs for the reference cases

Fuel Cycle Option Fuel Cycle Cost(mills/kwh)

NU 4.242

SEU 2.221

RU without reprocess 3.069

RU with reprocess 11.834

Table 4. Sensivity analysis for the fuel cycle costs

Param eter Range

Fuel Cycle Costs(mills/kwh)

NU SEU RU RU-reproces Uranium price ($/kgU) 10-100 3.98-6.37 1.97-3.87 2.42-3.87 11.83 Fabrication price ($/kgU) 50-100 3.73-5.01 2.03-2.51 2.75-3.54 11.52-12.31 Enrichment price($/kgSWU) 60-130 4.24 2.14-2.41 3.07 11.83

Spent fuel disposal ($/kgU) 60-150 3.96-5.23 2.12-2.59 2.90-3.66 11.66-12.43 Reprocess ($/kgHM) 50-1000 4.24 2.22 3.07 2.49-18.63 Escalation Ratio(%) 0 - 2 4.24-7.13 2.22 - 3.52 3.07 - 5.11 11.83-18.03 Discount Rate(%) 0-30 5.30-4.03 2.41-3.54 3.64-3.82 11.08-26.27

Table 5. Sensitivity analysis for variations o f fuel cycle costs from reference fuel cycle

costs

Param eter

Ref.

Value Range

Variation from reference fuel cycle cost(%)

NU SEU RU

RU-Reprocess

Min Max Min Max Min Max Min Max

U ran iu m price ($/kgU ) 20 10-100 -6 50 -11 74 -21 26 0 0 F ab ricatio n price ($/kgU ) 70 50-100 -12 18 -8 13 -10 15 -2 4 E nrichm ent price ($/kgSW U ) 80 60-130 0 0 -4 9 0 0 0 0 SF D isposal ($/kgH M ) 80 60-150 -6 23 -4 17 -5 19 -1 5 R eprocess ($/kgH M ) 600 50-1000 0 0 0 0 0 0 -78 57 E scalatio n R atio(% ) 0 0-2 0 68 0 58 0 66 0 52 D iscount R ate(% ) 5 0-30 25 -5 8 59 19 24 -6 122 5- CONCLUSION

NU, SEU, RU without reprocess and RU with reprocess fuel cycle options for CANDU reactor were proposed. Fuel cycle cost of each option was calculated using levelizing fuel cycle cost methodology. By using reference data for each option, SEU fuel cycle cost was calculated as the cheapest option for CANDU reactor. Fuel cycle costs are sensitive to uranium price, enrichment price, fabrication price, spent fuel disposal cost, reprocess cost, escalation ratio and discount rate. Fuel cycle costs were calculated to analyze the impact of variations in each parameter on fuel cycle costs for each option. The most economical option for CANDU reactor in the case of variations in each parameter could be determined using results of this study.

REFERENCES

1. HART S. Ralph, 1997, AECL, CANDU Technical Summary

2. CAMPBELL Ross, Ottowa, 1977, AECL Seminar On Plutonium Recycle and The Thorium Cycle In Canada

3. OECD, Paris, 1994, The Economics Of The Nuclear Fuel Cycle, Nuclear Energy Agency 4. Nuclear Issuee Briefing Paper 36, August 2000, Uranium Markets

5. Historical Ux month-end spot prices, October 2000, The Ux Consulting Company, LLC&The Uranium Exchange Company