3. Results and discussion

purchase, thorium purchase, conversion, and enrichment. The price of natural uranium, conversion, and enrichment is the average price of spot market from 2010 to 2015 (http://www. uxc.com/), respectively: 120.9 \$/kg U, 8.75 \$/kg U, and 126.5 \$/SWU. The fabrication cost of 777 \$/kg HM for HTR fuel blocks comes from the Oak Ridge National Laboratory (ORNL)'s

Figure 5. Time flow of nuclear fuel cycle cost with direct disposal option.

104 Recent Improvements of Power Plants Management and Technology

Table 2. Reactor operation data for the HTR.

Table 3. Fuel cycle cost data for the HTR.

Items Values Thermal output 600 MWth Electric output 240 MWe Plant lifetime 60 years Refueling cycle length 2 years Load factor 90%

Back-end options Direct disposal

Components Prices Lead or lag times

Natural uranium purchase 120.9 \$/kg U 24 months Thorium purchase 120.9 \$/kg Th 24 months Conversion 8.75 \$/kg U 18 months Enrichment 126.5 \$/SWU 12 months Fabrication 777 \$/kg HM 6 months Storage 230 \$/kg HM 5 years Disposal 610 \$/kg HM 40 years

Discount rate 8%

For real HTRs with multi-batch refueling and multi-block, both the spatial separation levels defined in Section 2 and refueling patterns influence the performance of fuel in the reactor core, so the latter is excluded by ideally adopting one-batch fixed-patternrefueling mode in order to focus on the spatial separation effect of the thorium/uranium fuel. Based on the onebatch fixed-pattern refueling mode, as shown in Figure 3, thorium and uranium in the different spatial separation levels with the same mass are loaded into the geometrically same reactor core and are discharged when the keff reaches the same specified value. The core performance difference is due to the spatial separation levels.

## 3.1. Configuration effects in one spatial separation level

In order to compare four different spatial separation levels in the core scale, the configurations or patterns of the thorium/uranium fuels in the channel-level separation and block-level separation are simply analyzedin this section because the former meets different configurationsor patterns in block scale and the latter does in core scale, as shown in Figure 2, and the configurations influence the performance of the reactor core. For the other two separation levels, that is no separation level and TRISO-level separation, the same the fuel blocks exist in the whole reactor core.

## 3.1.1. Spatial configuration effects in the channel-level separation

For the channel-level separation, the spatial configurations of thorium/uranium compacts in the fuel block are different even for the same thorium content because uranium and thorium compacts are located in different channels, that is, there are 2210 configurations theoretically. It is almost impossible to investigate all of them, but it is not necessary to do it because the performance is possibly similar for some similar configurations, and the comparison in the reactor core scale is of the main concern. Five typical configurations (SBU 1#–SBU 5#) are chosen and investigated for 46% of thorium content, as shown in Figure 6. The uranium fuel compacts are located in the central, middle, and outer regions of the fuel block, respectively, for the SBU 1#–SBU 3#, and are evenly located in the fuel block for the SBU 4# and SBU 5#. The uranium compacts are relatively concentrated in the former group of configurations and are relatively dispersed in the latter group of configurations. For other thorium content, the spatial separation of uranium/thorium compacts are located in the similar mode.

Figures 7 and 8 show the keff of the reactor core for 46 and 91% thorium contents, which are calculated based on the model described in Section 2.2. When the thorium content increases, the number of thorium compacts will increase and the number of uranium compacts will decrease. In order to maintain the same mass of U-235 in the fuel block or in the reactor core,

Figure 6. Five typical configurations of uranium/thorium fuels for 46% thorium content (red: uranium compacts, blue: Thorium compacts).

Figure 7. keff of five configurations for 46% thorium content.

the enrichment of U-235 has to be increased. For different thorium contents, when the enrichment of U-235 is less than 20%, the five different spatial configurations of thorium/uranium fuels achieve the similar keff. However, when the enrichment is higher than 20%, for example 80%, the keff of the five configurations is different obviously. Moreover, the keff of the SBU 4# and SBU 5# is always higher than the other three configurations, and the influence of the spatial configurations becomes important. In the channel-level separation, the dispersed uranium compacts are advantageous to transmute Th-232 to U-233 when the thorium content and, thus, the enrichment of U-235 are high. Based on the calculations and analysis, the suggested spatial configurations of thorium/uranium compacts in the fuel block are shown in Figure 9 for the channel-level separation. The uranium compacts are concentrated in the

Figure 8. keff of five configurations for 91% thorium content.

Figures 7 and 8 show the keff of the reactor core for 46 and 91% thorium contents, which are calculated based on the model described in Section 2.2. When the thorium content increases, the number of thorium compacts will increase and the number of uranium compacts will decrease. In order to maintain the same mass of U-235 in the fuel block or in the reactor core,

106 Recent Improvements of Power Plants Management and Technology

Figure 6. Five typical configurations of uranium/thorium fuels for 46% thorium content (red: uranium compacts, blue:

Thorium compacts).

Figure 7. keff of five configurations for 46% thorium content.

Figure 9. Configurations of thorium/uranium compacts for channel-level separation (red: uranium compacts, blue: thorium compacts).

central region of the fuel block for low thorium content and are dispersed for high thorium content.

## 3.1.2. Spatial configuration effects in block-level separation

For the block-level separation, the spatial configurations of thorium/uranium fuel blocks in the reactor coremeet the same problem as the channel-level separation. The uranium and thorium fuel blocks located in different positions in the reactor core lead to different configurations, that is, there are 2198 configurationsof the fuel blocks theoretically. Based on the same analysis method as presented in Section 3.1.1, every five typical configurations (WASB 1#–WASB 5#) were chosen and investigated for different thorium content. The results show that the dispersed thorium fuel blocks are advantageous to transmute Th-232 to U-233 in the block-level separation. Based on the results, the suggested spatial configurations of thorium/uranium fuel blocks in the reactor core are shown in Figure 10 for the block-level separation, which is chosen to be compared with other three separation levels.

## 3.2. Effective enrichment and initial inventory of U-235

As mentioned in Section 3.1, the highly-enriched U-235 possibly has to be used when the thorium content increases. Compared with thorium/uranium-fueled reactor, the uraniumfueled reactor only contains one fissile isotope, that is, U-235, and one fertile isotope, that is,

Figure 10. Configurations of thorium/uranium fuel blocks for block-level separation (red: uranium fuel blocks, blue: thorium fuel blocks).

U-238, in the fuel. The mass fraction of fissile isotopes, that is, U-235, is the so-called enrichment of U-235, as defined in Eq. (1).,

central region of the fuel block for low thorium content and are dispersed for high thorium

For the block-level separation, the spatial configurations of thorium/uranium fuel blocks in the reactor coremeet the same problem as the channel-level separation. The uranium and thorium fuel blocks located in different positions in the reactor core lead to different configurations, that is, there are 2198 configurationsof the fuel blocks theoretically. Based on the same analysis method as presented in Section 3.1.1, every five typical configurations (WASB 1#–WASB 5#) were chosen and investigated for different thorium content. The results show that the dispersed thorium fuel blocks are advantageous to transmute Th-232 to U-233 in the block-level separation. Based on the results, the suggested spatial configurations of thorium/uranium fuel blocks in the reactor core are shown in Figure 10 for the block-level separation, which is chosen

As mentioned in Section 3.1, the highly-enriched U-235 possibly has to be used when the thorium content increases. Compared with thorium/uranium-fueled reactor, the uraniumfueled reactor only contains one fissile isotope, that is, U-235, and one fertile isotope, that is,

Figure 10. Configurations of thorium/uranium fuel blocks for block-level separation (red: uranium fuel blocks, blue:

3.1.2. Spatial configuration effects in block-level separation

108 Recent Improvements of Power Plants Management and Technology

to be compared with other three separation levels.

3.2. Effective enrichment and initial inventory of U-235

content.

thorium fuel blocks).

$$\varepsilon = \frac{m\_{\text{U5}}}{m\_{\text{U5}} + m\_{\text{U8}}} \tag{1}$$

When another fertile isotope, that is, Th-232, appears in the reactor fuel, the mass fraction of fissile isotopes, as defined in Eq. (2)

$$
\varepsilon\_{\it eff} = \frac{m\_{\it \square 5}}{m\_{\it \square 5} + m\_{\it \square 8} + m\_{\it \square 2}} \tag{2}
$$

is different from the traditional enrichment of U-235, which is called effective enrichment of U-235[19]. If the concept of the effective enrichment of U-235 is adopted, the nominal mass fraction of fissile isotopes is far less than 20% for the thorium-fueled HTRs as shown in Figure 11, which is the limit enrichment of U-235 for low-enriched fuel. Since the enrichment of U-235 is no longer equal to the effective enrichment of U-235 in a thorium-loaded reactor, and the physical meaning of the latter is clearer, the latter is analyzed instead of the former.

As shown in Figure 11, the required initial effective enrichment of U-235 obviously decreases with the increase of separation level from Th/U MOX to SBU and WASB for the same 2-year

Figure 11. Initial effective enrichment of U-235 as a function of thorium content for different separation levels.

refueling cycle length, when the thorium content changes from 10 to 80%. When the thorium content is less than 10% or larger than 80%, the influence of spatial separation level on the initial εeff is weak because the fuel is nearly uranium or thorium and thus there is no obvious difference. Moreover, the initial εeff of SBU is nearly the same as that of theWASB when the thorium content is in the range of 20–70%. For the Th/U MOX fuel, the initial εeff increases when the thorium content increases from 0 to 30% and decreases with the further increase of thorium. For the SBT, SBU, and WASB, the initial εeff decreases with the increase of the thorium content.

Although the total initial inventory of heavy metal slightly decreases because of the density difference between ThO2 (9.4 g/cm<sup>3</sup> ) and UO2 (10.4 g/cm<sup>3</sup> ), as shown in Figure 12, the required initial inventory of U-235,as shown in Figure 13, nearly has the same trend as the initial effective enrichment of U-235, according to Eq. (2). Furthermore, the initial inventory of the enriched uranium also decreases with the increase of the thorium content, as shown in Figure 12, because more and more U-238 is replaced by Th-232 in the reactor core. The change of the initial inventory of U-235 or the initial effective enrichment of U-235 is a result of the difference of nuclear performance (e.g., initial effective multiplication factor and average conversion ratio) caused by the spatial separation levels, as furtherly discussed in Section 3.3.

Figure 12. Initial inventory of heavy metal as a function of thorium content for different separation levels.

Analysis of the Spatial Separation Effects of Thorium/Uranium Fuels in Block‐Type HTRs http://dx.doi.org/10.5772/intechopen.68671 111

Figure 13. Initial inventory of U-235 as a function of thorium content for different separation levels.

#### 3.3. Initial effective multiplication factor

refueling cycle length, when the thorium content changes from 10 to 80%. When the thorium content is less than 10% or larger than 80%, the influence of spatial separation level on the initial εeff is weak because the fuel is nearly uranium or thorium and thus there is no obvious difference. Moreover, the initial εeff of SBU is nearly the same as that of theWASB when the thorium content is in the range of 20–70%. For the Th/U MOX fuel, the initial εeff increases when the thorium content increases from 0 to 30% and decreases with the further increase of thorium. For the SBT, SBU, and WASB, the initial εeff decreases with the increase of the thorium

Although the total initial inventory of heavy metal slightly decreases because of the density

required initial inventory of U-235,as shown in Figure 13, nearly has the same trend as the initial effective enrichment of U-235, according to Eq. (2). Furthermore, the initial inventory of the enriched uranium also decreases with the increase of the thorium content, as shown in Figure 12, because more and more U-238 is replaced by Th-232 in the reactor core. The change of the initial inventory of U-235 or the initial effective enrichment of U-235 is a result of the difference of nuclear performance (e.g., initial effective multiplication factor and average conversion ratio) caused by the spatial separation levels, as furtherly discussed in

Figure 12. Initial inventory of heavy metal as a function of thorium content for different separation levels.

) and UO2 (10.4 g/cm<sup>3</sup>

), as shown in Figure 12, the

content.

Section 3.3.

difference between ThO2 (9.4 g/cm<sup>3</sup>

110 Recent Improvements of Power Plants Management and Technology

#### 3.3.1. Initial effective multiplication factor and average conversion ratio

Figure 14 presents the initial effective multiplication factors (keff) and average conversion ratios (ACRs) of four spatial separation levels as a function of thorium content. For each spatial separation level, the keff nearly decreases with the increase of thorium content. For the same thorium content, the initial keff usually increases when the spatial separation level increases

Figure 14. Initial keff and average conversion ratio of different spatial separation levels.

from Th/U MOX to SBU. When the thorium content is larger than 80%, the difference among them becomes small. The more interesting rule is that the trend of ACR is always opposite to the initial keff, that is, when the ACR is higher, initial keff is smaller, even for the WASB level.

Figure 15 presents the keff as a function of operation time for two different ACRs in one refueling cycle. In these calculations, the refueling cycle length is 657 effective full power days (EFPDs), which means the coordinate of end of cycle (EOC) is fixed, as the point B (657,1.005) in Figure 15. On the other hand, the keff is a nearly linear function of burnup for the thoriumfueled HTR as shown in Figure 15. Therefore, if the average conversion ratio of the refueling cycle is smaller, the reactivity drop is larger and thus the initial keff must be higher to guarantee a critical reactor at EOC.

If Figure 14 is compared with Figure 11 or Figure 13, it is interesting to find that when the spatial separation level of thorium/uranium fuels changes from SBU to Th/U MOX, the required initial inventory of U-235 is the most for the Th/U MOX in order to achieve the same operation time, but the initial keff is smallest and the main benefit is the most amount of Th-232 transmuted into U-233. Once the spatial separation of thorium/uranium fuels changes from SBU to WASB, the required initial inventory of U-235 decreases. Moreover, the ACR increases

Figure 15. keff as a function of EFPD for different average conversion ratios.

with increase of thorium content, the initial keff decreases, and thus the reactivity drop or swingdecreases in the refueling length.

## 3.3.2. Further discussion of initial effective multiplication factor

from Th/U MOX to SBU. When the thorium content is larger than 80%, the difference among them becomes small. The more interesting rule is that the trend of ACR is always opposite to the initial keff, that is, when the ACR is higher, initial keff is smaller, even for the WASB level. Figure 15 presents the keff as a function of operation time for two different ACRs in one refueling cycle. In these calculations, the refueling cycle length is 657 effective full power days (EFPDs), which means the coordinate of end of cycle (EOC) is fixed, as the point B (657,1.005) in Figure 15. On the other hand, the keff is a nearly linear function of burnup for the thoriumfueled HTR as shown in Figure 15. Therefore, if the average conversion ratio of the refueling cycle is smaller, the reactivity drop is larger and thus the initial keff must be higher to guarantee

If Figure 14 is compared with Figure 11 or Figure 13, it is interesting to find that when the spatial separation level of thorium/uranium fuels changes from SBU to Th/U MOX, the required initial inventory of U-235 is the most for the Th/U MOX in order to achieve the same operation time, but the initial keff is smallest and the main benefit is the most amount of Th-232 transmuted into U-233. Once the spatial separation of thorium/uranium fuels changes from SBU to WASB, the required initial inventory of U-235 decreases. Moreover, the ACR increases

Figure 15. keff as a function of EFPD for different average conversion ratios.

a critical reactor at EOC.

112 Recent Improvements of Power Plants Management and Technology

In order to analyze the influence of spatial separation levels and thorium content on the initial keff and thus the initial inventory of U-235, four control groups of typical reactor cores including spatial separation levels and thorium content are calculated and discussed. The group 1 is to compare the Th/U MOX with SBU with 50% thorium content, in order to explain the reason of the decrease of the initial inventory of U-235. The group 2 is to compare the SBU and WASB with 50% thorium content, in order to explain the difference between them. The groups 3 and 4 are to discuss the Th/U MOX with 0, 30, and 80% thorium content, respectively, in order to explain the influence of the thorium content.

Table 4 presents the five factors and initial keff of the reactor cores involved in the four control groups, which are calculated according to the validated method [10]. The five-factor formula [20] can be written as

$$k\_{\rm eff} = \eta f p \varepsilon P\_{\rm NL} \tag{3}$$

where η, f, p, ε, PNL are, respectively, the reproduction factor, fuel utilization factor, resonance escape probability, fast fission factor, and nonleakage probability. However, in the core physics calculation described in Section 2.2.1, the homogenization of fuel block will mix the fuel, moderator, and coolant into a mixture, making the η and f inseparable. Thus, the production of η and f is regarded as the reproduction factor of the core. More information about the fivefactor formula can be found in Ref. [10].

To quantitatively describe the contribution of each factor to the variation of the initial effective multiplication factor, its contribution [21] is defined in terms of the components of five-factor formula

$$
\Delta r = \left(\frac{1}{\frac{r\_1}{\overline{r}} \cdot \overline{k}} - \frac{1}{\frac{r\_2}{\overline{r}} \cdot \overline{k}}\right) \cdot 10^5, \qquad r = \eta f, \varepsilon, p, P\_{\text{NL}} \tag{4}
$$


Table 4. Five factors and initial keff of different reactor cores involved in four control groups.

where

$$
\overline{r} = 0.5(r\_1 + r\_2), \quad \overline{k} = \overline{\eta}\overline{f}\,\overline{\varepsilon}\,\overline{p}\overline{P\_{\text{NL}}} \tag{5}
$$

The subscripts 1 and 2 represent two different cases, respectively. Table 5 presents the contribution of each factor for the four control groups.

For the group 1, when the thorium content is 50% and spatial separation level changes from Th/U MOX level to SBU level, the nonleakage probability increases by 96 pcm and contributes þ6% to the initial keff, because the spatial separation level strengthens thorium absorption, and the microscopic absorption cross section of Th-232 (7.4 barns) is three times of U-238 (2.7 barns). Based on the same reason, more thermal neutrons are absorbed by Th-232 but cannot induce fission reactions, which leads to the reproduction factor of core (ηf) to decrease by 4167 pcm and contributes �285% to the initial keff. The fast fission cross-section of Th-232 (0.01 barns) is smaller than that of U-238 (0.04 barns), which causes the fast fission factor (ε) to decrease by 1112 pcm and contribute �76%. However, because the thorium fuel is lumped in the thorium compacts for the SBU-50%Th and a smaller resonance integral (RI) of Th-232 (85) compared to U-238's (275), the resonance escape probability (p) of the SBU-50%Th increases by 6649 pcm and contributes þ455%. As a result of all effects, especially the contribution of the increase of p, the initial keff of the SBU-50%Th is 1460 pcm larger than that of the Th/U MOX-50%Th. The spatial self-shielding effect is strengthened by Th-232 and spatial separation levels, and thus the increase of resonance escape probability leads to the decrease of 188 kg initial inventory of U-235.

For the group 2, when the spatial separation increases from SBU level to WASB level, although the resonance escape probability further increases by 655 pcm, the reproduction factor decreases by 2754 pcm because of the further lumping of the thorium fuel. As a result, the initial keff decreases by 1313 pcm causing a smaller reactivity swing in the refueling period. Moreover, the initial inventory of U-235 decreases 6 kg because of a larger ACR.

For the group 3, for a fixed spatial separation level, for example, Th/U MOX level, when the thorium content increases from 0 to 30%, more thermal neutrons generated by fission are absorbed by fertile Th-232 because of three-time microscopic absorption cross section of


Table 5. Contribution of each factor to variation of initial keff.

Th-232, which leads to the decrease of the reproduction factor by 1753 pcm. Moreover, because of harder neutron spectrum, the resonance escape probability decreases by 1677 pcm. As a result, the initial keff decreases by 2992 pcm and the required initial inventory of U-235 increases by 77.8 kg. If the thorium content further increases from 30 to 80%, as shown in the group 4, a large amount of U-238 is replaced by Th-232 in the reactor core. Because of a smaller resonance integral of Th-232 (85) compared to U-238's (275), the resonance escape probability (p) increases by 6236 pcm and contributes þ336% but the reproduction factor decreases by 7017 pcm and contributes �378%. As a result, the initial keff decreases by 1857 pcm. Moreover, the required initial inventory of U-235 decreases by 206 kg because of a larger ACR.

### 3.4. Fuel cycle cost analysis

where

inventory of U-235.

r ¼ 0:5ðr<sup>1</sup> þ r2Þ, k ¼ ηf ε pPNL ð5Þ

The subscripts 1 and 2 represent two different cases, respectively. Table 5 presents the contri-

For the group 1, when the thorium content is 50% and spatial separation level changes from Th/U MOX level to SBU level, the nonleakage probability increases by 96 pcm and contributes þ6% to the initial keff, because the spatial separation level strengthens thorium absorption, and the microscopic absorption cross section of Th-232 (7.4 barns) is three times of U-238 (2.7 barns). Based on the same reason, more thermal neutrons are absorbed by Th-232 but cannot induce fission reactions, which leads to the reproduction factor of core (ηf) to decrease by 4167 pcm and contributes �285% to the initial keff. The fast fission cross-section of Th-232 (0.01 barns) is smaller than that of U-238 (0.04 barns), which causes the fast fission factor (ε) to decrease by 1112 pcm and contribute �76%. However, because the thorium fuel is lumped in the thorium compacts for the SBU-50%Th and a smaller resonance integral (RI) of Th-232 (85) compared to U-238's (275), the resonance escape probability (p) of the SBU-50%Th increases by 6649 pcm and contributes þ455%. As a result of all effects, especially the contribution of the increase of p, the initial keff of the SBU-50%Th is 1460 pcm larger than that of the Th/U MOX-50%Th. The spatial self-shielding effect is strengthened by Th-232 and spatial separation levels, and thus the increase of resonance escape probability leads to the decrease of 188 kg initial

For the group 2, when the spatial separation increases from SBU level to WASB level, although the resonance escape probability further increases by 655 pcm, the reproduction factor decreases by 2754 pcm because of the further lumping of the thorium fuel. As a result, the initial keff decreases by 1313 pcm causing a smaller reactivity swing in the refueling period.

For the group 3, for a fixed spatial separation level, for example, Th/U MOX level, when the thorium content increases from 0 to 30%, more thermal neutrons generated by fission are absorbed by fertile Th-232 because of three-time microscopic absorption cross section of

Group number Group Δηf [pcm] Δε [pcm] Δp [pcm] ΔPNL [pcm] Δkeff [pcm] 1 MOX-50%Th �4167 �1112 þ6649 þ96 þ1460

2 SBU-50%Th �2754 þ367 þ655 þ419 �1313

3 MOX-0%Th �1753 þ430 �1677 þ9 �2992

4 MOX-30%Th �7017 �1250 þ6236 þ171 �1857

Table 5. Contribution of each factor to variation of initial keff.

SBU-50%Th (�285%) (�76%) (þ455%) (þ6%) (þ100%)

WASB-50%Th (�210%) (þ28%) (þ50%) (þ32%) (�100%)

MOX-30%Th (�59%) (þ14%) (�56%) (þ1%) (�100%)

MOX-80%Th (�378%) (�67%) (þ336%) (þ9%) (�100%)

Moreover, the initial inventory of U-235 decreases 6 kg because of a larger ACR.

bution of each factor for the four control groups.

114 Recent Improvements of Power Plants Management and Technology

Using the levelized lifetime cost methodology described in Section 2.2.2, the fuel cycle cost of the four types of reactor cores (Th/U MOX, SBT, SBU, and WASB) analyzed in Sections 3.2 and 3.3 as a function of thorium content is shown in Figure 16. Compared with Figures 11 or 13, the fuel cycle cost changes with the same trend as the effective enrichment of U-235 or the initial inventory of U-235 in the reactor cores. When the thorium content is constant, the fuel cycle cost decreases with the increase of the spatial separation level. However, the difference of the SBU level and WASB level is small. The fuel cycle cost decreases with the increase of the

Figure 16. Fuel cycle cost of different spatial separation levels.

thorium content for the SBT, SBU, and WASB levels. However, it increases in the range of thorium content from 0 to 40% and decreases when the thorium content is larger than 40%.

The tight relationship between fuel cycle cost and initial inventory of U-235 mainly results from the composition of the cost. Figure 17 presents the composition of fuel cycle cost for three different reactor cores, including natural uranium purchase, thorium purchase, uranium conversion, uranium enrichment, the fabrication of fuel blocks, and storage and the deposal of spent fuels. Because the required amount of natural uranium is 13 times of the inventory of heavy metal in the reactor core due to 0.7% U-235 in natural uranium, both the natural uranium purchase and uranium enrichment are 70% of total fuel cycle cost. The thorium purchase is only 2.5% because the thorium need not be enriched, and the required amount of thorium is by far less than the amount of the natural uranium. The fabrication cost is the highest unit price (777 \$/kg HM) and the fabrication involves all heavy metals, so it is about 20% of the total cost. Although the unit price of the disposal of spent fuel is also high (610 \$/kg HM), the disposal cost is only about 0.5% because of so-called time value.

The fuel cycle cost is mainly determined by natural uranium purchase (35–40%), uranium enrichment (32–35%), and the fabrication of fuel blocks (18–21%). The total of three items is 85– 96% fuel cost. The fabrication cost is the same for all reactor cores because the inventory of heavy metal in the reactor core is the same. The natural uranium purchase and uranium enrichment are directly related to the initial inventory of U-235. The more is the inventory of U-235, the more are the required natural uranium and the uranium enrichment. As a result, the fuel cycle cost is the same trend as the initial inventory of U-235 or the initial effective enrichment.

Figure 17. Composition of fuel cycle cost for some typical reactor cores.
