Economic value of uranium recovered from LWR spent fuel as fuel for HWRs 1. Introduction

The reprocessing of spent nuclear fuel from LWRs to extract plutonium with which to create a mixed oxide (MOX) fuel, typically depleted uranium mixed with plutonium, meets two important objectives of SYNERGIES:

it increases the sustainability of the global fuel cycle by generating more energy per mined uranium mass, and it increases proliferation resistance, since the quality of the plutonium in the overall fuel cycle decreases. However, previous work has shown that reprocessing is difficult to justify on the basis of economics alone [3.14].

TABLE 3.15. AIR: STRUCTURE OF THE LEVELIZED UNIT ENERGY COST

Cost component US $/MW h

Investment cost 38.73

Fixed cost 8.98

Variable cost 0.83

Uranium cost 3.00

Conversion cost 0.20

Enrichment cost 3.21

Fabrication cost 1.60

Spent fuel management cost 0.21

Transfer cost 0.027 4

Cost of the processing without the return of

processing waste 0.11

Levelized unit energy cost 56.92

TABLE 3.16. THE LEVELIZED UNIT ENERGY COST IN DIFFERENT SCENARIOS

Scenario US $/MW·h

Management of SNF without construction of small reactors SFDS

SNF export by rail

SNF transfer to geological disposal

53.69 53.82 53.73 Construction of small reactors, export of SNF from the NPP site to another country

(old fuel modification) SNF export by rail SNF export by road SNF export by air

57.153 57.157 57.18 Construction of small reactors, export of SNF from the NPP site to another country

(new fuel modification) SNF export by rail SNF export by road SNF export by air

56.893 56.896 56.92 Note: NPP — nuclear power plant; SFDS — spent fuel dry storage; SNF — spent nuclear fuel.

Heavy water reactors (HWRs) have a low parasitic neutron capture rate such that natural uranium can be used as fuel. The creation of ²³⁹Pu from neutron capture on ²³⁸U capture is very efficient, permitting a very high fissile utilization, which is the energy yield per initial mass of fissile material (the same as the burnup divided by the initial fissile mass fraction in the fuel). The fissile utilization increases with enrichment and reaches a peak at an optimized value of around 1.2wt% ²³⁵U/uranium [3.15]. The reprocessed (or recycled/recovered) uranium from LWR spent nuclear fuel, a by-product of the MOX fuel cycle, typically has a fissile content of 0.76–0.92wt%

235U/uranium and can therefore be used very efficiently in an HWR.

Reuse of reprocessed uranium in HWRs is a potential near term collaborative architecture which increases the proliferation resistance and sustainability of the fuel in the global NES. This work attempts to establish the economic case for such an architecture (or strategy). If a net economic benefit can be demonstrated, then the share of this benefit which accrues to the holder of the reprocessed uranium increases the economic motivation for MOX reprocessing. In this way, the benefits to the global NES would be improved substantially by this synergy.

This study falls within the framework of Task 1 on the evaluation of synergistic collaborative scenarios towards sustainable NESs — nuclear fuel cycle synergies. The complete case study can be found in Annex III on the CD-ROM accompanying this publication.

3.1.3.2. Objective and problem formulation

Currently, reprocessed uranium is considered a waste product without significant value. The reasons for this assessment are the following:

— Few reactor types can use it directly because its enrichment is too low.

— It contains sufficient residual radioactivity to contaminate process lines (i.e. in plants for uranium enrichment).

— The isotopic composition specifications of reprocessed uranium are variable, depending on its origin, so that more effort is required to qualify its use in fresh fuel.

Returning reprocessed uranium to the NES fuel cycle would help to increase global sustainability. The objective of this study is to explore the economics of burning reprocessed uranium and demonstrate that a synergy between an reprocessed uranium generator and user could be made mutually beneficial from this point of view.

The economic value of reprocessed uranium to a user is defined to be the cost of fuel which would otherwise have to be obtained to produce the same total energy. This cost defines the theoretical maximum that a user of reprocessed uranium should be willing to pay for it, and a synergy would be a situation when a user was paying less than this maximum (thus saving on fuel costs) while the reprocessed uranium producer was making a profit on a MOX reprocessing line by-product. For convenience, and because there is a large supply of this material, the focus has been placed on analysing the use of reprocessed uranium extracted from spent nuclear fuel from an LWR having approximately a burnup of 33 MW·d/kg. The uranium isotopic composition from this fuel, originally 3.25wt% ²³⁵U/uranium, is defined as [3.14]:

—²³⁵U: 0.924 2wt%;

—²³⁶U: 0.408 8wt%;

—²³⁸U: 98.667 0wt%.

The question to be answered is how does the value compare to other potential uses of the reprocessed uranium (i.e. recycling into an LWR of similar type, or use to support minor actinide disposal). The economic value of reprocessed uranium to an HWR will be assessed by its exit burnup relative to the exit burnup of natural uranium (the fuel which would otherwise be used). Higher exit burnup (assuming constant power) reduces the total fuel required (and hence the fuelling expense) in inverse proportion.

3.1.3.3. Assumptions, methods and data used

(a) Assumptions

The assumptions include the following:

TABLE 3.15. AIR: STRUCTURE OF THE LEVELIZED UNIT ENERGY COST

Cost of the processing without the return of

processing waste 0.11

Levelized unit energy cost 56.92

TABLE 3.16. THE LEVELIZED UNIT ENERGY COST IN DIFFERENT SCENARIOS

Scenario US $/MW·h

Management of SNF without construction of small reactors SFDS

SNF export by rail

SNF transfer to geological disposal

53.69 53.82 53.73 Construction of small reactors, export of SNF from the NPP site to another country

(old fuel modification) Construction of small reactors, export of SNF from the NPP site to another country

(new fuel modification) Note: NPP — nuclear power plant; SFDS — spent fuel dry storage; SNF — spent nuclear fuel.

(1) There is an active market (a balance between supply and demand) in reprocessed uranium. This ensures that the price obtainable for reprocessed uranium by the sellers is set closely by its economic value to buyers. It is assumed that natural uranium is part of this market.

(2) HWRs can be reconfigured easily to use reprocessed or natural uranium in different core loads, depending on availability and price, and switching fuels requires no other cost. However, long term fuel contracts are more likely. The economic analyses would be similar for such contracts, so these two assumptions are not strictly required.

(3) Shipping of reprocessed uranium from LWRs to HWRs via the reprocessing and fuel refabrication facility has a negligible relative cost.

(4) Reprocessed uranium is already available as a by-product of MOX production with no further costs. These extra costs, if included, would reduce its value, and would have to be considered in a more complete study.

(5) The HWR is assumed to run in an ‘advanced fuel cycle mode’ using a 43 element fuel bundle, and smaller numbers of fuel bundles per shift (e.g. a 2 bundle fuel shift with reprocessed uranium based fuels instead of an 8 bundle fuel shift with natural uranium fuel), and fewer core reactivity devices for adjusting reactivity and flux distributions. In this mode, the exit burnup is defined as that burnup for which the core losses in reactivity (due to neutron leakage and parasitic absorption) are 3–4% (0.1% = 0.001 dk/k = 1 mk = 100 pcm) depending on the fissile content of the reprocessed uranium. For comparison, when natural uraniumis used as fuel, reactivity devices such as liquid zone controllers and adjusters filled with light water serve the purpose of flattening the flux axially, and core reactivity losses due to neutron leakage are approximately about 4.5%

(45 mk = 4500 pcm).

(6) The central pin of the advanced fuel cycle bundle is assumed to be poisoned (containing a burnable neutron absorber) to achieve the same coolant void reactivity or slightly less than that of natural uranium (so that safety analyses do not have to be updated).

(b) Codes

The lattice physics code WIMS-AECL 3.1.2.3 [3.16] was used to determine the expected exit burnup of the reprocessed uranium. A burnup weighting of k_inf during a full burnup calculation gives an estimate of the k_inf of an infinite lattice core with a distribution of such bundles from fuel changes. When this k_inf is less than 1 + leakage + parasitic absorption (see assumption 5), the exit burnup has been reached.

(c) Input data

The burnup of natural uranium in an HWR is assumed to be 7500 MW·d/t (7.5 MW·d/kg). The fuel/coolant/

moderator temperatures and densities are similar to that of a CANDU-6 pressure tube heavy water reactor.

3.1.3.4. Summary presentation and analysis of the results

(a) Overall economic value of reprocessed uranium

The economic values of reprocessed uranium for direct recycling in an HWR and for re-enrichment (for use in an LWR) are shown in Fig. 3.14.

Since fuel bundles made of reprocessed uranium, or re-enriched reprocessed uranium, displace normal fuel bundles, in each case the cost of natural uranium and the cost of fuel bundle assembly hardware are parameters which affect its value. Estimates of US $105.26/kg and US $250/kg have been made for HWR and LWR hardware, respectively [3.14]. Reference [3.14] assumes an extra US $10/kg for the handling of the (slightly radioactive) reprocessed uranium fuel instead of natural uranium, this expense has been added to the cost of HWR fuel hardware for reprocessed uranium as well.

At the current (nominal) natural uranium cost of US $90/kg, it can be seen that reprocessed uranium from 33 MW·d/kg used nuclear fuel is worth about US $230/kg if recycled in an HWR, while US $100/kg if re-enriched for an LWR. Thus, a synergetic collaboration with an HWR is more attractive (to a MOX reprocessor) than with an LWR. The absolute value of the difference becomes larger as the price of natural uranium rises to US $300 (probably a natural ceiling, since at this price it starts to become economical to mine sea water [3.17]).

FIG. 3.14. Value of reprocessed uranium as a function of the cost of natural uranium, with fabrication costs held fixed.

FIG. 3.15. Reduction in the value of reprocessed uranium as a function of the presence of ²³⁶U.

(b) Effect of ²³⁶U

Reprocessed uranium naturally contains ²³⁶U from neutron capture, without fission, on ²³⁵U. No primordial

236U exists in natural uranium due to its relatively short half-life of 2 × 10⁷ years. The effect of ²³⁶U was investigated for the actual amount present in the nominal 33 MW·d/kg used nuclear fuel or 0.408 8wt%, and parametrically as a sensitivity case, and the results are shown in Fig. 3.15. Independent of the amount of ²³⁵U, the effect of ²³⁶U is seen to decrease the value of reprocessed uranium by around US $3 for every 0.1wt% increase. For 33 MW·d/kg used nuclear fuel, the reprocessed uranium value is reduced by about US $12/kg. Uranium-236 is more of a problem in LWRs because it is a better absorber in the somewhat harder neutron spectrum there. It is estimated in Annex III that ²³⁶U reduces the worth of reprocessed uranium to be used for re-enrichment by about twice this amount to about US $24/kg.⁴

4 This assumes centrifuge type re-enrichment, where a significant amount of ²³⁶U is carried along with the ²³⁵U. Laser enrichment techniques would presumably reduce the cost of ²³⁶U in reprocessed uranium to nearly US $0 for LWRs.

FIG. 3.14. Value of reprocessed uranium as a function of the cost of natural uranium, with fabrication costs held fixed.

FIG. 3.15. Reduction in the value of reprocessed uranium as a function of the presence of ²³⁶U.

(c) Coolant void activity

Because the coolant provides little moderation in a CANDU-6 pressure tube heavy water reactor, loss of coolant tends to result in a net positive reactivity insertion. The effects of this coolant void reactivity (CVR) insertion are considerably mitigated by the very long neutron lifetime in HWRs, about ten times that of a PWR. It can be seen that the positive effect of ²³⁶U on the (core averaged) CVR is greater than the negative effect of increasing enrichment. It is assumed that the reactor will be returned to a standard CVR (for fuel with ²³⁵U = 0.711wt% and

236U = 0wt%). In the case of reprocessed uranium with 0.924 2wt% ²³⁵U/U and 0.402 2wt% ²³⁶U/U, the effect is 0.13 mk. Increasing the neutron poison in the central element to reduce the CVR by this amount adds a burnup penalty worth approximately US $3.70/kg.

3.1.3.5. Conclusions

It is estimated that aqueous reprocessing of LWR fuel will cost US $1000–2000 per kg [3.11]), so the recovery of uranium with an enrichment near that of natural uranium (currently at ~US $90/kg) cannot be the main economic driver. However, low burnup reprocessed uranium supplied to an HWR in place of natural uranium has the potential to save the utility up to US $230/kg in fuelling costs (given current prices for natural uranium and HWR fuel bundle fabrication). Divided up between the reprocessor and HWR, this potential profit makes the reprocessing case more economically feasible, and therefore moves the world towards a more sustainable fuel cycle. The profit in other scenarios, such as re-enrichment of the reprocessed uranium for use in LWRs, or mixing the reprocessed uranium with depleted uranium to make a natural uranium equivalent fuel for HWRs (discussed in the full case study in Annex III on the CD-ROM accompanying this publication), result in an reprocessed uranium value much more closely tied to the price of natural uranium and are therefore less attractive as economic drivers.

Reprocessed uranium contains the neutron absorbing poison ²³⁶U. When re-enriched, the reactivity loss due to this poison has to be compensated by over-enriching the fuel slightly. It was estimated that this over-enrichment reduces the worth of the reprocessed uranium by about US $24/kg ²³⁶U. The burnup penalty imposed by ²³⁶U is less in an HWR, resulting in a value of US $12/kg ²³⁶U, but the effect of ²³⁶U on HWR CVR requires additional poisoning in the central pin which subtracts an additional US $4/kg from the reprocessed uranium value.

3.1.4. A reactor synergy: Using HWRs to transmute Am from LWR spent fuel