Fuel Cycles

susceptible to Loss of Flow (LOFA) and Loss of Heat Sink (LOHS) accidents and therefore require additional measures to assure their safety.

Moreover, the technology for the design of large accelerators and for their coupling with the reactor requires extensive development to improve the overall system reliability, capacity factors, and reduce the development and deployment costs. A 1999 DOE review of the US Accelerator Transmutation of Waste (ATW) program concluded that operation of an ATW prototype or demonstration unit might take as long as 20 years to implement and would cost as much as $11 billion. Full implementation of ATW in the United States for treating civilian spent nuclear fuel would require several decades and could cost hundreds of billions of dollars [Bresee J.C., 2003].

Production of electricity and improved resource utilization can partially offset the costs related to ADS technology. In addition, thermal spectrum molten salt fueled ADS, as pointed out in [Bowman C.D., 2000], may require up to 4 times smaller accelerator due to reduced neutron leakage and reduced reactivity control requirements which would also facilitate better economic performance of ADS.

On the other hand, critical fast spectrum systems with carefully designed safety features may appear to be more favorable candidates for TRU transmutation as they can potentially provide comparable to ADS transmutation capabilities but at lower cost and with greater reliability due to utilization of more mature technologies [Hejzlar et al., 2003], [Romano A., et al., 2002].

Fuel Cycles

The proposed fuel cycles for transmutation of waste differ in several respects. Depending on the main goal of transmutation Pu or Pu, MA, and LLFP can be considered. Suggested fuel cycle strategies also included single path with maximized deep burndown of TRU or multi-recycling of Pu and MA in one or multi-tier systems. Several examples of the proposed fuel cycles are presented in Figure 1.2.4.

The concern over growing Pu stockpile from commercial LWR spent nuclear fuel in addition to significant quantities of weapons grade (WG) Pu from dismantled nuclear warheads [Albright D. et al., 1997] resulted in extensive research effort aiming at the reduction of the excess amounts

of Pu. Numerous studies initiated by the US DOE confirmed the possibility of disposition of reactor grade (RG) and WG Pu in the form of mixed PuO2-UO2 oxide fuel in all major currently existing reactor types: [Westinghouse Electric Co., 1994] and [Combustion Engineering Inc., 1994] for PWRs, [GE Nuclear Energy, 1994] for BWRs, and [AECL Technologies Inc., 1996] for Canadian heavy water reactors (CANDU). Thorium and fertile free fuel matrices appear to be more effective for Pu burning than MOX because no secondary Pu is generated. Possibility of Pu burning in fertile free matrix fuel in PWRs peripheral assemblies and achieving the sustainable Pu inventory was demonstrated by [Chodak P., 1996]. The full fertile free fuel loaded PWR core for disposition of RG and WG Pu was analyzed in [Kasemeyer U. et al., 1998] and shown to be feasible.

RG and WG plutonium disposition using thorium based fuels has been widely studied as well. For example, potential feasibility of burning WG and RG Pu are discussed in [Lombardi C. et al., 1999] and [Phlippen P.W. et al., 2000]. They addressed several issues of reactor control and safety as well as issues concerning multi-recycling of Pu. Alternative fuel design for more efficient thorium assisted Pu disposition in PWRs based on Seed-Blanket fuel assembly concept was proposed and discussed in [Galperin A. et al., 1998]. Feasibility of using Th-Pu fuel in high conversion BWRs is discussed in [Downar T. et al., 2000]. Various other reactor systems such as High Temperature Gas-cooled Reactors [Ruetten H.J., 2000b] and fast spectrum reactors [Tommasi J. et al., 1998] with Pu-Th fuel were also suggested and examined. These studies showed that thorium based fuels can efficiently perform the task of RG and WG plutonium stockpile reduction while maintaining acceptable safety and control characteristics of the reactor systems studied. However, no significant improvement in the spent fuel repository characteristics can be achieved through single path Pu burning in thorium matrix [Reutten H.J., 2000a].

The recent and most comprehensive transmutation systems and fuel cycle strategy studies were reported in [OECD/NEA, 2002] as a part of a joint nuclear waste partitioning and transmutation effort by OECD member countries and in [Van Tuyle G.J., 2001] in the framework of formerly Advanced Accelerator Application (AAA) program sponsored by the US DOE. The most significan findings of these studies are summrized in [Finck P., 2002]. In both studies, the main goal of waste transmutation was the reduction of radiological impact of nuclear waste on the repository and surrounding environment to the extent that very long term storage would no longer be needed. The tradeoffs between single path burndown and multi-recycling as well as advantages and drawbacks of the single tier versus multi-tier approaches are discussed.

Once Through Fuel Cycle

Uranium

LWR Spent fuel to HLW repository

LWR Recycle

Uranium

LWR

Uranium Recycled Pu

LWR

Recycled Pu + MA FP + MA to HLW repository

U to LLW repository

FP to HLW repository

U to LLW repository

Multi-Tier Recycle

LWR Fast Burner

Recycled U, Pu, LLFP Recycled Pu + MA

Figure 1.2.4. Examples of Fuel Cycles

FP to HLW repository

In the multi-tier approach [Hill R.N. et al., 2002], Pu in the form of MOX is recycled once or multiple times in existing LWRs or advanced gas cooled reactors to achieve major TRU mass reduction using conventional technologies and therefore maximizing the economic benefits. While in the second tier, small amounts of residual Pu, MA, and LLFP are burnt in fast reactors or ADS via multiple recycling to the extent required to achieve the radiotoxicity reduction goal.

The single tier system considered in [OECD/NEA, 2002] was self supporting fast reactor system with closed fuel cycle and zero net generation of TRU. This strategy implies a distant future scenario where the entire fleet of nuclear reactors is of the described type.

The single path burndown fuel cycle strategy suggests reprocessing of the existing LWR spent fuel and subsequent once through deep burnup of Pu or TRU in various transmuter systems. For example, GT-MHR was proposed for disposition of Russian WG Pu and is said to achieve a destruction efficiency of up to 90% of fissile Pu per path [Kodochigov N., 2002]. However, none of the single path transmuter systems can achieve the degree of TRU destruction sufficient enough to satisfy the radiotoxicity reduction criteria (Figure 1.2.5 [Van Tuyle G.J., 2001]). This conclusion is consistent with the findings elaborated later in this thesis.

Pu MOX LWR

Pu FFF LWR

Pu GCR

TRU MOX LWR

TRU FFF LWR

TRU GCR

Figure 1.2.5. Radiotoxicity relative to NU ore for the single path burdown scenario [Van Tuyle G.J., 2001].

Additionally, both OECD and AAA studies came to a common conclusion that in principle, all analyzed fuel cycle options are capable of achieving the radiological and TRU mass reduction goal provided that multi-recycling of TRU with relative losses to the waste stream of less than 0.1% can be achieved.

Multi-recycling strategies to constrain generation of Pu and TRU in LWRs were also recently studied by several groups. Advanced Pu Assembly (APA) concept [Puill A. et al., 2001]

was proposed and possibility of Pu inventory stabilization in PWRs was confirmed. The APA combines large annular internally cooled fertile free fuel pins containing Pu and standard UO2 fuel pins as shown in Figure 1.2.6.

Such a configuration allows effective Pu consumption through high moderator to fuel volume ratio in Pu bearing region and fertile free matrix. Annular geometry also helps to accommodate high power peaking in Pu pins. However, APA cannot be combined with conventional assemblies in the same PWR core because of the differences in thermal hydraulic designs.

Annular Pu rod

UO2 rod

Guide tube

Figure 1.2.6. Advanced Plutonium Assembly [Puill A. et al., 2001]

The alternative CORAIL assembly concept [Aniel S. et al., 2001] was proposed. All fuel pins in the CORAIL assembly have identical geometry. However, the pins on the assembly periphery contain Pu or TRU [Taiwo T. A. et al., 2002] in the MOX form. The availability of existing technology for reprocessing and fabrication of MOX fuel is the main advantage of the CORAIL concept. The equilibrium TRU concentrations in the CORAIL fuel cycle are relatively high (about 36 kg per assembly) due to the presence of fertile U238 which results in additional TRU generation and therefore deteriorates TRU destruction efficiency.