D.L. Porter

Irradiation behavior of metallic fast reactor fuels

Abstract Metallic fuels were the first fuels chosen for liquid metal cooled fast reactors (LMRs). In the late 1960s worldwide interest turned toward ceramic LMR fuels before the full potential of metallic fuel was realized. However, during the 1970s the performance limitations of metallic fuel were resolved in order to achieve a high plant factor at the Argonne National Laboratorys Experimental Breeder Reactor II. The 1980s spawned renewed interest in metallic fuel when the Integral Fast Reactor (IFR) concept emerged at Argonne National Laboratory. A fuel performance demonstration program was put into place to obtain the data needed for the eventual licensing of metallic fuel. This paper will summarize the results of the irradiation program carried out since 1985.

In-reactor precipitation and ferritic transformation in neutron-irradiated stainless steels

Abstract Ferritic transformation (γ → α) was observed in type 304L, 20% cold-worked AISI 316, and solution-annealed AISI 316 stainless steels when subjected to fast neutron irradiation. Each material demonstrated an increasing propensity for transformation with increasing irradiation temperature between 400 and 550° C. Irradiation-induced segregation of Ni solute to precipitates was found not to be a controlling factor in the transformation kinetics in 304L. Similar compositiondata from 316 materials demonstrates a much greater temperature dependence of matrix Ni depletion by precipitation reactions during neutron irradiation. The 316 data establishes a strong link between such depletion and the observed γ → α transformation. Moreover, the lack of correlation between precipitate-related Ni depletion and the γ → α transformation in 304L can be related to the fact that irradiation-induced voids nucleate very quickly in 304L steel during irradiation. These voids present competing sites for Ni segregation through a defect drag mechanism, and hence Ni segregates to voids rather than to precipitates, as evidenced by observed stable γ shells around voids in areas of complete transformation.

Ferrite formation in neutron-irradiated type 304L stainless steel

Abstract Austenitic (γ) to ferrite (α) transformation was observed using transmission electron microscopy in type 304L stainless steel that had been irradiated at ~500°C to fast-neutron ( E > 0.1 MeV ) fluences greater than ~ 3 × 10 22 n / cm 2 . Previous studies on similar unirradiated stainless steels found no such transformation, indicating that the γ → α transformation was irradiation-induced. The α phase appeared to nucleate on stacking faults, indicating that the presence of large Frank loops was the critical step in the transformation. After an entire grain of austenite had transformed, the only remaining γ phase existed as shells around voids. Coincidence of rapid swelling behavior with γ → α transformation indicated that the two were related, perhaps by reaction of both phenomena to the effects of irradiation and temperature on microchemical segregation. A volume expansion of about 2.5% was found to be associated with the transformation. Inferences are drawn relating this behavior in type 304L steel to the effects of radiation on other reactor structural materials such as type 316 stainless steel, which is also a metastable austenitic composition.

Irradiation creep and swelling of AISI 316 to exposures of 130 dpa at 385–400°C

The creep and swelling of AISI 316 stainless steel have been studied at 385 to 400/sup 0/C in EBR-II to doses of 130 dpa. Most creep capsules were operated at constant stress and temperature but mid-life changes in these variable were also made. This paper concentrates on the behavior of the 20% cold-worked condition but five other conditions were also studied. Swelling at less than or equal to00/sup 0/C was found to lose the sensitivity to stress exhibited at higher temperatures while the creep rate was found to retain linear dependencies on both stress and swelling rate. The creep coefficients extracted at 400/sup 0/C agree with those found in other experiments conducted at higher temperatures. In the temperature range of less than or equal to400/sup 0/C, swelling is in the recombination-dominated regime and the swelling rate falls strongly away from the approx.1%/dpa rate observed at higher temperatures. These lower rates of creep and swelling, coupled with the attainment of high damage levels without failure, encourage the use of AISI 316 in the construction of water-cooled fusion first walls operating at temperatures below 400/sup 0/C. 23 refs., 8 figs.

Irradiation creep and swelling of annealed Type 304L stainless steel at ~ 390°C and high neutron fluence☆

Abstract The irradiation-induced creep and swelling of annealed AISI 304L in EBR-II at ~390°C have been investigated to exposures of ~ 80 dpa and compared with the behavior of AISI 316 and PCA stainless steels. While swelling depends on stress, displacement rate, composition and cold-work level, the creep rate directly depends only on the stress level and the instantaneous swelling rate. The creep-swelling coupling coefficient of austenitic alloys at ~ 400°C does not appear to be very sensitive to composition or cold work level.

Irradiation creep and embrittlement behavior of AISI 316 stainless steel at very high neutron fluences

Abstract The irradiation-induced creep and swelling of AISI 316 stainless steel have been investigated at two temperatures (400 and 550°C) to very high neutron fluences. It is shown that creep and swelling can be considered as interactive phenomena with several stages of creep related to the total amount of accumulated swelling. The final stage involves the apparent cessation of creep and has been observed only at the higher irradiation temperature. The development of a coincident and severe ex-reactor embrittlement problem after irradiation at 400° C appears also to be separately related to the development of substantial swelling. This latter phenomenom was not observed at 550° C. The mechanisms thought to be possibly responsible for each of these two phenomena are discussed in detail.

A third stage of irradiation creep involving its cessation at high neutron exposures

Abstract As swelling approaches 5–10% in AISI 316, the creep rate appears to rapidly decline and eventually vanish. There may be some correlation between this phenomenon and concurrent changes in failure mode that also appear to be related to void swelling. For some fusion-relevant applications, creep correlations derived from fast reactor data may lead to an overprediction of the creep strain.

Experience with advanced driver fuels in EBR-II☆

Abstract Several metallic fuel element designs have been tested and used as driver fuel in Experimental Breeder Reactor II (EBR-II). The most recent advanced designs have all performed acceptably in EBR-II and can provide reliable performance to high burnups. Fuel elements tested have included use of U-10Zr metallic fuel with either D9, 316 or HT9 stainless steel cladding; the D9 and 316-clad designs have been used as standard driver fuel. Experimental data indicate that fuel performance characteristics are very similar for the various designs tested. Cladding materials can be selected that optimize performance based on reactor design and operational goals.

Direct evidence for stress-enhanced swelling in type 316 stainless steel☆

Abstract Immersion density analysis of sections of Type 316 stainless steel tubes from He-pressurized in-reactor creep experiments has unequivocally demonstrated an enhancement of void swelling by an applied stress. In addition, the study has shown that in most cases of irradiation temperature and applied stress level, the void sizes and number densities are unaffected by stress at a given magnitude of swelling. Swelling versus irradiation dose relationships and microscopic examinations imply that the stress effect is to initiate swelling at a lower dose, while not affecting the nature of the mechanisms causing swelling.

Nuclear fuel considerations for the 21st century

Abstract There are many external influences that may control the path that nuclear power deployment follows. In the next 50 years several events may unfold. Fear of the consequences of the greenhouse effect may produce a carbon tax that would make nuclear power economically superior very quickly. This, in turn, would increase the rate at which uranium reserves diminish due to the increased rate of nuclear power deployment. However, breakthroughs in the extraction of uranium from the sea or deployment of fast breeder reactors would greatly extend the uranium reserves and, as well, utilize the thorium cycle. On the other hand, carbon sequestering technology breakthroughs could keep fossil fuels dominant for the remainder of the century. Nuclear power may only then continue, as today, in a lesser role or even diminish. Fusion power or new developments in solar power could completely displace nuclear power as we know it today. Even more difficult to predict is when the demand for mobile fuel for transportation will develop such that hydrogen and hydrogen rich fuel cells will be in common use. When this happens, nuclear power may be the energy source of choice to produce this fuel from water or methane. In a similar vein, the demand for potable and irrigation water may be another driver for the advent of increased deployment of nuclear power. With all these possibilities of events that could happen it appears impossible to predict with any certainty which path nuclear power deployment may take. However, it is necessary to define a strategy that is flexible enough to insure that when a technology is needed, it is ready to be deployed. For the next few decades there will be an evolutionary improvement in the performance of uranium oxide and mixed uranium oxide-plutonium oxide (MOX) LWR fuels. These improvements will be market driven to keep the cost of fuel and the resulting cost of nuclear power electricity as competitive as possible. The development of fuels for accelerator transmutation and for reactor transmutation with inert matrix fuels is in its infancy. A great deal of research has been initiated in a number of countries, which has been summarized in recent conferences. In Europe the work on these fuels is directed at the same problem as their utilization of MOX; namely to reduce the inventory of separated plutonium, minor actinides, and Long Lived Fission Products (LLFP). In the United States there is no reprocessing and thus no inventory of separated civilian plutonium. However, in the United States there is a resistance to a permanent spent fuel repository and thus accelerator transmutation presents a possible alternative. If nuclear power does have a long-term future, then the introduction of the fast reactor is inevitable. Included in the mission of the fast reactors would be the elimination of the inventory of separated plutonium while generating useful energy. The work that is ongoing now on the development of fuel concepts for assemblies that contain actinides and LLFP would be useful for fast reactor transmutation. There is still a great deal of work required to bring the fast breeder reactor option to maturity. Fortunately there is perhaps a fifty-year period to accomplish this work before fast breeders are necessary. With regard to fast reactor fuel development, future work should be considered in three stages. First, all the information obtained over the past forty years of fast reactor fuel development should be completely documented in a manner that future generations can readily retrieve and utilize the information. Fast reactor development came to such an abrupt halt world-wide that a great deal of information is in danger of being lost because most of the researchers and facilities are rapidly disappearing. Secondly, for all of the existing fast reactor fuels, and this includes, oxides, carbides, nitrides, and metallic fuels, the evolutionary work was far from being completed. Although mixed oxide fuels were probably the furthest advanced, there were many concepts for improved claddings and advanced fabrication methods that were never fully explored. Finally, with such an extended period before fast reactors are needed there is ample time for truly innovative fuels to be developed that are capable of performing over a wide range of conditions and coolants.