Sodium-cooled fast neutron reactors

VI. An Outline of Potential Hazards to the Population from Accidents to Sodium-Cooled Fast Neutron Reactors. 

VI. AN OUTLINE OF POTENTIAL HAZARDS TO THE POPULATION FROM ACCIDENTS TO SODIUM-COOLED FAST NEUTRON REACTORS... 

Figure 7.2 Calculated neutron spectrum of the reference MSFR green curve). For comparison, a typical sodium-cooled fast neutron reactor spectrum in red) and a typical PWR thermal spectrum in blue) are shown. MSFR, molten salt fast reactor SFR, sodium-cooled fast neutron reactor PWR, pressurized water reactor.

The choice of cladding material for fast reactors is less dependent upon the neutron absorption cross section than for thermal reactors. The essential requirements for these materials are high melting point, retention of satisfactory physical and mechanical properties, a low swelling rate when irradiated by large fluences of fast neutrons, and good corrosion resistance, especially to molten sodium. At present, stainless steel is the preferred fuel cladding material for sodium-cooled fast breeder reactors (LMFBRs). For such reactors, the capture cross section is not as important as for thermal neutron reactors. 

The SFR is a sodium-cooled fast-neutron-spectrum reactor designed primarily for the efficient management of actinides and conversion of fertile uranium in a closed fuel cycle. 

Operating with a sodium-cooled fast neutron core, ASTRID is expected to produce approximately 600 MW of electricity. Before the construction of a first-off commercial unit, a demonstration facility is needed to test innovations with respect to previous FNRs. The FNR greatly improves the amount of energy derived from depleted or reprocessed natural uranium, enables plutonium to be used and recycled several times and can recycle minor actinides if needed. Such reactors are currently being built or are on the drawing board in India, Russia, China, and Japan. 

Figure 2.5 SFR Molten sodium-cooled, fast neutron spectrum reactor with closed fuel cycle and outlet temperatures within 500—550°C (shown pool-type reactor with indirect steam turbine Rankine power cycle). 

Fig. 1.56 Comparison of neutron spectra of the Super FR, a LWR and a sodium cooled fast breeder reactor...

Properties. Most of the alloys developed to date were intended for service as fuel cladding and other stmctural components in hquid-metal-cooled fast-breeder reactors. AHoy selection was based primarily on the following criteria corrosion resistance in Hquid metals, including lithium, sodium, and NaK, and a mixture of sodium and potassium strength ductihty, including fabricabihty and neutron considerations, including low absorption of fast neutrons as well as irradiation embrittlement and dimensional-variation effects. Alloys of greatest interest include V 80, Cr 15, Ti 5... 

In fast (neutron) reactors, the fission chain reaction is sustained by fast neutrons, unlike in thermal reactors. Thus, fast reactors require fuel that is relatively rich in fissile material highly enriched uranium (> 20%) or plutonium. As fast neutrons are desired, there is also the need to eliminate neutron moderators hence, certain liquid metals, such as sodium, are used for cooling instead of water. Fast reactors more deliberately use the 238U as well as the fissile 235U isotope used in most reactors. If designed to produce more plutonium than they consume, they are called fast-breeder reactors if they are net consumers of plutonium, they are called burners . 

Large amounts of sodium waste arise from fast neutron reactors (Phenix and Superphenix in France, Dounreay in the UK, Monju in Japan), which are cooled by large amounts of liquid sodium, which is contaminated by 137Cs during its functioning. We shall see that it is possible to remove radioactive cesium after conversion of liquid sodium to sodium hydroxide. 

Because the thorium atom density is higher in thorium metal than in any thorium compound, metal is the preferred form of thorium where the hipest nuclear reactivity or hipest density is wanted. One likely nuclear application is in a sodium-cooled fast reactor where thorium would capture a neutron and be converted to... 

The first power-producing reactor in the United States was a sodium-cooled fast reactor, EBR-I. Fast neutron based reactors are able to utilize the LP as well as the U as fuel. In addition, they are able to produce more fissionable plutonium than they burn. In a cover letter to a study commissioned by President Kennedy in 1962, Atomic Energy Commission Chairman Glenn T. Seaborg wrote the following ... 

The Rapsodie experimental sodium cooled reactor was the first French fast neutron reactor. The construction was started in 1962 within an association of CEA and EURATOM. The reactor went critical on 28 January 1967, reaching 20 MW (th) power on 17 March 1967. The core and equipment were modified in 1970 to increase the thermal power level to 40 MW (th). The operating parameters were similar to those in large commercial size reactors. During 16 yets of operation 30 000 fill pins of the driver core were irradiated, of which -10 000 reached a bumup beyond 10% 300 irradiation experiments and more than 1 000 tests have been performed. The maximum bumup of the test fuel pins was 27% (173 displacement per atom). In 1971, the irradiations performed in the core revealed a phenomenon of irradiation swelling in the stainless steel of the wrapper and the fuel cladding in the high neutron flux. The R sodie results have been extrapolated in the Phenix reactor. 

The MBRU-12 is a modular nuclear power plant (NPP) with sodium cooled fast reactor. The name reflects the main engineering bases of the concept a fast spectrum of neutrons, metallic (sodium) coolant, NPP assembly from factory-built equipment modules and a nominal value of electric power. 

The technical basis for the RAPID includes general experience with sodium cooled fast reactors. Specifically, the RAPID concept includes no control rods but incorporates the passive lithium expansion modules, lithium injection modules and lithium release modules to enable an operator-free operation mode. These systems utilize Li as a liquid poison instead of B4C rods. To verify the reactivity worth of Li, the criticality test [XVII-5] using the fast critical assembly (FCA) of the Japan Atomic Research Institute (JAERI) has been conducted. Also, the manufacturing technology of the lithium modules was mastered, and the performance and neutron radiography tests of the lithium expansion and lithium injection module pilots were conducted. 

In fast neutron reactors, mostly sodium-cooled (SFR), the neutron spectrum leads to a limited neutron absorber materials choice [5]. The CEA generally incorporate high-density B4C boron carbide, most often enriched to improve its efficiency, as cylindrical pellets piled in stainless steel mbes. The use of large components (size identical to fuel assembhes) leads to an improved efficiency by a self-moderating effect. 

The neutron spectrum of the Super FR is compared with those of LWRs and the sodium cooled fast reactor in Fig. 1.56. The blanket assemblies of the Super FR are equipped with zirconium hydride layer for the negative coolant void reactivity. The spectrum near the layer is similar to that of LWRs. Both fast and thermal neutron spectra are available in the Super FR. Availability of both will be suitable for the transmutation of long-lived fission products as well as minor actinides [95,96]. The improved core design for the high power density was reported [87]. 

Full advantage of the neutron production by plutonium requires a fast reactor, in which neutrons remain at high energy. Cooling is provided by a hquid metal such as molten sodium or NaK, an alloy of sodium and potassium. The need for pressurization is avoided, but special care is required to prevent leaks that might result in a fire. A commonly used terminology is Hquid-metal fast-breeder reactor (LMFBR). 

Nuclear and magneto-hydrodynamic electric power generation systems have been produced on a scale which could lead to industrial production, but to-date technical problems, mainly connected with corrosion of the containing materials, has hampered full-scale development. In the case of nuclear power, the proposed fast reactor, which uses fast neutron fission in a small nuclear fuel element, by comparison with fuel rods in thermal neutron reactors, requires a more rapid heat removal than is possible by water cooling, and a liquid sodium-potassium alloy has been used in the development of a near-industrial generator. The fuel container is a vanadium sheath with a niobium outer cladding, since this has a low fast neutron capture cross-section and a low rate of corrosion by the liquid metal coolant. The liquid metal coolant is transported from the fuel to the turbine generating the electric power in stainless steel... 

---