Fast-neutron spectrum reactor

The international group has identified six Generation rV reactor systems for development. All of these reactors should be ready for deployment by 2030. The fast neutron spectrum reactors can use the fuel values of all of the fissile and fertile transuranic isotopes in reprocessed fuel. This does not occur in the current thermal spectrum reactors. Producing energy... 

Stainless-steel-clad UOj-PuO fuel elements have been assumed for the reference design of the gas-cooled fast reactor cores, although comparative evaluations of stainless-steel-clad carbide fuel elements have been made 14). In either case, the type of fuel element envisioned for the gas-cooled fast reactors is an assembly of approximately 1-cm-diam, stainless-steel-clad fuel rods. The subdivision of the fuel must be very great in order to achieve the high core power densities and fuel ratings required for reasonable performance with a fast-neutron-spectrum reactor. 

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. 

The LFR is a fast-neutron spectrum reactor cooled by molten lead or a lead-bismuth eutectic liquid metal. It is designed for the efficient conversion of fertile uranium and the management of actinides in a closed fuel cycle. 

In the case of fast neutron spectrum reactors, the efficacy of inherent and passive safety features to anticipated transients without scram (ATWS) has been well established by testing in the EBR-II, FFTF, and Phoenix reactors [5 and 6], So too has passive decay heat removal. The severe accident hazard of reactivity addition upon core rearrangement in small fast reactors is addressed by a range of approaches including ... 

The specific power, (kW/kg of TRU fuel), of long refuelling interval battery plants has been reduced by a factor of 5 or 6 relative to traditional fast neutron spectrum reactors. This allows for a 5 or 6 times increase in fuel residence time per refuelling interval (from 3 years to 15 to 20 years) within a given maximum achievable fuel discharge bum-up capability ... 

Figure 2.4 GFR Helium gas-cooled, fast neutron spectrum reactor with closed fuel cycle and outlet temperature of about 850°C (shown with direct gas turbine Brayton power cycle). Courtesy of Generation IV International Forum.

In line with the priority put on fast neutron spectrum reactors, the ESNII is supporting the design and construction of four demonstrators ... 

In a nuclear fuel, the fission chain reaction is maintained by fission of fissile elements, which are capable of sustaining the fission reaction with neutrons of all energy. As such, fissile nuclides are used in the fuel of both thermal neutron spectrum and fast neutron spectrum reactors. The fissile nuclides of importance for nuclear reactors are 233u 235 j 239p Amoug these fissile nuclides, only is a naturally... 

The atom density of uranium is another important factor, especially in fast neutron spectrum reactors because fission probability is significantly lower for fast neutrons compared to those of thermal neutrons. Both UN and UC have high uranium atom density, approximately 1.40 and 1.34 times that of U02- Hence, use of UC or UN leads to smaller core sizes compared to that of UO2 fuel. 

It is obvious that the neutron energy spectrum of a reactor plays an essential role. Figure 19.4 shows the prompt (unmoderated) fission neutron spectrum with 2 MeV. In a nuclear explosive device almost all fission is caused by fast neutrons. Nuclear reactors can be designed so that fission mainly occurs with fast neutrons or with slow neutrons (by moderating the neutrons to thermal energies before they encounter fuel). This leads to two different reactor concepts - the fast reactor and the thermal reactor. The approximate neutron spectra for both reactor types are shown in Figure 19.4. Because thermal reactors are more important at present, we discuss this type of reactors first. 

The guidance given in this publication is applicable to research reactors with limited hazard potential to the public and typical characteristics. For addressing the topic in research reactors with several tens of megawatts of power, fast neutron spectrum research reactors or small prototype power reactors, etc., other similar IAEA publications prepared for power reactors may be more appropriate for a number of aspects (see References). No specifications for such a transition to other guidance are presented. 

The supercritical-water-cooled reactor (SCWR) ( Fig. 58.21) system features two fuel cycle options the first is an open cycle with a thermal neutron spectrum reactor the second is a closed cycle with a fast-neutron spectmm reactor and full actinide recycle. Both options use a high-temperature, high-pressure, water-cooled reactor that operates above the thermodynamic critical point of water (22.1 MPa, 374°C) to achieve a thermal efficiency approaching 44%. The fuel cycle for the thermal option is a once-through uranium cycle. The fast-spectrum option uses central fuel cycle facilities based on advanced aqueous processing for actinide recycle. The fast-spectrum option depends upon the materials R D success to support a fast-spectrum reactor. 

Conventional LWRs alone cannot be used to transmute minor actinides because thermal neutrons are not as effective for inducing the fission reaction. As a result, nfinor actinides (especially, non-fissile even isotopes of plutonium) build up as a function of time. Therefore, thermal reactors tend to preferentially produce minor actinides. Fast reactors or fast neutron spectrum devices on the other hand tend to more effectively destroy the minor actinides because the probability of fission for both the even and odd isotopes of plutonium fission with fast neutrons is considerably higher than with thermal neutrons. Thus, last reactors are heavily preferred for the recycle of plutonium and the ultimate complete destruction of all of the minor actinides. 

For the AFC to be most effective and reduce the inventory of minor actinides at a reasonable rate, dedicated devices that produce a hard or fast neutron spectrum will be required. Such devices include the advanced liquid metal reactors (ALMRs), a fast reactor configured to operate as an actinide incinerator rather than breeder and accelerator-driven systems (ADSs). Fast reactor technology discussed earlier in this chapter is relatively mature whereas the development of ADS is in its infancy. Accelerator-based waste transmutation programs are ongoing in France, Japan, USA, and CERN. 

A fast neutron spectrum allows production of more fissile material than that consumed for heat generation. In a fast reactor liquid metal such as sodium is normally used to remove the heat, and it has a minimum effect on the moderation of fission neutrons. Sodium as a coolant has an excellent heat capacity, low operating pressure and natural convection capability. 

The balance-of-plant design (Figure 9.3) utilizes a relatively simple direct cycle power conversion system. The reference design for this concept is a 1700-MWe reactor operating at a pressure of 25 MPa with a reactor outlet temperature between 510°C and 550 C. This reactor can be designed as a fast neutron spectrum or thermal neutron spectrum reactor. The relatively simple design also allows for the incorporation of passive safety features. However, unlike the previously discussed concepts, the lower reactor outlet temperature... 

The gas-cooled reactor (GCR) has a gas as the primary coolant, usually helium. With a fast-neutron spectrum, the GCR has the ability to breed fertile uranium and to consume actinides. 

Of all small reactors without on-site refuelling in this report, about half are liquid metal cooled reactors with fast neutron spectrum they include the following concepts (see Table 5) ... 

The higher specific in-core fissile inventory of small reactors with fast neutron spectrum may help ensure that not only the LWR legacy inventories already in temporary storage are worked down, but even uranium dioxide spent fuel from current and future LWR discharges is used up. As a result, minor actinide free HLW forms, which can be more tightly packed because of smaller heat loading, could be accommodated in less repository space. 

RAPID (Refuelling by All Pins, Integral Design) is the abbreviation for a small sodium cooled reactor of 10 000 kW(th) (1000 kW(e)) with U-Pu-Zr metal fuel and fast neutron spectrum [XVII-1 and XVII-2]. It is one of the successors of the RAPID-L [XVII-3 to XVII-7] - the operator-free fast reactor concept designed for a lunar based power system. 

The strong coolant temperature-driven reactivity feedback in the fast neutron spectrum core enables autonomous load following whereby the reactor power self-adjusts itself to match heat removal from the primary coolant solely as a consequence of inherent physical phenomena. The system temperatures that are attained following an autonomous power change from the nominal steady state can be optimized through design of the core clamping... 

The strong reactivity feedback from the fast neutron spectrum core with transuranic nitride fuel and lead coolant results in passive core power reduction to decay heat power levels while system temperatures remain within structural limits, in the event of loss-of-normal heat removal to the secondary side through the in-reactor lead-to-C02 heat exchangers. 

The fast neutron spectrum with transuranic nitride fuel and lead coolant is fissile self sufficient with a core conversion ratio of unity. This enables a closed fuel cycle based upon a fertile feed stream of depleted or natural uranium and a minimal volume waste stream comprised only of fission products. All fissile material including minor actinides is recycled in the fabrication of new fuel cores and is burned as fuel in STAR reactors. 

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