Thermal-neutron spectrum

Future nuclear reactors are expected to be further progressed in terms of safety and reliability, proliferation resistance and physical protection, economics, sustainability (GIF, 2002). One of the most promising nuclear reactor concepts of the next generation (Gen-IV) is the VHTR. Characteristic features are a helium-cooled, graphite-moderated thermal neutron spectrum reactor core with a reference thermal power production of 400-600 MW. Coolant outlet temperatures of 900-1 000°C or higher are ideally suited for a wide spectrum of high temperature process heat applications. 

Because of epithermal resonance absorption of neutrons in Pa, its effective cross section in a thermal-neutron spectrum is much greater than the 2200 m/s cross section listed in Table... 

The fuel pins used in the core are a new type for OCR s. The vast majority of OCR reactors use a TRISO type fuel embedded in a graphite matrix and are thermal neutron spectrum reactors. TRISO fuels are small UC spheres coated with layers of silicon carbide and pyrolitic carbon. While this results in fuels that can be used to extremely high bumup, the uranium density of the fuel is low. A coolant hole is then drilled through the blocks of fuel and these blocks are put in a prismatic array. 

LWRs (with a thermal neutron spectrum) have a safety potential that should not be underestimated and which can be further exploited. This could allow them to fulfil all expected safety requirements in the foreseeable future. Exhausting this potential by innovative means (e.g. in PIUS) Is, however, coupled to considerable economic penalties, as t implies smaller plants, eventually with lower power densities. In order to further improve the use of resources and to reduce the amount of radioactive waste, the fuel cycle should be further closed, which is only partially possible with a pure LWR-strategy. 

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. 

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... 

In thorium fuel, TJ is produced in-reactor through neutron capture in and subsequent beta-decay of h and Pa. The concentration of U in the spent fuel is about five-times higher than that of Pu in spent natural uranium UO2 fuel. This isotope of uranium is a very valuable fissile material because of the many neutrons produced per neutron absorbed (g) in a thermal neutron spectrum. 

While LWR type thermal reactors using uranium dioxide fuel of low enrichment do not experience a reactivity insertion hazard due to compaction, it is noted that some small reactor concepts with thermal neutron spectrum that are based on cermet or TRISO fuel at enrichments approaching 20% might face fuel reconfiguration reactivity addition issues, which requires further examination. 

The undeclared production of weapon-grade nuclear materials in the reactor is essentially precluded by the thermal neutron spectrum, as well as by the associated necessity to change the design configuration of the distribution of fuel, absorber and graphite elements over the core volume, which, as mentioned above, can be performed only at a specialized factory with the use of the equipment that is not available on the site ... 

Fig. 4.27 Distortion in thermal-neutron spectrum due to the presence of absorbing materials ( normalized so that each system contains the same total number of neutrons).

As it can be seen from both Fig. 1 and Table 1, water cooled SMRs are the most suitable candidates for a near-term deployment. The high temperature gas cooled reactors with thermal neutron spectrum follow them closely. Small PWR designs from Russia are based on the experience of the marine reactors and are said to be deployable within a very short term, once the financing for a necessary limited amount of the Research, Design Demonstration (RD D) becomes available. 

The AHTR reactor core physics, general core design, and fuel cycle are similar to those of the proposed General Atomics Gas-Turbine Modular Helium Reactor (GT-MHR). The low-power-density graphitemoderated core also has the long neutron lifetime, slow kinetics, and thermal neutron spectrum characteristic of the proposed GT-MHR. The molten salt (Fig. 3) flows through the reactor core to an external heat exchanger (to provide the interface for the H2 production system), dumps the heat load, and returns to the reactor core. The molten salt can be circulated by natural or forced circulation. 

The calculation of this reaction rate becomes a complex matter when the cross section of the material varies rapidly with energy over the region of the thermal neutron spectrum. Fortunately, as mentioned in Section 1.7, most materials of interest have cross sections which have a 1/v dependence, or an approximate 1/v dependence, in this region. The absorption cross section of a 1/v absorber may be written as... 

Assuming that a thermal neutron spectrum is given by equation (3.15), show that the average neutron velocity is equal to (SkT/nmY. ... 

Figure 2.3 VHTR Helium gas cooled, graphite-moderated, thermal neutron spectrum reactor with core outlet temperature 900—1000°C (shown with hydrogen cogeneration).

Figure 2.8 SCWR Supercritical water-cooled, thermal neutron spectrum reactor with outlet temperatures within 510—625°C (shown with direct steam turbine Rankine power cycle). Courtesy of Generation IV International Forum.

The pressure-vessel type of SCWR may use UO2 in a once-through fuel cycle, with an enrichment of 5—7%, or mixed oxide (MOX) fuel if plutonium should be recycled in a closed fuel cycle. In the case of a thermal neutron spectrum, the use of MOX fuel is optional as in a conventional PWR or BWR. However, because the higher... 

Beyond 390°C, the coolant density is less than 200 kg/m, hardly enough to produce a thermal neutron spectrum. Therefore a moderator is needed for a thermal neutron spectrum, either as feed water running through moderator boxes inside of the fuel assemblies and in gaps between assembly boxes or as separate heavy water in case of a pressure tube concept. In any case the mass of structural material inside of the reactor core should be minimized to limit neutron absorption. 

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... 

Figure A1.18 Simplified iayout of typicai BWR NPP (courtesy of US NRC) geneiai basic features (1) thermal neutron spectrum (2) UO2 fuel (3) fuel enrichment about 3% (4) direct cycie with steam separator (steam generator and pressurizer are eiiminated), ie, singie-flow circuit (singie loop) (5) RPV with verticai fuel rods (elements) assembled in bundle strings cooled with upward flow of fight water (water and water—steam mixture) (6) reactor coolant, moderator, and power cycle working fluid are the same fluid (7) reactor cooiant outlet parameters pressure about 7 MPa and samration temperature at this pressure is about 286°C and (8) power cycle subcritical-pressure regenerative Rankine steam turbine cycie with steam reheat.

PWR NPP (Generation III+, to be implemented within next 1—10 years) thermal neutron spectrum moderator and reactor coolant—H2O P = 15.5 MPa (P at = 345°C) and Pout = 327° C indirect cycle (double loop, ie, water—water/saturated steam) Rankine power cycle with single steam reheat—primary steam (turbine inlet) Pin = 7.8 MPa Pin/sat = 293°C Up to 38... 

ABWR NPP (Generation ni+) thermal neutron spectrum direct cycle (single loop) moderator/reactor coolant/ working fluid in Rankine power cycle with single steam reheat—H2O primary steam (turbine inlet) Pin = 6.97 MPa Pin/sat = 286°C secondary steam Pin = 1.7 MPa (Paat = 204°C) Pin = 259°C Up to 34... 

VHTR NPP thermal neutron spectrum moderator graphite reactor coolant—He P — 1 MPa and = 640°C and Tout = 1000°C primary power cycle—direct Brayton gas turbine cycle possible backup—indirect Rankine steam cycle >55... 

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