AHTR reactor vessel

Table 5.1. Coated F-M or stainless steels, or monolithic alloys potentially snitable for AHTR reactor vessel needs... 

Coated F-M or Stainless Steels or Monolithic Alloys Will Likely Meet AHTR Reactor Vessel Needs... 

To meet the passive safety requirements of the NGNP, the AHTR uses a reactor vessel auxiliary cooling system (RVACS) similar to that of S-PRISM. It may also use a direct reactor auxiliary cooling system (DRAGS) similar to what was used in the Experimental Breeder Reactor II to supplement the RVACS and reduce the reactor vessel temperature. 

Earlier, a sealing analysis for the passive decay heat cooling system suggested that the AHTR could operate at a thermal power of 2400 MW(t). A more sophisticated analysis was performed that indicates that 2400 MW(t) can indeed be achieved with reasonable RVACS capacity. The analysis showed that for a loss-of-forced-cooling accident (with scram), significant natural convection of the molten salt is established and the eore temperature peaks at only 1160°C, which occurs about 30 hours after the accident. The reactor vessel temperature peaks at 750°C after about 40 hours. This analysis, which did not include a DRAGS, indieates that a 2400 MW(t) AHTR ean easily survive this type of transient. 

The AHTR appears to have excellent safety attributes. The combined thermal capacity of the graphite core and the molten salt coolant pool offer a large time buffer to reactor transients. The effective transfer of heat to the reactor vessel increases the effectiveness of the RVACS and DRAGS to remove decay heat, and the excellent fission product retention characteristic of molten salt provides an extra barrier to radioactive releases. The low-pressure, chemically nonreactive coolant also greatly reduces the potential for overpressurization of the reactor containment building and provides an important additional barrier for fission product release. The most important design and safety issue with the AHTR may be the performance and reliability of the thermal blanket system, which must maintain the vessel within an acceptable temperature range. 

If the AHTR-VT reactor vessel is built with the same dimensions as the 9.2-m-diam, 19.5-m-high S-PRISM reactor vessel, then the TBS will have an outside diameter of 8 m and a total surface area (including the bottom) of 500 m. For a t5 ical graphite thermal conductivity of 32 W/m°C, 0.64-m-thick graphite blocks would transfer 15 MW(t) at a TBS temperature difference of 600°C. At this temperature difference, a 6 1/s leakage flow through the blanket would also transfer 15 MW(t) across the blanket. 

While the potential for highly effective RVACS heat removal exists for the AHTR, detailed design will be required to optimize and maximize its heat removal capability. There are multiple RVACS cooling options, including operating the reactor vessel at lower temperatures than the molten salt coolant. Scaling of both the thermal inertia and RVACS heat removal capability of the AHTR suggest that reactor thermal... 

Reactor vessel heat up. After loss of decay heat cooling, the initial event is heat up of the reactor vessel. The AHTR thermal inertia per megawatt thermal in the reactor vessel exceeds that of the MHTGR that is, the peak fuel temperatures increase at a slower rate after loss of all cooling. 

This slower increase occurs despite the fact that the AHTR vessel volume [2400 MW(t), 9.2-m diam, 1260 m ] is almost identical to that of the MHTGR [600 MW(t), 8. 4-m diam, 1210 m ] and reflects the more efficient use of the thermal inertia of materials within the reactor vessel. 

The potential cost of the AHTR is estimated based upon cost information for the S-PRISM and the GT-MHR. The reference AHTR design produces 2400 MW(t) from a reactor vessel with the same diameter as the 1000 MW(t) S-PRISM and slightly larger diameter than the 600 MW(t) GT-MHR. 

Reactor vessel and building The 2400 MW(t) AHTR reactor building and vessel are assumed to have the same size as a single S-PRISM reactor vessel and building. The most important assumption for this cost estimate is that an AHTR can produce 2400 MW(t) with an S-PRISM size vessel and building, and deliver it to heat helium to temperatures between 750 and 900°C. 

Reactor vessel. The higher density of molten salt creates larger hydrostatic loads than sodium. While the AHTR will have smaller loads from reactor internals, because ihe fuel will be close to neutrally buoyant, it is anticipated that the AHTR will still require a thicker vessel. Thus the cost of the AHTR vessel cost is assumed to be equal to the cost of two S-PRISM vessels to account for a 10-cm-thick wall vs the 5-cm-thick S-PRISM vessel. 

Power level Preliminary analyses indicate that 2400 MW(t) is an achievable power level for an AHTR while maintaining the capability for passive safety. Additional evaluations are needed to establish the optimum power level for the AHTR considering core size, passive safety, and size of the reactor vessel. Because the economics are strongly tied to the reactor size, and the reactor size is tied to the passive decay heat cooling system, detailed design and heat transfer optimizations are needed to maximize the potential for passive safety performance. 

Thermal blanket system test facility (engineering). The AHTR requires a thermal blanket system to insulate the reactor vessel from the elevated core and coolant salt. Appropriate high-temperature tests for a variety of transient conditions are required to test alternative insulation... 

The AHTR facility layout (Fig. 2) is similar to that for the S-PRISM sodiiun-cooled fast reactor designed by General Electric. Both reactors operate at low pressure and high temperature thus, they have similar design constraints. The 9.2-m diameter vessel of the AHTR is the same size as that used by the S-PRISM. Earlier engineering studies indicated that this was die largest practical size of low-pressure reactor vessel. The vessel size determines the power output. For our initial studies, we assumed fuel and power densities (8.3 W/cm ) to be similar to those of MHTGRs. 

AHTR design basis. As a starting point, the S-PRISM facility design was used as a basis for the AHTR. This included using the same size reactor vessel. Because the AHTR is also a low-pressure liquid-cooled reactor, it is a reasonable starting assumption to assume the same fundamental limitations in facility and vessel design. 

The intermediate heat exchangers and SNF storage are removed from the reactor vessel to provide space for the larger AHTR core. The intermediate heat exchangers are moved to the compartment that in S-PRISM contains the sodium-water heat exchangers. 

SNF storage. The AHTR uses separate SNF storage, it does not use the reactor vessel for SNF storage. 

Decay heat removal. Several types of passive decay heat removal systems have been used in liquid-metal reactors. The AHTR, like S-PRISM, uses RVACs. There are other options such as DRACs, a secondary natural circulation loop to remove heat from the reactor vessel to the environment. This provides multiple longer-term cooling options including the options that may ultimately allow larger power outputs. 

Vessel volume. The larger core (see earlier) is possible because S-PRISM includes SNF storage and the intermediate heat exchangers in the reactor vessel while these are moved out of the vessel in the AHTR design. Equipment size. Pipes, pumps, and valves are similar in size because the volumetric heat capacity of molten salts is several times greater than sodium. Volumetric heat capacity sizes much of the equipment. 

Regarding the control of accidents within the design basis (DID Level 3 in Table 4), the design of AHTR incorporates a mechanical reactivity control and shutdown system based on control rods with the external drives, and two diverse decay heat removal systems, of which one is passive and one is active. The reference AHTR design uses passive reactor vessel auxiliary cooling (RVAC) systems similar to that developed for decay heat removal in the General Electric sodium cooled S-PRISM reactor. Different from its prototype, the RVAC system of the AHTR relies not only on the processes of convection and conduction but on the radiation also. 

Three peak coolant temperatures were evaluated 705, 800, and 1000°C, for the AHTR — Low Temperature (AHTR-LT), the AHTR — Intermediate Temperature (AHTR-IT), and the AHTR— High Temperature (AHTR-HT), respectively. The respective thermal-to-electric efficiencies are 48.0, 51.5, and 56.5%. The AHTR-LT uses existing code qualified materials, the AHTR-rr uses existing materials that have not been fully tested, and the AHTR-HT uses advanced materials. The AHTR-LT has a metallic blanket system that separates and insulates the reactor vessel from the reactor core so that the fuel and coolant can operate at higher... 

The AHTR has two coupled physical characteristics that may potentially enable the development of a low cost medium sized AHTR using factory assembled modular units with ease of transport. The low pressure liquid cooling for a medium sized reactor implies small reactor vessel and heat exchanger sizes relative to those for gas cooled reactors. The high temperature allows the use of Brayton power cycles. Brayton turbines have much higher power densities than steam turbines and are consequently much smaller in size per unit output. Brayton cycle turbines are typically manufactured and shipped as modular units and have lower costs than traditional steam cycles per unit output. These options have not yet been investigated. 

In terms of passive decay heat removal systems, a major difference is noted between the liquid cooled AHTR and gas cooled reactors. The AHTR can be built in very large sizes (>2400 MW(th)), while the maximum size of a gas cooled reactor with passive decay heat removal systems is limited to -600 MW(th). The controlling factor in decay heat removal is the ability to transport this heat from the center of the reactor core to the vessel wall or to a heat exchanger in the reactor vessel. The AHTR uses a liquid coolant, where natural circulation can move very large quantities of decay heat from the hottest fuel to the vessel wall with a small coolant temperature difference ( 50°C). Unfortunately, under accident conditions when a gas cooled reactor is depressurized, the natural circulation of gases is not efficient in transporting heat from the fuel in the center of the reactor to the reactor vessel. The heat must be conducted through the reactor fuel to the vessel wall. This inefficient heat transport process limits the size of the reactor to -600 MW(th) to ensure that the fuel in the hottest location in the reactor core does not overheat and fail under accident conditions. 

The AHTR facility design is similar to the General Electric S-PRISM thus, the AHTR uses the same seismic safety strategy. The nuclear island (Fig. XXVI-2) is a seismically isolated platform with seismic isolation pads to separate it from large ground accelerations during an earthquake. The reactor vessel is hung from the nuclear island platform. 

The AHTR has the potential for extraordinary capabilities to ensure protection of the public against severe accidents or sabotage. The combination of a high temperature fuel, the liquid salt coolant and an underground silo facility design implies that failure of the reactor vessel and other containment structures will not result in significant release of radionuclides to the... 

---