Good introductory document: "Gas Reactor (VHTR) Technology" by Dr. David Petti

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&cad=rja&uact=8&ved=0CEgQFjAD&url=https%3A%2F%2Finlportal.inl.gov%2Fportal%2Fserver.pt%2Fgateway%2FPTARGS_0_1646_66331_0_0_18%2F10%2520VHTR%2520fundamentals%2520for%2520science%2520teachers.pdf&ei=1UMpU4DNKKPy4QSCwoCQBA&usg=AFQjCNEH100NCwj72ID0b4kqaoDlwbr2pQ

It will be the first truly new reactor design to go into commercial service in the U.S. in decades; it is to be up and running by the mid 2020s, depending on the available investment. The high-temperature reference is to the reactor’s outlet temperature, about 750-925 °C, or very roughly three times higher than most of today’s reactors. That means HTGRs can be a source of low-carbon, high-temperature process heat for petroleum refining, biofuels production, the production of fertilizer and chemical feedstocks, and reprocessing coal into other fuels, among other uses.

A summary of my paper entitled "NEW TECHNOLOGIES ASSOCIATED TO THE CONSTRUCTION OF NUCLEAR POWER PLANTS"

The main goals for the Generation IV nuclear power reactors are the following:

• Sustainability:

• Economics:

• Safety and reliability:

• Proliferation resistance and physical protection:

According to different government sources, it is expected that Generation IV nuclear power reactors may be available for commercial application before 2050.

Future nuclear power reactors must be designed so that during normal operation or anticipated transients safety margins are adequate, accidents are prevented, and off-normal situations do not deteriorate into severe accidents. At the same time, competitiveness requires a very high level of reliability and performance. There has been a definite trend over the years to improve the safety and reliability of nuclear power reactors, particularly after the Three Miles Island, the Chernobyl and Fukushima Daiichi nuclear accidents, reduce the frequency and degree of off-site radioactive releases, and diminish the possibility of significant reactor damage.

Generation IV nuclear power reactors must ensure high levels of safety and reliability through further improvements in their designs that are safer and that can reduce the potential for severe accidents and their consequences to the environment and human health to the minimum. The achievement of these ambitious goals also requires high human performance and training as a major contributor to the plant availability, reliability, inspectability, and maintainability.

Withitn the Generation IV there is one prototype of graphite moderator reactor called Molten Salt Reactor (MSR). The MSR experiment demonstrated many features, including:

• A lithium/ beryllium fluoride salt;

• Graphite moderator;

• Stable performance;

• Off-gas systems;

• Use of different fuels, including uranium-235, uranium-233, and plutonium.

A detailed 1 000 MWe engineering conceptual design of a MSR system was developed. Many issues relating to the operation of MSRs as well as the stability of molten salt fuel and its compatibility with graphite and Hastelloy N were already resolved. Significant progress was achieved in 2009 in the development of the MSR system. This included:

• Development of MSFR pre-conceptual design and performance analysis of MSFR potential for starting with plutonium and minor actinides from PWRs wastes;

• Laboratory scale processing of Ni-W-Cr alloys was recently demonstrated. The alloys were found to have acceptable workability and very good high temperature hardness. The whole potentialities of these kinds of materials as well as Hastelloy N30 have yet to be tested and characterized over the full range of temperatures and in the presence of the fluoride salts;

• Corrosion tests of Ni-based alloys;

• Better understanding of the PuF3 solubility in various carrier salts by means of thermochemical modeling;

• The material property database for molten and liquid salts was extended through experiments and theoretical calculations. New experimental facilities were and continue to be developed;

• Significant improvement of fuel salt clean-up scheme;

• The optimal core configuration and salt composition of a moderated MSR system that maximize the power density while keeping the self-breeding capabilities were found. New breeding gain definitions were developed that account for the unique behavior of the MSR system;

• Better understanding of the transmutation capabilities, dynamics and safety-related parameters, for fertile and fertile-free fuel concepts;

• Demonstration of fluoride-cooled high-temperature reactor (FHR) performance and safety;

In a MSR system, the fuel is dissolved in a fluoride salt coolant. Prior MSR systems were mainly considered as thermal-neutron-spectrum graphite-moderated concepts. Since 2005, research and development has focused on the development of fast-spectrum MSR concepts (MSFR) combining the generic assets of fast neutron reactors (extended resource utilization and waste minimization) to those relating to molten salt fluorides as fluid fuel and coolant (favorable thermal-hydraulic properties, high boiling temperature, and optical transparency). In addition, MSFRs exhibit large negative temperature and void reactivity coefficients, a unique safety characteristic not found in solid-fuel fast reactors. MSFR systems have been recognized as a long-term alternative to solid-fuelled fast neutron systems with unique potential (negative feedback coefficients, smaller fissile inventory, easy in-service inspection and simplified fuel cycle, among others.).

Taking advantage of technology available since the 1960s, the MSR system has been designed for a plethora of uses. From commercial power plants to nuclear powered bomber aircraft, the MSR system has the advantage of low pressure operation with higher core heat transfer. This allows for a reduced reactor size with fewer pumps and pipes operating at higher efficiencies. There are two proposals for the MSR designs:

• Molten salt fueled reactors;

• Molten salt cooled reactors.

The chemical characteristics of molten salts demand constant reprocessing and purification. Fluoride salts react with water, creating hydrofluoric acid, which is incredibly corrosive. The reprocessing is advantageous in that it removes fission products, increasing the neutron economy of the reactor. The safety advantages (retention of fission products, lower risk of explosion, and less risk of departure from nucleate boiling), combined with the higher efficiencies associated with higher operating temperatures, encourages the new design proposals. The main advantages and disadvantages of the MSR system are the following:

Advantages

• Allows for small reactor size;

• Technology is researched and proven;

• Higher operating temperatures;

• Can use simple two fluid fuel processing without the “plumbing problem”;

• Very strongly negative fuel salt coefficients;

• Blanket will also have negative temperature/void coefficient as it acts as a partial reflector;

• Ease of graphite core fabrication (and replacement if necessary);

• Ease of modeling and prototyping;

• Fissile inventory of 400 kg per GWe or even lower is possible;

• Chemical retention of fission products.

Disadvantages

• High corrosion potential;

• Unknown material required for corrosion resistance.

General benefits of the MSR system are the following:

• Salts have a high boiling point and operate at low pressure;

• Fuel salt at the lowest pressure of the circuit, which is the opposite of a LWR;

• Volatile fission products continuously removed and stored, including xenon;

• Low fissile inventory;

• Very high thermal efficiency;

• Ability to use closed thorium cycle;

• Only consume 800 kg thorium per GWe/year;

• Transuranic waste production extremely low;

• Much lower long term radio-toxicity.

The main problems associated to the MSR system are the following:

• Limited graphite lifetime (four years);

• Fuel processing hindered by chemical similarity of thorium and rare earth fission products;

• Problem with temperature reactivity coefficient recently discovered;

• Possible improvements of the MSR system but at the expense of lower conversion ratios;

• Graphite pebbles as moderator: removes need for flux flattening; can go to smaller to higher power core; pyro-lytic coatings for increased safety;

• Carrier salt switch; NaF-BeF2 low cost, low melting point; NaF-ZrF4 low cost, no tritium production;

• Graphite free “tank of salt” core: retain thermal spectrum by having very low fuel concentration and let the carrier salt act as moderator (Be, Li, F).

Thank you Jorge Morales Pedraza sir.

I am very very grateful to you.

Thank you Florian Glodeanu sir

I am very very grateful to you.

More environmental friendly and can be used for coal gasification and liquifaction process?