How is high temperature reactor design different?

[Topher925]

I think its common knowledge that the next generation of nuclear reactors are going to operate at much higher temperatures than current designs in order to get away from the Rankin cycle and take advantage of thermochemical cycles, but what is it that makes high temperature reactors (HTR) different than low temperature reactors? Does it mostly only have to do with the materials used to construct the reactor or are different fuels and control rod materials used?

 

[Astronuc]

The working fluid temperatures in high temperature reactors are higher than those in conventional LWRs, and the materials are necessarily different.

Let's consider LWRs.

PWRs (pressurized water reactors) operate with a primary circuit temperature of about 285-293°C inlet and about 325-330°C outlet at a nominal pressure of ~2270 psia. The fuel is subject to a number of engineering constraints under normal and off-normal conditions (NUREG-0800, Chapter 4.2). Basically the fuel is not allowed to melt (no centerline melting), must have less than 1% strain, and must not allow cladding lift-off in the event of design basis transients.

BWRs operate with the coolant at saturation conditions with a nominal core pressure of about 1045-1055 psia.

In LWRs the cladding material is an alloy of Zr and usually Zr-Sn or Zr-Nb with other alloying elements, e.g. Fe, Cr, Ni and others in lesser quantities. The fuel is UO₂ or (U, Pu)O₂, with additions of gadolinia, erbia or ZrB₂ for reactivity control.

High temperature plants can use He (gas), Na or Pb (Pb-Bi) (liquid metal), molten salt or H₂O. With respect to H₂O, there is consideration of a supercritical water reactor which would have an operating conditions exceeding (374°C, 22 MPa, or 705°F, 3208 psia). Fuel materials could be cermet or metal fuel or UN, UC or (U,Pu)N or (U,Pu)C or possibly silicide.

See the concept here - http://nuclear.inl.gov/gen4/index.shtml

The operating environments of the Gen IV concepts are very challenging for the materials and engineering.

The idea is however to increase the temperature in order to increase the efficiency. In addition to producing electrical power, heat would also be used for chemical processes, e.g. hydrodgen production using the S-I process.

 

[Topher925]

So, high temperature reactors do in fact use different U-based fuels than the typically low temperature reactors? And really what it all comes down to then is just finding the right materials that can handle those temperatures then right?

In the case of an S-I or Cu-Cl cycle, would any of these chemicals ever make direct contact with the reactor itself or would a large heat exchanger be involved?

 

[Astronuc]

So, high temperature reactors do in fact use different U-based fuels than the typically low temperature reactors? And really what it all comes down to then is just finding the right materials that can handle those temperatures then right?

The high temperature reactors do use U-235 (or Pu-239), and perhaps U-233 if a thorium cycle is employed, and these fuels would likely be in a different form than UO₂, and would be clad in different alloys rather than the conventional Zr-alloys now employed in LWRs. And it is most definitely a matter of finding the right materials to resist degradation in the radiation environment at those temperatures and in contact with any working fluid that is not chemically inert, i.e. not He.

In the case of an S-I or Cu-Cl cycle, would any of these chemicals ever make direct contact with the reactor itself or would a large heat exchanger be involved?

My understanding is that there would be a heat exchanger involved and the chemical reactor would be located ex-core.