Per Peterson: key requirements for new reactor designs, what about thorium?

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More from the reddit.com Science AMA Series with members of the UC Berkeley Department of Nuclear Engineering. Aside from “Can I still eat fish from the contaminated Pacific” the dominant questions seemed to be variations of “is thorium the answer?”. I was surprised – I thought obsessing on the LFTR concept was a niche nerdy issue. The slant no doubt illustrates that the reddit crowd doesn’t represent an Anglo cross section.

 I guess I’ll start things off. What type of reactors should we be building? I know a big deal a few years ago was made about liquid flouride thorium reactors. Is that the way of the future, or are there superior alternatives?

Prof. Per Peterson replies (emphasis mine):

I do not think that we have the basis to determine or select the best coolant or fuel type to use in future reactors. But there are some attributes which we do need to make sure are used in future reactors.

The first is to use passive safety systems, which do not require electrical power or external cooling sources to function to remove decay heat after reactors shut down, as is the case with the AP-1000 and ESBWR designs, and with all of the light water reactor SMRs now being developed in the U.S.

The benefits of passive safety go well beyond the significant reduction in the number of systems and components needed in reactors and the reduced maintenance requirements. Passive safety systems also greatly simplify the physical protection of reactors, because passive equipment does not require routine inspections the way pumps and motors do, and thus can be placed in locations that are difficult to gain access to rapidly.

The second is to further increase the use of modular fabrication and construction methods in nuclear plants, in particular to use steel-plate/concrete composite construction methods that are quite similar to those developed for modern ship construction. The AP-1000 is the most advanced design in the use of this type of modularization, and the ability to use computer aided manufacturing in the fabrication of these modules makes the manufacturing infrastructure much more flexible. In the longer term, one should be able to design a new reactor building, transfer the design to a module factory over the internet, and have the modules show up at a construction site, so the buildings are, in essence, 3-D printed.

The final attribute that will be important for new reactors will be to make them smaller, and to develop a regulatory framework and business models that work for multi-module power plants. While there will likely always be a market for large reactors, creating an ecosystem that includes customers for smaller reactors (inland locations served only by rail, installations needing reliable power even if fuel supplies are interrupted, mature electricity markets that need to add new capacity in small increments).

On thorium, a question:

Hello! What do you think is the most important advantage that thorium has over uranium as a “fuel?”

Prof. Per Peterson’s reply

The thorium fuel cycle has clearly attractive features, if it can be developed successfully. I think that most of the skepticism about thorium emerges from questions about the path to develop the necessary reactor and fuel cycle technology, versus open fuel cycles (uranium from seawater) and closed, fast-spectrum uranium cycles.

The most attractive element of the thorium fuel cycle is the ability to operate sustainably using thermal-spectrum neutrons. This allows the design of reactor core structures that use high-temperature ceramic materials like graphite, which have substantial thermal inertia and cannot melt. Because these ceramic materials also provide significant moderation, it is difficult to use them in fast-spectrum reactors and thus the most plausible fast-spectrum reactor designs need to use metallic structural materials in their cores.

So thorium reactors are compatible with higher intrinsic safety (cores which do not suffer structural damage even if greatly overheated) and that can deliver heat at higher temperature, which enables more efficient and flexible power conversion.

Molten fluoride salts are compatible with these high-temperature structural materials, and given their very high boiling temperatures make excellent, low pressure heat transfer fluids. In the near term, the largest benefits in using fluoride salts come from the low pressure and high temperature heat they can produce. This can be achieved with solid fuel, which is simpler to work with and to obtain regulatory approvals.

But molten salt technologies also have significant challenges. One of the most important is managing the much larger amounts of tritium that these reactors produce, compared to light water cooled reactors (the quantities are closer to what heavy-water reactors, such as the CANDU, produce, but methods to control and recovery of tritium are much different for molten salts than for heavy water, and key elements remain to be demonstrated).

To repeat a critical point “…largest benefits in using fluoride salts come from the low pressure and high temperature heat they can produce. This can be achieved with solid fuel…”. This summarizes why Prof. Peterson’s lab is focused upon developing the PB-AHTR design, which will also prove out many materials and technologies required subsequently to implement the more challenging Liquid Fuel molten salt reactor concept (such as LFTR).

Energy density: the key to our no-carbon future

Your lifetime energy supply, a golf ball-sized lump of thorium or uranium – Credit Kirk Sorenson

Your personal lifetime energy supply, the golf ball-sized lump of heavy metal at left represents about 780 grams of thorium or uranium. Barry Brook recently used this to illustrate the energy density and cleanliness of fast neutron fission power relative to coal. And compared to solar or wind power options, coal is extremely energy dense. But the energy density of thorium or uranium is about 2.6 million times that of coal.

Bill Gates has described the solar/wind options as “energy farming“, which neatly captures the extraordinarily diffuse nature of these energy sources. For land use-friendly and economical energy, dense is good, diffuse is bad.

A golf ball of uranium would provide more than enough energy for your entire lifetime, including electricity for homes, vehicles and mobile devices, synthetic fuels for vehicles (including tractors to produce your food and jet fuel for your flights). Your legacy? A soda can of fission product was, that would be less radioactive than natural uranium ore in 300 years.

Tom Blees used the above graphic to illustrate the tiny volume of waste generated by fast neutron fission power: Your lifetime energy supply = 1 golf ball, your waste = 1 soda can. For more please see the conference paper Advanced Nuclear Power Systems to Mitigate Climate Change, or IFR FaD 4 – a lifetime of energy in the palm of your hand.

Blueprint for 100 new nuclear powerplants in 20 years

Your lifetime energy supply, a golf ball-sized lump of thorium or uranium – Credit Kirk Sorenson

Very interesting. Here is our second example (for June) of a legislator with real knowledge of energy policy. This is Senator Alexander Lamar [R-TN].

July 7 2009 – U.S. Senator Lamar Alexander (R-Tenn.) today told a panel of Administration officials that the United States should build 100 new nuclear plants in the next 20 years.

“Why are we ignoring the cheap energy solution to global warming which is nuclear power?” Alexander asked a hearing of the Senate Committee on Environment and Public Works (EPW), of which he is a member. “Over the next 20 years, if we really want to deal with global warming, we really only have one option and that is to double the number of nuclear power plants. There is no technological way to obtain a large amount of cheap, reliable, clean electricity other than nuclear power.”

And thanks to Iain McClatchie for the link to the white paper [PDF], which is a quite useful “executive summary” of why nuclear energy is the clear winning policy choice — provided that we can build the political support. The public support is already in place. Excerpt from the white paper:

All this does not automatically ensure success. For America to build 100 new reactors by 2030 a lot of things will have to be done right. Most important, the Nuclear Regulatory Commission will have to issue licenses that will stand up in court. Public Citizen, the Naderite Public Interest Research Groups (PIRG), and Greenpeace are all loaded for action, challenging regulatory decisions every step of the way. These groups cannot be taken lightly. PIRG has branches in every major state, fueled by its raid of student activity funds at colleges all over the country. Greenpeace International has an annual budget of $150 million, half again as large as the UN’s World Health Organization. All these opposition groups are staffed with skilled lawyers and eager young volunteers anxious to make their mark on the world. Stopping nuclear power has been a near-religious vocation for opposition groups in the past and will be again. Fortunately, the opposition is concentrated mostly in the Northeast and the West Coast, where not much manufacturing takes place and where opposition extends to all kinds of industrial activities.

In the Heartland, people are eager to embrace nuclear power. The people who understand industry and its needs should be allowed to make the decisions for industry. Nothing is more important for manufacturing than cheap and reliable electricity.

Highly recommended. Read more »

Our first example of a well-informed legislator was Chuck DeVore.