Molten salt reactors mean inherently safe nuclear electricity
In view of their potential safety
issues a shift away from PWR’s and fast breeders, designed in the 60's and 70's, towards new inherently safe plants would be very welcome. Ideally these should:
- operate at atmospheric pressure and so
couldn’t explode;
- be unable to overheat or meltdown;
- be designed to be walk-away-safe in the event
of power failure;
- have a negative temperature coefficient of
reactivity: meaning that as the reactor temperature increases its
output reduces;
- allow load following and rapid output changes;
- produce much less waste with a much shorter
radio-active half-life.
Of the various proposed Generation IV nuclear reactor designs there
is one which fulfils all of these criteria, the liquid fuelled thorium
reactor (LFTR).
On 14th July 2011, thanks to Ken Pottinger (now sadly deceased)
of French News Online, I became aware of an alternative to current PWR
technologies, which is based on liquid fuelled thorium reactors LFTRs
and the Molten Salt Reactor Experiment.
In this context a salt is the chemical term for a compound of a metal and a halide such as a fluoride or a chloride e.g. lithium fluoride. At the time I had no idea that nuclear reactors could have many design variants and even work in the liquid phase. I spent several
days researching the topic and wrote this piece on my blog summarizing my findings.
The molten salt reactor experiment
A prototype molten salt reactor,
operating at atmospheric pressure, ran at Oak Ridge National Nuclear Laboratory (ORNL), Tenessee in the 60’s and 70's. The film below, made at the time, shows how this pilot scale plant was designed,built and operated.
The Molten Salt Reactor Experiment (MSRE) ran for more than 13,000 hours at full power, without significant materials problems, and successfully demonstrated the viability of the concept. At the weekends, when they didn’t want to have staff on duty, they used to turn off the power, the fan cooling the freeze valve below the reactor vessel would stop, the plug of solid salt would melt and the reactor contents would drain by gravity into storage tanks. Since these did not contain a moderator to slow down the neutrons the chain reaction would stop. It was truly walk away safe!
To turn it into a commercial product there is more development work needed on the optimization of the waste processing stages, and also on the materials necessary to resist the high temperature and intense radiation environment over the long term. At the time that the project was shut down, ORNL had already started to work on these areas. We are fortunate that the work on the MSRE at Oak Ridge was very fully documented and these documents are in the public domain.
The fuel used for the MSRE was
Uranium 233 produced in a reactor at Hanford. It was intended that at a future
stage the reactor design would be modified to work as a breeder reactor
producing U233 from Thorium 232 in a self-sustaining way. This would require the
development of a chemical process to separate the U233 formed in the blanket
and send it to the reactor. Unfortunately the project was shut down before this
was fully progressed.
Kirk Sorensen has been actively promoting this technology and he explains in this video how he came to rediscover molten salt reactors and Thorium as an alternative fuel.
What about nuclear waste?
But the specific concern of many people is with the
management of nuclear waste.
The composition and quantity of nuclear waste depends on the fuel used in the
reactor and the degree to which the fuel is burned. Pressurised water reactors running on the
Uranium 235/238 fuel cycle can only burn about 4-5% of the fuel contained in
their fuel rods, which deteriorate and have to be replaced every 18 months. During this operation the reactor is taken
out of service.
Liquid fuelled reactors can burn a much larger
percentage of their fuel than solid fuelled reactors because, by bubbling inert gas through the molten salt, it’s so much
easier to remove gaseous fission products like Xenon 137, which poison nuclear
reactions by absorbing neutrons. Also,
by incorporating a side-stream, carrying the molten salt fuel for waste removal
and fuel addition, there is no need to shutdown the reactor for these processes.
Therefore, for the same amount of energy generated, molten salt reactors can
produce 35 times less waste.
LFTR versions running on Thorium also produce waste with a much shorter
half-life of 300 years as opposed to tens of thousands of years. This very clear video, again from Kirk
Sorensen with others, explains
how fission products are created and what they can be used for.
In this rather more detailed video Kirk Sorensen projects forward the results of radioactive decay over time on waste from the Uranium 235/238 fuel cycle. He expands on the idea of recycling nuclear waste and asks, is it really all waste?
But as Kirk Sorensen says in an earlier video, you
can also dispose of waste from PWR’s using liquid fuelled waste burning
reactors, which also generate power and this is described more fully later.
But without internationally agreed and binding carbon taxes, which would make fossil fuels more expensive,
safer nuclear power just won't happen unless it’s cheaper than other options.
Robert Hargraves develops the argument that in
order to replace fossil fuel burning power plants you must be able to generate
electricity by low carbon methods at an overall cost less than that of fossil
fuels.
In this detailed talk he examines the costs of generating electricity from different sources
including wind and solar. He also
proposes using liquid fuelled reactors and points out that the higher operating
temperatures of such reactors offer higher power generation efficiencies as
well as various options to use the high temperature heat to directly drive
chemical reactions.
And cheaper than unsubsidized renewables
At the beginning of the next video Ian Scott of
Moltex Energy reaffirms the importance of radical innovation to enable new
nuclear power investment to find a role in a commercial environment where the
penetration of renewables is significant. This means finding ways to reduce construction
and regulatory costs by modularisation and factory production techniques so
that new nuclear plants can still be profitable at 50% utilisation factors. In
this video he describes the Moltex Waste Burner which is being developed in New
Brunswick, Canada by the North American incarnation of his company, with the
full support of central and local government.
If you’ve got this far you are probably suffering from information overload but congratulations on your persistence!
Why haven’t molten salt reactors been developed before?
When I first researched liquid
fuelled reactors burning thorium (LFTRs) I found it hard not to think that
there was something that was being hidden from me. Such as some reasons that explained why such
obviously better technology hadn’t been developed! Finally I was convinced that there isn’t anything of the sort, and the
reasons why it has languished for 60 years are almost entirely political in origin. This Google Tech
Talk, again by Kirk Sorensen, explains the background.
Where will safer nuclear power happen first?
Nuclear power is, a complex subject difficult to
explain to non-specialists, or the general public, and it can’t be fitted into
a few tweets for people with short attention spans or other priorities. Unfortunately it’s much easier to invoke
fear among the public of accidental releases of radioactive
materials; so a sustained campaign of education is required to overcome decades
of nuclear scepticism and deliberate misinformation that has stuck in the minds
of the public. The success or otherwise
of such public education will be a factor in determining where new nuclear
power will be developed.
Western European countries especially France and
the UK have the necessary infrastructure, but I doubt that they have the
political will and in the case of the UK, the resources, to take any sort of
lead in developing new nuclear designs beyond the feasibility stage. Luckily
other countries are better placed. The following is by no means an exhaustive
list but only a selection of the most likely candidates to take new nuclear
designs to full scale commercialization.
The United States
There are many opportunities for the US government
to fund research into up-scaling advanced nuclear reactors and their much safer
technology.
For example, the US Department of Energy DOE is
financing research into several initiatives intended to
enable cost reductions in nuclear construction projects.
In June 2021, in response to the announcement by the
Biden-Harris administration of their policy to aim for net zero carbon
emissions by 2050, the Office of Nuclear Energy has requested 1.8 billion
dollars from Congress. To quote from
their press release,
In the case of the USA, a few years ago, it seemed
to me unlikely that these companies could persuade regulatory agencies to
reduce the one to two billion dollar cost and ten year timescale that the US Government Audit Office estimated
in July 2015 that it
would take to certify and license a fundamentally new design. At
present the US Nuclear Regulatory Commission requires a fully developed design
to review, so that companies would have to spend a very large amount of money
upfront. Faced with this situation private investors just won't bother or they
will migrate to jurisdictions which are more welcoming and which proceed by a
staged approval process.
More recently in the USA there has
been a change in the mood and, helped by international pressures and extreme
weather events around the globe, there is renewed impetus towards addressing
the climate crisis and a political will building around the development of carbon
free energy from innovative nuclear technology.
Small modular reactors - Executive Order 13972
One of the last actions of the outgoing Trump Administration
was to persuade him to sign Executive
Order 13972 of January 5, 2021.
Promoting Small Modular Reactors for National
Defense and Space Exploration”
Quote <<Sec.
3. Demonstration of Commercial Reactors to Enhance Energy Flexibility at a
Defense Installation. (a) Micro-reactors have the potential to enhance energy
flexibility and energy security at domestic military installations in remote
locations. Accordingly, the Secretary of Defense shall, within 180 days of the
date of this order, establish and implement a plan to demonstrate the energy
flexibility capability and cost effectiveness of
a Nuclear Regulatory Commission-licensed micro-reactor at a domestic military
installation.>> Unquote
A few years ago Kirk Sorensen was hoping to bypass national
nuclear regulatory procedures by installing an LFTR on a military site but unfortunately
this order invokes the NRC licensing process. Assuming that the order progresses through the
Biden Administration, perhaps Kirk can find a way to speed up and simplify
certification.
Canada
As a result of its history of nuclear innovation,
and years of operational experience with CANDU heavy water reactors, Canada has
an industrial, intellectual, regulatory and political infrastructure which is
favourable to advances in nuclear power. Its staged regulatory process is particularly
helpful to innovative technologies.
Quoting from their press
release of Tuesday, May 25, 2021
Quote <<Moltex
Energy is delighted to have completed Phase 1 of the Canadian Nuclear Safety Commission’s
(CNSC) Pre-Licensing Vendor Design Review (VDR) for Moltex’s 300 MW Stable Salt
Reactor – Wasteburner (SSR-W). The CNSC concluded that Moltex has a clear
understanding of the Canadian regulatory requirements and expectations. >> Unquote
Terrestrial Energy, another
Canadian-based company, is developing a Denatured Molten Salt Reactor (DMSR)
design called the Integral Molten Salt Reactor (IMSR). The
IMSR is designed to be deployable as a small modular reactor (SMR). Their
design currently undergoing licensing is 400MW thermal (190MW electrical). With
high operating temperatures, the IMSR has applications in industrial heat
markets as well as traditional power markets. The main design features include
neutron moderation from graphite, fuelling with low-enriched uranium and a
compact and replaceable Core-unit. Decay heat is removed passively using
nitrogen (with air as an emergency alternative). The latter feature permits the
operational simplicity necessary for industrial deployment.
Terrestrial Energy completed the first phase of a
pre-licensing review by the Canadian Nuclear Safety Commission in 2017, which provided a regulatory opinion that the
design features are generally safe enough to eventually obtain a license to
construct the reactor.
Indonesia
Thorcon has signed a memorandum of understanding with the Indonesian Defence Ministry to study the development of a 50MW thorium molten salt reactor.
Thorcon have already done a conceptual design of a 500MW system, based on thorium molten salt reactor technology, but built in a similar way to a large cargo ship. The basis of this interesting concept is to use a modular approach with each module being fabricated in shipyards and joined together prior to transport to the chosen permanent site.
This video summarises very clearly the background to and the conceptual design of the Thorcon system. (Sorry about the adverts).
Indonesia has the fourth largest population in the world at over 273 million and, like all developing countries, it has an expanding need for clean and cheap electricity. Currently this is being supplied by coal fired power stations and the aim is to compete with coal by using ship building techniques thus reducing construction costs and time to completion. Compared with Light Water Reactors, Molten Salt Reactors do not need heavy reactor vessels or containment buildings able to resist the pressure exerted by a rupture of the 300 bar pressurized reactor vessel. They are therefore a good fit with steel modular construction.
This detailed video from Lars Jorgensen gives a full explanation of the design concept.
This project is in its preparatory stages but the concept appears to have been well considered. Thorcon appear to be successfully navigating their way through any difficulties that might occur in securing the necessary funding and approvals. There appears to be a sense of urgency on behalf of both the Indonesian clients and Thorcon itself.
China
Due to the language barrier, and the Chinese
tendency to be cautious about public announcements, it’s difficult to
accurately follow China’s progress towards implementing molten salt reactors. Western
press reports are often sketchy, inaccurate and journalistic, but it does
seem that they are making good progress towards starting up a molten salt
reactor at their research facility in the desert region of Wuwei very soon.
In January 2011 Chinese Academy of Sciences initiated a thorium molten-salt reactor research project. A 100 MW demonstrator of the solid fuel version (TMSR-SF), based on pebble bed technology, was planned to be ready by 2024. Initially, a 10 MW pilot and a larger demonstrator of the liquid fuel (TMSR-LF) variant were targeted for 2024 and 2035, respectively. China then accelerated its program to build two 12 MW reactors underground at the Wuwei research facilities by 2020, beginning with the TMSR-LF1 prototype. The project sought to test new corrosion-resistant materials. By 2021 China stated that the Wuwei prototype Molten Salt Reactor could start-up in September. We are waiting to hear what they have achieved.
As a result of that investment, and less lengthy certification requirements, China is most likely to be the first to recreate the Molten Salt Reactor Experiment and develop it further to a commercial design for completion in 2030. They will then patent their designs and sell them internationally.
I wish them every success!
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