China’s faces technical challenges to develop thorium based nuclear power

By 2017 the Chinese are expecting to be running an experimental molten salt reactor, whilst other countries and commercial organisations, are being left behind. China’s first step towards liquid fuelled thorium reactors is, in essence, to reproduce the work done by Oak Ridge National Nuclear Laboratory (ORNL) in the sixties, when ORNL built and ran the Molten Salt Reactor Experiment MSRE. But what are the problems and challenges facing the Chinese as front runners in the drive to start up a liquid fuelled thorium reactor (LFTR)? 

Alvin Weinberg

Under Director Alvin Weinberg, back in the sixties and seventies a number of them were identified by the MSRE at Oak Ridge National Nuclear Laboratory.  In the test environment some of these challenges seemed to have answers, whilst others are still to be solved.  They are all written up in reports and published papers, now freely available on the internet, or at the ORNL archives.  You can be sure that they have been well studied by the many Chinese visitors to Oak Ridge.

Materials Hastelloy - N

The MSRE ran at around 670 deg C and the combination of high temperature and radiation made the selection of materials critically important.

The Molten Salt Reactor Experiment

Based on laboratory tests Hastelloy –N (originally known as INOR-8), a nickel alloy with molybdenum and chromium developed by ORNL, was chosen for the reactor containment vessel and for piping and heat exchangers. 

 Chemical Composition of Hastelloy N™

•     Carbon 0.04-0.08

•     Chromium 6.00-8.00

•     Molybdenum 15.00-18.00

•     Iron 5.00 max

•     Manganese 0.8 max

•     Aluminum + Ti 0.5 max

•     Boron 0.01 max

•     Sulfur 0.03 max

•     Nickel remainder

During construction questions arose concerning the stress-rupture life and fracture strain, which were found to be drastically reduced by thermal neutron irradiation. 

An out-of-pile corrosion test program was carried out for Hastelloy-N^[9] which indicated extremely low corrosion rates at MSRE conditions.  Capsules exposed in the Materials Testing Reactor showed that salt fission power densities of more than 200 W/cm³ had no adverse effects on compatibility of fuel salt, Hastelloy-N, and graphite.  With their chosen salt mixture (see below) and at the operating temperature, they found that Hastelloy N was adequately resistant against corrosion and embrittlement for its design life in the MSRE.  Since this was an experimental reactor, however, this is not the same as saying that it will last the 30 years necessary for a commercial reactor.

Later experience showed that the mechanical properties of Hastelloy – N do deteriorate as a result of exposure to thermal neutrons but that the addition of small amounts of titanium and hafnium significantly improve its performance. to select the most resistant alloy much more long term operational experience is needed.

MSRE core

Graphite Moderator
As in Light Water Reactors, the graphite moderator blocks were found to swell and crack in the high radiation environment.  As a minimum this creates a maintenance issue since the blocks have to be replaced.  More seriously, depending on the reactor design, it can block fluid pathways and lead to hot spots.

Research into graphite production methods and coatings was carried out by ORNL but it remained an unsolved problem in the early seventies.

There are many ways to design a nuclear reactor and other possibilities of moderating the neutron flux may need to be tried.  

Alternatively one could opt for an un-moderated molten salt reactor design working in the fast spectrum.  Such a reactor would be able to burn actinides and eliminate almost entirely the production of highly radioactive waste having a long half life.  This is the subject of research at Grenoble, France but it has never been attempted on the scale of an experimental reactor so far.

Tritium removal

Tritium is generated in small quantities within the molten salt as a breakdown product of lithium 6. Tritium, or hydrogen-3, is made by bombarding lithium-6 (⁶Li) with a neutron (n). This neutron bombardment will cause the lithium-6 nucleus to fission, producing helium-4 (⁴He) plus tritium (³T) and energy.

Tritium is highly radioactive and must be sequestered in a secure storage facility.  If, however, the lithium salt used is purified to 99.995% Lithium -7, then the quantities of tritium produced amount to only a few hundred grams per year from a 1GWe reactor. 

At the temperatures concerned, however, tritium can pass through the heat exchanger material and get into the secondary cooling loop from where it could escape into the environment.  ORNL developed a secondary loop coolant system that would chemically adsorb the few hundred grams of residual tritium to a less mobile form, so that it could be trapped and removed from the secondary coolant, rather than diffusing into the turbine power cycle.  ORNL calculated that this technique would, by itself, reduce tritium emissions to the environment to acceptable levels 

The tritium was then extracted and successfully oxidized over copper oxides and captured in a pair of water bubblers.  No tritium was found in the exhaust gases.  

The process of capturing the already low concentration of tritium from the molten salt was found to be not very straightforward and more work is need to develop this or other process options.

The Lithium Problem

ORNL chemists did extensive pioneering work to find the best mixture of salts to give the desired physical and chemical characteristics.  Many were tested but they opted for Flibe, a mixture of lithium and beryllium fluorides.

Present day research programmes may need to revisit the choice of lithium because it’s necessary to use only lithium 7 and to remove the 7.5 % of the naturally present lithium 6 isotope.  Otherwise, depending on the concentration, lithium 6 would either transmute in the reactor into tritium, as mentioned above, or prevent the reactor from starting up due to its high neutron absorption characteristics.

But this is only part of the lithium problem.   At present only two countries are carrying out lithium isotope separation, China and Russia.

Since lithium 6 can be used to make tritium its production is strictly monitored. 

Interestingly in the US Castle Bravo hydrogen bomb, tested at Bikini Atoll on 1^st March 1954, it was assumed that only the 30% of lithium-6 present in the lithium charge would react in a hydrogen bomb, but the contribution from an overlooked reaction due to the presence of the remaining 70% of lithium-7  caused an unexpected increase in yield of 250%,  making the 15 Megaton explosion the largest thermonuclear device tested at that date.

The US stopped separation of these isotopes, using the mercury amalgam method, a number of years ago due to concerns about its toxicity and the fact that large quantities of mercury had been lost from the Oak Ridge National Nuclear Laboratory inventory in an unknown manner.

So the lithium problem presents a difficulty to any private sector group not working in co-operation with a government having a form of lithium isotope separation technology. 

Other salt mixtures not using lithium are possible and many were tested at ORNL in the laboratory but so far they are completely untested in reactors and may affect the choice of reactor materials.

Storage of Solidified Salts

At the end of the MSRE programme the molten salts were stored as a solidified material.  It was found that fluorine and uranium hexafluoride were continuously released by radiolysis.   As a temporary measure the solidified contents were periodically reheated to induce recombination, but eventually the uranium was removed from the salts in an expensive and challenging cleanup programme.  A solution to this problem would be to remove the uranium prior to storage of the solidified salt by sparging the molten salt with fluorine gas, to create uranium hexafluoride which can be re-introduced into the reactor. 

Plutonium must also be removed to prevent radiolysis, which can be done by the addition of sodium carbonate to create plutonium dioxide. 

Conclusion

This is by no means a full list of the issues which arose during the operation of the MSRE. (Section 7 of the Weinberg Foundation's recent Report on Thorium-Fuelled Molten Salt Reactors gives several more). So there are plenty of technical challenges to address as the Chinese firstly repeat the MSRE experiment, and then extend it into larger reactor designs.  Personally I’m extremely pleased that the Chinese have the vision, the money and the forward thinking to restart and extend the pioneering work that was done at ORNL in the sixties and seventies   The use of thorium reactors running at atmospheric pressure will be much safer and will produce much less radioactive wastes than the current 50 year old designs of the existing Light Water Reactor fleet.  For me safe and abundant nuclear power is the only way of avoiding the prospect of runaway global warming, because conservation of energy, and the intermittent nature of the main renewable sources, cannot provide more than a part of the answer to the rapidly increasing global demand for energy.

Under Weinberg's leadership ORNL had a world leading  combination of nuclear scientists, engineers, chemists and metallurgists all working under one organisational umbrella.  They were capable of taking any brief concerning nuclear power generation and turning it into reality quickly and efficiently.  The USA allowed much of this expertise to dissipate when the Nixon administration fired Alvin Weinberg because he disagreed with the administration’s chosen programme of liquid metal cooled fast breeder reactors.  By the time that programme was closed down the USA  had forgotten about the highly promising liquid fluoride technology that is once again coming to the fore. 

It’s a pity that the US, like other western countries, has not yet found the courage or political will to overcome the inertia inherent in the nuclear establishment, which is committed to the Pressurised Light Water Reactor and the uranium–plutonium fuel cycle.   Coming from the UK, I’ve seen many examples over the years where pioneering research is done by underfunded organisations, only to be developed by other better placed countries.  It’s a symptom of the West’s debt-burdened economic decadence in the face of the challenge of the Far East, but in fifty years time, if all goes well, we’ll thank the Chinese for picking up and developing ORNL’s 50 year old Liquid Fluoride Reactor research, even if we have to pay the licence fees to China for our neighbourhood power station.

Although there are some signs of increasing openness, and their thorium programme is a good example, it’s also worrying that the Chinese, as communists, do not have a fully representative form of government.  I sincerely hope that the Chinese thorium programme doesn't get caught up in some sort of revolutionary “Chinese Spring” that could set their programme back decades.



Here are some other posts that might interest you if you have read this far.

An outline of the Chinese thorium program given by Kun Chen,Professor and Deputy Director, Department of Nuclear Safety and Engineering, Shanghai Institute of Applied Physics.
http://johnpreedy.blogspot.fr/2012/08/the-chinese-thorium-programme.html

A detailed review of Robert Hargraves' book "Thorium- energy cheaper than coal".
http://johnpreedy.blogspot.fr/2012/11/thorium-energy-cheaper-than-coal.html

China has a virtual monopoly on the production of rare earth elements of which process thorium is a byproduct.
http://johnpreedy.blogspot.fr/2012/09/the-thorium-problem.html

Horace Preedy - A Tribute



Horace Preedy on his 107th birthday

Dad was born on 17th January 1906, in East Ham, the third of four children. At the age of three he caught diphtheria and was taken to hospital in a hansom cab. He survived but contracted a heart murmur for which he was grateful later.

He left school at fourteen and worked for a while as an apprentice painter and decorator.  He progressed to working in the City, starting as a messenger then as a clerk for a firm of stockbrokers. Here he met Mum and she recalled the event in the reminiscences that she wrote at the age of 91. She says:-

“There was one customer who fancied the country girl who served him in the teashop at Stratford Broadway where she worked and asked if he could see her after work. The other girls asked his name. She said she didn’t know and they laughed and teased her saying, “It’s not Horace is it?” At their next meeting she asked him his name, “It’s not Horace is it?”, she said. He burst out laughing and said that it was. She said “I can’t call you that! Don’t you have any other names?”

He told her that his other names were Horace Robert Arthur, and so Bob was chosen, and he remained Bob in the office for years.”

They were married in Chingford in June 1935, and in the late thirties they lived in a bungalow in Billericay, owned by Horace’s parents, on a third of an acre, with a garden lovingly tended by Horace. They had two dogs and they used to play badminton and tennis at the local club. They’d achieved the dream life style of a young couple in the 1930’s and the only thing missing was a child or two.

A year and a half later the slump of the thirties had caught up with the City and Horace, always cautious, was concerned that he might be next to be sacked: so he took a part-time job with the GPO in the Brentwood Telephone Exchange. Some days he would come home from the City and then cycle six miles to the Exchange for an evening’s work. In August 1939, after a year’s probation, his status as a civil servant became permanent. He left his job in the Stock Brokers and went to work as a telephonist.

On 3rd September 1939 war was declared. He continued to work as a telephonist, frequently riding his bike to work at night whilst air raids were in progress. In 1942 he was drafted into the Signals Regiment and was well behind the front lines as the invasion advanced across France in 1944.

He liked to tell the story that in 1944 his unit was due to be sent to Burma. Dad was 38 at the time. The doctor who examined him at the medical looked at his papers, in which he was classified as A1, and said,

“You don’t want to go do you?” He certainly didn’t want to be sent to fight the Japanese in the jungles of Burma so he said “ No sir!” .
The doctor said “You have a heart murmur, but it’s well compensated and nothing to worry about,” and classified him C3. This meant that he was unfit for tropical service! It was a lucky escape because there were few survivors from his unit after their tour of duty!

The bungalow in Billericay was sold during the war and Mum, by now pregnant with Brenda, was sleeping on her brother’s sofa. Brenda was born in October 1944. Accommodation was very scarce by the end of the war and in the winter of 1944 Mum and Dad found a few rooms to rent in Westcliff on Sea, where they lived for the next 38 years.

At first the gloomy house, with navy blue wallpaper, was shared with two old ladies. In the scullery there was a stone sink, a copper to heat water over a gas ring, and a bath, which was not plumbed in and had to be bailed out. Needless to say, in my childhood, baths were only a weekly event.

After discharge from the Army he went back to the GPO, but soon he took the Civil Service Officers exam, passed and was allocated to the War Damage Commission. This meant getting up at 5 o’clock, walking to the station to catch the steam train to Liverpool street and then taking the tube to Acton. They still worked Saturday mornings so on Saturdays he had to travel for a total of six hours to work for four!

He applied for promotion, passed and was transferred to the National Assistance Board, which later became the Department of Social Security. His job was to visit claimant’s on Canvey Island to assess their cases. He had an auto-cycle, a sort of primitive moped, which meant dressing up in waterproofs and driving to work in all weathers. When he found old people struggling to get by he applied his own discretion and was able to give them some extra money for household items, but he was not at all sympathetic to men who wouldn’t work.

He bought a car in the early sixties. I remember that one day we took it to Canvey Island to fetch a little Yorkshire terrier from an old man who could no longer look after it. It was filthy and smelly, with matted hair and an aggressive nature. It was my job to restrain it on the back seat! When we got home Dad dressed up in a plastic mac and sou’wester, dumped the little dog in the bath, cut off most of his fur and gave him a good wash. In the next few months a beautiful silver-haired Yorkie, called Nobby, became one of the family, but he was definitely Dad‘s dog. Several years later, when Nobby was dying of heart trouble, my father would sleep in a deck-chair downstairs in order to stay with him at night, and reassure him when he was in pain.

Dad was a pioneer of DIY in the fifties and bought a table saw, which was dangerously plugged into an unearthed lighting fitting. He was very practical, he knew about carpentry and even how to "wipe" a lead pipe joint, but where he learnt it from I’ve no idea! As a child I used to watch him working and whenever I asked a question he patiently explained what he was doing.

While he worked steadily at modernising and improving the house, which by now he had bought, he sang songs from the 1930’s especially the Fred Astaire numbers that he used to dance to in his twenties. He always had a tune going on somewhere in his head. He‘d wanted to learn the violin when he was young but couldn’t persuade his parents to buy him one. He never did learn to play a musical instrument, but he was, none-the-less, very musical. When she was helping him with the decorating, Brenda remembers him singing “When Father Papered the Parlour” a hit from 1910. Less characteristically, she also remembers him dancing with her down the hill to the station to the tune of “ We’re off to see the Wizard…“ when we were going to a cousin’s wedding.

Normally though, he liked peace and quiet. In the evenings after work he was always outside in the small garden patiently chopping up things with his secateurs to feed the worms. Brenda and I both owe our love of gardening to him.

In 1982 Mum and Dad moved from Southend to Wantage to be nearer Brenda, and her husband Les. Here, as well as having a larger garden to keep him busy, he could still be seen painting the bungalow at the age of 92.

It was typical of him that he almost never complained and accepted what life sent his way. He was very patient, stoical and only rarely showed any anger. He didn’t need a lot of people around him but he had a strong sense of duty to his parents, his family and to a few close friends. He was polite right through to the end of his life, a real gentleman, and always said thank you when you took him a cup of tea or a meal.

In his later years, unlike my mother, he was still mobile, but he suffered progressively from memory loss, so he acted as my mother’s arms and legs while she told him what she wanted, and where to go to find it!

When Mum died from a massive stroke at the age of 97 we asked Dad, on the way back from the funeral, if he knew where we’d been and he said no. Sometimes Nature can be kind as well as cruel.

He never smoked and only drank alcohol at Christmas. He owes a good part of his very long life to these things, but Brenda has been his carer for the last ten years, whilst he slowly declined, and she has looked after him attentively. I’m quite sure that without her care he would have died some years ago.

She has also made it possible for him to die peacefully at home, in his own bed, at 107, the seventh oldest man in Britain. He died in his sleep on 2nd June 2013.

Thanks Dad for providing a stable and loving family home for us to grow up in. We’ll both miss you.

John Preedy