Tuesday, March 31, 2009

Controling Nuclear Costs: Part I Pebble Bed Reactors.

There is no reason why nuclear costs need be nearly as high as the currently quoted for new reactor builds in the United States. India's new generation IV Liquid Metal Fast Breeder Reactor cost $700 million for 500 MWe generating capacity. The problem of nuclear costs lies in no small measure with the choice of light water reactors as the design of choice for the nuclear power industry, and in its labor costs in advanced societies. The choice of the Light Water Reactors in turn leads to a series of design decisions that cause an excessively priced finished product. The LWR emerges from the design process, as a relatively large machine requiring even larger housing. By the time the cost of building a LWR is added up, you might as well buy the giant economy size as so that is the way most reactors are built.

The Indian LMFWR is cheaper primarily because the cost of labor in India is lower than in the United States, and because the reactor itself is somewhat simpler to build. The LWR is both very complex, and its construction is very labor intensive. Thus two potential ways to lower nuclear cost would be to adopt simpler reactor designs and to improve the efficiency of labor in reactor manufacture.

There are several Generation IV reactor designs that are relatively simple machines. The Liquid Metal Fast Breeder Reactor, and both the Indians, the Russians and Argonne Lab have gotten LMFBRs to work reliably. Yet I suspect there would always be strong opposition to LMFBRs because of the coolant safety issue. LMFBRs are cooled by sodium and it will burn on contact with air. Furthermore the sodium coolant of LMFBR/s is radioactive, so a coolant fire involves safety issues.

However, there are other Generation IV reactor options. No one who knows anything at all about nuclear safety would accuse the Pebble Bed Reactor of being unsafe. You can shut the whole thing off, leave it set with no coolant at all and the thing won't melt down. In fact the PBR is so safe, that you don't really need a safety containment dome. PBRs are not regarded as extreme technological challenges. They are currently under development for commercial applications in South Africa and China. Both PBR projects envision PBR manufacture involving factory construction. However, it is unlikely that either project will seethe whole reactor emerge from complete from a factory. Rather it would appear that the goal would be to manufacture rapidly assembled kits in factories and do final assembly on site. Both projects involve the manufacture of small reactors. The Chinese plan to produce a 100 MWe while the South Africans plan manufacture 165 MWe units. Both plan to cluster multiple units, in order to match the power outputs of middle size or even large reactors. The Chinese, for example plan to link heat output of 2 PBRs to one steam turbine. The South Africans plan to cluster as many as 8 small gas turbine PBMRs to match the power output of large reactors. Both the Chinese and the South Africans are confident that they can at least match large reactors on manufacturing costs.

There are some downsides to thePBR project. The Chinese appear to plan to manufacture construction kits for large reactors in factories as well. It is not clear that Chinese PBRs will
PBRs ether cost less per kW or requires less on site labor than Chinese kit built LWRs. One of the major problems with PBR manufacture is the size of its pressure vessel. Although not manufactured from a single huge ingot, as LWR pressure vessels are, the PBR pressure vessel, meant to contain pressurized helium, is twice as large as the pressure vessel of a large LWR. Thus transportation other PBR pressure vessel will be something of a problem.

The advantage of PBRs then may not be lower costs as much as their rapid manufacturing/set up times, and the fact that PBR units can be built sequentially and thus each can come online within a few months of its construction start. Rapid manufacture from the point of first investment to the point of first power delivery will lead to a considerable cost savings, an the ability to tailors project power output to existing demand means that surplus production capacity will not be paid out of a partial capacity retinue stream.

Although the PBR offers a solution to one of the traditional objections against the LWR, that of safety, its TRISO fuel poses a significant reprocessing challenge. Although the use of TRISO fuel technology makes the PBR virtually proliferation proof, it also potentially creates a significant nuclear waste issue. Indeed the volume of nuclear waste from PBRs would be much larger for unit of power output, than the nuclear waste from current LWRs, with significantly greater recycling difficulties.

Thus it would appear that PBR man nor present such decreased complexity and labor savings to offer a significant cost advantage over the LWR.

More information:

Wikepedia PBR page

Nucleaer China

South African Pebble Bed Modular Reactor home page

Robert Hargraves PBR page

The learning curve for serial reactor production

Brian Wang on Chinese PBRs


Anonymous said...

My belief in the efficacy of nuclear power began with the Pebble Bed Reactor (PBR). Realizing the basic systems flaws in the standard Light Water Reactor (LWR) design, I realized that the compensating complications and associated limitations in the engineering implementation of the LWR were unfortunate and sadly regrettable.

Although better than the LWR, the PBR has its own flaws; those being it low power density, it’s associated over large dimensions together with and extravagant use of structural construction materials.

The complications in the pebble fuel handling equipment that recycles the fuel pellets in and out of the reactor vessel rubs the esthetics of my engineering sensibilities wrongly; such designs are problem prone and an operational and maintenance (O&M) aggravation.

The use of high temperature and high pressure gas cooling also has many downsides.

But it good points do somewhat compensate for the downsides; factory manufacture, abundant graphite moderation, walk away fail safe operation, centralized computer controlled clustering, low proliferation danger, and the need for far fewer personnel than the LWRs for O&M.

It then struck me that the use of liquid fluoride molten salts as a replacement for high pressure helium could mitigate many limitations of the PBR and this idea was the subject of my first post here at this site; if the world could only realize the benefits provided by these salts.

But to my surprise, it had. Years later, I ran across a study about the Advanced High Temperature Reactors (AHTR). C. W. Forsberg and Per Peterson among others had perfected the concept to a great extent by the time I realized that it was happening.

Although much improved, the AHTR still has the intrinsic systems problems imposed by the use of TRISO fuel, being smaller than the PBR but still larger and more complicated than it should be, and having the same problematical and failure prone pebble handling as the PBR.

But wait, let us go farther, let us grab for the brass ring, let us go all the way. Why can’t an ideal reactor design spring from the marriage of the good parts of the TRISO fuel concept and superior coolant and operational characteristics of liquid fluoride salts?

Eliminate the hard silicon protective outer shell of the TRISO pebble, grind up the graphite moderator to nano particle size, and mix all together with liquid fluoride salts to form the ultimate nano fluid coolant. Load it into a Liquid Fluoride Thorium Reactor and you have an ultimate ideal reactor.

It has abundant ideal graphite moderation, a thermal spectrum, very small reactor size, very high power density, ideal heat transfer characteristics, minimal heat exchanger size, minimum fissile fuel loading, near proliferation proof, on the fly fuel reprocessing, no nuclear poison buildup, walk away fail safe operation, complete fuel burnup, on and on; if it can only be made to work.

I have posted about this regularly and in depth. Nobody has said that it can’t be done; but nobody has said that it should be done, either; maybe someday.

Engineering development is and evolutionary exercise; with the next improvement standing on the shoulders of the last system. Unfortunately, the ideal solution just does not spring whole cloth out of thin air and vault to the head of the line. It takes time and patients. But for me it takes too much time and I have not enough patients.


Charles Barton said...

Axil you have perhaps anticipated a problem before your peers have. As I have argued ifyou want to start 10,000 + LFTRs, you are probably going to need a graphite moderator. The traditional approach to graphite in MSRs leads to serious problems.

DW said...

Graphite aside, the biggest cost here is going to be the closed cycle brayton turbines, something no one has produced yet. It's the reason the Chinese are willing to take the efficiency hit by using traditional rankine cycle turbines. It's the reason the Chinese are able to break ground this year and the S. Africans don't even have a working model yet.


Charles Barton said...

You are probzably right David. L?FTR R&D will probbly need several Billion to develop the technology.


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