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XYLENE POWER LTD.

MODULAR REACTORS

COMMENTS BY PAUL ACCHIONE:
The paradigm shift for nuclear will come with the development of small modular reactors (SMRs) that are passively safe and operate at higher neutron energies (fast neutrons of typically around 2 MeV instead of thermal neutrons at typically 0.025 eV). Fast neutrons consume actinides just as well as U-235 (or U-233 from Thorium) and Pu-239.

There are several serious threats to present nuclear power technology that should go away if the SMRs prove to be as good as the physicists are suggesting. Here is a list of 9 major public and utility concerns:

(1) cost and schedule over-runs: SMRs can be built in an climate controlled factory like a Boeing 787 and shipped to site on a train. Costs per unit labor drops by 30 to 50% for manufacturing labor compared to site skilled trades. Also labour hours per MW will drop if mass production methods are used like airplane manufacturing. SMRs can be parked in a warehouse for a few months waiting for the utility to place the order so production can continue efficiently. A fixed delivery date and price is known to the utility when placing the order and the in-service date can be achieved in 2 years instead of 5 to 8 years after placing the order. Cost and schedule risks drop dramatically to levels comparable to combined cycle gas turbine (CCGT) plants.

(2) limited load following operation: Fast neutron SMRs do not have a Xenon poison problem following a power reduction so power maneuvering is as good as a CCGT plant. Because their capital and operating costs will be lower than present nuclear technology, they should be economic for load following duty.

(3) waste disposal concerns: Fast neutron SMRs with fuel reprocessing reduce long term storage times by 1000x and volumes by up to 100x. They utilize mined resources 100x better than current reactors and they can use existing spent fuel waste after it is reprocessed to remove fission products.

(4) accident risk concerns: Fast neutron SMRs are passively safe and operate at low pressure. They shut down if they overheat and the building can be designed to contain accident releases on site especially if the CANDU negative pressure containment feature we use in Ontario is added. It is essentially a big building under vacuum that acts as an oversized vacuum cleaner in the short term. In the longer term post accident filter can be used to keep off-site releases within acceptable limits that will not require evacuation by local populations.

(5) proliferation concerns: Fast neutron SMRs can be spiked with weapons defeating isotopes (eg: Pu240 in Pu 239 fuel) that make the fuel useless for bombs.

(6) high operating and maintenance labour costs: Fast neutron SMRs are passively safe so it should result in reduced manpower to operate and maintain the plants. Serious equipment problems can be dealt with by removing the fuel and shipping the module to a repair depot if the site is suitably designed to permit plug and play units.

(7) large unit size and large capital requirements: Fast neutron SMRs can be build in small incremental sizes to match load growth and the associated capital requirements are within the capital raising capabilities of existing private utilities without government support.

(8) large freshwater consumption: Fast neutron SMRs operate at higher temperatures than present reactors. The higher temperatures can permit closed loop air cooled condensers to be used in the power cycle. That reduces water consumption dramatically and reduces water vapour injection into the atmosphere that causes various local environmental problems like fog and icing on cold days.

(9) low natural gas prices: If global warming is an existential threat caused by CO2 emissions natural gas and other fossil fuel generation will not be cheap in the future as carbon dioxide emission penalties are imposed or their use will be outlawed like coal generation has been in Ontario. If global warming turns out not to be a problem we won't need nuclear fission power and we can wait for commercial scale fusion to be developed in the future.

Let's hope the physicists are right and the engineers can deliver on their promises.

Paul Acchione, P. Eng., FCAE
 

Further comments by Charles Rhodes, P.Eng., Ph.D.

SMALL MODULAR REACTOR (SMR) CONSTRUCTION:
During the 1980s many water cooled nuclear reactor based power stations were built. There was a false belief in economies of size. There was also lack of adequate appreciation of the cost of construction financing as a fraction of the total capital cost. Since then it has become apparent that the most economical nuclear generating stations are those that are assembled from truck or rail transportable modules that are factory built and tested. Doing as much work as possible in a factory environment accelerates reactor construction and commissioning and minimizes total cost.

In order for nuclear power to economically displace fossil fuels the reactors modules must be mass produced in factories in accordance with standard designs. The individual modules must be sufficiently small to be truck and rail transportable along main highways and rail routes without expensive measures such as moving overhead power lines or barge transport.

Reactors built in this manner are known as Small Modular Reactors (SMRs).

With SMRs there are practical limits to the physical sizes of individual components such as reactor pressure vessels, steam generators, turbines, heat exchange bundles, transformers, I beams, etc.

It is of paramount importance that each module be fully specified and factory tested for specification compliance so that when the modules arrive at the reactor site everything fits together easily and operates as intended.

Thus an economical nuclear generating station consists of an assembly of modest size modules.

In the case of a fast neutron power reactor the required sodium pool size is too large to transport as a single unit. However, the sodium pool can be assembled on site from factory fabricated subassemblies. The reactor fuel tubes and fuel bundles can be factory fabricated so that they are ready for on-site assembly and installation. The heat exchange bundles are prefabricated. Thus, subject to on-time module delivery, a modular fast neutron reactor can be rapidly assembled on site.
 

DESIGN RULES:
Some basic rules of thumb are:
1) Maximum length of an individual long rigid object such as an I beam is 20 m.
2) Absolute maximum diameter of a cylindrical object or module is 6 m but 5 m is much less expensive to transport;
3) Absolute maximum weight of a single object or module is 100 tons but 70 tons is much less expensive to transport;
4) Practical maximum weight of a highly radioactive objects such as FNR core rods is 10 tons in order to allow for 40+ tons of bio-safety lead shielding;
5) There are major cost savings in keeping individual module unit weights within the capacity of the site's central construction crane. The farther that the crane has to reach the less is its lifting capacity. There are large costs associated with requiring a special purpose crane to be at the site at a particular time.
 

PILOT PLANTS:
If there is a significant equipment design change that change should be tested in a pilot plant before the design change is incorporated into multiple power reactors. Use of a pilot plant minimizes the cost consequences of unforeseen or difficult to model engineering problems. In this respect the track records of governments are extremely bad. Governments chronically leave decisions about nuclear reactor construction to the last possible moment. That delay frequently has the effect of preventing design changes being tested in a pilot plant before those design changes are incorporated into multiple power reactors. A prime example of this problem were the hydraulic resonances which were encountered at the Darlington Nuclear Generating Station during the period 1990 to 1992. The interest cost alone of having four large units out of service while this problem was studied and remedied was over $3 billion.

Many of the cost overruns in past multi-unit nuclear power plant construction were directly attributeable to governments failing to fund pilot plants sufficiently ahead of required full scale plants that new knowledge gained from construction and operation of the pilot plants could be economically incorporated into the full scale plant design.
 

PERSONNEL TRAINING:
A related economic issue is providing continuous employment to nuclear engineering and supply chain personnel. It is far more economical to have a slow but steady nuclear construction program that builds modular reactors one after another than to have a boom and bust program that involves repeated expensive personnel retraining. All too often in boom and bust programs current personnel do not fully understand the reasons why certain design decisions were made in the past because the design engineers have long since retired.
 

DECOMMISSIONING:
Today there are large costs related to nuclear plant decommissioning. A fundamental flaw in past nuclear plant design was failure to provide for economical ongoing plant operation via module replacement. There is also continuing failure to pay attention to preventing fast neutron damage and neutron activation of materials such as graphite, salt, concrete and stainless steel where neutron activation produces the long lived radio isotopes C-14, Cl-36, Ca-41 and Ni-59.
 

EXTENDED WORKING LIFE:
New fast neutron reactors should incorporate an 2.8 m thick band of liquid sodium around the reactor core and blanket assembly so that surrounding components are not subject to either fast neutron damage or neutron activation. Then the working life of the major components is only limited by physical wear issues, not neutron impact and neutron activation issues. Then worn heat transport components or modules can be easily replaced without nuclear waste concerns. Provided that the equipment foundations are sufficiently robust, with ongoing module replacements the plant working life is almost unlimited and hence decommissioning waste disposal costs are vastly reduced or totally avoided.

Thus, quite apart from safety concerns, if possible it is desirable to build fast neutron reactors in igneous bedrock to provide a robust foundation that enables a nearly unlimited plant working life.

Concrete is not a good substitute for igneous rock. Portland cement, a critical component of concrete, sets by absorbing water of hydration. At FNR operating temperatures this water of hydration is gradually lost, which eventually leads to concrete structural failure. Concrete also contains calcium which when neutron activated becomes long lived nuclear waste.
 

This web page last updated May 14, 2017.

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