13. Summary and conclusions

The essence of this monograph is to make a convincing argument that there are abundant opportunities for reducing the capital cost of the LMFBR by designing for and realizing the inherent attributes of the metal coolant and the breeding principle. It should be possible to design a plant that is capable of continuous operation without refueling for periods of about ten years and operating base loaded or load following at the option of the owner. If the reactor is capable of operating continuously for ten year periods between refuelings, a simpler (but slower) refueling system can be adopted, which creates numerous opportunities for capital cost reduction.

As a result of the foregoing sections, the key reference design parameters turn out to be:

Electrical output (nominal)                 1200 MWe

Thermal output (nominal)                   3000 MW

Thermodynamic efficiency                 ~41%

Reactor outlet temperature                 1017°F

Reactor inlet temperature                    693°F

Number of PHTS loops                      2

Number of PHTS pumps                    4

PHTS pump concept                           electromagnetic

PHTS pump location                           cold leg

Primary pump head                             30-40 psig

PHTS total flow rate                           225,000 GPM

PHTS pump flow rate                         56,250 GPM

Number of IHTS loops                       2

Number of IHTS pumps                     4

IHTS pump location                           cold leg

IHTS pump concept                            EM

IHTS flow rate                                    225,000 GPM

IHTS head                                          36 psig

IHTS hot leg temperature                   977°F

IHTS cold leg temperature                 653°F

Number of IHXs                                 2

Number of steam generators               4

Steam temperature                              914°F

Steam pressure                                    2400 psig

Feedwater temperature                       490°F

Cycle length                                        10 years

Load following capability                   15-100% of full power

Refueling down time                          six months

Capacity factor between refuelings    95%

Maximum peak fuel burnup                300,000 MWD/MTU

Average fuel burnup                           150,000 MWD/MTU

Refueling concept                               Single rotating plug

Decay heat removal concept               PRACS + OHRS with IRACS or DRACS option

The heat output, thermal, and steam pressure conditions are the same as Superphénix since the same steam generator configuration is being proposed. Note that the above steam conditions are an improvement over CRBRP primarily because a once through steam generating system is selected rather than the recirculating system chosen for the CRBRP design. Within the context of the proposed design approach identified here, improvements can probably be made by either increasing steam pressure moderately and/or increasing PHTS and IHTS temperatures by about 50°F while maintaining 2400 psig steam conditions.

It is imperative that the LMFBR take advantage of its natural attributes and be designed to be economic. Approaches to design and licensing must be rethought with the goal of developing a concept that can compete on the marketplace with all alternatives, including renewables, natural gas, and especially light water reactors.

• The plant should be designed to be capable of load following from full power down to 15% power so as to be compatible with a power grid that is supplied with a substantial fraction of renewables. Base-loaded operation could be economically preferable, but the extent to which capital cost reduction is successful correlates with the facilitation and desirability of load following operation. I.e., a less expensive plant is a more attractive candidate for operation in a load following mode.
• The loop type design is more flexible and is likely to be more economic, easier to construct, and more reliable that the pool type design. (See Appendix 5)
• PHTS expansion loops should mostly be in the vertical direction to minimize containment volume.
• There should be two primary system loops with one IHX in each loop. The IXHs should be located as closely to the reactor vessel as can be reasonably achieved so as to minimize containment volume and the demands on the expansion loops.
• The primary pumps should be electromagnetic (EM) and located in the cold leg. The EM pumps should be capable of providing flow over all ranges continuously from 15% to 100%. The IHTS pumps may be centrifugal if that is the more economic option but EM pumps would be preferred so as to match the PHTS pump coastdown characteristics. The IHTS pumps should also be capable of operation down to 15% flow.
• There is no need for and there should be no pony motors, either on the PHTS or the IHTS.
• The reactor cover gas should be helium so as to enable better separation and removal of fission gases, but argon would be acceptable.
• There should be four helical-coil once through steam generators and four IHTS pumps located in the cold leg. The steam generators should be isolable on the water side. The capability of sodium side isolation is optional, but probably not necessary.
• The core should be designed for a ten year refueling interval with the whole core, internal blankets and the most if not all of the radial blankets replaced at each refueling. The refueling shutdown may require up to six months.
• The fuel should be vented to the primary coolant. The fuel should be capable of a 15 a/o average burnup and peak burnup of 30 a/o.
• Orificing of blanket assemblies should be either controllable from outside the reactor or self actuating. It would be highly desirable to be able to control the flow to both fuel and blanket assemblies as well to reduce the effects of thermal striping.
• The pressure drop across the reactor should be less than 20 psid.
• Hydraulic hold down of core assemblies may be eliminated as unnecessary with the reduced core pressure drop.
• The refueling system should be single rotating plug. The EVST should be sized to accommodate a full core load. The fuel handling system should be designed to permit loading the spent fuel cask five years after the fuel has been removed from the reactor through a wash station.
• The reactor head should have one centrally located rotating plug. The reactor vessel should have no in-vessel transfer position. Core component pots should be eliminated. The vessel should be about 30-35 feet in height and 28 feet in diameter.
• The containment should be confined to the refueling cell and the primary HTS vaults. Any provisions for HCDAs mandated by the regulator must be beyond design basis. Safety emphasis should be on plant simplification, highly reliable natural circulation decay heat removal, and enhancement of reactor shutdown system reliability.
• Decay heat removal should be through a PRACS with an OHRS for backup and normal shutdown operation. An IRACS or DRACS would be a reasonable alternative to a PRACS or supplement for PRACS if necessary.

Although the simplicity of the “design approach” is promising, it is expected that it will not be easy to obtain a reliably accurate cost estimate. Much of the cost of the plant will be in the major components — particularly the reactor vessel, and to a lesser extent the IHXs, EM pumps, and steam generators. Many of the specialty items e.g. cold traps will need to be “build to print”. Identifying potential vendors for LMFBR components who can make reliable cost estimates in the U.S. will not be a trivial undertaking. The primary reason the CRBRP reactor vessel was procured at such an early stage was the perception that there was only one credible vendor for such a task remaining in the U.S. at that time, and that vendor would shut down its capability for vessel manufacture if the CRBRP vessel were not procured early. The estimate for the structures, piping, and installation should be more straightforward given that the preliminary design is available.

Another area that is likely to be problematic is the state of current knowledge in sodium technology. There was a good deal of work done in this area at the Liquid Metal Engineering Center in the 1960s and 1970s, which continues to be somewhat available. Technical development activities with liquid metals were also performed at the Hanford Engineering Development Laboratory, Argonne National Laboratory, and the Knolls Atomic Power Laboratory. A five volume set on Sodium and NaK Engineering Technology was prepared during that period and the associated technical literature continues to be available, although not necessarily on the internet. It will likely turn out to be necessary to spend much time in technical libraries.

Once a believable cost estimate exists for this configuration, and the estimate shows total costs to be considerably less than an equivalently sized LWR as expected, consideration could be given to options which potentially improve the plant’s performance and operability. Although it may be tempting to do so, the cost of licensing should be known first. So long as the investment has been kept under control, there is no reason for seeking a limited work authorization or beginning construction immediately after a construction permit has been secured. The preliminary design can be modified if desired so long as the PSAR is kept up to date. For example, it may also be desirable to expand the containment volume to better accommodate growth items unforeseen during early design, alternative primary sodium treatment systems, or even the adoption of four IHXs rather than two. Any such expansions should be made only after assessing their cost impact and ensuring the resulting capital cost remains significantly below LWRs.