Sunday, February 24, 2008

WASH-1222 with Comments: Part 5

Introduction: This section of WASH-1222 focuses on the ORNL MSBR design of 1972. I must confess that I am not an admirer of the MSBR design. The flaws of the design were in part a direct result of the requirements of Milton Shaw. A better research and development plan would have involved a evolutionary follow up to the MSRE. The MSRE was a one fluid design. Scientist like my father felt that a 2 fluid design held greater promises for breeding. The Fluoride salt based reactor us not a high output breeder. It is a 1 for 1 breeder. It only breeds as much fuel as it uses, or slightly more. I call this type of breeder an auto-breeder.

A second form of molten salt breeder, the chlorine breeder, offers far more promise as a breeder of surplus reactor fuel. The 1972 MSBR was in many ways a mistake and not a fair test of molten salt reactor technology. I invite my readers to contemplate the distinction between a flawed concept, and a flawed design. The Molten Salt Reactor is a promising concept, but the 1972 MSBR was a flawed design. - CB



E. Reactor Equipment and Systems Development

While the MSBR would utilize some existing engineering technology from other reactor types, there are specific components and systems for which additional development work is required. Such work would have to take into account the induced activity that those components would accumulate in the MSBR system, i.e., special handling and maintenance equipment would also need to be developed. The previous discussion has already dealt with a number of these, such as fuel processing components and systems, but additional discussion is appropriate.

[True. – CB]

1. Components

As indicated in the Table 3, a number of components must be scaled up substantially from the MSRE sizes before a large MSBR is possible. The development of these larger components along with their special handling and maintenance equipment is probably one of the most difficult and costly phases of MSBR development. However, reliable, safe, and maintainable components would need to be developed in order for any reactor system to be a success.

The MSBR pumps would likely be similar in basic design to those for the MSRE, namely, vertical shaft, overhung impeller pumps.

Substantial experience has been gained over the years in the design, fabrication and operation of smaller salt pumps, but the size would have to be increased substantially for MSBR application. The development and proof-testing of such units along with their handling and maintenance equipment and test facilities are expected to be costly and time consuming.

The intermediate heat exchangers for the MSBR must perform with a minimum of salt inventory in order to improve the breeding performance by lowering the fuel inventory. Special surfaces to enhance heat transfer would help achieve this, and more studies would be in order. Based on previous experience with other reactor systems, it is believed that these units would require a difficult development and proof testing effort.

The steam generator for MSBR applications is probably the most difficult large component to develop since it represents an item for which there has been almost no experience to date. It is believed that a difficult development and proof-testing program would be needed to provide reliable and maintainable units. As discussed previously, the high melting temperatures of candidate secondary coolants, such as sodium fluoroborate, present problems of matching with conventional steam system technology. At this time, central station power plants utilize feedwater temperatures only up to about 550ºF (560 K). Therefore, coupling a conventional feedwater system to a secondary coolant which freezes at 725ºF (658 K) presents obvious problems in design and control. It might be necessary to provide modifications to conventional steam system designs to help resolve the problems. Because of these factors, a study related to the design of steam generators hash been initiated at Foster-Wheeler Corporation.

Control rods and drives for the MSBR would also need to be developed. The MSRE control rods were air-cooled and operated inside Hastelloy-N thimbles which protruded down into the fuel salt. The MSBR would require more efficient cooling due to the higher power densities involved. Presumably rods and drives would be needed which permit the rods to contact and be cooled by the fuel salt.

The salt valves for large MSBR's represent another development problem, although the freeze valve concept which was employed successfully in the MSRE could likely be scaled up in size and utilized for many MSBR applications. Mechanical throttling valves would also be needed for the MSBR salt systems, even though no throttling valve was used with the MSRE. Mechanical shutoff valves for salt systems, if required, would have to be developed.

Other components which would require considerable engineering development and testing include the helium bubble generators and gas strippers which are proposed for use in removing the fission product xenon from the fuel salt. Research and development in this area is currently under way as part of the technology program at ORNL.

[MSR technology is poorly matched to steam generation but well matched to Brayton cycle gas turbines. ORNL molten salt research began with an attempt to power aircraft jet engines with the output heat from reactors. The Aircraft Nuclear Propulsion program envisioned small molten salt reactors heated red hot by nuclear energy, channeling its coolant fuel salts into the jet engines of the atomic powered aircraft. Thus the molten salt reactor was known to be well matched to Brayton cycle turbines. Using a MSR for heat to convert sater into steam presents difficulties. – CB]

2. Systems

The integration of all required components into a complete MSBR central station power plant would involve a number of systems for which development work is still required. It should be noted that some components, such as pumps and control rod drives, would require their own individual system for functions such as cooling-and lubrication.

Given the required components and materials of construction, the basic reactor primary and secondary flow systems can be designed. However, the primary flow system would require supporting systems for continuous fuel processing, on-line fuel analysis and control of salt chemistry, reactor control and safety, handling of radioactive gases, fuel draining from every possible holdup area in components and equipment, afterheat control, and temperature control during non-nuclear operations.

The continuous fuel processing systems proposed to date are quite complicated and include a number of subsystems, all of which would have to operate satisfactorily within the constraints of economics, safety, and reliability. The effects of off-design conditions on these systems would have to be understood so that control would be possible to prevent inadvertent contamination of the primary system by undesirable materials.

The fuel drain system is important to both operation and safety since it would be used to contain the molten fuel whenever a need arises to drain the primary system or any component or instrument for maintenance or inspection. Thus, additional systems would be required, each with its own system for maintaining and controlling temperatures. The fuel-salt drain tank would have to be equipped with an auxiliary cooling system capable of rejecting about 18 MWt of heat should the need arise to drain the salt immediately following nuclear operation. The secondary coolant system would also require subsystems for draining and controlling of salt chemistry and temperature. In addition, the secondary loop might require systems to control tritium and to handle the consequences of steam generator or heat exchanger leaks.

The steam system for the MSBR might require a departure from conventional designs due to the unique problems associated with using a coolant having a high melting temperature. Precautions would have to be taken against freezing the secondary salt as it travels through the steam generator; suitable methods for system startup and control would need to be incorporated. ORNL has proposed the use of a supercritical steam system which operates at 3500 psia (240 bar) and provides 700ºF (640 K) feedwater by mixing of supercritical steam and high pressure feedwater. This system would introduce major new development requirements because it differs from conventional steam cycles.

[The argument here is that there were many developmental challenges confronting ORNL’s MSBR. Had the same criteria been applied to the AEC’s proposed LMFB, many of the same concerns would have emerged. Since the current MSR concepts envision the use of closed circulation gas turbines, rather than steam generation, so while the inclusion of concern about the steam generation system in 1972 was valid, it is not still valid today. However, the design and construction of primary and secondary heat exchange systems in any MSR power recovery system, do present technical challenges.]

F. Maintenance - A Difficult Problem for the MSBR

Unlike solid-fueled reactors in which the primary system contains activation products and only those fission products which may leak from defective fuel pins, the MSBR would have the bulk of the fission products dispersed throughout the reactor system. Because of this dispersal of radioactivity, remote techniques would be required for many maintenance functions if the reactor were to have an acceptable plant availability in the utility environment.

The MSRE was designed for remote maintenance of highly radioactive components; however, no major maintenance problems (removal or repair of large components) were encountered after nuclear operation was initiated. Thus, the degree to which the MSRE experience on maintenance is applicable to large commercial breeder reactors is open to question.

As has been evident in plant layout work on nuclear facilities to date, this requirement for remote maintenance will significantly affect the ultimate design and performance of the plant system. The MSBR would require remote techniques and tools for inspection, welding and cutting of pipes, mechanical assembly and disassembly of components and systems, and removing, transporting and handling large component items after they become highly radioactive. The removal and replacement of core internals, such as graphite, might pose difficult maintenance problems because of the high radiation levels involved and the contamination protection which would be required whenever the primary system is opened.

Another potential problem is the afterheat generation by fission products which deposit in components such as the primary heat exchangers. Auxiliary cooling might be required to prevent damage when the fuel salt is drained from the primary system, and a requirement for such cooling would further complicate inspection and maintenance operations.

In some cases, the inspection and maintenance problems of the MSBR could be solved using present technology and particularly experience gained from fuel reprocessing plants. However, additional technology development would be required in other areas, such as remote cutting, alignment, cleaning and welding of metal members. Depending to some degree on the particular plant arrangement, other special tools and equipment would also have to be designed and developed to accomplish inspection and maintenance operations.

In the final analysis, the development of adequate inspection and maintenance techniques and procedures and hardware for the MSBR hinges on the success of other facets of the program, such as materials and component development, and on the requirement that adequate care be taken during plant design to assure that all systems and components which would require maintenance over the life of the plant are indeed maintainable within the constraints of utility operation.

[The observations about potentially difficult maintenance problems are valid. The removal and replacement of core graphite was a realistic possibility with the MSBR, and indeed its graphite core might have requited frequent replacement. The best approach to maintenance is to eliminate or simplify maintenance problems as much as possible during the design phase. If pumps can be replaced with thermal siphoning this would eliminate problems attendant on the design of costly parts, and the problems and cost of maintaining them. Having said this, it still should be noted that requirement for automated maintenance of the MSBR was by no means beyond the capacity of 1972 technology.]

G. Safety - Different Issues for the MSBR

The MSBR concept has certain characteristics which might provide advantages relating to safety, particularly with respect to postulated major types of accidents currently considered in licensing activities. Since the fuel would be in a molten form, consideration of the core meltdown accident is not applicable to the MSBR. Also, in the event of a fuel spill, secondary criticality is not a problem since this is a thermal reactor system requiring moderator for nuclear criticality.

Other safety features include the fact that the primary system would operate at low-pressure with fuel salt that is more than 1000ºF (550 K) below its boiling point, that fission product iodine and strontium form stable compounds in the fluoride salts, and that the salts do not react rapidly with air or water. Because of the continuous fuel processing, the need for excess reactivity would be decreased and some of the fission products would be continuously removed from the primary system. A prompt negative temperature coefficient of reactivity is also a characteristic of the fuel salt.

Safety disadvantages, on the other hand, include the very high radioactive contamination which would be present throughout the primary system, fuel processing plant, and all auxiliary primary systems such as the fuel drain and off-gas systems. Thus, containment of these systems would have to be assured. Also, removal of decay heat from fuel storage systems would have to be provided by always ready and reliable cooling systems, particularly for the fuel drain tank and the 233Pa decay tank in the reprocessing plant where megawatt quantities of decay heat must be removed. The tritium problem, already discussed, would have to be controlled to assure safety.

Based on the present state of MSBR technology, it is not possible to provide a complete assessment of MSBR safety relative to other reactors. It can be stated, however, that the safety issues for the MSBR are generally different from those for solid-fuel reactors, and that more detailed design work must be done before the safety advantages and disadvantages of the MSBR could be fully evaluated.

[This last statement is quite disingenuous. The problem lies with the word complete. By 1972 the safety advantages of the MSR were well known. If there were missing components of the picture the limits of the safety problems were obvious. Uri Gat and H. L. Dodds were later to discuss the safety of the MSR in a paper titled MOLTEN SALT REACTORS - SAFETY OPTIONS GALORE. Gat also discusses MSR safety in another paper. This discussion was based on of information that would have been available to the authors of WASH-1222. It is clear from Gat and Dodds that WASH-1222 could and should have discussed the relative safety of the MSR. In summarizing the MSR’s safety features, they observe:

“The molten-salt reactor with fuel processing can be designed to be almost as safe as desirable. The basic features of fluoride based molten salts allow for a high temperature, and thus efficient, operation at low pressures. The molten salts are inert and well compatible with selected structural materials. The MSR is not subject to safety concerns from chemical or mechanical violent reactions or explosions. External cooling results in a simple design with few structural requirements that permits optimization of the design for safety-eliminating compromises. The on-line processing results in an equilibrium fuel that requires no excess reactivity for burn-up or poison compensation. The fission product inventory, and therefore the source term, is held low. The severe accidents of uncontrolled super-criticality or loss-of-cooling that fails to remove the after-heat can become a hypothetical accident.

The dreaded meltdown looses all its meaning in a fluid-fuel reactor. In an MSR, a spill may be self-containing by the freezing of the fuel upon cooling. Freeze valves are one more feature that can make an MSR PINT (passive, inherent, non-tamperable) safe.”

In addition Gat and Dodds believe that the MSR concept is capable of refinement into an reactor that is absolutely and ultimately safe, if that degree of safety is desired.

It is clear then, that WASH-1222 has largely discounted the known and highly desirable safety features of the MSR. This was a reflection of the problem that Milton Shaw brought to the AEC. As I have demonstrated in earlier posts, Milton Shaw believed that LWRs were completely safe. His anger caused by Alvin Weinberg insistence that LWR safety issues had not been resolved eventually lead to Weinberg’s firing. Shaw was determine to prevent adverse evaluations of the relative safety of his pet reactors to become a part of AEC project evaluations. Thus the safety related comments of WASH-1222 ware written to further Shaw’s agenda, and in no way reflected the state of reactor technology. This was part of the path which lead to Three Mile Island. In addition the safety features of the LMFBR compaired quiye unfaviorably with those of the MSR. Shaw was protecting another pet project by with holding a true safety evaluation of the MSR. - CB]

1 comment:

Engineering observer said...

Two more points regarding how the report distorts the MSR safety comparison:

1) Fluoride salts simply do not react with air or water, so when the report says that “the salts do not react rapidly with air or water”, it’s spinning the facts. Not too important in itself — but it points to an agenda.

2) Writing in the wake of the Fukushima events, I find it striking that the report speaks of “safety” without distinguishing between radiation exposure hazards inside the plant and massive release of radioactive materials into the environment. Avoiding massive external release is (a) more important, (b) the greatest MSR safety advantage, and (c) an unmentionable advantage in the already-risk-free Milton Shaw world.


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