Tuesday, July 29, 2008

Freeze valves in the MSRE

ORNL Freeze Valve Research

Freeze valves were very much a part of the design of the MSRE. ORNL-TM- 732 describes in some detail freeze plug operations. I did a brief review of ORNL technical reports to determine if passive safety freeze valves had been tested during the MSRE. ORNL-TM-128 (DEVEL0PMENT OF FREEZE VALVE FOR USE IN THE MSRE) offers a fairly detailed account of ORNL Freeze Valve research to 1962. ORNL Freeze valve research indicated that the system adopted was both safe and reliable.

ORNL-TM- 732 made numerous references to the Freeze Valve system in connection to safety considerations in operation of the MSRE.

No valves of the ordinary type are used in contact with salt. Flow in lines connecting the reactor vessel and the drain tanks is prevented by freezing salt in designated sections of pipe. The "freeze valves" thus formed can be thawed by stopping the flow of cooling air and heating the pipe. By this means, salt can be drained from the reactor vessel. (From Page 20-21)

Thus reeze valves were a part of the standard MSRE, and their reliable function was essential to its operations. Freeze valves were kept closed by cooling the outside of the valve.

Freeze Valves

Since no reliable mechanical valve was available at the start of the project, it was decided to use frozen plugs of salt to control the trans- fer of salt. Development of these "valves" produced a unit in which it is possible to establish and maintain reliably either the open or closed condition. For the normally open valve, approximately 10 min is required to thaw the frozen valve, even under the conditions of a total power failure. The normally closed valves, with the requirement that they remain closed during a power failure, are allowed to cool to ambient temperature, thereby insuring that there is not enough energy to open them, except on demand. The time required to open this type of valve is in excess of 1 hr. Both types of valves require an average of 25 min to freeze.

The freeze valves to be used in the reactor system were designed to ensure that the thawing operation would proceed from the molten zones at the ends of the valve to the frozen zone at the center. The purpose of this feature is to prevent expansion damage to the valve as a result of thawing at the center of an excessively long frozen zone. Prototype valves of this design have been operated through more than 100 freeze-thaw cycles without dimensional change.
(page 302)

ORNL-TM- 732

This report indicates that all valves on the MSRE were freeze valves, and that their operation was routine. Freeze valves were closed by cooling a pipe containing hot salt.  They were opened by interrupting the cooling, and applying heat to the pipe. It would appear from MSRE reports that freeze valves were operated frequently during the MSRE. I have not found a reference to a problem with freeze valves during the MSRE, and that the reactor salts were drained several times.

Given the method of valve operation developed by ORNL, freeze valve system was very amenable to passive safety.  The shutdown of freeze valve cooling and the external application of heat would be an easy engineering challenge as part of a fail safe emergency shut down system.   A fail safe freeze valve system should be subjected to extensive testing as part of a commercial LFTRM reactor design.    

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