Item A-16: Steam Effects on BWR Core Spray Distribution

DESCRIPTION

Historical Background

Prior to May 1978, tests conducted by GE showed that the presence of steam and/or increased pressure in and above the upper core region of BWRs could adversely affect the distribution of flow from certain types of core spray nozzles. These nozzles are arranged to distribute water over the top of the reactor core in the event of a LOCA.

These new test data were collected from a reactor core spray system with a single nozzle spraying downward. However, spray flow in most domestic BWR core spray systems comes from many nozzles spraying approximately horizontally over the core from a sparger (or spargers) surrounding the core. Therefore, the degree of applicability of the new data to domestic BWRs was not known. As a result, this issue was included

in NUREG-0371[1]to provide results which could be verified as being applicable to the size and design of each BWR in operation. In order to justify the preliminary acceptability of core spray cooling designs in operation,

GE presented test results and calculations that were based on the separability of hydrodynamic phenomena (droplet-to-droplet interaction where spray patterns from two or more nozzles intersect) and thermal phenomena (steam condensation).

Safety Significance

If BWRs are to strictly conform to the post-LOCA requirements established by 10 CFR 50.46 to ensure the health and safety of the public, then their core spray systems must supply a specified minimum amount of coolant to each fuel bundle in their respective reactor cores. Therefore, core spray assumed in the LOCA analyses must be actually supplied in the post-LOCA team environment. This issue was a topic in the SEP for both Millstone 1 and Dresden 2.

Possible Solution

The solution recommended in NUREG-0371[2]called for: (1) a series of tests on operating BWR/6 core spray distribution systems; and (2) a full-scale test of a 30 sector of a BWR/6 upper plenum, complete with spargers. Test results were to be reviewed by the NRC for acceptability of the analytical and experimental techniques used to determine the safety margin present in core spray distributions for all BWRs in operation and under construction.

CONCLUSION

Test results issued by GE in August 1979[3]compared favorably with the pre-test prediction, within defined acceptance limits, and confirmed the capability of the methodology to handle steam environment effects on spray performance. The results substantiated the key assumption of separability of thermodynamic and hydrodynamic effects. These test results were reviewed by the NRC and determined to "constitute an adequate confirmation of the GE spray distribution methodology for BWR/6-type spargers."[4]However, the NRC required additional tests to be performed to confirm the design methodology for other sparger designs. As a result, a test program was initiated to provide core spray distribution data in a steam environment for a 30-degree sector of the BWR/4 and BWR/5 design. In March 1981, test results for this BWR design were published in NUREG/CR-1707.[5]These data demonstrated the applicability of the core spray methodology in this design which had nozzle types and sparger evaluations that were different from the BWR/6 design tested in 1979.

The BWR/1 core spray design was reviewed by DSI/NRR in 1979 and found to be acceptable. Following the review of GE test data for the BWR/3 core spray design, DSI/NRR concluded in March 1983 that the core spray

distribution adequacy was not a safety concern for all BWR/3 reactors.[6]

MPA D-12 was established by DL/NRR for the review of the BWR/2 core spray system design, and for the preparation of an SER for each of the two domestic reactors of this design: Oyster Creek and Nine Mile Point,

Unit 1.[7],[8]Based on the plant-specific reviews that were undertaken by the NRC, this issue was RESOLVED with no new requirements for licensees.