ML20024H852
| ML20024H852 | |
| Person / Time | |
|---|---|
| Issue date: | 08/06/1993 |
| From: | Thadani A Office of Nuclear Reactor Regulation |
| To: | Mower C AFFILIATION NOT ASSIGNED |
| References | |
| NUDOCS 9308260168 | |
| Download: ML20024H852 (3) | |
Text
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AUG 0 61993 Mr. Charles A. Hower 5501-5509 Wayne Avenue Apartment 406
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Philadelphia, PA 19144
Dear Mr. Hower,
Thank you for your letter of May 23, 1993, regarding the safety of nuclear power plants with pressure suppression containments designed by the General Electric Company.
We understand your concern to be the adequacy of a pressure suppression containment during or following a core melt accident.
As you may know, there are three different designs of pressure suppression containments used with boiling water reactors in this country.
These are designated. Mark I (the earliest design), Mark II (a later design, used, for example, on the i_imerick reactor in Pottstown, Pennsylvania which you may be familiar with), and the Mark III design, which is the most recent design in operation.
The NRC considers the pressure suppression containment to be a proven concept.
Boiling water reactors have used the pressure suppression containment since the very earliest designs. The physical mechanisms involved in steam condensation in a pool of water are well understood.
Equations have been derived to describe the process. Calculations have demonstrated the ability of this type of containment to adequately meet all its design requirements.
In addition, and most important, many tests performed both in this country and a;ound the world have verified the theory and demonstrated the ability of the pressure suppression containment to limit the pressure to > %in design limits. Major test programs were carried out in this country, for example, at the Massachusetts institute of Technology and at General Electric test facilities.
Full scale tests have 'een performed at the LaSalle and Monticello commercial nuclear power reactors.
Other calculations have shown the ability of the pressure suppression containment to withstand loads well in excess of the design pressure without failure.
For example, the design pressure of a Mark II containment is in the range of 45 to 53 pounds per square inch. Calculations have shown a factor of greater than 2% above this value before catastrophic failure.
Some problems were identified as a result of the extensive testing and calculations.
For example, loads on containment due to the discharge of larp quantities of steam to the suppression pool were identified as an issue in 1972. These loads had not been taken into account during licensing of individual plants up to that time. The issue came to light as a result of
, O proof testing of the Mark III design. A major program of testing and analysis solved this problem. Also, when it was discovered that the pressure
'f) suppression containment was vulnerable to damage from hydrogen burning, the p/
NRC issued a regulation requiring these containments to use nitrogen gas as 9308260169 930006
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Mr. Charles A. Mower 2
the containment atmosphere to eliminate oxygen and to therefore preclude hydrogen combustion.
With regard to the nuclear accident at the Chernobyl reactor in the Ukraine in 1985, the NRC has studied this accident extensively including the implications to reactors of the types utilized in this country. There are important points to be made regarding the suppression pool at Chernobyl.
First, the suppression pool had no effect on the course of the accident. Second, the containment at Chernobyl and the suppression pool are very different from the containments and suppression pools in U.S. boiling water reactors. The Chernobyl reactor did not have a containment in the sense that we would use the term in this country. Only the lower portiens of the reactor piping were in a housing which we would term a containment in the sense that the enclosure was designed to prevent the release of fission products during accidents. The remaining portions of the system were enclosed in a building which is not designed to the same degree of structural strength or leak tightness. Also, U.S. boiling water reactors contain much more water in the suppression pool, and this is very beneficial in determining the course of a severe accident.
In evaluating the likelihood _f a particular response of a nuclear reactor and its containment to core melt conditions, the NRC relies on an analysis technique called probabilistic risk analysis or PRA, together with more conventional experimental and calculational methods. PRA puts various postulated accidents into a safety context by calculating the probability of various outcomes. The evaluation of severe accident vulnerability using PRA involves three distinct evaluations. The first involves estimatir.g the probability of core (nuclear fuel) damage, the second involves estimating the likeithood of containment failure and the third involves an assessment of the r.aiological consequences.
These studies have been performed on boiling water reactors with each of the three types of containments. Although these studies are inherently chav e terized by large uncertainties, the analyses have all shown that the
- ty of core damage, containment failure and severe radiological pro 3
con. g. s as to the public are quite low. The results of these studies suppor; 6,e overall conclusion of low severe accident risk for boiling water reactors with suppression pool containments.
The NRC has established several programs to assure the adequacy of containments for U.S. reactors to adequately meet challenges caused by severe accidents. These include efforts following the Three Mile Island Unit 2 accident in this country in 1979, the implementation of the Commission's Policy Statement on Severe Accidents, a Containment Improvement Program and an Independent Plant Examination effort, which is an NRC-required, utility-performed probabilistic risk assessment to identify severe accident veo erabilities. One result of these programs has been the requirement for a
%ned vent." This is a vent from the containment to the atmosphere which m able the containment to be depressurized to maintain containment i
EE O 61993 Mr. Charles A. Mower 3
integrity during a severe accident. The containment pressure may otherwise cread the design pressure. At the same time, harmful radioactive substances are removed by filters from the discharged gas. All BWRs with Mark I containments are required to have a hardened vent.
I hope that the information I have provided will give you an increased confidence in the safety of the pressure suppression containment used on BWRs.
As a result of these efforts the NRC is confident that the response of a BWR pressure suppression containment to a severe accident is well understood and that the consequences to the public. of a severe accident or meltdown in a BWR are acceptably low.
Sincerely, AshoR'pcip'h[adani, Director 9GNED 3Y K.C.TiiADANI n
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