October 19, 2000

 

 

The Honorable Richard A. Meserve

Chairman

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

 

Dear Chairman Meserve:

 

We are writing to you in reference to a serious issue concerning the safety of pressurized-water reactors (PWRs) with ice condenser containments.

 

This issue, the high susceptibility of ice condenser containments to early failure from hydrogen combustion during a severe accident, has come to our attention through documents released by the NRC in the last few months. In particular, a new technical study conducted for the NRC by Sandia National Laboratories (SNL), Assessment of the DCH Issue for Plants with Ice Condenser Containments (NUREG/CR-6427), finds that "ice condenser plants are at least two orders of magnitude more vulnerable to early containment failure than other types of PWRs." [1] In view of these alarming new findings, which call into question the ability of ice condenser containments to provide adequate protection of the public health and safety, we urge the Commission to take immediate action to mitigate the risk of catastrophic, loss-of-containment accidents at the nine U.S. ice condenser units.

 

The vulnerability of ice condenser plants to early containment failure in a severe accident is of particular concern in view of the impending use of mixed plutonium-uranium (MOX) fuel at the McGuire and Catawba plants, operated by Duke Energy. A study by the Nuclear Control Institute (NCI), released last year and scheduled for publication in a peer-reviewed journal, calculates that implementation of Duke Energy's MOX plan would significantly increase (by 25%) the number of latent cancer fatalities resulting from a severe accident with early containment failure at McGuire or Catawba, because of the greater quantities of plutonium and other actinides that would be released into the environment. In addition, many issues remain to be resolved regarding the potential for MOX fuel use to increase the probability of a severe accident. Given these additional risks and uncertainties, the use of MOX in the type of PWR that is most vulnerable to early containment failure will make an already risky situation even worse and should no longer be considered.

 

The report NUREG/CR-6427 finds that the lower volume and strength of ice condenser containments render them less resistant than other types of PWR containments to the overpressures that would be generated by credible hydrogen combustion events. While the presence of hydrogen control systems (glow plug igniters) can mitigate this risk, they require AC power supplies and thus would be inoperable in station blackouts (SBOs). Consequently, the calculations presented in NUREG/CR-6427 (Tables 4.21, 4.24 and 7.4) indicate that if an SBO occurs, the early containment failure probability ranges from 27% to 97% for U.S. ice condenser plants. For Catawba and McGuire, the probabilities are 34% and 59%, respectively. If an SBO occurs and the vessel ruptures at high pressure, the failure probabilities are 100% and 98%, respectively. These results indicate that defense-in-depth is seriously compromised at ice condenser plants.

 

We are aware that the Commission has recently received an NRC staff paper that recommends that, as part of a "risk-informed" revision to the NRC's rule on combustible gas control (10 CFR 50.44), a requirement should be added for ice condenser plants to maintain a capability for hydrogen control during all risk-significant core-melt accidents, including SBOs. [2] However, since such a requirement would be voluntary on the part of licensees, we would go further than this recommendation and urge the NRC to transfer this issue to the generic safety issues (GSI) program for determining on a high-priority basis the need for imposition of mandatory backfits at ice condenser plants. There should be a separate determination of this need for Catawba and McGuire, comprehensively taking into account the additional safety risks associated with the planned use of MOX fuel at these plants.

 

It is highly unlikely that the unacceptably large risk of early containment failure at ice condenser plants can be mitigated merely by an attempt by licensees to show that the SBO frequency is low, a possibility that the NRC staff has suggested. SBOs can result from a wide range of external events, such as seismic events, that are hard to predict accurately. Moreover, an important precursor to an SBO, a loss of off-site power, can be caused by a sabotage attack outside the protected areas of the plant. Such attacks cannot be prevented with high assurance, and their likelihood cannot be quantified. In this scenario, a robust containment is the major line of defense.

 

Even if the NRC takes action to reduce the risk of early containment failure at ice condenser plants during SBOs, it remains to be seen whether the resulting plant risk will be small enough to offset the increased risk associated with MOX use. The fact will remain that ice condenser containment buildings are only half as pressure-resistant as other types of PWR containments. Moreover, the assumption made by NUREG/CR-6427 that the ice condensers themselves will function properly to mitigate the threat of steam explosions is itself open to question. In our view, these plants will never be safe enough to accommodate MOX, and we are hopeful that the NRC will eventually reach a similar conclusion when it faces the challenge of approving license amendments to permit the use of MOX at Catawba and McGuire.

 

Since the results of NUREG/CR-6427 clearly constitute significant new information pertaining to the safety of using MOX fuel in Catawba and McGuire, we have requested that the Department of Energy (DOE) prepare a supplement to its November 1999 Surplus Plutonium Disposition Final Environmental Impact Statement (EIS) to take the new findings into account. The reactor safety analysis in the Final EIS was based on the Catawba and McGuire Individual Plant Examinations, the results of which are called into question by the new SNL analysis. While DOE would be the lead agency in preparing such a supplement, we expect that the NRC will play a key role in providing DOE with the most recent technical information necessary for the analysis.

 

We urge the NRC to act in a timely manner to address the critical safety issues posed by the inadequacies of ice condenser containments and the synergistic impacts of MOX use in such plants. In the event that the NRC fails to do so, we are reviewing our options for action under the applicable regulations. We look forward to receiving your prompt reply.

 

 

Sincerely,

 

 

Edwin S. Lyman, PhD

Scientific Director

 

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