Page 136 - Dynamics and Control of Nuclear Reactors

132 CHAPTER 11 Nuclear reactor safety

Nuclear Regulatory Commission (NRC) rule requires that these analyses be con-
ducted for all nuclear plants – new reactor builds, reactors requiring life extension,
and operating reactors.
The NRC considers information provided by an applicant in a Safety Analysis
Report (SAR). The SAR must address NRC requirements published in 10CFR50,
a code of federal regulations (CFR) document [11]. The NRC reviews the SAR using
procedures defined in NUREG800 (NUREG stands for Nuclear Regulation). Orig-
inally, applicants had to submit two SARs, a Preliminary SAR (PSAR) for permis-
sion to construct and a Final SAR (FSAR) to operate. Subsequently, the NRC
simplified the application process, requiring only a single SAR. The applicant’s pro-
posal is reviewed by the NRC staff and an independent group of reactor safety
experts (The Advisory Committee on Reactor Safeguards or ACRS). The proposal
also undergoes a public hearing where opponents are given an opportunity to express
their concerns.
The NRC can impose new requirements after the reactor goes into operation
through Nuclear Regulatory Guides. These are imposed as a result of further
NRC study, new developments or experience. For example, NRC decided that it
is necessary to confirm that the response time of safety system sensors is as short
as assumed in the SAR. The Electric Power Research Institute (EPRI) sponsored
a research project that resulted in an in-situ response time testing of resistance ther-
mometers [12] or resistance temperature detectors (RTD). The NRC approved the
test and it is used routinely in PWRs.
Probabilistic risk assessment (PRA) provides the calculated likelihood of an acci-
dent [13]. Likelihood of failures in safety-related components are combined to pro-
vide the probability of an overall failure and an accident. PRA results are presented
as the likelihood of an accident in Y years. PRAs are done very carefully, but the
possibility of inaccurate component failure probabilities or failure to realize and
include an important component failure means that PRA results cannot be judged
as perfect. Also, even a low probability does not mean that an event could not occur.
A likelihood of one failure in a million years means that a failure in the first year is
very unlikely, but not impossible. So, the issue is “How good is good enough?”

11.5 Accidents in Generation-II power reactors
11.5.1 Three mile Island [14]
On March 28, 1979, an accident occurred at Three Mile Island unit 2 near Harrisburg,
PA. TMI-2 was an 800 MWe PWR supplied by Babcock and Wilcox. The initial fac-
tor in causing the accident was a problem introduced during routine maintenance of a
component in the secondary system called the condensate polisher. It is a filter used
to purify the secondary water. Forcing water through the filter to clean it found its
way into other secondary systems. The crucial effect was disabling the feedwater
pumps that fed water into the steam generators.