Page 135 - Dynamics and Control of Nuclear Reactors

11.4 Analysis of potential reactor accidents 131

boundary (with the possibility of causing cancer in exposed residents). These
occurred because of an inadequate safety culture, design deficiencies, and unsafe
operation (often because of failure to understand how to deal with abnormal reactor
behavior). These events were disasters, but they were an unfortunate part of the path
to better understanding of how to build and operate reactors safely. An analogy is the
accidents that occurred during the age of early space exploration. A slower pace of
early reactor implementation might have avoided the accidents, but there was a
strong motivation to hurry in the interest of producing weapons materials or advanc-
ing nuclear power for generating electricity.

11.4 Analysis of potential reactor accidents
Analyses are performed to assess reactor safety before they are licensed for construc-
tion and operation. These analyses include deterministic assessments and probabilis-
tic assessments. The deterministic assessments involve simulation of postulated
reactor accidents. The probabilistic assessments involve evaluation of the likelihood
of component failures resulting in an accident.
The deterministic simulations use all of the modeling methods described in pre-
vious chapters, but also model phenomena that do not occur in normal operation
(such as loss of coolant or rapid control rod ejection). The reactor safety simulations
must address extreme conditions such as boiling in normally liquid coolant, fuel
melting, pressurization in the primary system or the containment building, and struc-
tural failures. Great effort at a number of government and private organizations to
prepare various computer codes to simulate reactor accidents has occurred. For
example, the reactor safety computer codes at the U.S. Nuclear Regulatory Commis-
sion (NRC) include the following [8]:

• Probabilistic risk assessment codes
• Fuel behavior codes
• Reactor kinetics codes
• Thermal-hydraulics codes
• Severe accident codes
• Protection codes
• Radionuclide transport codes
• Materials performance codes

Safety analysis involves analysis of a Design Basis Accident (DBA) [9]. The DBA
analysis is a simulation of the worst conceivable accident (typically a loss of cool-
ant). The assumption is that if the reactor can tolerate a DBA, then it can tolerate
other accidents considered less serious than the DBA. But experience has shown
us the accidents that are worse than the DBA can occur because of unanticipated nat-
ural events (like a larger than anticipated tsunami), equipment failure (like a stuck
valve), or operator errors. Such events are called Beyond Design Basis Accidents
(BDBA) [10]. There are now active analyses of BDBAs underway and the U.S.