shutdown, reactor core cooling and confinement of radioactivity.

For example, in the major pipe break accident of “loss of reactor coolant/¹ a single failure in the emergency core cooling system (ECCS), which is required to achieve the function of core cooling, is assumed so that the single failure causes the most severe consequences, taking account of the combination of the ECCS and the associated safety protection systems, power supply systems, etc. needed for starting the ECCS. And from the viewpoint of confinement of radioactivity, single failure of an emergency diesel generator is assumed.

4. Operation by operating personnel

In safety evaluation it is stipulated that an adequate time margin should be considered for operating personnel to accomplish manual operations required in coping with the event occurred. That is to say, evaluation is based on a necessary time allowance of at least 10 minutes before starting required actions after the operator obtains adequate information for diagnosis and operation. Some countties — like Germany, adopt the design concept that during the initial period of several minutes to 30 minutes from abnormal event occurrence, operators are directed not to undertake any operational actions and to rely on the automatic response of the plant In Japan, on the contrary, operations by operating personnel to mitigate abnormal events are allowed, but those are not considered during the time margin stipulated in the safety evaluation.

5. Treating control systems

Among the systems of NPPs, control systems are indispensable for normal operation. Examples are the control rod control system, pressurizer pressure control system, steam generator water level control system, and turbine control system. In general, control systems function to mitigate abnormal conditions, but some specific parameters may be influenced in a severer direction. (For example, from the viewpoint of minimum departure from nucleate boiling ratio (DNBR), operation of the pressurizer pressure control system causes severe results.)

Therefore in the safety analysis for PWR, a control system is assumed to function if it makes

the result worse, but if it does not make the result worse, it is assumed not to function. This is because of the policy that the mitigation effects of control systems are not given credit from the standpoint of safety evaluation that is confirmation of the pertinence of the basic policy of safety design of reactor facilities.

2. Abnormal Transients during Operation

1. Postulation of Events

Events postulated for “abnormal transients during operation” are those which may occur one or more times during the life of the nuclear reactor facility, and could result in excessive damage to the reactor core or the reactor coolant pressure boundary if left uncontrolled. For PWR plants the following possible events in three categories are analyzed:

(D Abnormal changes in the reactivity or power distribution in the reactor core

©-1 Uncontrolled control rod withdrawal at reactor startup

(D-2 Uncontrolled control rod withdrawal at power

(D -3 Control rod drop and misalignment

Q) 4 Inadvertent dilution of boron concentration in the reactor coolant

©Abnormal changes in heat generation or heat removal in the reactor core

(2) -5 Partial loss of reactor coolant flow

(2) -6 Startup of an inactive reactor coolant loop

(2) -7 Loss of off-site power supply

@ -8 Loss of normal feedwater flow

(2) -9 Excessive increase in steam flow

2. -10 Inadvertent depressurization of

secondary cooling system

(D -11 Excessive increase in feedwater flow

(3) Abnormal changes in reactor coolant pressure or reactor coolant volume.

2. -12 Loss of external electrical load

(3) -13 Inadvertent depressurization of reactor coolant system

(3) -14 Inadvertent actuation of the ECCS during power operation

The acceptance criteria for “abnormal transients during operation” are as indicated in Table 7.1.1. Three acceptance criteria are related to the integrity of

NSRA, Japan

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