Risk Based Maintenance for Nuclear Power Plant HVAC System

1.Matsudo Research Laboratory,
Hitachi Plant Technologies, Ltd., Matsudo, Japan


2.Nuclear System HVAC Construction Department,
Hitachi Plant Technologies, Ltd., Tokyo, Japan

3.Nuclear System HVAC Construction Department,
Hitachi Plant Technologies, Ltd., Tokyo, Japan

Abstract

1. Object
Nuclear power plant HVAC
(Heating, Ventilating, Air-Conditioning)
system aims at keeping suitable environment for
main machinery, and suppressing the release of
radioactive substances into unpolluted area.
Therefore, the stable operation is always necessary
whether the nuclear reactor is operating or stopping,
and more reliable maintenance is requested.

There is rotating machinery, such as the fan pump,
and static machinery, such as the duct damper,
in HVAC system. In traditional maintenance,
we maintain the rotation machinery on the basis
of recommended frequency by vendor. And we maintain
static machinery on the basis of visual check at daily
surveillance patrol and experts' assessments.

More enhanced and more efficient maintenance than
this traditional maintenance is necessary. It is necessary
for rotation machinery to introduce condition based
maintenance (CBM). On the other hand, it is necessary
for static machinery to plan appropriately preventive
maintenance.

In view of these facts, we applied risk based maintenance
(RBM) to static machinery in Nuclear power plant HVAC
system to catch experts' knowledge and make traditional
maintenance more enhanced.

2. Method
A risk consists of the effect of breakdown and the
degrees of degradation.

First, we clarified the factors that estimate the
effect of breakdown. As a result, the factors were
repair cost, the damage of system stop, the damage
of loss of function, and mechanical downtime.
And using these factors, we statistically analyzed
the effect of breakdown.

Second, we clarified the factors that estimate the
degrees of degradation. As a result, the factors were
degradation state (corrosion and fatigue), the past
replacement track record, the use environment
(corrosive environment, wind velocity, etc)
and design data (equipment types, wall thickness, etc.).
And using these factors, we statistically analyzed the
degree of degradation　based on the model which
increases linearly along with the duration of service.

3. The main result
(1)Based on the results of questionnaire made to
three experts, the method for quantification of
the degree of influence of evaluation factors in the
evaluation formula for the magnitude of damage
by using the Analytic Hierarchy Process (AHP)
was established.

(2)By pigeonholing the fluctuation factors of service
life and changing the service life depending on the
operating conditions of equipment, the degree of
deterioration was calculated in the ratio of years
lapsed to service life.

(3) Risk evaluation of static equipment in the actual
plant was made by the established quantitative evaluation
method with the result that the equipment of high risk
evaluation was almost coincident with the equipment
having the maintenance results by the judgment of expert
engineers. The adequacy of RBM was thus confirmed.

References
[1] Takayuki Aoki .et al,
“The literacy of maintenance,” 2007