Experiences in Nuclear Power Generation and Safety Research

atomic bombs. This experiment provided important suggestions and notices for reactor design including the fuel and control rods of LWRs, which has contributed to the basis of current nuclear reactor designs.

The second subject is about the experiments and researches on loss of coolant accidents, which is one of the two most important reactor safety researches, the other being the reactivity insertion accident researches by the BORAX and SPERT. The experiments at a semi-scale testing facility simulating a nuclear reactor at Idaho in the US draw special attention, because they showed that the emergency core cooling water injected from accumulators was lost through the break of the reactor coolant pipe and could not reach the reactor core. The testing was not sufficient to conclude that similar phenomena as observed could occur in a real reactor, since the testing facility did not precisely simulate the real system functions of the reactor coolant loop configuration. Stimulated by the testing, researches on LOCAs began being conducted however eagerly using large-scale testing facilities inside and outside Japan. At present, the effectiveness of emergency core cooling system (ECCS) functions has been verified by the LOFT (Loss of Fluid Test) program in the US and the ROSA (Rig of Safety Assessment) program of the former Japan Atomic Energy Research Institute.

More than half a century has passed since the first nuclear reactor was constructed. During the years some accidents and problems have occurred due to the highly sophisticated technologies in use and they have been reported on in detail. Some of the serious accidents and the lessons learned from them are described in the following.

Although many experiments had been done that focused on reactivity insertion accident (RIAs) and accidents with loss of coolant, from the beginning of the development of nuclear power reactors as already described, we experienced severe accidents representing typical types of these accidents occurred. The Three Mile Island (TMI) accident which occurred in March 1979. was a LOCA. It was triggered by heat removal failure from the secondary side of the steam generators and it eventually resulted in core damage.

Many lessons were learned from the TMI

NSRA, Japan

1-4

accident. In the accident, all the exit valves of the auxiliary feedwater pumps were in the closed position and a pressurizer power-operated relief valve failed in the stuck open position; these indicated problems in equipment control after restoring normal operation following maintenance work. There was also the operator’s mal-operation by terminating the ECCS operation too early and problems on man-machine interfaces. All of these occurred and enlarged a small or intermediate LOCA event into a core damage accident. In addition, the accident gave us lessons learned on the inadequate information systems, including off­site organizations, and of emergency response measures. The lessons learned were related to interfaces with the environment, in a broad sense in the vicinity of NPPs, as well as being related to design, operation and maintenance work, and its control.

Beside we have to mention the severe accident occurred in April 1986 at the Chernobyl Nuclear Power Reactor (RBMK-type; Graphite Moderated Light Water Boiling Reactor) in the former USSR and was an RIA. In the accident, because priority was being given to testing of the turbine generator inertia, the unique characteristics of the reactor at low power operation caused a rapid reactor power increase, which brought on thermal shocks and over-pressure shocks and destroyed the reactor and the buildings largely. A fire of the graphite in the core followed and finally large amounts of radioactive materials were released to the environment

The scale of the radioactivity releases and the number of victims due to acute radiation were large and had never been experienced in the past. The root-causes of such an expanded accident were pointed out as: (a) reactor power instability at low power; (b) deficiency of the reactor containment; and (c) many actions of the operators which were against the rules.

We learned that safety consciousness and safety culture of the personnel in charge of operational management is the last fort of defense. The fact that severe accidents such as the TMI and Chernobyl accidents, which were regarded as beyond design events for evaluation at the beginning, could occur and gave us big shocks really. Since then, severe

accidents have been elaborated on worldwide and nowadays, operating manuals and countermeasures in design to cope with beyond design basis events (socalled accident management; AM) are prepared.

As mentioned, lessons learned from experienced accidents during development of nuclear power generation have always been reflected onto ensuring safety. It should be noted that operational management is important in a broad sense. Operators make decision and actions based on their knowledge of accidents and their experiences as is the case in any situations. The more experiences that are accumulated as well as knowledge, the more adequately decisions and actions can be made.

However the actual events experienced by individuals who work at a power plant are limited.

If failures experienced at other power plants anywhere in the world are acknowledged and thought about as each operator’s own experience, adequate decisions and actions can be expected. For these purposes, the US Institute of Nuclear Power Operations (INPO), founded after the TMI accident, and the Japan Nuclear Technology Institute (IANTI) are gathering, processing and compiling data bases of operating experience information on NPPs and transmitting them rapidly to operators. The World Association of Nuclear Operations (WANO), founded in 1987 after the Chernobyl accident, and the JANTI in Japan are carrying out peer reviews by specialist teams organized from members of these organizations to extract issues and good cases through site visits, examination of documentations and interviews for the purpose of opinion exchanges in specialties. Those activities help to promotion voluntary safety enhancement by their members.

On the other hand, as a post-TMI activity, efforts have been made to grasp reactor safety quantitatively by probabilistic safety assessment (PSA) and to reflect the results of the evaluation to safety design. The approach was already studied by the famous Dr. Norman C. Rasmussen in the “Reactor Safety” report (WASH-1400, 1975) and drew attention after the TMI accident. The accumulated operating experiences with NPPs have exceeded 12,000 reactor years during about 50 years until now, but the number of the accidents that have occurred is not so many.

However some people exaggerate the risks of

1-5

NSRA, Japan

• 1) Based on U.S. population as of 1986

• 2) Mean value from 1953 to 1971

• 3) Mean value from 1901 to 1972

• 4) Individual risk for population of 15 x 10⁶ within about 40 km from a NPP

NSRA, Japan

1-6