Are Indian nuclear power plants earthquake resistant?

Date:26/07/2007 URL: http://www.thehindu.com/thehindu/seta
/2007/07/26/stories/2007072650021500.htm



Are Indian nuclear power plants earthquake-resistant?

Indian nuclear power plant design follows internationally accepted seismic safety criteria

The plants at Kakrapar, Narora, and Rawatbhata operated normally during the Bhuj quake

Nuclear power plants are the most seismically hardened structures in the country

Earthquake resistance of nuclear power plants (NPPs) received well deserved attention worldwide because the epicentre of the recent (July 16, 2007) earthquake in Japan was 19 km from the seven-unit Kashiwazaki-Kariwa nuclear power station. The units which were operating (3, 4 and 7) shut down automatically as per the design intent.

The media ‘storm’ rightly covered the delay in releasing the information and some inaccuracies in the reports on the minuscule releases of radioactivity and other dramatic events (waste drums toppled, exhaust ducts displaced etc.) which in reality had a limited impact on safety.

Two reassuring facts

However, nobody noticed two reassuring facts: the plant creditably withstood the earthquake which had twice the power of its design basis; the reactor, turbine building structures or major components did not suffer any damage.

The nuclear power plants at Kakrapar, Narora, and Rawatbhata operated normally during the Bhuj earthquake (6.9 on the Richter scale) on January 26, 2001.These plants experienced levels of vibration much below those for which they have been designed (Warudkar, NuPower, 2001).

The public may have concerns about Tarapur Unit 1&2 which were designed as per earlier standards. Specialists re-evaluated the seismic safety of these reactors and remedied the shortfalls by following the practices and guidelines in the Safety Report Series No 28 titled ‘Seismic Evaluation of Existing Nuclear Power Plants’ of the International Atomic Energy Agency (IAEA, 2003). Specialists are re-evaluating the seismic safety of other older reactors

Safety review is a continuous process. IAEA recommends reassessment if there is any evidence of higher seismic hazard than considered for the design of the plant. Re-evaluation will cover systems, structures and components required for safe shutdown, for maintenance of the plant in safe shutdown state, for removal of decay heat generated and for confinement of radioactive materials. The designers of Indian nuclear power plants follow internationally accepted seismic safety criteria and guidelines.

The plants shall withstand maximum credible earthquakes at their sites. The designers followed seismic safety principles even when a high intensity event has a low probability of occurrence.

Seismic sensors

The Nuclear Power Corporation of India Limited (NPCIL) has installed seismic sensors at all plants as stipulated by the Atomic Energy Regulatory Board (AERB)

The Earth is made up of a mosaic of tectonic plates which move constantly. They may collide with each other. Strains develop gradually over a large mass of rock at the plate boundaries (Bhardwaj, NuPower, 2001); when accumulated strain exceeds the strength of the rocks, the rock masses rupture suddenly releasing stored energy which gets transmitted as seismic waves. The vibrations generated get attenuated as they travel through the intervening media.

Earthquake vibrations contain a mixture of frequencies. They are rich in the frequency range of 1 to 15 Hz. Above 15 Hz the energy reduces continuously and is very less beyond 33 Hz (Warudkar, NuPower, 2001). The force induced in a structure gets amplified if the excitation frequency is close to its natural frequency.

Earthquake magnitude

The effect of earthquake-induced vibrations depends on the magnitude of the earthquake, the depth of focus, the distance from the epicentre and the strata on which the NPPs stand.

Specialists accept a site for constructing a nuclear power plant after analyzing the seismic inputs from agencies such as the Oil and Natural Gas Commission (ONGC), National Geophysical Research Institute (NGRI), India Meteorological Department, and Atomic Minerals Directorate for Exploration and Research. NPPS shall not be constructed at sites falling above Zone 4. AERB also prohibits construction of NPPs at sites with a fault located within 5 km.

Seismic parameters

The designers estimate the seismic parameters for nuclear power plant structures conservatively. The analysis and design of these structures follow internationally accepted standards.

They subject the designs to the requirements of vigorous quality assurance and safety review at multiple levels (Warudkar, NuPower, 2001)

I know that the nuclear power plants are the most seismically hardened structures in the country. Only the ignorant will call me foolhardy, if I rush to a nuclear power plant to save myself from an imminent earthquake!

K.S. PARATHASARATHY
FORMER SECRETARY, AERB

( ksparth@yahoo.co.uk)

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