Fukushima crisis: Anatomy of a meltdown - March 13, 2011

This is a great article which offers understanding of Japan's Nuclear Issue - Reproduced from Internet.

Fukushima crisis: Anatomy of a meltdown - March 13, 2011

As Japan struggles with its worst nuclear disaster in decades, I thought it might be useful to provide a few more technical details about the reactors at Fukushima, and the most likely scenario for what is happening there.

The plant
The Fukushima Daiichi power station operates six boiling water reactors all completed during the 1970s. Details of the reactors vary but the concept is the same: the core consists of a pill-shaped pressure vessel filled with several hundred fuel assemblies. Each fuel assembly is in turn filled with about a hundred fuel rods. A fuel rod is a long, narrow tube of zirconium alloy filled with pellets of uranium which has been enriched to around 3-5% of the energy-producing isotope U-235. (In the case of unit 3, Plutonium-239 is also an active part of the fuel).
When enough fuel is brought together at the core, a chain reaction begins that generates heat, and ultimately power. The core of a modern reactor can hum along for a year or more before the fuel needs to be changed.

The accident
The key to the crisis is water. In addition to the uranium fuel rods, the fuel assemblies have channels which carry highly purified water between the fuel. The water acts as both a moderator for the nuclear reactions and a coolant for the reactor core. On top of it all, it makes the electricity: as it is heated by the reactor, it turns into steam that drives the power turbines. Once the water passes through the turbines it is cooled and re-injected into the core to do it all again.
It all goes great unless the water stops flowing, and that's exactly what it appears has happened in the wake of a massive magnitude 9.0 earthquake that shook the region on 11 March. Diesel generators designed to keep feeding water to Fukushima Unit 1 apparently shutdown about an hour after the quake. Yesterday, the water supply to Unit 3 was interrupted. In both cases, the cores began to heat up.

Meltdown

Immediately after the earthquake, the Fukushima reactors, and many others, went into an automatic shutdown mode. Special rods of neutron-absorbing material, known as control rods, were inserted between the fuel assemblies, halting the power-producing nuclear reactions. But power-producing reactions are not the only ones happening at the core: as nuclear fuel burns it creates new elements that themselves generate a great deal of heat through their radioactive decay. A small but significant amount of the core's heat is generated by these elements, and there is no way to turn them off.

So, without emergency cooling, the temperature at the core of both reactors began to rise. As it did, what water that remained began to boil off, increasing the pressure inside the pellet-shape pod.

When temperatures reached around a thousand degrees Celsius, the zirconium alloy holding the fuel pellets probably began to melt or split apart. As it did, it reacted with the steam and created hydrogen gas, which is highly volatile.

Operators may or may not have known what was happening when they decided to release some of the pressure from Unit 1 on Saturday. The hydrogen apparently caused a massive explosion which blew the roof off of the fuel hall, though the reactor's primary containment vessel appears to have remained intact (see diagram, from NEI).

If, as it appears, the zirconium came apart, then some of the uranium and plutonium pellets in the fuel rods may have become loose or melted and sunk to the bottom of the pressure vessel. In that case, the cores of units 1 and 3 are now a volatile test tube filled with radioactive fuel, melted zirconium and water.

The real danger is the fuel. If enough fuel gathers at the bottom of the reactor, it could burn through the concrete containment vessel. In a worse case scenario, the fuel could again gather to form a critical mass outside the fuel assembly. The loose fuel would restart the power-producing reactions, but in a completely uncontrolled way. This, if it happened, would lead to a full-scale nuclear meltdown.

Emergency procedures

To prevent such a catastrophe, plant operators have decided to swamp both units with seawater. The decision is not made easily: the impurities in the seawater will contaminate the reactor cores, effectively ruining them. But it should allow the temperature inside to again drop, preventing further melting of the zirconium rods and the fuel elements. On top of this, the reactors are being filled with boronic boric acid. Boron is an excellent neutron absorber, and it should hamper nuclear reactions, even if fuel pellets are loose inside the core.

The reactors are also venting excess steam, reducing the pressure inside.

What's next?
It is very difficult to say. In the best case scenario, the fuel will be sufficiently cooled to stabilize the situation. But its important to understand that there's no way to “shut off” the residual heat inside these reactors. Unless the fuel can be moved, which seems unlikely for now, they will need to be actively cooled for weeks in order to prevent a crisis. (Although the half-life radionuclides in the fuel mean that cooling will become less urgent with time). Even after the immediate crisis is past, decommissioning the reactors could take decades.

Nuclear Power Plant - Potential Disastor - Risk Assessment?

Japan Nuclear Potential Disastor - Knowing the fact that Japan is prone to Tsunami and Earthquake - I doubt the Layout Analysis / Plant Sitting Workshop / HAZID / Risk Assessment were undertaken adequately? If yes, how was "Natural Disastor" risk was taken into consideration and analysed? It sad to learn that Nuclear Power Plant with well known threats are sited facing the open sea which led to distruction of main utility system - cooling plant. Would like to know more about the Risk Assesment performed for Japan's Nuclear Power Plant? Any COMAH reports or at least Safety Cases?

Can "Partial Stroking" Facility for ESDV taken credit in PFD calculations for SIL Verification?

I penned down some of my thoughts based on my experiences in SIL studies and verification.

1)      The Norwegian Guideline - Application of IEC 61508 and IEC 61511 In the Norwegian Petroleum Industry Rev 02, Dated 29th October 2004,  Page 46, clearly states the proof testing requirements. (i.e. whereby partial stroking is acceptable practice to ensure reliability). This section also clearly indicates what should adopted as a procedure if partial stroking is applied on the facility.

Cut the long story short, if an integral test is not possible due to safety or operational reasons, a non-integral (partial) test may be performed for each sub-system comprising the SIS loop. Some sub-systems may be tested under normal operation by providing inhibit of the input signal or override of an output action. Testing of other sub-systems such as e.g. valves often causes process shutdown and may therefore be performed during planned shutdown periods.

“It should be noted that although partial function testing reduces the need to fully test the SIS loop, a complete integral test should still be performed at certain intervals”.

 In standard practice, “Planned Shutdown Period” will be used to test all SCEs and this needs to be mentioned in Performance Standard. 

2)      GS-371 (i.e. TOTAL Specification) does recognize ESDV partial stroking which is quite norm in O&G facility. Once again based on my experience - Often PERIODICAL SHUTDOWN is utilized for full FUNTIONAL TEST and during normal production we live with PARTIAL STROKING.

3)      Last but not the least, for SIL verification purpose, in line with GS-371, IEC 61508/61511 “partial stroking” can be taken into consideration for PFD calculation.

Donut Personal Control Descent Device vs Scramble Nets

Despite Piper Alpha tragedy in 1988 which claimed  the lives of 160+ persons, Scramble Nets are still largely being used in Offshore Oil and Gas facilities throughout the world.

Whilst, Scramble Nets has been considered as tertiary means of escape to sea, DONUT Personal Control Descent Device is a better alternative and certainly would improve Evacuation, Escape and Rescue during unwanted incidents in Oil and Gas facilities.

Below are some discussions on DONUT system compared to conventional methods of Evacuation.

Conventional methods of Secondary/Tertiary Evacuation:

• Scramble nets are awkward and difficult to use and require periodic inspection.
• Knotted ropes are both physically and mentally very demanding to use, especially if the evacuee is wearing a Survival Suit or Life Jacket
• Chutes are fixed entity, which may be rendered inoperative if the cause of the emergency is in the area where the chute is situated.

Advantages of DONUT over conventional methods of Evacuation:

• DONUT is both lightweight and portable. It can be deployed from any safe area on the Platform or Installation
• It can be used with any other combination of Survival equipment found onboard the Platform
• Users do not have to wait their turn to use the equipment
• It can be used during an escape to Liferaft, where a strop is attached to the Liferafts Painter Line allowing a guided descent to the Liferaft, thus preventing the risk of being washed once in the water.
• The user can lock off the DONUT device above the water, awaiting Rescue remaining visible and more importantly dry.
• The DONUT is fitted with a non-spark chemical Lightstick allowing the user to illuminate their descent or attract attention.

Process Safety - Asset Integrity

"Nearly four years after the disaster in Texas City, there continues to be a disturbing number of fires, explosions and releases at the nation's refineries. These events endanger workers and public and can distrupt the supply of needed transportation fuels". A quote extracted from Chairman John Bresland.

"A sudden release of flammable liquid from a flare or blow down stack poses a potential risk to people, equipment and the environment and warrants a close look"

The above extracts are related to Fire at the Tesoro refinery in Salt Lake City, Utah, following a power outage earlier in the day.

Liquid hydrocarbons were released froma flare stack during an effort to restart the refinery's crude unit. The hydrocarbons were ignited in a pool fire that extended from the base of the stack and damaged a trailer and other equipment that were positioned nearby.

Inquiry to seek to determine if there are any similiarities to the 2005 accident at the BP Texas City refinery are being conducted/investigated by CSB.

All the investigation is currently on its way, nonetheless the following issues needs a particular attention;

1) Management of Change - Was a proper Risk Assessment done for the restart. If yes was it done with proper attendees? Competency? Did they highlight all possible scenarios?

2) API 752 application. Temp and Permanent Occupied Buildings. What we have learned from Texas City? Are repeating the same mistakes?

3) Process Safety and Assest Integrity Issue. Safety Critical Elements and Performance Standards? Was this properly identified and managed?

4) The list goes on.......

Regards
Siva

Thought of the day

The end of education is character - Baba

Ignition sources of explosion - Electromagnectic Waves emiited from Telecommunication Tower/Antenna

Besides explosive atmosphere made of air (oxygen) and flammable matter in the form of dust, gas or mist, also ignition source is necessary for the explosion. It is the third apex of the explosion triangle (combustible matter, oxygen, ignition source). Ignition source is, generally said, such effect, which produces energy to its surroundings and this energy is sufficient to ignite the explosive atmosphere in which this source occurs. Energy necessary for the ignition is depending on the type of the explosive atmosphere and characteristics of the dust or gas. Generally can be said, that for ignition of gaseous atmosphere only 10-1000 times lower energy is sufficient, comparing to dust atmosphere.

Despite of the fact, that there can be unlimited reasons for forming the ignition source (and telling them after the massive explosion is the most complicated part of the investigation), there is only limited number of ignition sources and many of them is even not sufficient for the ignition of majority of the matters. The complete list of the ignition sources with the information if the source is or is not relevant in normal conditions for gases or dust is in the following table.

ignition source	common reason	relevant for dusts	relevant for gases and vapours
hot surface	friction of the rotating parts, failures of the devices working with high temperature, damages of heat insulations, processing of the materials, failures of brakes, barings, rotating mechanisms	YES	YES
flames, glowing particles, sparks	furnaces, burning chambers, devices for drying, devices working with open fire,...	YES	YES
mechanical sparks	friction, impacts, abrassive processes (grinding, polishing), foreign objects in the technology, cutting, breakes, bearing, rotating mechanisms,...	YES	YES
electric devices (discharge sparks, hot surfaces)	switching on and off the electric circuits, releasing of the contacts and connections, electric arc and short connections, heating, damaged lightings, bulbs,...	YES	YES
stray electric current	reverse current, damaged insulations of high voltage cables and devices, failures in electric installations, magnetic induction, back current circuit failures,...	rarely (depending on the type of the dust and energy of the ignition source)	probably yes
static energy	failures of grounding, fast separation processes, friction	rarely (only the most sensitive dusts)	almost always
lightning	lightning itself, heating of the lightning conductor, static energy induction in the close surrounding of the lightning	YES	YES
high-frequency electromagnetic waves	TV and radio broadcasting transmitter failurs, measuring, army and medical devices failure, high frequency generators (drying, hardening, welding, cutting),...	rarely	sometimes (depending on the energy of the source)
electromagnetic waves	concentrated rays and waves (lenses, reflectors), lasers (including measuring devices), strong sources of radiation,...	yes, but depending on the energy	YES
ionizing radiation	X-rays, radioactive materials, chemical reaction caused by radiation (disruptive exotermic reactions), microwaves,...	yes, but depending on the energy	yes, but depending on the energy
ultrasound	absorption of ultrasound waves	depending on energy, but in normal conditions unrealistic	rarely
adiabatic compression and shock waves	heat produced during fast and strong adiabatic compressions, heat caused by pressure shock wave going through the pipeline	no, unrealistic in normal conditions	rarely
exotermic reactions (including autogenous ignition)	chemical reactions, flammable matters, biochemical processes, polymeration reactions,...	YES	YES

This table should never be understood as definitive and for all kinds of explosive atmospheres and conditions, because minimum ignition energy may vary in great range. Also these ignition sources can reach various energies depending on their origin (for example electromagnetic waves). If there is "rarely" or "depending on the conditions" in the table, it means during normal circumstances and in commonly used devices. Always keep in mind that theoretically any of these ignition sources may ignite any type of explosive atmosphere, during certain circumstances, so no ignition source can be fully ignored if there is at least theoretical chance of its appearance.