Nuclear emergency diesels: some current issues1 February 2014
As well as having to meet increasing customer requirements in a post-Fukushima world, emergency diesel generator suppliers face challenges relating to fuel supply, emissions and control systems.
In terms of specific Fukushima-related repercussions for recent emergency diesel generator specifications, the impact to date has been more on building design and location of the engines, and their related supply tanks, than on the basic design of the diesel generator itself. For example we have had requests for water tight buildings up to a certain water level with the possibility for external fuel supplies to be connected above this water level.
We have also seen increasing severity in specifications deriving from the post- Fukushima stress tests carried out at nuclear power plants all over the world. Examples include the following:
- Specification of ever more severe
- environmental conditions, eg in terms of climatic conditions. There are examples where emergency diesels have to be able to work with an intake air temperature range of up to 100°C.
- More detailed specification of environmental conditions to be designed against (eg, tornado characteristics).
- Specification of increasingly severe seismic conditions.
This tendency to stipulate more and more severe specifications leads to a routine over-dimensioning of equipment in terms of amounts of materials used, footprint and performance. It also entails the introduction of new systems operated by new control systems (eg, air intake heaters/coolers, fuel oil coolers) that have the negative effect of making emergency diesel systems more and more complex.
Other current challenges relate to fuel supply, emissions regulations and control systems.
The following trends in fuel supply for emergency diesels have been observed worldwide: reduction in sulphur content (to not more than 0.1% in Western countries) and an increase in percentage of fatty acid methyl ester (FAME); requirements for flexibility of fuel supply, such as the possibility of using aviation blends; widening of the specified range of ambient conditions: eg, -50°C to +50°C.
The sulphur content of diesel fuel has been reducing in recent years, but sulphur compounds act as natural lubricants and thus a decrease in their content reduces the natural lubricity of the fuel. This can lead to problems with reliability of injection equipment, for example. One possible countermeasure is the introduction of additives and/or FAME. In France, for example the percentage of FAME incorporated into automotive diesel fuel has risen to as much as 7%.
However, this comes with its own consequences, including: hydrophilic properties; microbiological growth; and questionable stability under long-term storage conditions, with no oil supplier today ready to guarantee stability within storage tanks for more than six months (although stability can be improved with further additives).
Another trend, the use of aviation fuel blends in emergency diesels, also comes with various consequences. Generally speaking, emergency diesels use fuels corresponding to marine distillate fuels specified in ISO 8217. Aviation blends can have less lubricity, leading to equipment reliability problems. Second, they may be lower quality in terms of cetane index, affecting start-up time, load pick-up and cold-start ability. They can also have a lower flash point (the lowest temperature at which a volatile substance can vaporise to form an ignitable mixture in air). This can impact the equipment specification and its compliance with Europe's ATEX directive, which regulates equipment and work in an environment with an explosive atmosphere.
The significantly wider range of specified ambient temperature conditions that one encounters today (eg, -50°C to +50°C as already noted) also impacts fuel. Factors that must be considered include the cetane index for cold conditions, reflecting the fact that engines are more difficult to start in cold weather as gelling/freezing can occur. In contrast, high temperatures increase the potential for low fuel viscosity, which can affect injection equipment reliability. Potential solutions could include engineering measures, and, again, additives.
Trends in fuel supply are driven by two conflicting factors: the oil market, which is itself driven by the automotive sector; and safety requirements in the nuclear industry relating to the operation of emergency diesel generators, which have to be guaranteed over the lifetime of a nuclear power plant.
A potential solution could be to create a specific worldwide standard for fuel for nuclear power plant emergency diesel applications (with a precise fuel specification, as in the automotive industry). However, this would require the standards to be ratified and imposed by nuclear safety authorities worldwide.
Some environmental regulators around the world aspire to limit gaseous emissions from all diesel engines, including those of emergency diesel gensets, with less than 500 running hours per year. In certain countries, it remains unclear if emergency diesels in nuclear applications should be subject to such limitations. One clear exception is the USA, where the Environmental Protection Agency (EPA) has decided to impose less severe emission limits for engines used in stationary emergency applications when they run less than a certain number of hours per year.
Elsewhere it is often not clear which authority prevails: the nuclear safety body or the environmental regulators.
Emergency diesel gensets for nuclear applications have only two potential operating scenarios: periodic testing; or use in an emergency.
Periodic testing of emergency diesel functionalities such as start-up is typically conducted every 30 days. Cumulative running hours are typically less than 40 hours per year for each emergency diesel genset, and operation duration is very seldom longer than two hours. Furthermore, when they do operate, most of the time the emergency diesels are running at idle or at very low output. Emergency diesel gensets used for nuclear applications are typically only tested about once a year at rated output. Therefore the annual average emissions from a nuclear emergency diesel generator are minimal and fitting of external emissions control technologies is inappropriate, and potentially counterproductive. Indeed, the complexity of emissions control systems could threaten the overall reliability of the emergency diesel, while their start-up speed cannot match that of the emergency genset, rendering them totally ineffective or, at best, seriously degrading performance.
In the other operating scenario, application in an emergency, the safety of the nuclear facility is of course paramount; nobody will care about the exhaust gas emissions of the generating sets during a nuclear emergency.
This raises the question of whether there should be exemptions from environmental regulations. As already noted, the USA is an example of a country that does this. Shouldn't all countries have similar exemptions?
The functions of the control systems for emergency diesel generators include the following: engine speed control; power control; sequence management (for example, start/stop); management of mechanical and electrical genset safety protection systems; and interfaces with operators and the main nuclear plant control system. These control functions are fulfilled by equipment that is integrated with the emergency diesels (such as actuators, sensors, cables etc) or by equipment in control cabinets (such as regulators, relays or safety-dedicated devices).
Control system requirements can be split into three categories:
- quality, including highly demanding
- requirements for design document management, traceability and validation throughout the development and manufacturing of each module, testing and validation procedures and reports, and also selection of qualified and validated suppliers;
- reliability and availability, necessitating use of well-proven equipment, with qualification of hardware and software, redundancies in mechanical/electronic speed control, two-out-of-three logic for management of safety protection systems and physical separation of classified and non-classified function; and
- performance, eg, response time and stability.
Perhaps the key challenge in the area of control systems, faced with the rigorous requirements of nuclear emergency applications, is keeping it simple.
Authors: Laurent Tessier and Eric Huet, MAN Diesel & Turbo, France
Further reading: Nuclear Engineering Internatonal, Sept 2013 and TüV conference, Emergency Power Systems at Nuclear Power Plants, Munich, 11-12 April, 2013.
(Originally published in MPS February 2014)