It can, therefore, be seen that, if properly designed automatic controls were used, the cascade master demand controller operating inside a safety envelope would have kept ORM above 30 and the positive void coefficient (PVC) influence within safe limits. None of these conditions were met. In addition, the test was conducted under manual control and all automatic safety systems (both the emergency protection system and the emergency core cooling system) were disabled, which is a recipe for disaster.
Design errors also contributed to the disaster. The plant had no containment building. Consequently, only the zirconium cladding and the reactor walls insulated the uranium fuel rods from the outside surroundings. On top of that, an ignitable graphite moderator was used and xenon poisoning increased as the load on the reactor was reduced.
Furthermore, the designers did not understand that once the core starts melting, the zirconium cladding will burn and thereby generate hydrogen as the oxygen in the steam is used up. In addition, they did not understand that the produced hydrogen will not only displace the cooling water (and thereby reduce heat removal), but this extremely hot hydrogen will also quickly rise, increasing the pressure in the vapor space of the reactor. At Chernobyl, as this pressure increased, it lifted the top of the reactor, and as it entered the atmosphere, it formed oxy-hydrogen, initiating a detonation.
The lessons learned at Chernobyl include that (while there is no such thing as a safe nuclear power plant) understanding process dynamics and providing redundant automatic controls to match them can minimize the probability of accidents. To maintain such safe operation, the use of manual must be minimized, and the redundant automatic safety interlocks must not be bypassed. An even more important lesson is that designing a safe control system requires the in-depth understanding of the process by experienced process control engineers, and that safety will not be improved by relying only on the advice of manufacturer’s representatives alone. The designers of Chernobyl did not realize that in designing the plant controls, process control professionals (not salesman) must play a primary role, if nuclear safety is to be improved.
The relative features of nuclear, fossil and solar-hydrogen power plants are tabulated in Table 1.
Definitions of Terms Used:
Clading - Thin-walled metal tube that forms the outer jacket of a nuclear fuel rod
Control Rods – Absorber rods (in this case 211 boron carbide rods were used, 139 manual, 72 automatically controlled), which took 18 seconds for full insertion
ECCS – Emergency Core Cooling System
EPS – Emergency Protection System
Fuel Rods – Zirconium-clad uranium oxide having a concentration of 2% of U235
Graphite Followers – These followers were 1.25 m long and hung on the end of the control rods. When the control rod is raised, they reduce reactivity in the lower part of the core by replacing the water (positive scram effect), while at the top of the core, reactivity is increased by the lifting of the absorber. As the control rod is inserted (right side of figure above) the reactivity at the top of the core is reduced, while at the bottom it is increased as water is displaced by graphite.
ORM - Operating Reactivity Margin - The ratio: (extra reactivity obtained if all control rods are withdrawn divided by the effect on the total reactivity of one rod). In this case, ORM should have exceed 30 and it was 7. In addition, ORM calculation was intermittent, took 15 minutes and was done 150 feet away from the control console.
PDDC – Power Density Distribution Control
PVC - Positive Void Coefficient – Increased replacement of water by steam increases reactivity, which increases temperature that further increases boiling
RBMK - Reactor Bolohoj Moshosztyl Kanalnyj
RCS – Reactor Control System
Reactivity – Portion of nuclear energy that is available to generate steam.
Voiding – Proportion of steam bubbles in the cooling water
VC - Void Coefficient is a measure of the influence of voiding on reactor power generation. The Chernobyl design had a positive VC, meaning that an increase in core temperature (more boiling) increased power generation. Most (but not all!) present reactors are designed with a negative VC, so that the reactor shuts down if core temperature rises (cooling is lost).
VF - Void Fraction The portion of the coolant volume that is made up by steam bubbles. An increase in VF can either increases or decreases core reactivity, depending on the design.
Xenon burn out – Xenon poisoning occurs at low power output when Xenon135 formation inhibits the fission reaction