jimmyy said:
Hi,
Which are the major falws of an RBMK reactor.
I have found three:
1. fixed graphite moderator that doesn't act as the LWR reactor to slow down the chain fissions when the temprature in the core gets to hot
jimmy,
You are wrong about the above. As it heats up, graphite ALSO expands and lowers the reactivity just
as water does. The graphite may not be in direct contact with the fuel so that it doesn't respond as fast
as the coolant does - but just because the RBMK had a fixed graphite moderator is NOT in and of itself
a design defect. The graphite moderator at Chernobyl DID respond - but in the wrong direction.
The problem with the RBMK is that it was "over moderated". The RBMK is actually a scaled up version
of a Soviet nuclear weapons production reactor. The RBMK was meant to provide both power and more
nuclear weapons material.
Essentially an RBMK is 8 of these production reactors stacked in a 2 X 2 X 2 cube. The problem is that
the Soviet engineers didn't redesign the fuel. Because of its larger size, the RBMK has less neutron
leakage and therefore requires less moderation.
You want a reactor to be "under moderated". That way if you lose moderation - the reactivity of the
reactor will go down - as will the power. In the RBMK, it was "over moderated" - it had more than
enough graphite moderator to do the job - and the excess actually interferes with the reaction. In such
a reactor, when the graphite heats up, and the effective moderation goes down - you are only getting
rid of excess moderator - and reactivity goes UP, not down; and the power also goes UP.
The design flaw is NOT that the reactor uses fixed graphite as a moderator. The USA's Hanford
"N Reactor" was a large fixed graphite moderator reactor that provided both power and nuclear
weapons material. However, the N Reactor was properly designed and was stable.
2. the missing containment building ,or inapropiate one . Can someone explain what wasthe issue of the RBMK containment building, cause on wikipedia it is said that the RBMK has a caontainment building , and the documentaries over Chernobyl that I've seen show a containment building with a seeiling of many hundreds -thousand tones of concrete
Many of the anti-nukes like to claim that Chernobyl had a containment building. On a recent radio program
on KGO Radio 810 in San Francisco; the guest Harvey Wasserman claimed that the Chernobyl RBMK
had a containment building which failed.
A true containment building has thick walls of concrete. Here is a graphic of the RBMK courtesy of the
International Nuclear Safety Center [ INSC ] at Argonne National Laboratory:
http://www.insc.anl.gov/sov_des/rbmk.php
See the big thick concrete wall [ speckled yellow ] to the right of the number "8" denoting the refueling
machine. Follow that up. You see it STOPS just above the bridge crane. The whole top of the
reactor building is just a regular industrial building. A containment building has thick concrete walls
that completely surround the reactor.
3. the control rods had a graphite head, which were increasing the reactivity in the first miliseconds when the control rods were introduced.
Yes - that is another design defect.
However, the NUMBER ONE reason that Chernobyl had an accident - is that they conducted their
experiment in the middle of a Xenon Transient.
When you shutdown or lower the power of a nuclear reactor; it undergoes a temporary transient
build up of Xenon-135. Xenon-135 is an EXTREMELY potent neutron absorber.
The operators had lowered the power of the reactor in preparation for an experiment that they were
going to run. The load controller in Kiev called and asked for the plant to remain online because the
power was needed. It was about 12 hours later that the load controller released the plant from the grid.
That time delay allowed the buildup of Xenon-135 that those who planned the experiment hadn't
anticipated. The reactor was very UNSTABLE in the Xenon-poisoned state at low power.
The operators had to BYPASS the safety features of the reactor and withdraw more control rods
than normally allowed in order to keep the reactor going.
With the reactor in this VERY UNSTABLE condition - they should have canceled their experiment.
However, they pressed on under UNPLANNED FOR conditions - and the result is history.
Dr. Gregory Greenman
Physicist
