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Re: Nuclear reactors



Larry Woolf wrote:

William Whittemore, one of the developers of the TRIGA reactor, has provided
the following answers to Ludwik's questions:

"There are more facts involved in the simple explanation than alluded to by
the questioner. All the statements in the Web article are correct but they
do not stand alone. At least three important, fundamental nuclear facts
underlie the presentation.

(1) The fission cross section (i.e., the probability of fissioning) caused
by low energy neutrons DECREASES as the energy of the thermal neutrons
increases.

TRUE, EXCEPT IN THE REGION OF RESONANCES.

(2) In an alloy such as the uranium-zirconium hydride fuel of a TRIGA
reactor, the low energy neutrons rattling around are in thermal equilibrium
with the fuel matrix at the temperature of the uranium fuel element. This
assures that the average energy of these low energy thermal neutrons will
INCREASE as the fuel temperature rises, the basis fo the so-called "warm
neutron" effect.

I MISSED THIS IMPORTANT POINT WHILE READING
THE ORIGINAL TEXT.

(3) The high energy neutrons from uranium fission moderate (lose energy)
from about one million ev to about 1 ev over a physical space of a few
centimeters. Once the neutron energy is around 1 ev., further energy
moderation takes place predomonantly in the zirconium hydride in the fuel
matrix and not in the surrounding water. This is because there is at least
twice as much hydrogen in the fuel element as there is in the water
surrounding each fuel element.

ARE FUEL CYLINDERS SIGNIFICANTELY LARGER THAN IN
A COMMON POWER REACTOR? I SUSPECT SO.

With the above three facts, we can clarify the statements that have been
questioned.

Why does the water-cooled and water-moderated reactor undergo a catastrophic
accident ressulting in fuel meltdown when all control rods are suddenly
removed?

Following the logic of the three, fundamental facts stated above, we see
that the complete energy moderation of the neutrons takes place in the water
whose temperature does not rise much above 100 degrees C until the steam
explosion occurs and removes the water moderator. During this time before
the explosion, the average energy of the neutrons in equilibrium with the
water does not rise significantly (i.e., the neutrons are relatively "cold"
and remain "cold") and the probability of further fission continues to
remain high. With this continued high rate of fissioning, the heat from the
fission in the fuel element raises the fuel temperature rapidly until the
fuel actually melts, causing the water simultaneously to evaporate
explosively. Only then with the loss of water (and its hydrogen) does the
fissioning of uranium decrease as the thermal neutrons disappear.

In contrast to the above situation, let us follow the logic with the TRIGA
fuel and the sudden removal of the control rods. Since two-thirds of the the
final moderation of the neutrons takes place inside the fuel elements, the
rising fuel temperature assures that most of the low energy neutrons will
gain energy as stated in fact (2) above. As stated in fact (1), the rate of
fissioning will rapidly DECREASE as the neutrons gain energy. The time scale
for these effects is milliseconds. Long before the fuel temperature can
reach dangerously high levels, the rate of fission (hence, power production)
has decreased to tiny values. Thus, the TRIGA reactor is inherently safe
because of the "warm neutron" effect.

THIS EXPLANATION MAKES SENSE TO ME.

Two other questions were posed.

Can power reactors be provided with the "warm neutron" protection? The
answer in principle is yes but the fuel cost would be prohibitively high and
the efficiency of electricity production would be very inefficient. The
manufacturing proceedure for the uranium-zirconium hydride is far more
complicated than that for the present power reactor fuel.
Power reactors with very special purposes with power levels up to 100 MW
have been designed using the TRIGA fuel principle.

THANKS FOR CLARIFYING THIS.

Is the 22,000 MW power production of a pulsed TRIGA present for only a short
time? The answer is yes. This pulse lasts for only one or two milliseconds,
but can be repeated several times per hour as desired."

THANKS AGAIN, LUDWIK KOWALSKI

****************************************

Larry Woolf;General Atomics;6995 Flanders Dr.;MS 78-107;San Diego CA
92121-2975; Ph:858-526-8575;FAX:858-526-8568; www.ga.com; www.sci-ed-ga.org

-----Original Message-----
From: Ludwik Kowalski
Sent: Saturday, August 17, 2002 2:06 PM

Subject: Re: Nuclear reactors

Larry Woolf wrote:

Some relevant information about inherently safe nuclear reactors.

http://www.ga.com/triga/frtyears.html

Thanks for sharing. To distinguish my comments and questions
from what is quoted I will use upper cases.

1) "TRIGA, the most widely used research reactor in the world"

RESEARCH REACTORS ARE DIFFERENT FROM POWER
REACTORS. WHEN THEY REFER TO A 22,000 MW REACTROR
THEY PROBABLY MEAN THE POWER DURING A SHORT
PULSE, NOT THE AVERAGE POWER OVER MANY DAYS
OR WEEKS. CAN A 1000 MW POWER REACTOR BE MADE
"INHERENTLY SAFE" USING THE TRIGA APPROACH?

I HAVE TROUBLE WITH WHAT FOLLOWS BELOW.

2) "In meeting this challenge [of extreme safety], the idea of the
"warm neutron principle" was introduced as a first step towards
the design of an inherently safe reactor. In a water-cooled reactor,
the general result from suddenly removing the control rods is a
catastrophic accident, leading to a melting of the fuel."
SO FAR ALL IS CLEAR

"This is because the neutrons from the fission reaction remain
"cold" from interacting with the cold water around the fuel and
maintain their ability to cause further fissioning of uranium
atoms in the fuel."

WHY IS THE SUDDEN REMOVAL OF CONTROL RODS
CAUSING NEUTRONS TO "REMAIN COLD?" IT WOULD
BE BETTER TO SAY THAT NEUTRONS WHICH WOULD
BE ABSORBED IN CONTROL RODS ARE NOW ABLE TO
CAUSE FISSION? BUT THAT IS ONLY A SIDE COMMENT.

" This in turn results in the temperature of the fuel continuing
to increase rapidly until it finally melts." OK SO FAR.

" However TRIGA is no ordinary light water reactor because
much of its "moderation" of neutrons is due to the hydrogen
that is mixed in with the fuel itself. Therefore, as the fuel
temperature increases when the control rods are suddenly
removed, the neutrons inside the hydrogen-containing fuel rod
become warmer than the neutrons outside in the cold water."

FISSION NEUTRONS DO NOT ":BECOME" WARMER,
THEIR ENERGIES CAN ONLY GO DOWN AFTER
COLLIDING WITH HYDROGEN ATOMS. THE "BECOME
WARMER BECAUSE" MAKES NO SENSE TO ME.

"These warmer neutrons inside the fuel cause less fissioning in
the fuel and escape into the surrounding water."

LESS THAN WHAT? IS IT NOT TRUE THAT NEUTRONS
IN THE RODS WOULD HAVE HIGHER ENERGIES IF
HYDROGEN WAS NOT PRESET? NEUTRONS OF HIGHER
ENERGIES ARE LESS ABLE TO CAUSE FISSION (ON THE
AVERAGE) THAN WHEN HYDROGEN IS PRESENT. IN
OTHER WORDS, CONTRARY TO WHAT THEY SAY,
PRESENCE OF HYDROGEN MAKES NEUTRONS MORE
FISSIONING, NOT LESS FISSIONING.

" The end result is that the reactor automatically reduces power
within a few thousandths of a second, faster than any engineered
device can operate. In other words, the fuel rods themselves act
as an automatic power regulator, shutting the reactor down
without engineered devices."

I BELIEVE THEM BUT I DO NOT UNDERSTAND THEIR
EXPLANATION. AM I THE ONLY ONE TO WHOM THIS
TEXT SEEMS TO BE AN EMPTY TALK BY A PUBLIC
RELATION PERSON WHO IS NOT A PHYSICIST?
Ludwik Kowalski