in
ordinary water there is some heavy water. I forget the exact figures,
but I think that, by weight, there are five thousand parts of ordinary
hydrogen in water and only one part of deuterium."

"That's right." Steve Ames nodded. "Five thousand to one. Now tell us
what is peculiar about all isotopes?"

Rick thought furiously and came up with what he hoped was the answer. "I
think it's that isotopes aren't as stable as the basic elements. Some
are pretty stable, but some are pretty shaky. That's why some of the
isotopes of uranium can be split wide open in a chain reaction to make
an atomic bomb, and ..."

A chill ran through him. His mouth opened. He knew! He knew why heavy
water had Steve Ames all excited. He choked:

"Hydrogen bombs!"

Scotty and Barby gasped. Steve Ames and Hartson Brant smiled.

"It's true that one of the possibilities in building a hydrogen bomb
concerns deuterium," the scientist said. "But I scarcely think that's
the case here. How about it, Steve?"

"Possible, but extremely improbable," Steve agreed. "What I'm most
interested in is a use for heavy water Rick hasn't mentioned. Know what
a nuclear reactor is, Rick?"

Rick nodded. "It's what the newspapers usually call an 'atomic pile.' We
have quite a few in this country, I think. The Atomic Energy Commission
said quite a while ago that they used a nuclear reactor with uranium as
a fuel to make plutonium, which is the artificial element that can be
used in atomic bombs. Besides uranium itself, that is."

"That's right. What I'm interested in is the fact that heavy water can
be used as a neutron moderator in a reactor."

Rick looked blank. Steve was talking way over his head. Hartson Brant
saw his son's bewilderment and explained: "You've probably heard that
the uranium in a reactor is encased in blocks of graphite, which is
simply carbon, Rick. It prevents the neutrons from the uranium from
simply running wild. Well, heavy water can be used for the same
purpose."

"Exactly," Steve said. "So you see, I'm not afraid of the possibility of
hydrogen bombs as much as I am of the possibility that somewhere in Asia
is a nuclear reactor. Until we get international agreement on atomic
weapons, we simply have to keep track of atomic developments everywhere
for our own protection. If there's a new country going in for atomic
research, and it can build a reactor, it might also be able to build an
atomic bomb. Now, don't forget I said heavy water