This isotope is an alpha emitter with half-life of 138 days;it is produced by neutron bombardment of bismuth. Though it occurs naturally in pitchblende, it is too rare to extract chemically. Po-210 is used for one of the most important components of plutonium A-bomb which is called "initiator." The purpose of this device is to produce 10-20 initial neutrons for the chain reaction, within < 1 us at the peak of Pu core compression during the implosion. The neutrons are produced when Po-210 is fused with berillium (Be) foil in the initiator. This is a very difficult technical problem (that does not exist for projectile U bomb). The designer of the first initiator (called "Abner") for US A-bomb was Hans Bethe. So much secrecy surround this device that it remains closely guarded state secret (unlike many other details of the Manhattan Project) even now, 60 years after the invention. The initiator is not absolutely necessary, even in the implosion (Pu) bomb. It was included in the design very late in the project, against Oppenheimer's objections. Even without "Abner," the probability that the first bomb would have failed to explode within 200 ms was only 1:1000. The initiator is needed primarily to increase the gain and make certain that the bomb explodes.
The design has never been passed to the Soviets; it had to be reinvented in Russia. The countries that make the initiators consider their design one of the top national secrets, because the optimization of this device takes multiple explosion tests and requires massive calculations of implosion dynamics. Modern US initiators are said to be D-D neutron pulse tubes; perhaps. The countries that recently joined the club (Pakistan and India) spent many years (10+) on their initiator designs, which are all of the Po-210/Be kind. Iran produced Po and obtained Be. There are many countries that would pay any money for the initiator. Other countries would do whatever it takes to prevent the design of the initiator to fall into the wrong hands.
The initiator is the size of a walnut.
Poisoning by extremely expensive, difficult to handle Po-210 (that is fairly radioactive, lethal to inhale or injest and tends to disperse against air currents) is unprecedented. I can think of only one reason for such an extravagant choice of poison.
Guess what this Russian ex-spy must have stolen.
The early story of the "initiator" is given by R. Rhodes in "The making of the atomic bomb", Simon & Schuster 1986, pp. 578. http://www.amazon.com/Making-Atomic-Bomb-Richard-Rhodes/dp/0684813785
see also http://nuclearweaponarchive.org/Nwfaq/Nfaq4.html