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Schematic Design of a Fusion (Hydrogen) Bomb

Evolution of Explosion

Assume 10 kt trigger / 20 Mt total yield

1. Detonation of HE

    

2. Implosion and initiation of fission trigger (t = 0)

    

3. t = 1/2 microsecond: ~50 generations, E ~ 10⁸ calories

    

4. t = 1 microsecond: fission stops
   E ~ 10¹³ calories
   T ~ 20 x 10⁶ K
   80% of energy in thermal X-rays
   Pressure P ~ 10 ⁶ atmospheres

    

5. Radiation and plasma compress and heat LiD

    

6. LiD fuses, releases energetic neutrons + ~10Mt

    

7. Neutrons cause fission in U-238 blanket + ~10Mt

    

8. Weapon innards and casing vaporized, forming fireball (still has not moved much)

   	80%	photons
   Yield ~	15%	debris
   	5%	prompt nuclear radiation

    

9. Fireball expands
   At first, v_exp ~ 10⁷ cm/s ~ 100 miles/sec

(Lecture addendum: In class was a mock-up of a MK 12A reentry vehicle with a 335 kt warhead. It was roughly man-sized. Its yield would break down as follows: primary ~20 kt, secondary ~150 kt, casing ~150kt)

Energy From a Single Fission

n + (fissile nucleus) --> (fission fragments) + (2 or 3 n)

* Only this 172 MeV is counted in the explosive "yield" of nuclear weapons

Fusion Reactions

Thermonuclear

		+		-->		+		+	Energy	Probability:
1.	D	+	D	-->	3He	+	n	+	3.2 MeV	Int.
2.	D	+	D	-->	T	+	1H	+	4.0 MeV	Int.
3.	T	+	D	-->	4He	+	n	+	17.6 MeV	High
4.	T	+	T	-->	4He	+	2n	+	11.3 MeV	Low

Need T ~ 20 x 10⁶ K to react

Most energy comes off in neutron kinetic energy

==> can cause fission of U-238 ("fission blanket" concept)

Typically, yield is 50% fission / 50% fusion

Yields

Reactions 1+2+3: 5D (10 amu) yield 24.8 MeV

Recall: 1 fission (235 amu) yields 200 MeV

Fusion = 24.8/10 ~ 3 x 200/235 ~ 3 x fission yield per kg

Catalysis--

⁶Li + n --> ⁴He + T + 4.8 MeV

--> Fusion of 20 g of deuterium yields 1 kt <--

Terrorist Nuclear Bombs

Some problems confronting terrorist organizations wishing to construct a nuclear explosive --

• Assembling a team of technical personnel

   

• Substantial financial costs

   

• Radiation and chemical hazards

   

• Possibility of detection

   

• Acquisition of fissile material

Requirements for Making a Fission Bomb

1. Know nuclear physics of fission

    

2. Have needed data on the physical and chemical properties of weapon materials

    

3. Build technical facilities to fabricate and test devices and components of the chosen design

   All these requirements are now met in any significantly industrialized country

    

4. Obtain the needed fissile material

    

5. Allocate the necessary resources

Thermonuclear Weapons

Use deuterium (D) and tritium (T) - almost as plentiful as water!
The trick is to ignite them...

"Boosted" fission weapons
Add a small amount of D and T; get D-D and D-T fusion

Fusion contributes little to yield, but many fast neutrons n + 238U --> (fission products) + energy Increases efficiency of fission weapon

True fusion weapons

First attempts

Used liquid D ("Mike", 1952)
Experiments, not yet bombs

 

"Improved" method

Catalyze D-T burning with ⁶Li
(Natural Li is ~7.5% ⁶Li

⁶Li + n --> ⁴He + T + 4.8 MeV

Usually use solid LiD; then get:

⁶LiD + n --> ⁴He + T + D + 4.8 MeV

T + D --> ⁴He + n + 17.6 MeV

Net effect is:

⁶LiD + n --> 2 ⁴He + n + 22.4 MeV

Alıntı: http://wug.physics.uiuc.edu/courses/phys180/spring98/lectures/07/slide05.html