为什么边界条件会变成零

In[21]:= s =
NDSolve[{D[u[t, x], t] == 35/(7800*460)*D[u[t, x], x, x], u[0, x] == 200, D[u[t, 0], x] == -100/35*(u[t, 0] - 1000), D[u[t, 100000], x] == -100/35*(u[t, 0] - 1000)}, u, {t, 0, 10000}, {x, 0, 100000}]